EMG Findings in Monomelic Amyotrophy (Hirayama Disease)
In a patient with suspected Hirayama disease, EMG will demonstrate chronic and ongoing denervation in the C5-C7 (particularly C7-T1) innervated muscles, showing fibrillation potentials, positive sharp waves, and polyphasic motor unit action potentials with reduced recruitment, consistent with anterior horn cell disease affecting the lower cervical segments. 1
Characteristic EMG Pattern
The electrodiagnostic findings in Hirayama disease reflect anterior horn cell pathology at the lower cervical cord level:
Chronic denervation changes including fibrillation potentials and positive sharp waves in affected muscles, indicating ongoing motor neuron loss 2, 1
Polyphasic motor unit action potentials (MUAPs) with increased duration and amplitude, reflecting chronic reinnervation as surviving motor neurons sprout to reinnervate denervated muscle fibers 2
Reduced motor unit recruitment with a decreased interference pattern during maximal voluntary contraction, demonstrating loss of functional motor units 1
Root lesion pattern involving the fifth to seventh cervical segments, with the most severe changes typically in C7-T1 innervated distal upper extremity muscles 1
Distribution of Abnormalities
The EMG abnormalities follow a characteristic anatomical pattern:
Distal upper extremity muscles (hand intrinsics, finger flexors, wrist flexors) show the most severe denervation changes, corresponding to the clinical "oblique atrophy" pattern 3, 4
Brachioradialis is characteristically spared, which helps distinguish Hirayama disease from other motor neuron disorders 3
Proximal muscles may occasionally show abnormalities in atypical cases, though this is less common 1
Unilateral or asymmetric involvement is typical, though bilateral cases can occur 5, 6
Key Distinguishing Features
EMG helps differentiate Hirayama disease from other conditions:
Fasciculations may be present but are less prominent than in amyotrophic lateral sclerosis, where they occur irregularly and sound like "raindrops on a tin roof" 2
No sensory nerve conduction abnormalities, as Hirayama disease is a pure motor disorder affecting anterior horn cells 1, 5
Absence of myopathic features such as the short duration, low amplitude polyphasic potentials seen in inflammatory myopathies 2
Clinical Correlation
The EMG findings must be interpreted alongside imaging:
Cervical MRI with neck flexion is the definitive diagnostic test, showing anterior displacement of the posterior dural sac with cord compression at C4-C7 levels 1, 5, 4
EMG confirms the neurogenic pattern and helps localize the level of anterior horn cell involvement, guiding the interpretation of MRI findings 1
Serial EMG studies can document disease progression during the active phase or stabilization after the disease arrests 5, 4
Important Caveats
Do not confuse with radiculopathy: While both show denervation in myotomal distributions, Hirayama disease affects multiple adjacent myotomes without sensory involvement and has characteristic MRI findings 1
Upper motor neuron signs are atypical: If present (hyperreflexia, Babinski sign), consider alternative diagnoses or atypical presentations with myelopathic features 1
The EMG pattern reflects chronic, stable denervation after disease arrest, distinguishing it from progressive motor neuron diseases like ALS where ongoing active denervation continues 5, 4