Electrodiagnostic testing in pain medicine

Nerve conduction studies and needle EMG localize large-fiber lesions: radiculopathy spares sensory responses and shows paraspinal and myotomal denervation, plexopathy does not, and small fiber neuropathy needs skin biopsy because conduction studies are normal.

1. The dorsal root ganglion explains preserved SNAPs. The ganglion sits between the sensory root and spinal nerve, outside the central nervous system, so root compression proximal to it leaves distal sensory fibers intact and sensory nerve action potentials normal despite sensory loss ¹².
2. Needle EMG is specific, not sensitive. For cervical radiculopathy EMG sensitivity is about 50–71% with specificity approaching 100%; MRI is more sensitive but finds many clinically irrelevant abnormalities ¹.
3. Sample enough muscles. An adequate study samples at least six muscles from different myotomes, one of them a paraspinal muscle ¹.
4. EMG estimates chronicity. Acute lesions show fibrillations and positive sharp waves with reduced recruitment but normal motor units; chronic lesions show larger, longer, polyphasic units from reinnervation ¹.
5. Localize foot drop by competing levels. Weak inversion or hip abduction points to L5 rather than the fibular head; the short head of biceps femoris separates a proximal fibular-division sciatic lesion from common fibular neuropathy ².
6. Small fibers escape conduction studies. Nerve conduction is typically normal in small fiber neuropathy; distal leg skin biopsy for intraepidermal fiber density is the Level A test ³⁴.

Abnormal sensory responses spanning several nerves and root levels without paraspinal denervation suggest plexopathy, not radiculopathy; radiation, surgery and trauma are important causes ².

• Fibrillation potentials and positive sharp waves indicate active denervation and ongoing axonal loss ¹.
• Electrodiagnosis complements MRI by grading severity, localizing myotomes and detecting comorbid neuropathy ¹.
• Fibular motor studies stimulate across the fibular head to look for focal slowing or conduction block ².
• Common fibular neuropathy at the fibular head is linked to habitual leg crossing, prolonged positioning, substantial weight loss, knee trauma and ganglion cysts ².
• Ultrasound and MRI add structural information when focal nerve pathology is suspected ².
• Not all foot drop is peripheral; parasagittal cortical lesions can cause it ².
• Skin biopsy fiber density must be compared with age-matched norms ⁴.
• Small fiber involvement can also be shown with quantitative sensory testing, alongside compatible symptoms and signs ³.

• Expecting low sensory nerve action potentials in radiculopathy; they are usually preserved even with sensory loss ¹².
• Ordering repeat conduction studies for burning feet with normal reflexes and vibration ³.
• Treating a normal EMG as excluding radiculopathy, given its modest sensitivity ¹.
• Relying on MRI alone; imaging finds abnormalities that are not always clinically relevant ¹.