Electrophysiologic evidence of spinal accessory neuropathy in patients with cervical myofascial pain syndrome.

OBJECTIVE: To evaluate whether or not spinal accessory neuropathy exists in patients with cervical myofascial pain syndrome (MFPS).
DESIGN: Prospective study.
SETTING: A neurophysiologic laboratory in a university hospital. PARTICIPANTS: Patients with cervical MFPS (n=25) and healthy controls (n=20).
INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: We performed nerve conduction studies (NCSs) in bilateral spinal accessory nerves, and electromyography and stimulated single-fiber electromyography in the trapezius muscles of all patients and controls. Parameters including nerve conduction velocities (NCVs), amplitudes and areas of compound muscle action potentials (CMAPs), and mean consecutive differences (MCDs) in single-fiber electromyography were measured, analyzed, and compared with the disease durations of the patients.
RESULTS: Spinal accessory NCSs showed normative NCVs but with prominently reduced CMAP amplitude in the patients with cervical MFPS, which is recognized as an axonal neuropathy of the spinal accessory nerves. Electromyography showed prominent evidence of denervation and reinnervation patterns in 48% of the MFPS patients. The abnormal MCDs in single-fiber electromyography indicated a synaptic delay of motor endplates in the motor units, and may signify evolving instability of neuromuscular transmission in the spinal accessory nerves innervating trapezius muscles of the patients.
CONCLUSIONS: This study demonstrates electrophysiologic evidence of neuroaxonal degeneration and neuromuscular transmission disorder in a significant proportion of patients with cervical MFPS. We suggest that spinal accessory neuropathy may be associated with cervical MFPS.