Introduction: The presence of muscle-specific receptor tyrosine kinase (MuSK) defines a subset of patients with generalized myasthenia gravis. The diagnosis of this disease, due to its clinical distribution and negativity of the acetylcholine receptor (AchR) antibodies, is mo re complicated, especially when linked to other autoimmune diseases.
Case report: A 41 year woman with 3 month long symptoms consisting in diplopia, dysarthria and gait instability. On examination, only mild paresis of the right external rectus without fatigability. The complementary tests performed were evoked potentials were normal including multimodal except for the brain magnetic resonance imaging that detected six-eight hyperintense periventricular nodular lesions in both semioval centers, with impairment of corona radiata and corpus callosum without contrast uptake. Given these findings, she was diagnosed of demyelinating disease and treated with megadoses of methylprednisolone for 5 days with mild clinical improvement. At one month, her condition deteriorated, presenting dysphagia and respiratory failure. The neurophysiological study was extended, and very pathological Jitter was detected in the frontal muscle. Despite the negative AchR antibodies, treatment was initiated with pyridostigmine bromide with poor response, admission to the intensive care unit and plasmapheresis due to a new respiratory episode being required. The torpid course and positive outcome of the MuSK antibodies have confirmed the diagnosis.
Conclusions: The detection of these antibodies and performance of neurophysiological tests in clinically deficient muscles are required for the diagnosis of these clinical forms, especially when the neuroimaging-based pathological findings do not justify the clinical course of the patient.