A 30-year-old G2POA1 African American female developed diplopia and fatigue during the late second trimester of pregnancy. A year earlier, she had experienced similar symptoms briefly, but they improved spontaneously. She had one previous pregnancy resulting in a spontaneous abortion at about 10 weeks. During the second pregnancy, her weakness worsened and she underwent a Cesarean section at 34 weeks gestation after polyhydramnios was discovered. The baby had Apgar scores of 3 at 1 minute and 6 at 5 minutes. He was floppy and areflexic and had to be intubated. Blood chemistries including serum creatine kinase were normal.
The mother remained weak after delivery and initial neurology consultation documented mild dysarthria and decreased neck flexion. She had positive edrophonium testing and characteristic decrement was demonstrated on repetitive stimulation on both the facial and the peroneal motor studies. She was started on pyridostigmine but subsequently decompensated and was admitted to the hospital. On examination she had bifacial weakness, ophthalmoplegia, nasal speech, and 4/5 strength in the proximal extremities with 4+/5 strength distally. She demonstrated labored respirations and had a forced vital capacity of 0.95 L. By that time, AChR antibody testing had returned negative.
What to do Next?
Muscle-specific kinase (MuSK) antibody titers were drawn and found to be present on radioimmunoassay immunoprecipitation performed by Athena Diagnostics. She was treated initially with intravenous immunoglobulin (IVIG) with little improvement, but she responded well to subsequent plasmapheresis.
Although he did not have electrophysiological testing, the baby received 5 mg/kg of pyridostigmine on days 10–14 when the mother’s workup appeared consistent with myasthenia gravis. He initially showed little improvement and underwent exchanged transfusion on day 11. He was then switched to intravenous neostigmine on days 15–39 with minimal response. IVIG was initiated on day 23 and given for 4 days. He improved during the course of IVIG with increased spontaneous respirations and movements. On day 40, he was weaned off neostigmine and was extubated on day 44. At 9 weeks, he appeared normal. At 2 years of age, he had normal development and neurological examination was unremarkable.
The mother was treated with steroids and later azathioprine which she did not tolerate and mycophenolate, but she required intermittent IgG infusions. She later was treated with rituximab and she responded very well and is very stable.
Summary
A young woman with MuSK antibody-positive myasthenia delivered a child with transient weakness. She responded to plasmapheresis and the baby responded to IVIG. The baby has been asymptomatic for 6 years and the mother is being treated with immunotherapy. This case is an example of the difficulties in myasthenia during pregnancy. This case also demonstrates that MuSK antibody myasthenia can manifest in the baby as a transient myasthenia.
Discussion
The patient had a MuSK antibody-positive myasthenia and her baby had transient weakness likely secondary to the antibodies crossing the placenta. New-onset myasthenia may occur in pregnancy; 41% of myasthenic patients have an exacerbation during pregnancy and 28% have an exacerbation in postpartum. Patients with MuSK myasthenia and their baby should be observed carefully as they develop weakness. Fortunately for the baby this weakness is transitory.
About 10% of patients with myasthenia are acetylcholine receptor antibody-negative and among them about 40% have antibodies against MuSK, a disorder that manifests with somewhat different phenotypes. These patients also do not seem to respond to thymectomy and anticholinesterase drugs. In myasthenia, there could be transitory weakness in the baby and this can occur before delivery and may result in polyhydramnios. There is an association of severity with an increased ratio of fetal to maternal antibody titers. These have not been demonstrated in MuSK myasthenia.
Anti-MuSK myasthenia can be difficult to recognize because patients manifest with bulbar signs which can be confused with signs of a motor neuron disease. Patients respond to plasma exchange and IVIG, but the response to anticholinesterase drugs is not very good. It appears that rituximab is the best treatment for MuSK-positive myasthenia.
Important Points
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MuSK antibody myasthenia gravis antibodies can be transmitted to the newborn causing transitory weakness similar to acetylcholine receptor-positive myasthenia.
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Patients’ babies should be treated symptomatically and if necessary using immunotherapy, but weakness is transitory.
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MuSK antibody-positive myasthenia responds well to rituximab.
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MuSK antibody-positive myasthenia might present with prominent bulbar signs.
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The response to anticholinesterase drugs and thymectomy is not good in MuSK-positive myasthenia.
This case was reported in O’Carroll P, Bertorini TE, Jacob G, Mitchell CW, Graff J. Transient neonatal myasthenia gravis in a baby born to a mother with new-onset anti-MuSK-mediated myasthenia gravis. J Clin Neuromuscul Dis. 2009;11(2):69–71.
References
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