Unmasking Bilateral Inferior Rectus Restriction in Thyroid Eye Disease: Using Degree of Cyclotropia

Abstract

Purpose Thyroid eye disease (TED) frequently causes strabismus and diplopia. Though the condition involves multiple extraocular muscles in both eyes, the inferior rectus (IR) muscle is particularly susceptible. TED may be so asymmetric as to mask the involvement of the contralateral IR. The purpose of this study was to determine if the degree of preoperative cyclotropia measured with an objective test could be used to predict the extent of bilateral IR disease and assist in surgical planning. Methods Adults with TED involving the IR and demonstrating cyclotropia were enrolled. Preoperative and postoperative sensorimotor exam included quantification of cyclotropia using double Maddox rod test (DMR). Forced ductions were done intraoperatively. Degree of cyclotropia was compared to results of duction tests and surgical outcome. Results Twelve participating patients demonstrated excyclotropia on DMR test preoperatively (mean 16 ± 9°). Unilateral IR recession was planned based on the results of preoperative ocular rotations and forced duction testing in eight cases. The remaining four underwent bilateral IR recession. Four of the unilateral recession cases presented with reversal of the hypotropia, obvious limitation of elevation in the unoperated eye, and persistent symptomatic excyclotropia at the first postoperative visit. The mean excyclotropia of the bilateral cases was 21 ± 5°, compared to 5.5 ± 3° in the unilateral group (P < 0.005). Conclusions Prism diopters of hypotropia, limitation of elevation, and forced duction testing evaluate muscle function relative to its yoke and may underestimate contralateral IR involvement in the case of asymmetrical disease. Duction testing and forced ductions are qualitative and subjective. Results suggest that degree of excyclotropia correlates well with severity of IR restriction, and that amounts exceeding 15° infer bilateral IR involvement.