What does an axonal pattern on an electromyography (EMG) test indicate?

Axonal Pattern on EMG

An axonal pattern on EMG indicates peripheral nerve damage affecting the axons themselves, characterized by long-duration, large-amplitude motor unit action potentials (MUAPs) with reduced recruitment, along with signs of denervation and subsequent reinnervation. 1

Key EMG Findings in Axonal Neuropathy

Acute Phase (First Week)

• Abnormal motor unit recruitment patterns are the earliest finding, occurring before denervation develops 2
• Spontaneous discharges initially occur only with direct needle stimulation of muscle fibers 1
• Conventional nerve conduction studies may be normal early in the disease course 2

Subacute Phase (After 1 Week)

• Fibrillation potentials emerge as spontaneous short-duration muscle fiber action potentials (MFAPs less than 5 ms) with regular discharge patterns 1
• Positive sharp waves appear as spontaneous short-duration MFAPs in the positive (downward) direction, also indicating denervation 1
• These denervation changes typically appear 2-3 weeks after nerve injury 3

Chronic Reinnervation Phase (Weeks to Months)

• Long-duration, large-amplitude MUAPs develop as intact neighboring axons sprout to reinnervate denervated muscle fibers 1
• Polyphasic potentials with multiple baseline crossings (at least four zero crossings producing five phases) result from asynchronous activation due to thin, poorly myelinated axonal sprouts 1
• Reduced recruitment with fewer motor units firing at higher rates to compensate for lost units 1
• Complex repetitive discharges (CRDs) may occur in chronic neuropathies, starting and ending abruptly with a harsh, machinery-like sound 1

Distinguishing Axonal from Other Patterns

Axonal Neuropathy vs. Myopathy

• Axonal neuropathy produces long-duration, large-amplitude MUAPs, while myopathies produce short-duration, small-amplitude MUAPs 1
• The American Thoracic Society emphasizes distinguishing between axonal and demyelinating patterns in peripheral neuropathies 3

Additional Diagnostic Features

• Fasciculations (spontaneous motor unit discharges with irregular pattern) suggest neuropathy or motor neuron disease 1
• Presence of denervation (fibrillations and positive sharp waves) indicates axonal interruption, a major determinant of recovery completeness and speed 2

Clinical Correlation and Timing

Important Caveats

• Early EMG may be falsely normal: Denervation changes require 2-3 weeks to develop after nerve injury, so EMG performed too early may miss pathology 3, 2
• The American Academy of Neurology notes that early in disease processes, EMG results may be normal or near-normal 3

Prognostic Value

• The presence and extent of denervation indicates the severity of axonal interruption 2
• Serial EMG examinations can track disease progression or recovery over time 3
• Quantitative EMG can objectively measure disease severity 3

Practical Application

The American Academy of Neurology and American Association of Neuromuscular and Electrodiagnostic Medicine recommend EMG to detect motor neuron diseases through characteristic patterns of denervation and reinnervation 3. Needle EMG has 90% sensitivity for detecting radiculopathy, making it the most useful electrophysiologic test when evaluating nerve root lesions 4, 2.