Lower Motor Neuron Disease Misclassified As Inflammatory Myopathy in a Young Woman With Myositis-Specific Antibodies.

A 30-year-old woman presented with slowly progressive neck and proximal limb weakness over three years. Initial evaluation showed moderate creatine kinase elevation, anti-Mi-2β and anti-glycyl-tRNA synthetase (EJ) positivity, and electrophysiological and muscle biopsy findings interpreted as inflammatory myopathy. She was diagnosed with polymyositis and received multiple immunosuppressive therapies without clinical improvement. Despite normalization of creatine kinase, weakness and muscle wasting progressed. Re-evaluation revealed profound generalized atrophy, hyporeflexia, widespread limb and tongue fasciculations, and preserved sensation. Nerve conduction studies showed markedly reduced motor amplitudes with relative preservation of sensory responses, while repeat muscle biopsy demonstrated severe chronic neurogenic atrophy. Genetic testing identified a heterozygous FIG4 p.Arg197Cys variant of uncertain significance. Lower motor neuron dysfunction was demonstrated in multiple anatomical regions, and the overall clinical and electrophysiological findings were consistent with the Gold Coast diagnostic framework for amyotrophic lateral sclerosis with a lower-motor-neuron-predominant phenotype. Immunosuppression was withdrawn, weakness continued to progress, and mechanical ventilation was required approximately six months later. This case highlights the need to reconsider the diagnosis when presumed inflammatory myopathy progresses despite biochemical improvement and adequate immunosuppressive therapy, particularly when neurogenic features emerge.