Positive Cytosolic 5-Nucleotidase 1A Antibodies in Motor Neuron Disease

Inclusion body myositis (IBM) is the most common acquired myopathy in adults older than 50 years. Muscle biopsy remains the gold standard for diagnosis. Recently described serum antibodies against cytosolic 5-nucleotidase 1A (cN1A) are considered highly specific for IBM. However, positive cN1A antibodies in diseases other than IBM are recently reported. We review 2 cases in which serum antibodies were positive but ancillary testing revealed motor neuron disease. A 68-year-old man presented with asymmetric quadriceps and handgrip weakness prompting concern for IBM. However, electromyography showed purely chronic neurogenic abnormalities, and muscle biopsy was consistent with post-polio syndrome. A 60-year-old woman reported a history of progressive muscle weakness. Despite positive antibodies, examination and electromyography were indicative of amyotrophic lateral sclerosis. Serum cN1A antibodies are not 100% specific for the diagnosis of IBM. Careful clinical, electrophysiologic, and histopathologic correlation is required in workup of individuals with neuromuscular weakness and positive antibodies.