Abstract
Autoimmune myasthenia gravis (MG) is a well-characterized post-synaptic disorder of neuromuscular transmission. Immunologically, there is complement activation with autoantibodies binding to the acetylcholine receptor (AChR), leading to cross-linking and internalization of the receptor. The diminished functional clustering leads to impaired folding of the post-synaptic membrane. The antibodies generated by the autoimmune process are directed at the various components of the post-synaptic membrane and its scaffolding, including the AChR, muscle-specific tyrosine kinase (MuSK), low-density lipoprotein receptor-related protein 4 (LRP4), and the other recently described epitopes including the extracellular membrane proteins agrin and collagen Q (ColQ). MuSK MG is phenotypically different from classic AChR-antibody-mediated MG by a more frequent presentation of bulbar weakness, less responsiveness to symptomatic therapy with acetylcholinesterase inhibitors, the absence of a thymoma, and a better therapeutic response to a cluster of differentiation (CD-20) B-cell therapy such as rituximab. The pleiotropic ocular findings of ocular MG include ptosis, fluctuating and variable involvement of cranial nerves III, IV, and VI, pseudo-internuclear ophthalmoplegia (INO), near-complete or complete ophthalmoplegia, and variable gaze palsies. To our knowledge, we present one of the very few reported cases of MuSK MG presenting as isolated sixth nerve palsy. The localization of a sixth nerve palsy with lateral rectus muscle weakness can be due to disease anywhere along its path from the abducens nucleus, coursing at the skull base through Dorello's canal, through the cavernous sinus, and along its path through the superior orbital fissure and into the orbits. A painless sixth nerve palsy should alert the clinician to MuSK-MG as we outline in this case report.
Highlights
Ocular myasthenia gravis (MG) is characterized by fluctuating weakness of the ocular skeletal muscles which worsens with activity and improves with rest, a cardinal feature of MG
The antibodies generated by the autoimmune process are directed at the various components of the post-synaptic membrane and its scaffolding, including the acetylcholine receptor (AChR), muscle-specific tyrosine kinase (MuSK), low-density lipoprotein receptorrelated protein 4 (LRP4), and the other recently described epitopes including the extracellular membrane proteins agrin and collagen Q (ColQ)
We present one of the very few reported cases of MuSK MG presenting as isolated sixth nerve palsy
Summary
Ocular myasthenia gravis (MG) is characterized by fluctuating weakness of the ocular skeletal muscles which worsens with activity and improves with rest, a cardinal feature of MG. Triple-negative cases are seronegative for MuSK, AChR, and LRP-4 antibodies. A few such patients harbor agrin and ColQ antibodies [7]. With eye-straining, he developed a dull bitemporal headache He denied any vision loss, dysarthria, dysphagia, chewing difficulty, neck weakness, or breathing difficulty. His condition remained static until he presented to the neurology clinic one month later. Past medical history was significant for bilateral below-knee amputations due to peripheral vascular disease secondary to smoking, which he quit many years ago Otherwise, he was in relatively good health. The patient was advised about the potential for bulbar weakness and myasthenic crisis
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