EMG Flashcards
While performing an EMG, you are listening to the right deltoid muscle at rest. You hear a regular sound that sounds like raindrops falling onto a tin roof. What is the most likely clinical implication for this muscle?
A Active denervation
B This is normal at rest
C Previous denervation followed by reinnervation
D Radiation plexopathy
A
The “rain drops on a tin roof” sound, when occurring at regular intervals while the muscle is at rest, is classic for fibrillations. Fibrillations indicate active denervation.
A 40 year-old female develops 4 days of progressive lower limb weakness. She has a history of recent upper respiratory tract infection. Bilateral lower limb EMG/NCS reveals normal sural nerve SNAPs (sensory nerve action potentials), but prolonged latency of bilateral superficial fibular nerve SNAPs. The bilateral CMAPs to the EDB and AH (compound muscle action potentials to the extensor digitorum brevis and abductor hallucis) show prolonged latency but normal amplitude. EMG reveals decreased recruitment in the bilateral EDB, AH, tibialis anterior, gastrocnemius, and semimembranosus, but normal activity in rectus femoris and adductor longus. Which of the following additional EMG/NCS findings would most likely be discovered in this patient?
AAll of these findings are equally likely to occur
BPersistent A-waves
CAbsent F-waves
DDecreased temporal dispersion
Answer: C
• This patient presents with Guillain-Barre Syndrome (GBS), also referred to as AIDP (acute inflammatory demyelinating polyradiculopathy). GBS is caused by a recent infection (typically GI or URI) causing your immune system to confuse a foreign protein with a protein on your own myelin. Thus, your body destroys your own peripheral nerve myelin (demyelinating the nerves and prolonging the latency).
• The first EDX finding of GBS is delayed or absent F-waves.
• NCS reveals prolonged latency and sometimes reduced amplitude of SNAPs with sural nerve sparing, as the sural nerve is larger with more myelin than other peripheral sensory nerves, and thus is less affected than the other nerves.
• CMAPs show prolonged latency, decreased conduction velocity (CV), and typically normal amplitude (this is a demyelinating disease primarily, not axonal loss).
• EMG shows decreased recruitment but otherwise there should be NO active denervation usually, but this can occur as a secondary feature of GBS weeks after onset.
• Treatment involves PT and early plasmapheresis/IVIG.
• Abnormal/increased temporal dispersion is also a feature of this disease on NCS, due to the widespread and more uniform demyelination of nerves taking place
• The presence of A-waves indicates that some reinnervation has taken place in the past, and would NOT be a criterion in diagnosing GBS.
During an EMG study, you notice on the screen that with forceful muscle contraction by the patient, there are 2 motor units displayed: one firing at 40 Hz, and the other firing at 50 Hz. Which of the following is the most likely diagnosis?
APolymyositis
BPost-polio syndrome
CDuchenne muscular dystrophy
DMyotonia congenita
Answer: B
Explanation:
• This question describes a decreased recruitment pattern, which can be found in cases of conduction block or axonal loss: essentially neuropathies.
• This is sometimes called a neuropathic recruitment pattern for this reason, as decreased recruitment is generally NOT seen in myopathies (the remaining answer choices).
• Post-polio syndrome is the death of anterior horn cells due to “burning out” over time, leaving the patient with few remaining active motor units, which leads to decreased recruitment and, thus, increased firing rate of existing motor units.
A 34 year-old female long distance runner presents to you with gradual onset bilateral anterior knee pain that she has had for several years. Her pain is worse when descending stairs. She has occasional knee stiffness after prolonged sitting. Knee x-rays demonstrate a shallow lateral patellofemoral contour; you initiate physical therapy involving vastus lateralis stretching and vastus medialis strengthening. After 5 months of physical therapy, kinesiotaping, relative rest, and NSAIDs, the pain is still not improving. What is the next best step?
A Referral to major academic center
B Surgery
C MRI
D Repeat x-rays
Answer: C
Explanation:
• This patient presents with classic patellofemoral pain syndrome (PFPS). This is abnormal patellar tracking due to muscle weakness and imbalances; specifically this involves vastus lateralis and IT band tightness in combination with vastus medialis weakness.
• Treatment involves vastus medialis strengthening with vastus lateralis and IT band stretching, along with hip girdle strengthening for stability.
• Sometimes patellar knee sleeves are used to assist with the proprioception of proper patellar tracking, as is kinesiotaping.
• The patellar grind test is described here, which is a test for PFPS.
• If rest, physical therapy, NSAIDs, and bracing do NOT improve the pain, MRI and surgery may be considered. The purpose of the MRI is to look for cartilage damage, e.g. chondromalacia patella, which is a sequela of PFPS in which the patellofemoral cartilage softens and degenerates due to improper patellar tracking.
• Note: typically this is the muscular pattern of weakness and tightness for PFPS, but it is possible to have a weak vastus lateralis and tight vastus medialis and perform essentially the opposite PT as for typical PFPS.
A 79 year-old female with history of diabetes mellitus (well controlled) presents to your clinic with complaints of right groin pain. She denies trauma. It has developed gradually over the past several months along with low back pain. She denies numbness or tingling, but feels weak in her right leg. On exam, strength is neurologically intact. There is no tenderness to palpation of the anterior, lateral, or posterior right hip. FABERE reproduces her right groin pain, but not back pain. What is the most appropriate next step?
A Corticosteroid injection
B Orthopedics consult
C Hip x-rays
D Physical therap
Answer: D
Explanation:
• This patient presents with classic gradual onset osteoarthritis of the hip.
• True hip pain manifests as groin pain. An aging individual with a history of presumably knee OA leading to total knee replacements and gradual onset groin pain without trauma is most likely indicating “wear and tear” femoroacetabular joint arthritis.
• The first step in suspected OA management is physical therapy. If physical therapy fails to improve function and/or pain, x-rays would be appropriate to determine the extent of femoroacetabular disease.
• A corticosteroid injection into the hip joint could also be considered at that point.
• Orthopedics should be consulted if this becomes a surgical case; i.e. if physical therapy and/or corticosteroid injection fail the patient and the patient has significant osteoarthritic joint disease on x-rays.
Which of the following is the most likely cause of a patient’s lumbosacral plexopathy?
A Pregnancy
B Rapid weight gain
C Diabetes mellitus
D Hypertension
Answer: C
Explanation:
• Of the following choices, diabetes is the most likely to cause lumbosacral plexopathy.
• Rapid weight gain and hypertension are typically not causes of plexopathy.
• Rapid weight gain or even loss can contribute to lateral femoral cutaneous neuropathy, however.
• Pregnancy in the third trimester can cause plexopathy due to pressure from the fetus, but this would not be nearly as common as diabetes.
• In particular, diabetes causes a condition called diabetic lumbosacral radiculoplexopathy.
• Poor blood sugar control in relation to a rapid weight loss (not gain!) causes this condition.
• Remember, plexopathies show variable NCS/EMG findings depending on which nerves are affected.
• Treatment involves blood sugar control and rehabilitation.
During nerve conduction studies, raising the low frequency filter will cause which of the following effects?
A No effect
B Decreased amplitude
C Increased peak latency
D Increased amplitude
Answer: B
Explanation:
• Raising the low frequency filter will decrease the amplitude, as will lowering the high frequency filter.
A patient’s right lower limb EMG/NCS results are as follows. Routine fibular and tibial motor/sensory nerve conduction studies are normal. EMG reveals decreased recruitment and polyphasic potentials in tibialis anterior, extensor hallucis longus, short head of biceps femoris, tensor fascia lata, fibularis longus, and tibialis posterior, but normal activity in vastus medialis, adductor longus, gastrocnemius, and abductor hallucis. Which of the following is the most likely diagnosis?
A L4 radiculopathy without reinnervation
B L4 radiculopathy with reinnervation
C L5 radiculopathy without reinnervation
D L5 radiculopathy with reinnervation
Answer: D
Explanation:
• L5 radiculopathy with reinnervation is the best answer.
• All L5-innervated muscles are affected (with polyphasic potentials, indicating early reinnervation taking place), but all muscles that do NOT share any L5 innervation are spared.
• Please refer to the innervation chart at the beginning of the Upper and Lower Extremity Peripheral Nervous System Diseases chapters for a detailed innervation guide.
During an EMG, you ask the patient to very lightly begin contracting their extensor indicis proprius. Instantly you notice the EMG screen become flooded with many small, short-duration MUAPs. Which of the following is the most likely diagnosis?
A Severe radial neuropathy at the elbow
B PLS
C ALS
D Polymyositis
Answer: D
Explanation:
• MUAPs = Motor Unit Action Potentials
• This question describes a myopathic recruitment pattern, or increased recruitment pattern.
• This is typically seen in myopathies, not neuropathies; thus, polymyositis is correct.
Which of the following is the key NCS/EMG finding, found in radiculopathies, that helps to differentiate a radiculopathy from more peripheral nerve lesions?
A Abnormal compound muscle action potentials
B Normal compound muscle action potentials
C Normal paraspinal resting activity
D Normal sensory nerve action potentials
Answer: D
Explanation:
• Normal SNAPs (sensory nerve action potentials) are the key NCS finding demonstrated in a radiculopathy.
• Because the dorsal root ganglion contains the cell bodies of the sensory neurons of a dermatome of a given limb, and these cells are bipolar neurons residing outside the spinal cord, any nerve injury proximal to these cell bodies (e.g. radiculopathy) will NOT affect the distally directed axons of these bipolar cells (i.e. the projections extending down the arm or leg from the dorsal root ganglion).
• Thus, these unaffected axons will conduct impulses quite normally, and SNAPs will be entirely normal in a pure radiculopathy.
• Paraspinal muscles corresponding to the affected nerve root level will theoretically show EMG changes (increased irritability, possible active denervation).
• CMAPs will be expected to potentially be abnormal, but this would NOT be the key differentiating feature of a radiculopathy.
A 67-year-old male with a history of cervical spondylosis presents with right hand paresthesias. His internist orders an EMG/NCS of his right upper extremity, which reveals absent median nerve SNAP (sensory nerve action potential) and positive sharp waves in the APB (abductor pollicis brevis) muscle during the needle exam. He undergoes surgical treatment. Following surgery, which of the following orthoses is most appropriate?
A Aspen collar
B Thumb spica splint
C Gel shell splint
D Soft cervical collar
Answer: C
Explanation:
• The patient has right hand paresthesias in the setting of severe median neuropathy findings on EMG (absent median nerve SNAP and active denervation of Abductor Pollicis Brevis (Positive Sharp Waves)) which indicates median neuropathy as the likely diagnosis.
• The most appropriate surgical intervention would be carpal tunnel release given the severity of the disease based on electrodiagnostic findings.
• Following carpal tunnel release, a gel shell splint is used; this is a nonarticular brace (doesn’t cross joint meaning it doesn’t restrict ROM) used to help prevent hypertrophy of the surgical scar by maintaining pressure against the healing incision.
• A thumb spica splint limits thumb ROM, and can be used for:
○ DeQuervain tenosynovitis
○ Skier’s thumb: an acute partial or complete rupture of the ulnar collateral ligament (UCL) of the thumb’s MCP joint due to a hyperabduction trauma of the thumb
○ Distal 2/3 scaphoid fractures
○ 1st CMC arthritis
The nerve that provides cutaneous sensation to the medial leg below the knee arises via which of the following ways?
A As a branch of the tibial nerve
B As the continuation of the femoral nerve
C As the continuation of the obturator nerve
D As a branch of the sciatic nerve
Answer: B
Explanation:
• The saphenous nerve is described here, and is the continuation of the femoral nerve after it has innervated all its muscles.
You are reviewing EMG/NCS results of a patient. The results demonstrate decreased proximal and distal amplitude with normal latency. EMG reveals decreased recruitment. Which of the following is the most likely etiology of these findings?
A Axonal loss
B Conduction block
C Demyelination
D Normal variant
Answer: A
Explanation:
• Decreased amplitude throughout an entire nerve’s length suggests axonal loss.
• EMG will show decreased recruitment in these cases.
• Conduction block is seen with proximally reduced amplitude, but stimulating distally beyond the spot of conduction block can demonstrate a normal “repaired” amplitude.
A primary difference between patients with spinal muscular atrophy (SMA) type 2 and patients with SMA type 3 is which of the following?
A Nusinersen is approved for type 2 disease, but not type 3 disease
B Patients with SMA type 3 can walk independently
C Type 2 disease shows long-duration, large amplitude motor unit action potentials (MUAPs)
D A different gene is affected between the two types
Answer: B
Explanation:
• Spinal Muscular Atrophy (all types) is caused by mutations in the SMN1 gene; all types can show a neuropathic EMG pattern (long duration, large amplitude MUAPs).
• Nusinersen is FDA-approved for all types of SMA.
• Patients with SMA 2 can sit but NOT stand independently.
• Patients with SMA 3 can stand and walk independently, at least for some time.
During an upper limb EMG, you notice prolonged SNAP latency to digit 5, dorsal ulnar cutaneous nerve, and medial antebrachial cutaneous nerve (MAC). Which of the following diagnoses do these findings indicate? AUlnar neuropathy at the wrist BMedial cord plexopathy CC8 radiculopathy DUlnar neuropathy at the elbow
Answer: B
Explanation:
• Of the answer choices, only medial cord plexopathy could explain an abnormal MAC.
• Radiculopathy produces normal SNAPs.
The ulnar nerve is formed distally to the medial cord, thus the DUC (dorsal ulnar cutaneous nerve) and digit 5 SNAP could be abnormal in medial cord plexopathy OR ulnar neuropathy at the elbow, but ulnar neuropathy at the elbow would produce a normal MAC (medial antebrachial cutaneous nerve) SNAP.
During a nerve conduction study you realize that the patient’s limb being studied is cold. Which of the following effects will this have on the waveform?
AIncreased amplitude, prolonged latency, increased duration
BIncreased amplitude, prolonged latency, decreased duration
CDecreased amplitude, shortened latency, decreased duration
DDecreased amplitude, prolonged latency, increased duration
Answer: A
Explanation:
• A cold limb will result in increased amplitude, prolonged latency (slow conduction velocity), and increased duration.
• When the limb is cold, channels stay open longer, causing a larger, heftier amplitude, and longer time of depolarizing.
During repetitive nerve stimulation (RNS), which of the following percentage decreases in the compound muscle action potential (CMAP) amplitude indicates that a neuromuscular junction (NMJ) disease is present? A>40 B>30 C>20 D>10
Answer: D
Explanation:
• A greater than 10% decrement in the CMAP amplitude during RNS (repetitive nerve stimulation) is a positive finding for NMJ disease (myasthenia gravis, Lambert-Eaton Syndrome, botulism).
Which of the following muscles receives its innervation from all roots (C5-T1) of the brachial plexus? AFlexor digitorum profundus BPectoralis major CLatissimus dorsi DTriceps brachii
Answer: B
Explanation:
• The pectoralis major is innervated by C5-T1 roots of the brachial plexus.
• Triceps and Latissimus dorsi are innervated by C6, C7, C8 roots.
• FDP (Flexor digitorum profundus) is innervated by C7, C8, T1 roots.
A 62 year-old male with a past medical history of hypertension presents with progressive onset weakness and swallowing difficulties. He is a retired professional football player. He denies numbness/tingling, bowel/bladder dysfunction, or a family history of this problem. EMG/NCS reveals normal sensory nerve action potentials (SNAPs) and compound muscle action potentials (CMAPs) in 3 limbs. Needle exam reveals long-duration, large amplitude motor unit action potentials (MUAPs) with decreased recruitment and 2+ fibrillations in 3 limbs. Which of the following treatments is most likely appropriate? ARiluzole BNusinersen CAntitoxin DCorticosteroids
Answer: A
Explanation:
• This patient’s presentation and EMG findings are classic for ALS (amyotrophic lateral sclerosis). This is the death of anterior horn cells due to a not-yet clearly defined cause.
• It has been associated with a professional athletics history.
• Recall that anterior horn cells are entirely separate from sensory neurons, and, thus, the SNAPs of these patients will be normal.
• CMAPs can be normal vs. abnormal in these patients.
• EMG shows a “neuropathic” pattern of MUAPs as described.
• Riluzole has been shown to prolong survival in ALS patients.
• Rehabilitation with submaximal exercise is recommended.
• ALS carries a poor prognosis, and most patients experience progressive disability and death within a few years of onset.
• Nusinersen is indicated in patients with Spinal Muscular Atrophy.
Median nerve innervated muscles
- Pronator teres
- Palmaris longus
- Flexor carpi radialis
- Flexor digitorum superficialis
- Flexor digitorum profundus (only the lateral half)
- Flexor pollicis longus
- Pronator quadratus
- 1st & 2nd Lumbricals
- Flexor pollicis brevis (superficial head)
- Abductor pollicis brevis
- Opponens pollicis
Ulnar nerve innervated muscles
- Flexor carpi ulnaris
- Flexor digitorum profundus (medial half)
- Palmaris brevis
- 3rd & 4th Lumbricals
- Interossei
- Adductor pollicis - thenar
- Flexor pollicis brevis (deep head) - thenar
- Abductor digiti minimi - hypothenar
- Opponens digiti minimi - hypothenar
- Flexor digiti minimi - hypothenar
- During an EMG, you decide to stimulate the ulnar nerve at the wrist and record the CMAP (compound muscle action potential) over the first dorsal interosseous (FDI), whose amplitude appears normal. You then stimulate the ulnar nerve at the elbow and notice a significantly decreased amplitude compared to the wrist, even when adjusting for the patient’s skin conditions and stimulator placement. You decide to leave the recording electrode in place on the FDI. Which of the following is the next best step?
A Stimulate the anterior interosseous nerve in the forearm
B Stimulate the ulnar nerve above the elbow
C Stimulate the median nerve at the elbow
D Stimulate the ulnar nerve in the forearm, “inching” along to find the location of conduction block
Answer: C
Explanation:
• The Martin-Gruber anastomosis (MGA) involves median nerve fibers in the forearm crossing over and joining the ulnar nerve in the forearm.
• This means essentially that the median nerve innervates the abductor digiti minimi (ADM) and first dorsal interosseous (FDI), which are ulnar nerve muscles, in addition to innervating its own classic median nerve muscles (abductor pollicis brevis - APB, etc.).
• The ulnar nerve CMAP (compound muscle action potential) when stimulating at the elbow will show a low amplitude when recording over a distal ulnar muscle such as FDI. The ulnar CMAP will be “repaired”/normal if you stimulate the ulnar nerve at the wrist. Thus, it appears that there is a conduction block somewhere in the forearm when there actually is NOT (ulnar nerve conduction block in the mid to distal forearm is also very uncommon).
• When you stimulate the ulnar nerve at the wrist, at this point the median nerve fibers have finally joined the ulnar nerve, so you are finally stimulating all the motor axons that are supplying the ulnar nerve hand muscles.
• When you stimulate the ulnar nerve at the elbow, you are NOT stimulating the median nerve fibers that have yet to contribute to the ulnar innervations, thus you only generate part of the full amplitude, which is what makes it look like there is a conduction block in the ulnar nerve, when in reality there is NOT.
• When suspecting a Martin-Gruber anastomosis, the physician should perform further testing by recording over the ADM or FDI and stimulating the median nerve at the elbow to see if a sizeable CMAP can be generated (this will NOT be the full “normal” CMAP, just a small “bump” that represents the small portion of median nerve axons that are destined to cross over and supply the ADM, whereas the lion’s share of median nerve fibers will innervate the classic median nerve muscles and thus will NOT produce any signal here, as we are recording over the FDI, an ulnar nerve muscle).
• If Martin-Gruber anastomosis (MGA) is present, the small median CMAP amplitude you generate with this will “add up” with the proximal ulnar CMAP to create a full normal-amplitude CMAP, just like the normal CMAP when stimulating the ulnar nerve at the wrist.
In a case of axonotmesis of the median nerve at the wrist on EMG/NCS, 1 month after injury you detect decreased CMAP amplitude proximally and distally to the site of injury in addition to decreased recruitment. 2 years later, which of the following findings would you reasonably expect to discover?
A Decreased CMAP amplitude proximally and distally
B Normal CMAP amplitude distally, but not proximally
C Normal CMAP amplitude proximally, but not distally
D Normal CMAP amplitude proximally and distally
Answer: D
Explanation:
• In axonotmesis, even though the axons have died due to crush/stretch injury (leading to decreased CMAP distally and proximally) the epineurium is still intact, which will serve as a guide path for the axons to regenerate along and ultimately find their target muscle fibers again.
• Thus, months to years later, you may detect a repaired, normal CMAP due to axonal regeneration.
You are reviewing EMG/NCS findings. You note abnormal superficial fibular and sural SNAPs (sensory nerve action potentials), normal medial and lateral plantar nerve SNAPs, and abnormal CMAP to the EDB (compound muscle action potential to extensor digitorum brevis). The needle EMG results demonstrate decreased recruitment in tibialis anterior (TA), EDB, and fibularis longus, with normal activity in abductor hallucis, gastrocnemius, semimembranosus, biceps femoris, rectus femoris, tensor fascia lata, and lumbar paraspinals. Which of the following is the most likely diagnosis?
A Sciatic neuropathy
B Superficial fibular neuropathy
C Common fibular neuropathy
D Deep fibular neuropathy
Answer: C
Explanation:
• Common fibular neuropathy is the best answer.
• Deep fibular neuropathy would spare the fibularis longus (innervated by superficial fibular nerve).
• Superficial fibular neuropathy would spare the extensor digitorum brevis and tibialis anterior (innervated by deep fibular nerve).
• Sciatic neuropathy would show abnormalities in gastrocnemius, abductor hallucis, and plantar SNAPs potentially, as well as potentially hamstring muscles.
Which of the following can cause an ulnar neuropathy?
A Arcade of Frohse
B Arcade of Struthers
C Ligament of Struthers
D Bicipital aponeurosis
Answer: B
Explanation:
• The Arcade of Struthers is a piece of fascia connecting the brachialis to the triceps, and the ulnar nerve can become entrapped here.
• The Bicipital aponeurosis and Ligament of Struthers cause median neuropathy at the elbow.
• The Arcade of Frohse causes posterior interosseous neuropathy (a branch of radial nerve).
A patient with weakness presents for repetitive nerve stimulation (RNS). You decide to evaluate the compound muscle action potential (CMAP) of the abductor digiti minimi (ADM). On routine studies, you notice an abnormally low amplitude of the CMAP. During RNS while stimulating the muscle at a rate of 2 Hz, you notice a 20% decrement in the CMAP amplitude when comparing the 1st and 4th waveforms. However, during RNS while stimulating the muscle at a rate of 50 Hz, you notice a surge in the CMAP amplitude, increasing its size by 300%. Which of the following diseases do you suspect?
A Amyotrophic lateral sclerosis (ALS)
B Lambert-Eaton Myasthenic Syndrome (LEMS)
C Myasthenia Gravis (MG)
D Botulism
Answer: B
Explanation:
• Lambert-Eaton Myasthenic Syndrome (LEMS) is due to antibodies against presynaptic calcium channels, and presents with proximal muscle weakness that improves with exercise.
○ LEMS is also commonly secondary to a paraneoplastic process brought on by small cell lung carcinoma (malignant growth of cells).
○ Treatment involves Rehabilitation, Corticosteroids, Anticancer therapy, IVIG/Plasmapheresis.
• Myasthenia gravis (MG) is due to antibodies against postsynaptic neuromuscular junction (NMJ) acetylcholine receptors.
○ Classically symptoms (proximal muscle weakness) worsen with exercise, as the day progresses, and are associated with diplopia.
• Botulism is the final NMJ disease to know, and it is due to inhibition of presynaptic acetylcholine vesicle release into the synapse by botulinum toxin.
• All three of these NMJ diseases can be studied using RNS.
• During low-rate RNS (repetitive nerve stimulation) (2-3 Hz), all of them will show a CMAP decrement by at least 10% when comparing the 1st and 4th waveforms.
• During high-rate RNS (reptitie nerve stimulation) (10-50 Hz) all of them will show a CMAP amplitude increase (CMAP repair), but Lambert-Eaton Myasthenic Syndrome alone will show a massive increase compared to the other two diseases, and can even increase by 300%, which the other diseases simply cannot perform.
• High-rate RNS is one way to determine if the patient is suffering from Lambert-Eaton Myasthenic Syndrome vs. Myasthenia gravis especially.
Which of the following muscles does not receive its innervation from the lateral cord of the brachial plexus?
A Flexor carpi ulnaris
B Pronator teres
C Flexor carpi radialis
D Biceps brachii
Answer: A
Explanation:
• Biceps brachii, flexor carpi radialis, and pronator teres all receive lateral cord innervation. Flexor carpi ulnaris receives medial cord innervation.
Raising the low frequency filter during NCS (nerve conduction studies) will cause which of the following?
A No effect
B Increased amplitude
C Prolonged peak latency
D Shortened peak latency
Answer: D
Explanation:
• Raising the low frequency filter will shorten the peak latency and decrease the amplitude.
A 23 year-old male sustained an injury while playing soccer 2 weeks ago. He presents for an EMG evaluation due to foot drop. X-rays of the right lower limb are negative. On exam, he demonstrates intact lower limb strength except for 3+/5 right ankle dorsiflexion. Tinel at the fibular head is positive for reproduction of numbness and tingling he is experiencing. Nerve conduction studies demonstrate normal sural sensory nerve action potential (SNAP), prolonged latency of the superficial fibular SNAP, and normal compound muscle action potential (CMAP) of the tibialis anterior (TA) when stimulating below the fibular head. However, when stimulating within the popliteal fossa, the TA CMAP amplitude drops 50% compared to the below fibular head stimulation site. Needle EMG of the TA and extensor digitorum brevis (EDB) reveals decreased recruitment. Needle EMG of remaining muscles is normal. Which of the following is the most appropriate response to these findings?
A MRI is recommended
B Ultrasound-guided intervention is recommended
C Surgical opinion should be sought
D Prognosis for recovery is good
Answer: D
Explanation:
• This patient’s results demonstrate neurapraxia of the common fibular nerve, likely due to some physical injury sustained during the soccer match.
• Neurapraxia can be differentiated from axonotmesis (axon loss due to crush/stretch injury) and neurotmesis (axon loss due to complete nerve transection) due to the normal CMAPs present below the level of injury at 2 weeks out from injury.
• There is also the lack of active denervation (fibrillations and positive sharp waves) that suggests neurapraxia rather than axon loss.
• Neurapraxia, you recall, manifests as a conduction block, as in this case (normal amplitude CMAP distal to the lesion, decreased amplitude CMAP proximal to the lesion).
• Neurapraxia/conduction block represents a focal demyelination event; thus the axons are intact, and all that is required is remyelination for the CMAP, recruitment, and patient’s strength to recover.
• If this were axon loss, the CMAP amplitude would also be decreased distally at this point (2 weeks; recall that within ~10 days Wallerian degeneration is complete for both sensory and motor fibers); thus, the intact distal CMAP suggests the axons are alive and healthy, and just awaiting remyelination.
• Thus, this patient’s prognosis for spontaneous recovery is good. Thus, surgery would be inappropriate, MRI is unnecessary, and US-guided intervention is unnecessary.
The first sign of Guillain Barre Syndrome on electrodiagnostic studies is which of the following?
A Prolonged M wave
B Prolonged A wave
C Prolonged F wave
D Prolonged H reflex
Answer: C
Explanation:
• Prolonged or absent F waves are the first sign of GBS on EDX studies.
Which of the following muscles is not a shoulder adductor?
A Teres minor
B Teres major
C Latissimus dorsi
D Pectoralis major
Answer: A
Explanation:
All of the above are shoulder adductors, except the teres minor, which is part of the rotator cuff group and is an external rotator.
Which of the following muscles is innervated by the listed neural pathway? S1, S2; sciatic nerve; tibial nerve; medial plantar nerve.
A Flexor digitorum longus (FDL)
B Flexor hallucis brevis (FHB)
C Abductor digiti quinti pedis (ADQP)
D Flexor hallucis longus (FHL)
Answer: B
Explanation:
• The Flexor hallucis brevis (FHB) and the Abductor hallucis brevis (AHB) are medial plantar nerve-innervated.
• The Flexor hallucis longus (FHL) and Flexor digitorum longus (FDL) are tibial nerve-innervated.
• The Abductor digiti quinti pedis (ADQP) is lateral plantar nerve-innervated.
The purpose of EMG/nerve conduction studies is to diagnose disorders of which of the following?
A Resident’s EMG knowledge base
B Musculoskeletal system
C Peripheral nervous system
D Central nervous system
Answer: C
Explanation:
• The purpose of EMG/NCS is to diagnose peripheral nervous system diseases.
• It tells us nothing of the central nervous system, other than possibly poor activation of muscles.
• Despite how painful learning EMGs can be, its purpose is not to torture residents.
A patient presents with deep fibular neuropathy. Which of the following areas of the foot do you expect to demonstrate sensory abnormalities?
A 5th webspace, sparing the dorsum of the foot
B 1st webspace, sparing the dorsum of the foot
C Dorsum of foot, sparing the 5th webspace
D Dorsum of foot, sparing the 1st webspace
Answer: A
Explanation:
• Deep fibular nerve innervates the skin between the 1st and 2nd toes (the 1st webspace).
• Superficial fibular nerve innervates the skin over the entire dorsum of the foot, except the 1st webspace (which is deep fibular nerve territory).
A trauma results in complete severing of the median nerve at the elbow. Immediate EMG/NCS reveals absent CMAP proximally, and normal CMAP distally. Which of the following is the most likely diagnosis?
A Conduction block
B Neurotmesis
C Neurapraxia
D Axonotmesis
Answer: B
Explanation:
• Neurotmesis is the severing (transection) of a nerve all the way through the epineurium, due to trauma.
• Initially we see an absent CMAP proximally and normal CMAP distally.
• Once Wallerian degeneration is complete (roughly 5 days for motor fibers), we see absent CMAP both distally and proximally to the site of injury.
A 47 year-old male presents to your musculoskeletal clinic with complaints of 3 months of left shoulder pain. He works in the post office handling heavy packages. On exam, you note no pain with empty can, and no pain with resisted external or internal rotation of the shoulder. When abducting the arm 90 degrees and flexing the elbow 90 degrees, and then performing resisted external rotation of the shoulder, this reproduces the patient’s pain. What is the innervation of the muscle being tested?
A Upper subscapular nerve
B Axillary nerve
C Lower subscapular nerve
D Suprascapular nerve
Answer: B
Explanation:
• The physical exam maneuver described is how one isolates and tests the teres minor, which is innervated by C5,C6, upper trunk, posterior cord, axillary nerve.
• The suprascapular nerve innervates the supraspinatus and infraspinatus, and these muscles are not tested in the way this question describes.
• The upper and lower subscapular nerves innervate the subscapularis and teres major (lower subscapular nerve → teres major), tested differently from how this question describes.
A patient presents with left foot drop. On EMG/NCS, you note normal superficial fibular and sural nerve SNAPs (sensory nerve action potentials). EMG reveals 3+ positive sharp waves and decreased recruitment in the tibialis anterior (TA), extensor digitorum longus (EDL), extensor hallucis longus (EHL), and extensor digitorum brevis (EDB). EMG of the fibularis longus, abductor hallucis (AH), medial gastrocnemius, hamstrings, rectus femoris, thigh adductors, tensor fascia lata, and lumbar paraspinals is normal. Which of the following is the most likely diagnosis?
A Deep fibular neuropathy
B L4 radiculopathy
C Distal sciatic neuropathy
D Superficial fibular neuropathy
Answer: A
Explanation:
• Deep fibular neuropathy is the best answer.
• Superficial fibular neuropathy would affect fibularis longus and superficial fibular SNAP, and spare all the affected muscles in this question.
• Distal sciatic neuropathy would affect gastrocnemius and AH, as well as fibularis longus, and potentially superficial fibular SNAP.
• L4 radiculopathy would indeed show normal SNAPs, but would also be expected to affect other muscles receiving L4 innervation, such as rectus femoris, hamstrings, and thigh adductors.
Duchenne muscular dystrophy (DMD) usually begins with weakness in which of the following muscle groups?
A Knee extensors
B Plantarflexors
C Hip extensors
D Neck flexors
Answer: D
Explanation:
• Duchenne Muscular Dystrophy (DMD)’s earliest signs of muscle weakness are classically found in the neck flexors.
The sternocleidomastoid (SCM) shares its innervation with which of the following muscles?
A Splenius capitis
B Rhomboid major
C Rhomboid minor
D Trapezius
Answer: D
Explanation:
• The SCM and trapezius are innervated by C3, C4; spinal accessory nerve.
• The rhomboids are innervated by C4, C5; dorsal scapular nerve.
• It’s not important for you to know splenius capitis’ innervation, but it’s not the spinal accessory nerve.
A patient’s left upper limb EMG/NCS results are as follows. Routine median, ulnar, and radial sensory nerve conduction studies are normal. EMG reveals decreased recruitment and 4+ fibrillations in triceps, extensor indicis proprius, abductor pollicis brevis, and first dorsal interosseous, but normal resting activity and recruitment in deltoid, biceps, brachioradialis, and pronator teres. Which of the following is the most likely diagnosis?
A C8 radiculopathy
B C7 radiculopathy
C C6 radiculopathy
D C5 radiculopathy
Answer: A
Explanation:
• All muscles supplied by the C8 nerve root are affected, but those not involving the C8 nerve are spared; thus, C8 radiculopathy is the best answer.
• Please refer to the innervation chart at the beginning of the Upper and Lower Extremity Peripheral Nervous System Diseases chapters for a detailed innervation guide.
While examining a patient you notice that their right scapula sits more medially than the left scapula. Needle EMG would most likely reveal abnormalities of musculature belonging to which nerve?
A Thoracodorsal nerve
B Spinal accessory nerve
C Dorsal scapular nerve
D Long thoracic nerve
Answer: D
Explanation:
• The two nerves associated with winged scapula (this patient has a medially winged scapula) are:
• Long thoracic nerve (innervates the serratus anterior -> protracts scapula): injury can cause a medially winged scapula
• Spinal accessory nerve (innervates the trapezius -> retracts scapula): injury can cause a laterally winged scapula
• Dorsal scapular nerve (innervates the rhomboids -> retracts scapula).
• Thoracodorsal nerve (innervates the latissimus dorsi): adduct, internally rotate and extend arm
While performing a nerve conduction study (NCS) you apply current to the nerve and detect a small amplitude. You then move distally on the limb and apply current again to the nerve. To your surprise, the amplitude is normal. What is the most likely explanation for this finding?
A Total axonal loss
B Partial conduction block
C Total conduction block
D Partial axonal loss
Answer: B
Explanation:
• Axonal loss is seen with decreased amplitude throughout an entire nerve’s length. EMG will show decreased recruitment in these cases.
• Conduction block is seen with proximally reduced amplitude, but stimulating distally beyond the spot of conduction block can demonstrate a normal “repaired” amplitude.
Which of the following nerves is associated with the spinoglenoid notch?
A Thoracodorsal nerve
B Suprascapular nerve
C Axillary nerve
D Dorsal scapular nerve
Answer: B
Explanation:
• The suprascapular nerve innervates the supraspinatus, then passes through the spinoglenoid notch to innervate the infraspinatus.
• This is an important anatomical consideration when performing an EMG involving both supraspinatus and infraspinatus.
• The axillary nerve passes through the quadrilateral space of the axilla.
Which of the following correctly states the innervation of the flexor digitorum superficialis of the forearm?
A C6, C7, upper and middle trunk, medial and lateral cord, median nerve
B C7, C8, middle and lower trunk, medial and lateral cord, median nerve, AIN (ant. interosseous nerve)
C C7, C8, T1, middle trunk, medial cord, median nerve
D C7, C8, middle and lower trunk, medial and lateral cord, median nerve
Answer: D
Explanation:
“C7, C8, middle and lower trunk, medial and lateral cord, median nerve” correctly states the FDS innervation.
When a muscle recruits its first motor unit during initial contraction, it tends to fire at \_\_\_\_\_ Hz, and subsequent motor units are recruited for every \_\_\_\_\_\_ Hz increase in firing rate. A 10, 15 B 5, 5 C 10, 10 D 5, 10
Answer: B
Explanation:
• The initial motor unit is recruited at 5 Hz.
• Every 5 Hz an additional motor unit is added to the recruitment pattern.
• Thus, your EMG screen should at some point show you 4 motor units firing at once with gradually increased muscle contraction by the patient: these units will be firing at 20, 15, 10, 5 Hz respectively.
During an upper limb EMG/NCS study, you detect prolonged SNAP latency to digit 2, digit 5, and the dorsal ulnar cutaneous nerve. EMG reveals decreased recruitment to the abductor digiti minimi and flexor digitorum profundus to digits 4 and 5. Which of the following is most likely to narrow down the diagnosis?
A Needle EMG sampling of flexor carpi ulnaris (FCU)
B Testing the lateral antebrachial cutaneous nerve SNAP
C Comparing the DUC between left and right sides
D Testing the medial antebrachial cutaneous nerve SNAP
Answer: D
Explanation:
• These EMG/NCS results suggest either ulnar neuropathy at the elbow or medial cord brachial plexopathy.
• The medial antebrachial cutaneous nerve (MAC) branches off the medial cord of the brachial plexus to supply cutaneous innervation to the medial forearm just before the medial cord continues down to form the ulnar nerve.
• Thus, an abnormal MAC would indicate medial cord plexopathy (or lower trunk plexopathy), indicating that the nerve problem is somewhere proximal to the ulnar nerve, NOT at the ulnar nerve.
• However, a normal MAC would indicate that the medial cord of the plexus is intact, thus localizing the problem distally (likely the ulnar nerve at the elbow in this case).
• Comparing the dorsal ulnar cutaneous (DUC) nerve between both sides is NOT helpful, as we already know that ours being studied is abnormal.
• Obtaining the lateral antebrachial cutaneous SNAP (arises as a continuation of the musculocutaneous nerve from the lateral cord of the plexus) will NOT inform us on the status of the medial cord of the plexus, thus making this less helpful to us in this case.
• Finally, needling the flexor carpi ulnaris (FCU) may show abnormalities in this muscle, which could be caused by medial cord plexopathy or ulnar neuropathy at the elbow; thus, this would NOT help us narrow our diagnosis.
Years following a neurotmesis injury in the forearm, what is the most reasonable expectation on EMG/NCS?
A Equally any of the above
B Normal CMAP
C “Repaired” CMAP
D Absent CMAP
Answer: D
Explanation:
• Neurotmesis is the complete transection of a nerve.
• Thus, axons have NO connective tissue nerve path through which they may find their former muscle fibers, and the CMAP most likely will NOT be normal or partially repaired.
A Martin-Gruber anastomosis is an anatomic variant consisting of which of the following nerve findings?
A Ulnar to median anastomosis
B Median to posterior interosseous anastomosis
C Ulnar to anterior interosseous anastomosis
D Median to ulnar anastomosis
Answer: D
Explanation:
• Martin-Gruber anastomosis involves median nerve fibers in the forearm crossing over and joining the ulnar nerve in the forearm.
• This means essentially that the median nerve innervates the abductor digiti minimi (ADM) and first dorsal interosseous (FDI), which are ulnar nerve muscles, in addition to innervating its own classic median nerve muscles (abductor pollicis brevis - APB, etc.).
• The ulnar nerve CMAP (compound muscle action potential) when stimulating at the elbow will show a low amplitude than when recording over a distal ulnar muscle such as FDI.
• The ulnar CMAP will be “repaired”/normal if you stimulate the ulnar nerve at the wrist.
• Thus, it appears that there is a conduction block somewhere in the forearm when there actually is NOT.
• When you stimulate the ulnar nerve at the wrist, at this point the median nerve fibers have finally joined the ulnar nerve, so you are finally stimulating all the motor axons that are supplying the ulnar nerve hand muscles.
• When you stimulate the ulnar nerve at the elbow, you are NOT stimulating the median nerve fibers that have yet to contribute to the ulnar innervations, thus you only generate part of the full amplitude, which is what makes it look like there is a conduction block in the ulnar nerve, when in reality there is NOT.
• When suspecting a Martin-Gruber anastomosis (MGA), the physician should perform further testing by recording over the ADM or FDI and stimulating the median nerve at the elbow to see if a sizeable CMAP can be generated (this will NOT be the full “normal” CMAP, just a small “bump” that represents the small portion of median nerve axons that are destined to cross over and supply the ADM, whereas the lion’s share of median nerve fibers will innervate the classic median nerve muscles and thus will NOT produce any signal here, as we are recording over the FDI, an ulnar nerve muscle).
• If MGA is present, the small CMAP amplitude you generate with this will “add up” with the proximal ulnar CMAP to create a full normal-amplitude CMAP, just like the normal CMAP when stimulating the ulnar nerve at the wrist.
What does a polyphasic potential represent during an EMG?
A Active reinnervation
B Prior denervation
C Prior reinnervation
D Active denervation
Answer: A
Explanation:
• Polyphasic potentials represent ongoing reinnervation due to collateral sprouting of existing axons.
• The polyphasic property arises due to non-uniform myelination of these collateral sprouts during this early reinnervation stage.
Which of the following muscles is NOT innervated by the anterior interosseous nerve?
A Flexor digitorum superficialis
B Flexor digitorum profundus
C Flexor pollicis longus
D Pronator quadratus
Answer: A
Explanation:
• FDS is innervated directly by the median nerve, NOT the anterior interosseous nerve (which arises from the median nerve).
An EMG potential is described as polyphasic if it crosses the baseline a minimum of how many times?
A 6
B 5
C 4
D 3
Answer: B
Explanation:
Polyphasic potentials are defined as a baseline crossing of 5 or more times
Neurapraxia can be defined as which of the following?
A None of these answers is correct
B Complete severance of a nerve through the epineurium, causing paralysis of “downstream” muscles
C Focal pressure on a nerve, leading to focal demyelination and conduction block
D Nerve injury leading to axonal loss with the epineurium still intact
Answer: C
Explanation:
• “Focal pressure on a nerve, leading to focal demyelination and conduction block” describes neurapraxia, which is essentially a focal conduction block.
• This can become remyelinated over 2-3 weeks, resolving the conduction block.
• The pathologic compression stimulus must be removed for this healing to occur, however.
A 60 year-old female presents with proximal hip and shoulder weakness, periorbital erythematous rash, and erythematous papules over her metacarpophalangeal (MCP) and interphalangeal (IP) joints of the hands. Which of the following EMG findings is most likely present in this patient?
A Decreased amplitude sensory nerve action potentials (SNAPs)
B Giant motor unit action potentials (MUAPs)
C Decreased recruitment
D Early/increased recruitment
Answer: D
Explanation:
• This patient presents with classic dermatomyositis (proximal muscle weakness with heliotrope rash and Gottron papules).
• This is a myopathic disease; thus, the EMG will show signs of myopathy:
○ Short-duration
○ Small amplitude MUAPs with
○ Early/Increased recruitment
• Decreased recruitment is found in nerve disorders, not muscle disease.
• Giant MUAPs are found in post-polio syndrome.
• Decreased SNAP amplitudes can be found in any sensory nerve disease that affects axons.
During an EMG, you decide to stimulate the ulnar nerve at the elbow and record the CMAP (compound muscle action potential) over the abductor digiti minimi (ADM). You then stimulate the ulnar nerve at the wrist and notice a much higher amplitude at the wrist than when stimulating at the elbow. Which of the following is the most likely etiology of these findings?
A Axonotmesis
B Martin-Gruber anastomosis
C Conduction block
D Riche-Cannieu anastomosis
Answer: B
Explanation:
• The Martin-Gruber anastomosis involves median nerve fibers in the forearm crossing over and joining the ulnar nerve in the forearm. This means essentially that the median nerve innervates the abductor digiti minimi (ADM) and first dorsal interosseous (FDI), which are ulnar nerve muscles, in addition to innervating its own classic median nerve muscles (abductor pollicis brevis - APB, etc.).
• The ulnar nerve CMAP (compound muscle action potential) when stimulating at the elbow will show a low amplitude when recording over a distal ulnar muscle such as FDI.
• The ulnar CMAP will be “repaired”/normal if you stimulate the ulnar nerve at the wrist. Thus, it appears that there is a conduction block somewhere in the forearm when there actually is not (ulnar nerve conduction block in the mid to distal forearm is also very uncommon).
• When you stimulate the ulnar nerve at the wrist, at this point the median nerve fibers have finally joined the ulnar nerve, so you are finally stimulating all the motor axons that are supplying the ulnar nerve hand muscles.
• When you stimulate the ulnar nerve at the elbow, you are not stimulating the median nerve fibers that have yet to contribute to the ulnar innervations, thus you only generate part of the full amplitude, which is what makes it look like there is a conduction block in the ulnar nerve, when in reality there is not.
• When suspecting a MGA, the physician should perform further testing by recording over the ADM or FDI and stimulating the median nerve at the elbow to see if a sizeable CMAP can be generated (this will not be the full “normal” CMAP, just a small “bump” that represents the small portion of median nerve axons that are destined to cross over and supply the ADM, whereas the lion’s share of median nerve fibers will innervate the classic median nerve muscles and thus will not produce any signal here, as we are recording over the FDI, an ulnar nerve muscle).
• If MGA is present, the small CMAP amplitude you generate with this will “add up” with the proximal ulnar CMAP to create a full normal-amplitude CMAP, just like the normal CMAP when stimulating the ulnar nerve at the wrist.
A 45 year-old female presents with 3 months of right foot drop and leg pain. On exam, tinel at the fibular head is positive. Nerve conduction studies of the right lower limb (NCS) reveal prolonged latency of sural and superficial fibular SNAPs (sensory nerve action potential) and decreased amplitude of extensor digitorum brevis (EDB) CMAP (compound muscle action potential). EMG reveals decreased recruitment of tibialis anterior (TA), EDB, abductor hallucis (AH), fibularis longus, and semitendinosus. EMG of lumbar paraspinals, gluteus maximus, tensor fascia lata (TFL), rectus femoris, and thigh adductors is normal. Which of the following is the most likely diagnosis?
A L5 radiculopathy
B Proximal sciatic neuropathy
C Distal sciatic neuropathy
D Common fibular neuropathy
Answer: B
Explanation:
• Proximal sciatic neuropathy is the best choice, as all sciatic nerve-innervated muscles (some hamstrings and the gastroc/soleus are untested) are affected on EMG, as well as both tibial and fibular SNAPs being affected as well, indicating that most parsimonious explanation is a proximal sciatic neuropathy.
• A distal sciatic neuropathy, such as at the knee, would spare the hamstrings, which are affected in this case.
• A radiculopathy would show normal SNAPs (abnormal in this case).
• Common fibular neuropathy would spare the hamstrings and the abductor hallucis, both of which are affected in this case.
Normal upper and lower extremity conduction velocities are at least which of the following, respectively, in meters per second?
A 50, 50
B 50, 40
C 40, 50
D 40, 40
Answer: B
Explanation:
• Normal upper and lower limb CV is at least 50 m/s and 40 m/s respectively.
During a diagnostic ultrasound scan, you notice a structure that appears like a honeycomb. Which of the following are you most likely viewing?
A Tendon
B Nerve
C Vein
D Artery
Answer: B
Explanation:
• On ultrasound, in a short axis view (cross-sectional view), a nerve looks like a honeycomb.
• Veins and arteries look like hollow (dark/anechoic) tubes that are compressible by the transducer.
• A tendon appears more uniformly hyperechoic (bright) than a nerve.
A patient’s right lower limb EMG/NCS results are as follows. Routine fibular and tibial motor/sensory nerve conduction studies are normal. EMG reveals decreased recruitment and polyphasic potentials in tensor fascia lata, rectus femoris, adductor longus, semimembranosus, and tibialis anterior, but normal activity in short head of biceps femoris, fibularis longus, gastrocnemius, and abductor hallucis. Which of the following is the most likely diagnosis? AS1 radiculopathy BL5 radiculopathy CL4 radiculopathy DL3 radiculopathy
Answer: C
Explanation:
• All muscles receiving some degree of L4 innervation are affected here, but all those without L4 innervation are spared.
• Note that the short head of biceps femoris is innervated by L5-S1, common fibular division of sciatic nerve, thus differentiating it from the medial hamstrings and long head of biceps femoris, which have L4-L5-S1, tibial division of sciatic nerve innervation.
Note also that typically an S1 radiculopathy question will refer to an abnormal H-reflex as partial evidence of an S1 radiculopathic process.
A 66 year-old male develops right leg pain and weakness following a hospital stay for acute coronary syndrome requiring cardiac stenting. On exam he demonstrates isolated knee extension weakness. On EMG/NCS he demonstrates abnormal CMAP (compound muscle action potential) to the rectus femoris. Needle EMG reveals decreased recruitment and 1+ fibrillations in the vastus medialis. In order to cinch the diagnosis, you decide to perform additional studies, and expect to find which of the following findings? SNAP: sensory nerve action potential.
AProlonged latency of saphenous SNAP
BAbnormal EMG activity in adductor longus
CAbnormal EMG activity in extensor digitorum brevis
DDecreased amplitude of sural SNAP
Answer: A
Explanation:
• This patient presents with femoral neuropathy due to cardiac catheter trauma (inserting the catheter into the femoral artery - next to the femoral nerve).
• The femoral nerve terminates as the saphenous nerve, supplying skin innervation to the medial leg below the knee.
• Thus, the saphenous nerve SNAP being abnormal would nicely complete the diagnosis of femoral neuropathy in this patient.
Regarding the dorsal and palmar interossei of the hand, which of the following statements is true?
AThey share the same innervation
BThey have ulnar and anterior interosseous nerve innervation, respectively
CThey are not routinely needled during an EMG study
DThey have ulnar and median nerve innervation, respectively
Answer: A
Explanation:
• The dorsal and palmar interossei of the hand are all innervated by the ulnar nerve.
• The first dorsal interosseous (FDI) is routinely needled in an EMG study as a valuable, accessible ulnar nerve-innervated muscle in the hand.
You are performing an EMG and decide to advance the needle into a muscle at rest when you hear what sounds like a “divebomber”. Which of the following is the most likely diagnosis? AAll of the listed choices are correct BHyperkalemic periodic paralysis CMyotonia congenita DMyotonic dystrophy
Answer: A
Explanation:
• A divebomber sound is classic for myotonic discharges. This can be seen on the screen as a steadily decreasing amplitude as the muscle fiber continues to fire.
• Essentially anything with “myotonia” in its name can demonstrate myotonic discharges on EMG, as can hyperkalemic periodic paralysis and acid maltase deficiency.
