Nerve Conduction Studies:

NCV  STUDIES : There are a number of physical parameters that require correction or allowance for. The most important is temperature. The fastest motor nerve conduction velocity (FMNCV) is reduced by approximately 1 m/s per  ̊C temperature fall. Conventionally, studies are performed as close to a surface recorded temperature of 34  ̊C. If that is not achieved by adequate heating or the limb, rarely a temperature correction must be applied. Some measures of conduction require correction for limb length or height. Finally nerve conduction data alter with age. The motor conduction slows by 0.4– 1.7 m/s per decade after 20 years and the sensory by 2–4 m/s.

M o t o r  N C S   in   A x o n a l   L o s s : The most striking abnormality is a reduction in CMAP amplitude as fewer functioning motor axons are connected to muscle fibres. Since myelin is unaffected, the remaining axons conduct normally and one would expect latencies and conduction velocities to remain normal. However, with increasing motor axon loss some of the largest fastest conducting fibres will be lost. Therefore distal motor latency may be slightly prolonged (, 120% of normal limit) and conduction velocity slightly slowed (. 80% of normal limit). The dynamics and timing of an axonal insult can affect the abnormalities seen. Immediately after a traumatic complete transection of the nerve, the portion of the nerve distal to the lesion will be normal as there has not been time for axonal degeneration to occur. The CMAP amplitude will only start to fall a few days later. Conversely, if there is a very slow loss of axons in a generalised neuropathy, the remaining unaffected axons may have time to sprout new connections to muscle fibres that have lost their innervation (collateral reinnervation) and the CMAP may remain within the normal amplitude range even though the total number of nerve axons is smaller. However, the immature regenerating fibres have slower velocities due to the effect of the short internodal distances and this produces a more dispersed CMAP.

M o t o r  N C S   i n   D e m y e l i n a t i o n :  With loss of myelin thickness nerve conduction is slowed and, if severe enough, saltatory conduction fails (conduction block). NCS shows severely prolonged motor latencies and notably slowed conduction velocities. The precise changes seen depend on the site and extent of demyelination. If demyelination is very proximal then distal motor latency and conduction velocity may be normal in which case only F waves may show abnormalities.

                                  Brain and Spine Center, Williamsville, NY 14221 tel (716) 631-3555