Accessory nerve latency to the middle and lower trapezius.

Trapezius muscle dysfunction occurs following injury to the spinal accessory nerve from a variety of causes. The trapezius consists of three distinct segments: upper, middle, and lower. Since previous studies of accessory nerve latency measure conduction to the upper trapezius segment only, this present study describes a standardized technique for determining accessory nerve latency to the middle and lower trapezius, a particularly important measurement for accurate examination for those who are in pain or unable to cooperate. Mean normal latency in 21 nerves to the middle trapezius is 3.0 +/- 0.2 msec and to the lower trapezius is 4.6 +/- 0.3 msec. Accessory nerve latency measurement of the middle and lower trapezius segments is valuable in a case of surgical injury in the posterior cervical triangle, especially during recovery from accessory nerve injury when reinnervation of the upper segment precedes the middle and lower segments.