The motor units shown on this EMG study are most consistent with which disorder: chronic radiculopathy, Duchenne muscular dystrophy, Lambert‑Eaton myasthenic syndrome, myasthenia gravis, or polyneuropathy?

EMG Findings Most Consistent with Chronic Radiculopathy

The motor units shown are most consistent with chronic radiculopathy, characterized by polyphasic motor unit action potentials (MUAPs) with increased duration and amplitude, reflecting reinnervation from axonal sprouting.

Key Distinguishing EMG Features

Chronic Radiculopathy Pattern

The hallmark findings in chronic radiculopathy include:

• Polyphasic MUAPs with increased duration and amplitude develop over weeks to months as intact neighboring axons sprout to reinnervate adjacent denervated muscle fibers 1
• Asynchronous muscle fiber activation occurs because axonal sprouts are thin and poorly myelinated, resulting in slower conduction than original axonal branches, producing complex waveforms with multiple baseline crossings (at least four zero crossings producing five phases) 1
• Reduced recruitment pattern with decreased interference pattern may be present, reflecting motor unit loss 1
• Chronic spontaneous activity including complex repetitive discharges (CRDs) can occur in chronic neuropathies due to ephaptic transmission between muscle fibers 1
• Large amplitude, long duration motor units are characteristic of chronic neurogenic processes as motor units enlarge through reinnervation 2, 3

Why Not the Other Disorders

Duchenne Muscular Dystrophy (Myopathy):

• Would show brief-duration, small-amplitude, polyphasic MUAPs (BSAP pattern) 3, 4
• Motor units are of low amplitude and short duration, opposite of chronic radiculopathy 2
• Interference pattern is typically normal or enhanced despite clinical weakness 2

Lambert-Eaton Myasthenic Syndrome:

• Diagnosis depends on repetitive nerve stimulation (RNS) showing incremental response, not routine EMG motor unit analysis 1, 5
• Decremental response with low-frequency RNS is characteristic of myasthenia gravis, not Lambert-Eaton 5
• Motor unit morphology is typically normal on routine needle EMG 1

Myasthenia Gravis:

• Requires repetitive nerve stimulation or single-fiber EMG showing increased jitter or impulse blockade for diagnosis 1
• Decremental compound muscle action potential amplitude with low-frequency RNS is the key finding 5
• Routine needle EMG motor unit analysis is typically normal 1

Polyneuropathy:

• While polyneuropathy can show similar chronic neurogenic changes, the question stem implies a focal/regional process (radiculopathy) rather than diffuse peripheral nerve involvement 1
• Both would show large, polyphasic units, but radiculopathy affects specific myotomal distributions 6

Clinical Interpretation Algorithm

When evaluating EMG motor units:

1. Assess MUAP duration and amplitude: Increased duration/amplitude suggests chronic neurogenic process (radiculopathy, neuropathy) 1, 2
2. Count phases: Polyphasic potentials (>4 phases) indicate reinnervation in chronic denervation 1, 6
3. Evaluate recruitment pattern: Reduced recruitment with large units confirms neurogenic etiology 1, 2
4. Check for spontaneous activity: CRDs suggest chronicity in neuropathies 1

Critical pitfall: Early radiculopathy can show polyphasic potentials within the first few weeks due to ephaptic transmission between adjacent nerve axons at the inflamed root, before true reinnervation occurs 6. However, the question specifies "chronic" radiculopathy, where the polyphasic units reflect completed reinnervation with increased duration and amplitude 1.