🔬 Nerve Conduction Studies

Nerve Conduction Studies (NCS) are used to evaluate the function of peripheral nerves. By stimulating nerves electrically and measuring the resulting signals, clinicians can determine if there is damage to the nerve axons or the myelin sheath. It is a key tool for diagnosing entrapment neuropathies and systemic polyneuropathies.

Indications include symptoms of numbness, tingling, or weakness that suggest a peripheral nerve lesion. Common uses include diagnosing carpal tunnel syndrome, ulnar neuropathy at the elbow, and peripheral polyneuropathy (e.g., due to diabetes or B12 deficiency). It is also used to evaluate suspected radiculopathy (pinched nerves in the neck or back) and more complex neuromuscular conditions like Myasthenia Gravis (using repetitive nerve stimulation).

Surface electrodes are placed over a muscle (for motor studies) or along a nerve path (for sensory studies). A brief, mild electrical pulse is applied to the nerve at one point, and the resulting electrical response (Muscle Action Potential - MAP, or Sensory Nerve Action Potential - SNAP) is recorded at another point. The distance between the stimulation and recording sites is measured to calculate the conduction velocity. The procedure can be slightly uncomfortable due to the electrical sensations.

A normal study shows nerve impulses travelling at appropriate speeds (usually >50 m/s in the arms and >40 m/s in the legs) with robust amplitudes. Latencies (the time for the impulse to travel from the stimulus to the recorded response) should fall within established laboratory norms. There should be no evidence of conduction block (a sudden drop in amplitude along a nerve segment) or abnormal temporal dispersion.

Interpretation involves analysing three main components: Latency (the time taken for the fastest fibres to respond), Amplitude (reflecting the number of functioning axons), and Conduction Velocity (calculated over a distance). For example, in carpal tunnel syndrome, there is a characteristic prolongation of the sensory and motor latency across the wrist. Comparison with the contralateral side or reference values for age and temperature is essential.

Abnormal findings are broadly categorised into axonal or demyelinating patterns. Axonal loss (e.g., in diabetic neuropathy or vasculitis) is characterised by a reduction in the amplitude of the MAP or SNAP, while conduction velocity remains relatively preserved. Demyelination (e.g., in Guillain-Barré syndrome or Chronic Inflammatory Demyelinating Polyneuropathy) is characterised by significant slowing of conduction velocities, prolonged distal latencies, and conduction block. Focal slowing at specific sites (e.g., the wrist) indicates entrapment.

NCS is essential for the objective diagnosis of peripheral nerve disorders, helping to distinguish between polyneuropathies, mononeuropathies, and plexopathies. It is particularly useful in surgical planning for carpal tunnel syndrome and ulnar neuropathy. In the acute setting, it helps confirm a diagnosis of Guillain-Barré syndrome. When combined with Electromyography (EMG), it can differentiate between primary nerve disorders and primary muscle disorders (myopathies) or motor neurone disease.

Technical errors, such as incorrect electrode placement or inaccurate distance measurements, can lead to false calculations of conduction velocity. Failing to warm a cold limb can lead to an erroneous diagnosis of a demyelinating neuropathy. In patients with significant oedema, it may be difficult to stimulate the nerves or record a clear signal, leading to potentially misleading results.

NCS primarily assesses large-diameter myelinated fibres; it cannot detect disorders of small-diameter unmyelinated fibres (small-fibre neuropathy), which often present with pain and temperature loss. Results can be affected by the temperature of the limb (cold limbs show slower conduction), which must be controlled. It is also limited in the very early stages of a nerve injury (e.g., Wallerian degeneration takes several days to manifest on NCS).

MLA students should know: NCS/EMG is the 'gold standard' for carpal tunnel diagnosis. Distinguish between axonal (low amplitude) and demyelinating (slow velocity) patterns. Be aware that most 'routine' NCS only tests large fibres; a patient with burning pain but normal NCS may have a small-fibre neuropathy (often diagnosed by skin biopsy).