EMG Blind Spots:
What Your Study Can Miss
Disorders of the NMJ
W. David Arnold, MD
AAPMR 2014
Outline
• Review of Prototypical NMJ Disorder,
Myasthenia Gravis, and the General Approach
to NMJ Disorders
• Review a series of 4 Cases to Highlight Potential
Pitfalls and Key Concepts in NMJ Edx studies
Case 1: “Proximal Limb Weakness”
• 18 year old woman presents with a progressive
proximal limb weakness is sent to the EMG Lab
Limb Girdle Muscular Dystrophy?
• Examination:
– 4 grade proximal limb weakness
– Normal bulbar strength, sensation, and reflexes
Case 1: NCS and EMG
• Normal median sensory
and motor response
• EMG: short duration,
small amplitude motor
unit action potentials
with early recruitment
without fibrillations
Case 1: Proximal Limb Weakness (Cont.)
• EMG and NCS: Non-irritable myopathy
• Diagnostic consideration was given to a possible
limb girdle muscular dystrophy
• Muscle Biopsy: normal
• Thoughts?
Case 1: Additional Studies
• Repetitive ulnar nerve
stimulation at 3 Hz
~50% decrement
• SFEMG
Increased jitter and
blocking
Follow up
Lab testing:
– Elevated acetylcholine receptor antibodies
Diagnosis: acetylocholine receptor antibody positive
myasthenia gravis (MG)
Treatment: Excellent response to pyridostigmine and
immunomodulatory treatments
Clinical Pearl of Case 1:
Ask the Correct Question
338 charts of patients with NMJ disorders
6 patients underwent muscle biopsy for
possible myopathy
2 with coexistent inflammatory muscle dz
4 with NMJ disorders mimicking
myopathy (based on normal muscle bx)
Consider RNS in patients with weakness
MG
• MG is the most common NMJ disorder
• Prevalence of 20 in 100,000 and incidence of 2 per
100,000. (Phillips Ann N Y Acad Sci. 2003)
• Antibodies
– Ach receptor antibodies (80%)
– Muscle specific tyrosine kinase (MuSK) (10%)
– Antibody negative (5%-10%)
• Treated with cholinesterase inhibitors and
immunomodulatory treatments
NMJ Disorders: History
• Weakness: proximal muscles with fatigability
– Severity of involvement:
Ocular>bulbar>limb>respiratory
• Sensory: No sensory loss, paresthesia, or neuropathic
pain
• Autonomic: dry mouth, blurry vision, impotence,
constipation, and difficulty with urination
– Some disorders such as Lambert Eaton myasthenic
syndrome (LEMS) and botulism
Generalized fatigue ≠ NMJ disorder
NMJ Disorders: Examination
• Weakness in proximal limb, ocular, and bulbar
muscles
– Distal or focal weakness may occur in 12% of MG
(Wirtz et al 2002)
– Extraocular and bulbar weakness are early and
prominent in MG and botulism
– Less prominent extraocular weakness in LEMS
• Reflexes are normal in postsynaptic disorders such as
myasthenia gravis but characteristically absent or
reduced in presynaptic NMJ d/o
• Sensory function is normal
• Pupillary responses are usually reduced in botulism
Post-synaptic NMJ disorders
• Sensory NCS
-normal
• Motor NCS
-usually normal CMAP amplitude
• EMG
-Possibly short duration, low
amplitude, unstable MUAP’s if
severe (Jiggle)
• Slow RNS (3 Hz)
-decrement
(60-80% of generalized MG)
• CMAP with 10 sec exercise
(or Rapid RNS, 20-50 Hz)
-no increment
• CMAP with Exercise
-no increment
• SFEMG
-Jitter and blocking
(95-98% MG)
(Nl jitter in weak muscle ≠ MG)
Post-synaptic NMJ disorder
Pre-synaptic NMJ disorder
Sensory NCS
-normal
Motor NCS
-usually low CMAP amplitude
EMG
-Possibly short duration, low
amplitude, unstable MUAP’s if
severe
-some disorders with
fibrillations (botulism)
Slow RNS (3 Hz)
-decrement
CMAP with 10 sec exercise
(or Rapid RNS, 20-50 Hz)
-increment
300% in one muscle or 100% in three
(diagnostic for Lambert Eaton MS)
SFEMG
-Jitter and blocking
(jitter and blocking decrease with
increasing firing rate)
Presynaptic NMJ
Pre-exercise
Post-exercise
350% increment following 10 seconds exercise
Case 2: Dysarthria
• 64 year old woman with symptoms of progressive
dysarthria and facial weakness is sent to the EMG
Lab
Possible myasthenia gravis?
• Examination
–
–
–
–
Mild facial and moderate tongue weakness
Tongue fasciculations
Normal limb strength
Generalized Brisk reflexes and jaw jerk
Case 2: Nerve Conduction Studies
Median and ulnar sensory
responses are normal
Median and ulnar motor
responses are normal
Repetitive nerve stimulation
at 3 Hz
-Orbicularis Oris with
>20% decrement
Myasthenia Gravis?
Case 2: Electromyography
Single Fiber EMG
-Increased jitter and
blocking in frontalis and
extensor digitorum
EMG
Fibrillations/fasciculations/
dec recruitment/enlarged
motor unit action potentials
(mentalis and tongue)
Limb muscles normal
Jitter and Blocking
Example: Post-synaptic
NMJ disorder
Case 2
Clinical Pearl of Case 2:
Not all that Jitters is Myasthenia
the “False Positive” test
Amyotrophic Lateral Sclerosis & NMJ transmission defect
• Severe decrement frequently seen on RNS (reported up to 35% of
CMAP amplitude) (I have seen up to 50%)
• Increased decrement correlates with faster progression and worse
prognosis
Daube 2004; Wang 2001
• SFEMG: increased jitter and block may be early finding, prior to
denervation (also seen in early reinnervation)
• Axonal Stimulation SFEMG: lack of facilitation with increasing
frequency of stimulation
Arimura 1995
Case 3: Seronegative MG
A 58 year old woman with a 30 year history of
seronegative myasthenia gravis
PMH: poliomyelitis at age 6 requiring ventilation
support in an iron lung for a month
PE: normal CN exam, severe proximal limb
weakness with fatigability, normal sensation and
reflexes
Case 3: Seronegative MG
• 3Hz RNS with up to 70%
decrement in proximal
muscles
• She describes dramatic
improvement with
pyridostigmine
• Worsening with prednisone
• No improvement with
IVIG or PLEX
• Ideas?
Clinical Pearl of Case 3:
Hoof beats (aka decrement)? Don’t
assume it is a horse (MG)
– Multiple NMJ disorders may fall within this clinical
presentation
• Seronegative (autoimmune) MG
• Congenital Myasthenic syndromes
• Other neuromuscular disorders with secondary
NMJ effects or clinical patterns similar to NMJ
disorders
Congenital Myasthenic Syndrome
• Historical points in case 3
suggesting this possibility:
– No clear response to
immunomodulation
– Atypical phenotype (Severe
disease with minimal to no
bulbar involvement)
– PMH of “polio” after which
she states she was “normal” in
every way until developing
weakness in her 20’s
– On more direct questions she
described being a “blue baby”
at birth
• Electrodiagnosis of CMS
Case 4: MG second opinion
45 year old man with progressive ptosis, dysphagia
FH: mother with “seronegative myasthenia gravis”
Examination: bilateral ptosis and extraocular muscle weakness
without clear fatigability; mild proximal limb weakness
Antibody testing: AChR and MuSK Antibodies (-)
EMG and NCS: decrement on repetitive nerve stimulation studies
Diagnosed with seronegative myasthenia gravis but then had
minimal response to immune therapies and pyridostigmine?
Ideas?
Case 4: Repeat EMG and NCS
• NCS: multiple mononeuropathies c/w the
patient’s known HNPP
• RNS: normal
• SFEMG: mildly increased jitter in frontalis with
no blocking
• EMG: Short duration small amplitude motor
unit action potentials with sparse fibrillations in
proximal limb and facial muscles
Case 4: Pre and Post Surgery Images
Clinical Pearl of Case 4:
When in doubt, trust your exam… double
check technique
Movement artifact is significant challenge in repetitive nerve
stimulation and can lead to false positive tests
Our patient:
• Lacked clinical features of fatigability or response to treatment
• Diagnosed with oculopharyngeal muscular dystrophy (OPMD)
–
–
–
–
Autosomal dominant slowly progressive hereditary muscle disorder
GCG repeat expansion (8-13)
Often mistaken for seronegative MG
Typically later/less involvement of extraocular muscles
Thanks!
Questions?
William.arnold@osumc.edu
References
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