Presentation
Ocular and bulbar muscle weakness.
Patient Data
The thymus is of normal size and density for age. The AP diameter measures 2 cm, with each limb measuring <1 cm in size. No discrete thymic mass is identified.
No anterior superior mediastinal mass is seen. The lungs and pleural angles are clear. The heart, mediastinum, pulmonary vessels, and diaphragm all appear normal.
Status post thymectomy. Surgical staples are visualized within the mediastinum. No other interval changes from prior.
Case Discussion
Myasthenia gravis (MG) is an autoimmune disorder characterized by weakness and fatigability. The annual incidence is roughly 7 to 23 new cases per million; the prevalence is about 70 to 320 per million and has a bimodal distribution.
Most patients presenting with generalized myasthenia have detectable autoantibodies. Over 50% of all patients with MG initially present with isolated ptosis and/or diplopia. However, only 15% of all MG patients manifest as ocular MG, which is an isolated form of disease that does not generalize to involve other muscle groups.
The extraocular muscles express fetal acetylcholine receptors at the neuromuscular junction, a unique feature to this skeletal muscle group. More than 85%of patients with generalized MG have autoantibodies against the acetylcholine receptor. However, the sensitivity for testing these autoantibodies in ocular MG is low. Thus, electrodiagnostic studies play an important role in confirming a diagnosis in seronegative patients.
Treatment involves both pharmacological and non-pharmacological approaches. There is evidence that thymectomy may benefit MG patients regardless of the presence of a thymoma. In nonthymomatous MG, thymectomy has shown to reduce immunosuppression requirement and number of hospitalizations for myasthenia crises 2. Our patient underwent thymectomy for nonthymomatous MG at age 13 years. Since thymectomy, she has presented with few episodes of myasthenia crisis in the last 8 years.
This case was submitted with supervision and input from:
Soni C. Chawla, M.D.
Associate Professor
Department of Radiological Sciences
David Geffen School of Medicine at UCLA
Olive View - UCLA Medical Center
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