Co-Occurrence of Multiple Sclerosis and Amyotrophic Lateral Sclerosis in an FUS-Mutated Patient: A Case Report

A concomitant presentation of relapsing remitting multiple sclerosis (RRMS) and amyotrophic lateral sclerosis (ALS) is quite rare. However, a review of the literature showed an increased co-occurrence of both diseases, including in genetically determined cases. We report the case of a 49-year-old woman with a history of RRMS who developed a progressive subacute loss of strength in her left arm. The patient's father died from ALS, and her paternal uncle had Parkinson's disease. Brain and cervical MRIs were performed, and new demyelinating lesions were excluded. Electromyography (EMG) of the upper limbs showed fibrillations and fasciculations in distal muscles of both arms. In the following months, the patient presented a progressive loss of strength in the proximal and distal muscles of the right arm and hyperreflexia in the lower limbs. EMG and central motor conduction were consistent with ALS. A genetic test was carried out, revealing a mutation in the FUS gene (exon 15; c. 1562 G>A). To our knowledge, the co-occurrence of MS and ALS in patients with FUS mutation is extremely rare. We hypothesize a common pathway for both diseases based on the possibility of a shared oligodendroglial dysfunction due to FUS mutation.