Radiotherapy in thyroid eye disease: the effect on the field of binocular single vision

Thyroid eye disease (TED) can be a functionally disabling condition if ocular muscle involvement causes diplopia. The extraocular muscle restriction creates a reduced or eccentric field of binocular single vision (BSV). Orbital radiotherapy is now widely used in the treatment of TED, and although it has been reported as improving ocular motility, there have been few quantitative studies of the effect of treatment on ocular motor function.

Retrospective case note review of patients undergoing orbital radiotherapy for TED between 1992-1998 identified 79 case records. A total of 27 patients had diplopia in primary position or a significantly reduced binocular field before undergoing radiotherapy. The fields of BSV were analyzed pretreatment and at 3, 12, and 24 months after therapy to assess any improvement in function. We used the field of BSV as an outcome measure because it can be quantified and is a good indicator of functional ability.

None of the 12 patients with double vision in primary position pretreatment regained a central binocular field with radiotherapy alone. Of the 15 patients with a central but reduced binocular field, 8 (53%) remained unchanged with treatment. In 4 patients (26.6%), there was an improvement in the field, while in 3 (20%) the field deteriorated. In all, 12 patients (44%) went on to require strabismus surgery.

Orbital radiotherapy alone is ineffective in treating restrictive thyroid myopathy and improving binocular function.