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.