Saccadic eye movement velocity measured with the infrared reflection and search coil eye-tracking systems in patients with thyroid-associated ophthalmopathy.

BACKGROUND: Assessment of changes in saccade velocity may be useful in the early detection of thyroid-associated ophthalmopathy (TAO). Two eye-tracking systems were used to measure the velocity of saccadic eye movements in patients with TAO. METHOD: Fourteen patients with active TAO and 14 healthy controls were enrolled for recordings with two eye-tracking systems: the magnetic scleral search coil (MSC; Skalar Medical) and the infrared reflection systems (IR; Orbit XY-1000). The MSC is generally considered the "gold standard" method for tracking of rapid eye movements. The IR system uses novel computer technology and is based on sampling of reflected infrared light from the surface of the eyes. Main sequence plots constructed from the recorded saccadic peak velocity and amplitude were analyzed for differences between patients and healthy controls. RESULTS: There were no significant differences between patients with TAO and healthy controls in the constructed main sequence plots of maximum velocity (V(MAX)) and the slope constant (C). CONCLUSIONS: Main sequence analysis of saccadic eye movements was not useful for detecting TAO in this experimental setting with either of the two eye-tracking methods. This is hypothesized to be due to compensation for the early orbital changes in TAO by neural adaptation of the saccades in the brain stem. The contradictory results between this and previous studies cannot be easily explained. We assume that there is a large heterogeneity in the saccadic eye movement performance in both the normal population and the TAO-population. Also, differences in the study design may cause dissimilar outcomes and hence incoherent conclusions. A non-invasive recording system that is able to generate a minimum amount of intra- and inter-individual variability and a study design where normal variability can be reduced to a minimum may be useful for future identification of early eye muscle changes in TAO.

Intra-individual variability of saccadic velocity measured with the infrared reflection and magnetic scleral search coil methods.

BACKGROUND: The infrared (IR) and the magnetic scleral search coil (MSC) systems for eye tracking were studied with regard to the intra-individual variability in saccadic eye movement recordings. METHOD: Three healthy subjects performed similar saccadic eye movement tasks at five different occasions with both the IR (Orbit XY-1000) and the MSC (Skalar Medical) techniques. The maximum velocity (VMAX) and slope constant (C) of the main sequence plots were analyzed with regard to the coefficient of variation (CV) and the intraclass correlation coefficient (Ricc). In addition, the possible reasons for variability in the IR recordings, especially different causes for noise, were analyzed and discussed. RESULTS: The main sequence data showed intra-individual variation with both recording systems, but the coefficient of variation was higher for VMAX with the IR compared to the MSC method. Ricc analysis showed that 36% of the variance of VMAX and 49% of the variance of C resulted from intra-individual variability in recordings of the IR system. The corresponding results for the MSC recordings regarding VMAX and C were 48% and 88%. CONCLUSIONS: Saccadic eye movement recordings yielded a larger intra-individual variability with the IR system than with the MSC system. The effect that the MSC annulus may have on the ocular motor command signal and the possible low pass filter caused by the coil slipping on the surface of the eye may partly explain the relatively lower velocity in the MSC recordings. Also, noise in the IR recordings induces peaks of eye velocity, which can be reduced considerably by filtering. However, the variability in the recordings, which was larger in the IR than in the MSC recordings, did not seem to be decreased by filtering. The basic level of noise in the recordings was not clearly associated with the amount of reduction of VMAX when the IR recordings were filtered. We suggest that artefacts of the saccadic signal, which can be related to changes in the reflecting surface of the eyes and eyelids, are important factors for explaining the variability and high-velocity peaks in the IR recordings. Lighting conditions was confirmed as a cause for noise, but temperature and air humidity changes in the goggles were not suspected to influence data in the normal experimental setting. Although noise, shortcomings of the recording technique and procedure may offer explanations for the intra-individual variability, the calibration procedure and changes in attention and fatigue of the subject should also be considered.

A comparison between the magnetic scleral search coil and infrared reflection methods for saccadic eye movement analysis.

BACKGROUND: A non-invasive eye tracking system, based on pulsed infrared light (IR), was compared with the magnetic scleral search coil method (MSC) for saccadic eye movement recordings. METHODS: Ten normal subjects performed horizontal and vertical saccades recorded with both methods. Eight recordings were complete and analysed for maximum peak velocity (V(MAX)) and constant (C) of the main sequence curve. RESULTS: The IR recordings showed significantly higher peak velocity values than the MSC system and generally more inter-individual variability. No significant difference regarding peak velocity was detected between abducting and adducting saccades or between upward and downward saccades with either of the systems. Horizontal saccades had higher peak velocities with both techniques. CONCLUSIONS: Comparison of the main sequence plots of the IR and MSC eye tracking techniques reveals that the IR system yields higher values of maximum peak velocity and the constant, the differences being similar for eye movements in different directions. There are various possible explanations for the lower maximum velocity of the MSC recordings, e.g. slipping of the coil annulus on the surface of the eye and a change of the oculomotor command signal induced by wearing the coil. Also, artefacts associated with the IR recording system may cause over-estimations of the saccadic velocity and, furthermore, contribute to the higher variability of the IR recordings.

Surgical treatment of hyperthyroidism: a ten-year experience.

Hyperthyroidism is treated either by antithyroid drugs, radioiodine (I131) or surgery. In Sweden, surgery is often performed in patients with large goiter or severe hyperthyroidism with infiltrative endocrine ophthalmopathy. To evaluate indications and results of surgical treatment, data from 380 patients operated on for hyperthyroidism at our department during 1986-1995 were analyzed. Twenty-six percent were referred for surgery because of failure of treatment with antithyroid drugs or I131. Ninety-one percent were subjected to subtotal thyroidectomy with a median remnant weight of less than 2 g. In the remaining patients, total thyroidectomy was performed. Transient vocal cord affection occurred in 2.6%, none of which was permanent. Prolonged postoperative hypocalcemia occurred in 3.1%, and permanent hypoparathyroidism in 1%. There was no difference in complication rate between subtotal or total thyroidectomy. In patients with Graves' disease, 5% worsened with regard to ophthalmopathy initially after surgery but later improved. Recurrent disease occurred in 2% of the patients, all of whom had undergone subtotal thyroidectomy. Surgery is not first-line therapy in all patients with hyperthyroidism. However, in experienced hands, surgery is a good therapeutic alternative that can be carried out with no mortality, few complications, and, provided that a minimal remnant is left, very few recurrences.

Thyroid associated ophthalmopathy: botulinum toxin A in the treatment of upper eyelid retraction--a pilot study.

PURPOSE: Eyelid retraction in patients with thyroid associated ophthalmopathy is a common cause of eye discomfort and a disfigured facial appearance. The aim of this pilot study was to evaluate the effects and safety of inducing a temporary partial ptosis of the eyelid through injection of botulinum toxin A into the levator palpebrae superioris muscle. METHODS: Nine patients were treated. They were followed up with ophthalmologic examinations and self-assessment questionnaires until the point at which they either required renewed treatment or had no residual effect of the first injection. Maximum follow-up time was twenty weeks. RESULTS: In all eyes but one there was a lowering of the eyelid position one week after treatment and all these patients reported an improvement of eye appearance and symptoms. There was variability in the degree of lowering of the eyelid with the same dose of injected botulinum toxin A. It was also difficult to predict the time interval needed for re-treatment. Ptosis impairing visual acuity was not a consequence of the treatment in any of the patients. One week after treatment two patients experienced increased diplopia but only one showed transient change in Lees screen measurement of eye motility. CONCLUSION: Botulinum toxin A injection may be used to lower the upper eyelid position in patients awaiting eyelid surgery. The treatment safely relieved symptoms and improved eye appearance. However, treatment was short term and difficult to predict. There is also an inherent risk of transient diplopia.