Aims:We compared myopathological features in myasthenia gravis (MG) patients with antibodies against AChR (seropositive) and muscle-specific tyrosin-kinase (MuSK). While the immunopathogenesis of seropositive MG is well known, there is a lack of pathological studies in anti-MuSK antibody-positive (MuSK+) MG. Methods: We analysed skeletal muscle biopsy features of 13 MG patients: 6 MuSK+ (all women) and 7 anti-AchR antibody-positive (AChR+) (2 women and 5 men). In our histopathological examination, we quantified the atrophy factor of both fibre types, and the extent of minicores, myofibrillar disarray, cytochrome c oxidase (COX)-negative fibres, mitochondrial aggregates and fibre type grouping. Results: Mean muscle fibre atrophy factor was higher in AChR+ MG than MuSK+ MG, both in type I fibres (494 vs. 210) and particularly in type II fibres (1023 vs. 300). Fibre type grouping was observed in AChR+ MG whereas COX-negative fibres were common in MuSK+ MG. Bulbar muscles were more severely affected in MuSK+ MG and the disease was more severe: the onset was usually earlier (39 years) with Myasthenia Gravis Foundation of America score III in MuSK+ MG, and score II was found in AChR+ MG (62 years). Conclusions: Muscle biopsies of MuSK+ MG show myopathic signs with prominent mitochondrial abnormalities, whereas neurogenic features and atrophy are more frequently found in AChR+ MG. The mitochondrial impairment could explain the oculo-bulbar involvement in MuSK+ MG.

Autoimmune myasthenia gravis (MG) is a neuromuscular disorder caused by autoantibodies directed against the acetylcholine receptor (AChR). Current symptomatic therapy is based on acetylcholinesterase (AChE) drugs. The available long-term current therapy includes steroids and other immunomodulatory agents. MG is associated with the production of a soluble, rare isoform of AChE, also referred as the “read-through” transcript (AChE-R). Monarsen (EN101) is a synthetic antisense compound directed against the AChE gene. Monarsen was administered in 16 patients with MG and 14 patients achieved a clinically significant response. The drug is now in a Phase II study. Further investigations are required to confirm its long-term effects.

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.

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