Nerve Conduction Studies
Nerve conduction studies (NCS) allow the stimulation and recording of electrical activity from peripheral nerves. NCS is used to diagnose focal and generalised peripheral nerve disorders; aid in the differentiation between primary muscle and nerve disorders; classify peripheral nerve conduction abnormalities as being due to demyelination, axonal degeneration, or conduction block; and provide a prognosis on treatment effect and clinical course.
Anatomy and Normal Neurophysiology
Most peripheral nerves are mixed, comprising motor, sensory, and autonomic functions. There are certain primarily motor (e.g. anterior interosseous branch of the median nerve, posterior interosseous branch of the radial nerve) and primarily sensory fibres (e.g. sural nerve, peripheral peroneal nerve, superficial radial nerves) that allow neurophysiologic investigation.
Individual nerves are generally classified based on their diameters, conductivity, and myelin properties.
- A-alpha (or A-alpha beta): large myelinated fibers, 6 to 15 microns in diameter. The largest muscle afferent fibers are sometimes classified as 1a fibers.
- A-delta: small myelinated fibers, 3 to 5 microns in diameter.
- C fibers: unmyelinated fibers, 0.5 to 2 microns in diameter.
Most efferent motor fibres are A-alpha (large myelinated). A-alpha fibres also include sensory fibres providing touch, vibration, and position senses. Cold and pain sensations are mediated by A-delta fibres (small myelinated). Warm, itch, and pain sensations are mediated by C fibres (unmyelinated). C fibres also include efferent postganglionic sympathetic autonomic fibres.
Small-fibre neuropathy - where strength, vibration, and position are normal, but pain and temperature are abnormal - poses a problem with nerve conduction studies. This is because NCS can only evaluate the largest A-alpha fibres. In other words, NCS is normal in pure small-fibre neuropathies. A skin biopsy may be required for diagnosis in this case.
Compound action potentials
A-alpha fibres generate sensory nerve action potentials (SNAPs) and compound muscle action potentials (CMAPs). SNAPs and CMAPs are compound potentials representing summated electrical activity of individual nerve fibres with simultaneous activation of nerve stimulation. A-delta fibre electrical activity can be visualised through special techniques.
