EMG/NCS for Systemic Nerve & Muscle Diseases

Question 1

critical illness neuropathy

Answer 1

an axonal sensorimotor polyneuropathy

NCS:
- Abnormal SNAPs and CMAPs (low amplitude is key feature due to the axon loss)

EMG:
- LDLA, decreased recruitment, fibs and sharps

Question 2

charcot-marie-tooth neuropathy

Answer 2

the most common subtype is CMTIA, a duplication of the PMP-22 gene
* As a young child, gradual distal › proximal weakness with lesser sensory abnormalities
- Thus, this is a motor and sensory disorder
* Bilateral foot drop, Champagne bottle legs
* Deletion of the PMP-22 gene gives you hereditary neuropathy with liability to pressure palsy (HNPP)

CMT2 gives you an axonal polyneuropathy

CMT1: prolonged latency, decreased CV, without conduction block or abnormal temporal dispersion

CMT2: decreased amplitude SNAPs/CMAPs; active denervation/reinnervation on
EMG

Question 3

HIV neuropathy

Answer 3

Typically a distal axonal, symmetric sensory > motor polyneuropathy due to HIV/AIDS; Patient has numbness and tingling with burning in toes and fingers

NCS:
- Abnormal amplitude of SNAPs/CMAPs

EMG:
- Abnormal spontaneous activity in distal > proximal muscles

Question 4

mononeuritis multiplex

Answer 4

Essentially an axonal peripheral nerve injury (eg. median neuropathy) combined with other single peripheral nerve injuries
- For example, left median neuropathy at the wrist, right ulnar neuropathy at the elbow, left radial neuropathy at the elbow, all in the same patient

Can present acutely and asymmetrically with weakness, numbness/tingling

NCS and EMG show abnormalities in the respective peripheral nerves that are damaged, with axonal features generally

Question 5

myasthenia gravis

Answer 5

Proximal muscle weakness with diplopia that worsens as the day goes on

NCS:
- Normal routine studies
- Abnormal repetitive nerve stimulation (RNS)

EMG:
- Essentially normal; may see increased jitter and blocking on single-fiber EMG

Question 6

lambart-eaton syndrome

Answer 6

Proximal muscle weakness that is improved with exercise and does not involve diplopia

NCS:
- Low amplitude CMAPs
- Abnormal RNS

EMG:
- Increased jitter and blocking on single-fiber EMG

Question 7

botulism

Answer 7

Dysphagia, diarrhea, diffuse paralysis, respiratory compromise, areflexia due to ingestion of botulinum toxin (home canning, raw meat, raw honey)

NCS:
- Decreased/absent CMAP amplitude
- Abnormal RNS

EMG:
- Active denervation with decreased recruitment
- Increased jitter and blocking on single-fiber EMG

Question 8

anterior horn cell disease

Answer 8

Pure motor disease resulting in progressive weakness with normal sensation, due to death of alpha motor neurons (which reside in the anterior horn of the spinal cord)
- Patients typically exhibit LMN signs, but sometimes UMN signs are present as well
- ALS, PLS, HSP

In general, SNAPs are normal, and latency/CV are also normal, since this is not demyelinating. This NCS picture can be variable.

Must demonstrate abnormalities in 3 of 4 spinal segments
- Brainstem, cervical, thoracic, lumbosacral

Question 9

spinal muscular atrophy

Answer 9

SMA1: Werdnig-Hoffman Disease
- Floppy infant, never sits independently, dies of respiratory failure early

SMA2
- Weak/floppy 1 year-old, wheelchair as toddler, can sit independently and stand with ADs, dies of respiratory failure
SMA3: Kugelberg-Welander Disease
- Gradually weakening ~10 year old, can walk independently, normal life expectancy

EDX:
- Normal SNAPs, abnormal CMAPs, LDLA MUAPs with decreased recruitment

Question 10

amyotrophic lateral sclerosis

Answer 10

progressive pure motor weakness, dysphagia, respiratory compromise without sensory abnormalities

NCS:
- Normal SNAPs; normal CMAPs (due to reinnervation)

EMG:
- LDLA MUAPs, decreased recruitment, fibs and sharps, increased jitter and fiber density
- Show denervation in 3 of 4 spinal regions (brainstem, cervical, thoracic, lumbosacral)

Question 11

poliomyelitis

Answer 11

progressive, pure motor weakness, respiratory compromise without sensory abnormalities

NCS:
- Normal SNAPs
- Decreased amplitude CMAPs

EMG:
- LDLA MUAPs, fibs and sharps, decreased recruitment

Question 12

post-polio syndrome

Answer 12

New-onset progressive weakness in a patient who had fully recovered from polio 15 years ago, with no other etiology to explain the new weakness
- Essentially the anterior horn cells that are remaining after the patient initially recovers from polio just get burnt out from doing all the work and picking up the slack that the dead motor neurons used to carry

NCS:
- Normal SNAPs
- Abnormal CMAPs (low amplitude)

EMG:
- Giant MUAPs, decreased recruitment, fibs and sharps

Question 13

myopathy

Answer 13

weak with atrophy/hypertrophy and tone/grip issues (hypotonia, myotonia)

NCS:
- Normal SNAPs
- Abnormal CMAP amplitudes due to atrophied muscles

EMG:
- Small amplitude, short duration, polyphasic MUAPs
- Early recruitment pattern (ie. increased recruitment)
- Sometimes see spontaneous myotonic discharges (divebomber sound)

Question 14

dermatomyositis/polymyositis

Answer 14

Proximal hip and shoulder weakness +/- heliotrope rash (dermatomyositis) and Gottron papules (purple patches) over MCPs/PIPs/DIPs

NCS:
- Normal SNAPs and CMAPs

EMG:
- Abnormal (SDSA MUAPs with increased recruitment (early recruitment))

Question 15

statin myopathy

Answer 15

Proximal muscle pain (myalgias)

NCS:
- normal SNAPs and CMAPs

EMG:
- normal

Question 16

steroid myopathy

Answer 16

Proximal muscle weakness

Causes Type II muscle fiber atrophy
- EMG generally just analyzes Type I fibers since it takes a maximal contraction to start activating those Type II white fibers

NCS:
- normal SNAPs and CMAPs

EMG:
- normal

Question 17

critical illness myopathy

Answer 17

weakness and atrophy in both proximal and distal muscles, with normal sensation

NCS:
- Normal SNAPs
- Low amplitude CMAPs

EMG:
- SDSA MUAPS, sometimes fibs and sharps, with normal/early recruitment

Question 18

diabetic peripheral neuropathy

Answer 18

These findings are typically symmetric
- If demyelinating features are present, you may see conduction block and/or increased temporal dispersion on NCS
- Make sure you evaluate at least 3 limbs

NCS:
- Decreased amplitude and prolonged latency of SNAPs
- Decreased amplitude and prolonged latency of CMAPs
- Absent F waves

EMG:
- If axonal process, fibs and sharps and decreased recruitment in distal › proximal muscles
- Polyphasia

Question 19

guillain barre syndrome/AIDP

Answer 19

NCS:
- The first changes are abnormal (delayed or absent) F waves
* H reflexes also become abnormal early on
* F waves and H waves being abnormal early clues you in to the lesion being proximal
- “Polyradiculopathy”
* Prolonged latency and reduced amplitude of SNAPs with sural sparing
- Sural nerve is larger, with more myelin than e.g. median and ulnar nerves
* Prolonged latency, decreased CV, with usually normal amplitude of CMAPs
- Conduction block, abnormal temporal dispersion
- Distal CMAP amplitude is the key prognostic factor in AIDP
- If < 20% upper limit of normal, bad prognosis

EMG:
- Decreased recruitment; otherwise normal since no axon loss has taken place
- However, several weeks after onset, you may see fibs and sharps (active denervation) due to secondary axonal loss, which can occur in AIDP