thyroid myopathy

Thyroid myopathy is the restrictive myopathy of thyroid eye disease. Ocular motility is restricted first by inflammatory edema and later by fibrosis, and 30% to 50% of patients develop an ophthalmoplegia that may be permanent.¹ Optometrists are more likely to see the inactive fibrotic phase, which produces a mechanical incomitancy. The condition is bilateral but can be asymmetrical, and diplopia typically comes on gradually.²

Limitation of elevation is the most common motility defect, caused by fibrotic contracture of the inferior rectus, and it may mimic a superior rectus palsy.¹ Limitation of horizontal and then vertical gaze follow in frequency, from myopathy of the medial and superior rectus, and the lateral rectus is least commonly involved.³ Medial rectus fibrosis produces an abduction defect that may simulate a sixth nerve palsy.¹

The ocular myopathy does not necessarily reflect the state of thyroid activity. Evaluation by an internist may find hyperthyroidism, euthyroidism or even hypothyroidism.³ Smoking is the most important risk factor for thyroid eye disease.²