Differential involvement of forearm muscles in ALS does not relate to sonographic structural nerve alterations

We aimed to assess whether differential peripheral nerve involvement parallels dissociated forearm muscle weakness in amyotrophic lateral sclerosis (ALS).

The analysis comprised 41 ALS patients and 18 age-, sex-, height- and weight-matched healthy controls. Strength of finger-extension and -flexion was measured using the Medical Research Council (MRC) scale. Radial, median and ulnar nerve sonographic cross-sectional area (CSA) and echogenicity, expressed by the hypoechoic fraction (HF), were determined.

In ALS, finger extensors were significantly weaker than finger flexors. Sonographic evaluation revealed peripheral nerve atrophy, affecting various nerve segments in ALS. HF was unaltered.

This systematic study confirmed a long-observed physical examination finding in ALS - weakness in finger-extension out of proportion to finger-flexion. This phenomenon was not related to any particular sonographic pattern of upper limb peripheral nerve alteration.

In ALS, dissociated forearm muscle weakness could aid in the disease's diagnosis. Nerve ultrasound did not provide additional information on the differential involvement of finger-extension and finger-flexion strength.