A study of dynamic F-waves in juvenile spinal muscular atrophy of the distal upper extremity (Hirayama disease)

Abstract

Objective: The study aimed to analyse changes in the upper limb F-waves during neck flexion in patients with Hirayama disease (HD).

Material and methods: This study included 41 healthy subjects, 38 HD patients and 24 patients with amyotrophic lateral sclerosis (ALS). Bilateral F-waves were consecutively recorded 20 times with the neck both in the standard position and after persistent neck flexion for 30min. The persistence, minimal latencies, chronodispersion, F/M ratios and amplitudes of the F-waves and repeater F-waves were compared between the standard neck and neck flexion positions.

Results: During neck flexion, repeater F-waves were found in more HD patients, the percentage of both the ulnar and median repeater F-waves increased significantly, and higher F/M ratios were observed on the symptomatic side (P<0.05). No differences in the F-waves were observed in the healthy subjects or the ALS patients during neck flexion (P>0.05).

Discussion: HD might be more likely to present as a position-related dysfunction rather than a spinal cord-intrinsic disease. Thus, HD patients could be counselled to avoid neck flexion for long periods of time to prevent further damage, especially in the progressive stage of the disease.

Keywords: Amyotrophic lateral sclerosis; F-wave; Hirayama disease; Neck flexion.