Muscle histopathology in upper motor neuron-dominant amyotrophic lateral sclerosis

The distinction between primary lateral sclerosis (PLS) and amyotrophic lateral sclerosis (ALS) still remains debated. Recently, PLS patients displaying lower motor neuron (LMN) signs have been defined as 'upper motor neuron (UMN)-dominant ALS', using 'clinically pure PLS' diagnosis to those with no LMN signs. To further characterize the LMN involvement in UMN-dominant ALS we investigated the presence and the extent of neurogenic abnormalities in the skeletal muscle of patients affected with a pyramidal syndrome consistent with UMN-dominant ALS. A total of nine patients affected with UMN-dominant ALS were analysed. In all cases, muscle biopsies showed the presence of scattered or clustered atrophic angulated fibres in small groups, and a mild to moderate fibre type-grouping. Target and targetoid fibres were detected in two cases only. Three patients had a second muscle biopsy which demonstrated a roughly unchanged pattern of chronic denervation with still moderate reinnervation phenomena. This study suggests that in UMN-dominant ALS muscle denervation may be characterized by an early chronic impairment of a restricted number of LMNs. The extent rather than the presence of LMN signs may allow to categorize patients with motor neuron disease involving mainly UMN into distinct entities.