Anti-musk antibody after thymectomy in a previously seropositive myasthenic child.

We report a young girl with myasthenia gravis (MG) who was first seen at age 2 for unexplained falls, diplopia, and ptosis. The patient was born as a full-term spontaneous vaginal delivery and spoke her first words and started walking at the age 10 months. The child was physically active initially and loved to play and run. At age 27 months, the patient started having difficulty rising up while she was lying or sitting. She also experienced increasing difficulty swallowing without any dyspnea. Diagnostic workup at age 2 revealed a positive neostigmine test with improved muscle strength. Electromyography showed a decrement of 20 to 28% with 3and 10-Hz repetitive stimulation in two nerve muscle pairs; anti-acetylcholine receptor antibody (anti-AChR Ab) levels were 0.6 and 1.3 on consecutive testing (N 0 to 0.4). Chest radiograph and CT scan did not reveal any additional findings. Laboratory analysis including thyroid profile and metabolic and hematologic panels was unremarkable. The young girl’s mother had a history of Hashimoto thyroiditis. Her paternal grand mother had primary biliary cirrhosis, and her paternal aunt had a mixed connective tissue disorder. The patient was started on treatment with pyridostigmine (7 mg/kg/day) in divided doses with minimal response. Her family elected to use IV immunoglobulin (IVIG), given a choice between plasmapheresis and IVIG. The patient did demonstrate better response to IVIG (2 g/kg) and subsequent low-dose methylprednisolone compared with the initial regimen with improvement in weakness. Thymectomy was advised at an earlier age because of her overall poor response to other treatment modalities. Following surgery the patient’s weakness and balance problems gradually improved and anti-AChR Ab became negative. Steroid requirements were decreased, and the patient lost her cushingoid appearance and improved furrowing of tongue, which was initially observed. Her symptoms were relatively controlled subsequently for another 18 months. At age 57 months, the patient began experiencing exacerbations of weakness. Detailed laboratory workup revealed the presence of anti-MuSK Ab and the absence of anti-AChR Ab. The patient was subsequently started on IVIG; later, low-dose methylprednisolone was added, to which patient showed improved response. Currently the patient is 7 years old and continues to respond well to the immune modulation therapy.