To raise awareness of vitamin B12’s role in respiratory dysfunction.

Vitamin B12 deficiency causes numbness, sensory ataxia, gait dysfunction, hypotonia, psychosis, dementia and a myriad of other symptoms, however, it’s rarely implicated as the culprit in isolated respiratory muscle weakness.

A 67-year-old male with a history of asthma, atrial fibrillation and diabetes mellitus presented with progressively worsening breathing difficulty over 1 year. Family history included amyotrophic lateral sclerosis, dermatomyositis and Charcot-Marie-Tooth (CMT) type 1. Pulmonary function testing showed a forced vital capacity at 41% of predicted value indicating moderate restrictive lung disease. Examination revealed subtle weakness of shoulder abduction and hip flexion with mild hyporeflexia of the triceps, patellar and achilles tendons.

Computed tomography of the thorax showed no evidence of lung pathology. Electrodiagnostics including phrenic nerve testing and repetitive nerve stimulation showed no evidence of neuromuscular junction disease, motor neuron disease, neuropathy, or myopathy. Diaphragm electromyography was not performed due to anticoagulation. Creatinine kinase and aldolase levels were normal. Antibody testing for acetylcholine receptors and voltage gated calcium channels was negative. Lyme serology, Hemoglobin A1c, serum protein electrophoresis and thyroid function testing were unremarkable. Urine Glc4 testing (quantitative urine glucose), lactate dehydrogenase, aspartate aminotransferase and GAA enzyme activity were normal. Genetic testing for CMT1 was negative. Vitamin B12 level was < 150 pg/ml (Ref: 211 - 946 pg/ml). Methylmalonic acid and homocysteine levels were elevated. Anti-intrinsic factor and anti-parietal cell antibodies were normal.

Repletion of vitamin B12 with intramuscular injections resulted in gradual improvement of respiratory dysfunction with repeat forced vital capacity at 72% on repeat testing 8 months later. At 6-month follow up, our patient continued to be at baseline respiratory function. This is a rare case of predominantly respiratory muscle dysfunction that resolved with vitamin B12 repletion. B12 deficiency should be in the differential for isolated muscle weakness.