A quantitative analysis of eye movement characteristics during the cover test
The cover test is probably the most widely used clinical test of oculomotor status. It is surprising therefore, that there has been only one previous study which has attempted to provide a quantitative analysis of the eye movements during the test. There is also a dearth of information concerning the relationship between eye movements during the cover test and symptomatology and the correlation between cover test results and the outcome of other tests of binocular function. For the investigations described in this thesis, apparatus was developed to provide precise measurements of eye movements during a computer-controlled cover test, with subjects fixating a distant (3.4m) and near (0.4m) target. In the first study, this apparatus was employed to assess a group of asymptomatic emmetropes aged between 18 and 35 years (n = 100). The pattern of eye movements recorded was more complex than is often assumed. Eye movements during dissociation followed various patterns, with some subjects reaching a position of equilibrium within a few seconds while 20% had not reached a stable position at the end of the 10 s occlusion period. It was concluded that the 'standard' procedure of occluding an eye during the cover test for only about 2s is not adequate. The mean phoria after 10 s occlusion was 0.000 for distance fixation and -1.38° (exo) for near fixation. The pattern of eye movements during the recovery phase consisted of a variety of saccadic and vergence movements. There was a statistical difference between exophores and esophores for frequencies of initiating saccades and vergence eye movements (p < 0.001) with esophores more commonly commencing recovery with a saccade and exophores with a vergence eye movement. Recovery movements were often associated with movements of the "fixating" eye. There was a poor correlation between phoria amplitude and recovery time. In the second study, the relationship between the nature of eye movements during the cover test and the results of other common tests of binocular function was investigated. There was very little correlation between any aspect of the eye movements and the results of the other tests, or indeed between any of the tests of binocular function. In the third study, a group of symptomatic individuals were characterised using the automated cover test and a battery of other tests of binocular function. In the fourth study, the association between eye movement characteristics during the cover test and symptomatology was investigated. Results gave some support to the long-held view that a slow multi-stage recovery tends to be associated with a symptomatic binocular vision problem. In the fifth study, various aspects of binocular function were monitored throughout the course of orthoptic therapy. Not all subjects responded to treatment and none of the clinical tests assessed were found to be good discriminators of subjects who were likely to benefit. While several aspects of binocular function were found to parallel the amelioration of symptoms, the results were very variable.