Smooth pursuit eye movements in twins discordant for schizophrenia

Abstract

IN A SERIES of studies we reported that between 65 and 80 % of schizophrenic patients have disordered pursuit eye movements, compared with about 15 “/;, in other psychiatric populations and 6 % in the normal population. Of equal interest was the finding that about 45-50 “/,; of first-degree relatives of the schizophrenics also showed the same kind of eye-tracking impairments, compared with a prevalence in the families of other psychotic patients that was no greater than that found in the normal population. 1,~ This association of disordered pursuit eye movements with schizophrenia and with members of the families of schizophrenics suggested that deviant pursuit eye movements may represent a genetic indicator of schizophrenia. This hypothesis would obtain confirmation if monzygotic (MZ) and dizygotic (DZ) twins who are discordant for schizophrenia were concordant for deviant eye tracking. This paper describes such a study. Smooth pursuit eye movements are those attendant upon following a moving target, and the speed of the target determines almost exactly the speed of the eyes. Rapid or saccadic movements are those that occur when fixation shifts, as when a person looks about a room from one object to another. In saccadic shifts, the eye can move at very rapid speeds, at times up to 600 degrees a second. In contrast, following movements break down into saccadic shifts if the target moves faster than about 40 degrees a second. A simple test of pursuit eye movements simply requires a subject to follow a pendulum. The pendulum, of course, oscillates at a continuously varying velocity, at first accelerating to a maximum and then decelerating to zero velocity. Electronystagmographic recording permits the investigator to obtain a permanent record of eye movements during the pursuit task.‘,2 In good pursuit eye movements the eyes should reproduce the sinusoidal wave form of the pendulum. Figure 1 shows four examples of pursuit movements. Normal eye tracking is represented in Figs la, lb. Deviations in eye tracking, such as those illustrated in Figs lc and Id are found in persons with some hemispheric and brain-stem lesions, multiple sclerosis, Parkinson’s disease, and some drug intoxications, particularly barbiturate and alcohol.3 In our studies we found that deviant eye tracking patterns in young schizophrenic patients occurred without the presence of organic disease and in a large number of their first-degree relatives the poor tracking occurred without either the presence of clinical schizophrenia or of organic pathology. Indeed, none of the relatives examined had visited a psychiatric facility for treatment for themselves and all were functioning quite adequately in the social