EMG and Nerve Conduction Studies in Traumatic Peripheral Nerve Injury
DNB Orthopaedics • October 2024 • Paper I • 10 Marks
Introduction
Electrodiagnostic studies, principally nerve conduction studies (NCS/NCV) and electromyography (EMG), are important adjuncts in the evaluation of traumatic peripheral nerve injury. They help determine the anatomical level of injury, physiological severity, presence of axonal loss or demyelination, and evidence of subsequent reinnervation.
Core distinction
NCV/NCS evaluates conduction through a peripheral nerve.
Needle EMG evaluates electrical activity within muscle
supplied by that nerve.
Basic Definitions
| Term | Definition |
|---|---|
| Latency | Time interval between electrical stimulation of a nerve and onset of the recorded response. |
| Conduction velocity | Speed at which an electrical impulse travels along a segment of peripheral nerve. |
| Evoked response | Electrical response recorded following stimulation; usually a CMAP in motor studies or SNAP in sensory studies. |
| CMAP | Compound Muscle Action Potential — summed electrical response of muscle fibres supplied by stimulated motor axons. |
| SNAP | Sensory Nerve Action Potential — summed electrical response of sensory axons. |
Physiological Basis of Nerve Conduction
An electrical stimulus depolarises peripheral nerve axons. The resulting action potential propagates along the nerve and is recorded either from another point on the nerve or from a muscle…