Focal epilepsy expands the phenotype of FUS-associated amyotrophic lateral sclerosis.

Epileptic seizures are an exceedingly rare manifestation in classic amyotrophic lateral sclerosis (ALS). Emerging case reports suggest a potential link between ALS associated with FUS gene mutations (FUS-ALS) and seizures, expanding the disease's phenotypic spectrum. A 25-year-old man presented with a 5-month history of rapidly progressive limb weakness, atrophy, and both upper and lower motor neuron signs, consistent with ALS. Notably, he developed two types of epileptic seizures during the disease course, including focal motor seizures and focal seizures evolving to bilateral tonic-clonic seizures. Electroencephalography revealed epileptiform discharges over the right frontotemporal region. Brain 18F-fluorodeoxyglucose positron emission tomography-computed tomography showed focal hypometabolism in the right frontal cortex. Genetic testing identified a de novo pathogenic FUS variant (c.1574C>T, p.Pro525Leu). The patient was diagnosed with FUS-ALS with comorbid focal epilepsy. Treatment with levetiracetam partially controlled the seizures, but motor function continued to deteriorate. This case highlights focal epilepsy as a comorbid feature in FUS-ALS, broadening the recognized phenotypic spectrum. Genetic testing for FUS mutations should be considered in young-onset ALS patients presenting with seizures.