A 48-year-old woman with thyroid eye disease reports vertical diplopia. Examination shows limitation of elevation of the right eye, with raised intraocular pressure in upgaze and normal pressure in downgaze. Forced duction testing shows restriction to passive elevation. These findings are best explained by:
- A Paralysis of the superior rectus
- B Superior oblique overaction
- C Contracture of the enlarged inferior rectus muscle ✓
- D Myasthenia gravis associated with Graves' disease
Correct answer: C. Contracture of the enlarged inferior rectus muscle
Explanation
In thyroid eye disease, glycosaminoglycan deposition and later fibrosis enlarge the inferior rectus, making it inelastic. A tight inferior rectus limits elevation of the globe, producing pseudo-paresis of the ipsilateral elevators, a positive forced duction test, and pressure rise on attempted upgaze because the contracted muscle compresses the globe. Myasthenia and true rectus palsy give negative forced ductions, which excludes them.
Reference: Kanski's Clinical Ophthalmology, 9th ed.
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