Normative data of saccadic eye movements for routine diagnosis of ophthalmoneurological disorders

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.

Increase in saccadic peak velocity with increased frequency of saccades in man

Twelve normal subjects (aged 22-80 yr, mean 47 yr) performed three blocks of 20 saccades made to LED targets stepped back and forth. The first and last blocks were performed at a (slow) rate of 0.18 Hz, while the middle block was performed at the faster rate of 1.15 Hz. Mean saccadic amplitude was unaffected by saccade rate, but latency and duration became shorter at the higher frequency. Most interestingly, the peak velocity increased by approx. 6% when saccades were performed at the higher rate. This increase was statistically significant, even after normalization for saccade amplitude. That saccadic frequency may affect saccadic peak velocity must be considered as a potential variable when analysing saccades.

The contribution of oculography to early diagnosis of myasthenia gravis. A study of saccadic eye movements using the infrared reflection method in 22 cases

Saccadic eye movements were examined by infrared reflection oculography in 22 patients with suspected myasthenia gravis. In all of them the diagnosis was confirmed either by electromyography, by the presence of acetylcholine-receptor antibodies (15 cases), by a positive clinical Tensilon test (4 cases), or by the course of the disease (3 cases). The aim of the study was to find out which of the saccade disorders described in myasthenic patients can be found in the early stage of the disease, and to discover their diagnostic value. Typically, saccade abnormalities were also seen in clinically uninvolved eyes. Intrasaccadic disorders mainly consisted of "decrescendo" of the velocity profiles (two-thirds of the cases) and "hypometria" (half of the cases). Postsaccadic "waver" was present in about two-thirds of the patients. The most frequent finding was "intersaccadic variation" (i.e., variation between repeated corresponding saccades) which occurred in 18 of the 22 patients, and particularly affected the velocity profiles. A standardized fatigue test proved to be useful in accentuating mild, or provoking latent pathology in most of the patients. A standardized combined examination of saccades with Tensilon yielded pathological results less frequently than hitherto described in the literature. In 4 of the 9 patients with purely ocular myasthenia gravis oculography led to the diagnosis. The problems in attempting to delimit the observed saccade abnormalities from those found in other eye movement disorders are discussed.