Dr. Frankel’s Case of the Week:
Myasthenia Gravis
A 90 year old woman presented to clinic for horizontal diplopia for the past few months, that resolved with monocular occlusion. She was seen by my colleague a few weeks ago for the same complaint and it was thought to be due an 8 mm aneurysm abutting the oculomotor nerve. This aneurysm was longstanding and had been monitored by a neurologist. She was measured to have a 11 PD exotropia at that visit, with no lid ptosis. She had spectacles made with the appropriate base -in prismatic correction and was returning because her diplopia had not resolved with the new correction. Upon examination, I noted that she had a 10-11 PD concomitant esotropia. Due to this variability in diplopia, she was sent for a thyroid panel and AchR AB. Her TSH and TSI was normal with a slightly elevated T4, however she had a pre-exsisting thyroid abnormality. Her acetylcholine receptor antibodies were also elevated. When I called to patient to inform her of her lab results she stated that she recently started having shortness of breath. Concerned that this may be a Myasthenic crisis, I advised her to go to the ER immediately.
Myasthenia gravis (MG) is an autoimmune disorder affecting neuromuscular transmission, leading to generalized or localized weakness. There is an annual incidence of 0.25-2 patients per 100,000. Myasthenia is most commonly associated with antibodies against acetylcholine receptors in the post-synaptic motor end plate. There are 2 main forms of MG: ocular myasthenia and general myasthenia. Ocular myasthenia gravis only affects the muscles that move the eyes and eyelids. The symptoms of ocular myasthenia gravis include diplopia and ptosis, that usually varies and is worse at the end of the day. On the other hand, generalized myasthenia gravis affects muscles throughout the body. In addition to visual symptoms, generalized myasthenia gravis may cause trouble speaking, trouble swallowing, trouble breathing and weakness in the arms or legs.
All myasthenic patients should have a CT scan of the chest to rule out thymoma. About 7-26% of patient with myasthenia gravis have such a tumor.Thymic hyperplasia is common in young MG patients with positive AchR antibodies, especially women. Thymic tumors, found in 15% of patients with MG and in 32% of patients with myasthenic crisis , should be treated with thymectomy.
Myasthenic crisis is a complication of general myasthenia characterized by worsening muscle weakness, resulting in respiratory failure that requires intubation and mechanical ventilation. Fifteen to 20% of MG patients are affected by myasthenic crisis at least once in their life.
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