"Seronegative" anti-MUSK positive myasthenia gravis presenting during pregnancy

Myasthenia gravis is an autoimmune disease characterized by weakness and fatigue of the skeletal muscles of the face and extremities. Pregnancy is associated with 19-41% of exacerbations. We present a 19-year-old woman who was seen at the neurology clinic 10 days after her last cesarean section with diplopia and trouble swallowing liquids and solids for the previous 3 months. On examination she had slurred speech, bilateral palpebral ptosis, and mild droop of her shoulders. An arterial blood sample showed a PCO2 of 50 mmHg and a PO2 of 70 mmHg. The tensilon test was positive, Intravenous methylprednisolone and immunoglobulin and oral pyridostigmine effectively relieved her myasthenic signs and symptoms. Five assays for acetylcholine or anti-striational antibodies in her serum were negative. A test for muscle-specific tyrosine kinase antibody was positive. In 10-20% of patients with myasthenia gravis no antibodies to acetylcholine receptors are detectable. Forty percent of this group has an antibody to muscle-specific tyrosine kinase.