Motor unit number index (MUNIX) in the quantitative assessment of severity and surgical outcome in cervical spondylotic amyotrophy

Abstract

Objective: To assess the feasibility of motor unit number index (MUNIX) in the quantitative assessment of the cervical spondylotic amyotrophy (CSA).

Methods: MUNIX was recorded bilaterally on the abductor pollicis brevis, abductor digiti minimi, biceps brachii and middle deltoid in 41 normal controls and 47 patients with CSA (distal-type to proximal-type ratio: 25 to 22). Additionally, patients were assessed on handgrip strength (HGS), the disabilities of arm, shoulder and hand (DASH) and Medical Research Council (MRC) scales. These examinations were re-evaluated approximately 18 months after surgery in 37 of these CSA patients.

Results: MUNIX values were noticeably lower in the mainly affected muscles of CSA patients than those in controls (P < 0.05), and 49.0% (51/104) of the tested muscles with abnormal MUNIX measurements showed normal muscle strength. Significant correlations between MUNIX measurements and both DASH and MRC scores were observed in both CSA patient groups (P < 0.05). Postoperative longitudinal follow-up analysis identified significant increase in motor unit number in both CSA patient groups within approximately 18 months (P < 0.05), with or without improved measures of motor function.

Conclusions: A significant reduction in MUNIX values related to motor impairment was observed in CSA patients, even in the subclinical stage. Compared to measures of motor function, the MUNIX measurements in the patients with CSA improved more noticeably after surgical intervention.

Significance: MUNIX may serve as an available supplementary test to quantitatively evaluate the motor dysfunction in CSA and to track its progression, that is complementary to conventional electromyography.

Keywords: Cervical spondylotic amyotrophy; Disease progression; Motor dysfunction; Motor unit number index; Quantitative assessment; Surgical treatment.