Thyroid associated ophthalmopathy : treatment for hyperhyroidism and evaluation of methods for measuring saccadic eye movements

Abstract

Thyroid associated ophthalmopathy (TAO) is the orbital manifestation of autoimmune thyroid disease that clinically affects about 30-50 % of patients with Graves disease (GD). The clinical features of TAO are in most patients mild and transient. In some cases though, a severe orbital immune reaction develops, which may lead to permanent exophthalmos and double vision. Optic neuropathy in TAO is rare and denotes very severe and sight-threatening disease.

The objectives of the herein presented studies include first, the evaluation of techniques for the measurement of the velocity of saccades in normal subjects and patients with TAO and second, the assessment of the potential effect of treatment with anti-thyroid drugs or radioactive iodine for Graves hyperthyroidism on worsening or development of TAO.

Objective assessment of TAO and the detection of a potential subclinical extraocular muscle involvement in TAO may be difficult. Previous studies from our eye-movement laboratory have given way to optimism about using saccade velocity measurements in the detection of early and subclinical TAO. The magnetic scleral search coil (MSC) system has long been considered as a gold standard method for measuring saccadic velocity in the laboratory milieu. A modern development of the infrared reflection (IR) method was considered as a more suitable method for eye-tracking in the clinical setting. The technique had though until now not undergone evaluations for saccadic velocity measurements.

In the present studies saccadic eye movements were recorded in healthy subjects and patients with TAO with both the MSC and IR methods. The data were analysed regarding the characteristic amplitude-velocity relationship of the saccades (the main sequence). The results showed that the IR method generated saccadic velocities that were higher compared to the MSC method in healthy subjects. Intra individual as well as inter individual variability of the main sequence was shown with both methods, but was more pronounced with the IR recordings. No significant differences were shown for the main sequence relationship between patients with TAO and healthy controls with either of the recording systems.

The primary treatment for most patients with Graves hyperthyroidism is either radioactive iodine or anti-thyroid drugs. The results from the here presented study showed that the patients who were randomized to treatment with anti-thyroid drugs had a significantly lower risk for development or worsening of TAO than those randomized to treatment with radioiodine. Deterioration of the eye disease was not found to be related to either therapy among the patients who had TAO already before treatment for hyperthyroidism, whereas patients with no clinically detectable eye involvement from the start showed a significantly higher proportion of de novo development of TAO in the radioiodine treatment group. Smoking was confirmed as a risk factor for TAO. In the subgroup of smokers, the mode of treatment for hyperthyroidism did not significantly influence the outcome of worsening or development of TAO.