Myasthenia gravis can frequently be complicated by respiratory failure. However, myasthenic involvement limited solely to the muscles of ventilation has not been reported in well-characterized patients. In retrospective series of 22 myasthenics requiring mechanical ventilation reported by Gracey et al, only four presented in this manner. None was described as having isolated respiratory failure. The conclusions of two earlier series by Ferguson et al (31 patients) and Ashworth and Hunter (13 patients) were similar. The latter specifically stated that, “respiratory failure was never the first symptom of the disease.” Factors leading to respiratory failure in these reports included myasthenic crisis, cholinergic crisis, brittle crisis, steroid induced crisis, postoperative state, and other medical conditions typically requiring mechanical ventilation not unique to myasthenia gravis. Only a single case report was found describing a patient who initially presented with ocular myasthenia and later returned with isolated respiratory failure.
Unlike previously described cases, our patients myasthenic involvement was limited to the ventilatory musculature, and multiple tests commonly used to confirm clinical suspicion of myasthenia gravis were negative. It is possible that 10 mg of edrophonium was an inadequate test dose for assessing improvement in respiratory mechanics—though Osserman and Genlans’ 1966 review of the edrophonium test stated less than 0.5 percent of cases require more than 10 mg to produce a response. Standard repetitive nerve stimulation and electromyography also were not helpful. Ultimately, serologic studies confirmed the diagnosis.
Despite conventional treatment with acetylcholinesterase inhibitors and steroids, respiratory insufficiency persisted and extubation was not possible. Plasmapheresis, as reported by Pinching and Peters, and others, was then employed with success.
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This case of myasthenia gravis was unusual in three respects: the patient presented with what appeared to be primary respiratory failure of unknown etiology; associated symptoms of myasthenia were suggested by history but could not be verified by objective means. Diagnosis was difficult despite a correct clinical impression in that several edrophonium trials as well as other standard tests for myasthenia gravis were negative. Treatment following diagnosis with accepted firstline agents, acetylcholinesterase inhibitors and corticosteroids, failed to produce an adqe- quate response such that plasmapheresis was required before the patient could be weaned from ventilatory support.
This case illustrates the need to consider myasthenia gravis, as well as other motor neuron disorders, in evaluating individuals presenting with acute respiratory failure. The former should be aggressively pursued beyond conventional firstline tests, since diagnosis can be difficult and effective treatment is available.