The spatio-functional cortical representation of muscles innervated by the accessory nerve using nTMS

The accessory nerve is frequently affected by injury which causes disability due to a loss of function in the trapezius muscle and the sternocleidomastoid muscle. Navigated transcranial magnetic stimulation (nTMS) may be a valuable tool to quantify aspects of cortical motor reorganization.

nTMS was used to stimulate the motor cortex in 15 healthy volunteers and 3 accessory nerve lesion patients to determine the representation of the sternocleidomastoid muscle, trapezius, deltoid, biceps, first dorsal interosseus and zygomatic major muscles.

The center of gravitiy (CoG) of the sternocleidomastoid muscle was located between the face (zygomatic major muscle) and hand (first dorsal interosseus muscle). CoG of the trapezius muscle was located superiorly to the hand. Distances to the lateral sulcus differed significantly (median: trapezius muscle 54.9 [9.4] mm, sternocleidomastoid muscle 30.1 [15.7] mm, p < 0.001). The radius (in mm) of the area of the sternocleidomastoid muscle was significantly larger compared to the trapezius (14.3 [5.7] and 11.5 [6.9], p = 0.004).

This study describes the detailed spatio-functional representation of trapezius and sternocleidomastoid muscle in comparison to four other muscles.

Our data may serve as a baseline for future studies on patients with accessory nerve injuries and a decision-making tool for the indication and timing for reconstructive surgery.