The presence of spontaneous EMG activity in sternocleidomastoid is associated with ventilatory dysfunction in ALS

Summary

We investigated electromyography (EMG) of the sternocleidomastoid (SCM) in 128 patients with amyotrophic lateral sclerosis (ALS) including correlation with forced vital capacity (FVC) and ALS Functional Rating Scale scores. The presence of fibrillation potentials and positive sharp waves in the SCM was significantly more frequently observed in patients with an FVC < 80% (31/49, 63%) than in patients with an FVC ≥ 80% (34/79, 43%). This study suggests that the SCM could be concomitantly involved with primary respiratory muscles in ALS. This is of value since needle EMG is a less invasive procedure in SCM than in the diaphragm or intercostal muscles.

Résumé

Nous avons étudié l’activité électromyographique (EMG) des muscles sterno-cléido-mastoïdiens (SCM) chez 128 patients atteints de sclérose latérale amyotrophique (SLA), y compris les corrélations avec la capacité vitale forcée (CVF) et les scores des échelles d’évaluation fonctionnelle de la SLA. La présence de potentiels de fibrillation et de potentiels positifs à front raide dans les SCM était significativement plus fréquente chez les patients avec une CVF < 80% (31/49, 63 %) que chez les patients avec une CVF ≥ 80% (34/79, 43 %). Cette étude suggère que l’atteinte des SCM pourrait être concomitante de celle des muscles respiratoires principaux dans la SLA. Ceci est à souligner car la détection EMG à l’aiguille est une procédure moins invasive dans le SCM que dans le diaphragme ou les muscles intercostaux.

Introduction

The sternocleidomastoid (SCM) muscles are currently regarded as alternative muscles for investigating the bulbar area in patients with amyotrophic lateral sclerosis (ALS) [8], [9]. The SCM muscles are innervated by lower motor neurons of the lower medulla oblongata and the C2–C3 anterior horn [2], [7], and the diaphragm is innervated by lower motor neurons of the C3–C5, which are primarily located in the C4 anterior horn [11]. The two regions are tightly adjoined, and their motor neurons may be concomitantly involved in ALS. As axial muscles, the lower motor neurons of the SCM are more medially situated in the anterior horn, similar to those of the diaphragm, compared with those innervating the distal muscles of the limbs [5]. Pinto et al. found that SCM compound muscle action potential (CMAP) amplitudes, diaphragm CMAP amplitudes and many respiratory function measurements, including forced vital capacity (FVC) and ALS Functional Rating Scale (ALSFRS), were significantly different in a subgroup of ALS patients with neck weakness compared with a group without neck weakness. Their findings suggest that there is a parallel loss of motor units in the SCM and diaphragm [12].

To further investigate denervation of the SCM and its potential role in enabling judgment of ventilatory dysfunction and prognosis in Chinese patients with ALS, we studied needle electromyography (EMG) changes of the SCM and their correlation with FVC and ALSFRS.