A 56-year-old woman presented with 4 months of progressive dyspnea. Six months before presentation, she recalled an episode of upper respiratory infection. Over the next 2 months, bilateral arm pain and weakness of proximal muscles developed in all four extremities. As the pain improved, she noted the onset of dyspnea.
The patient was afebrile with room air oxygen saturation of 96%. The thorax appeared normal on inspection. She had reduced motor strength 4/5 bilaterally in the biceps, triceps, and deltoid muscles without atrophy. The chest was clear to auscultation. An admission chest radiograph is shown in Figure 1
. Pulmonary function tests showed FVC of 1.79 L (55% of predicted); FEV1, 1.48 L (60% of predicted); FEV1/FVC, 83%; total lung capacity, 3.07 L (60% of predicted), Expiratory reserve volume, 0.39 L (36% of predicted); and functional residual capacity, 1.76 L (76% of predicted). Maximum inspiratory pressure and maximum expiratory pressure were 14 cm H2O (19% of predicted) and 24 cm H2O (15% of predicted), respectively. Basic metabolic panel, CBC count, thyroid, and hepatic function test results were normal. An autoimmune panel including antinuclear antibody, Westergren sedimentation rate, rheumatoid factor, creatine kinase, and aldolase was negative. Serum protein electrophoresis was normal. Acetylcholine receptor and calcium-channel antibodies were not detected. Phrenic nerve conduction studies showed bilateral phrenic axonopathy with severely diminished action potential amplitude of 30 mV on the right and 100 mV on the left (normal, 160 to 500 mV). Electromyography of shoulder muscles was normal. Nerve conduction studies of the median, ulnar, and radial nerves were normal. A brain MRI was normal.