Eye Movements During Perimetry and the Effect that Fixational Instability Has on Perimetric Outcomes

Fixation of subjects is assumed to be stable during perimetry, although few studies have addressed the nature of fixation over prolonged test periods. In this study, the horizontal eye movements of 12 normal subjects were measured using an infrared eye monitor while they performed automated thresholds. Our results indicate that no observer can maintain steady fixation within +/-3 of the fixation marker for the entire period of a threshold test. Fixational instability in the order of +/-5 is found in all observers, whereas some show substantially greater fixation losses on occasion (up to 10-15). We also consider the effect that fixational losses have on perimetric outcomes by performing automated thresholding in the presence of intentional eye movements (5) on four trained observers. These effects were studied as a function of the presence and absence of a scotoma (physiologic blind spot) within the field. Our results indicate that eye movements increase the local short-term fluctuation and false negative rate. However, this increase is only significant in the presence of a scotoma. We conclude that fixational instability may "blur" a small isolated scotoma, making it less apparent. As a consequence, clinicians should consider abnormal short-term fluctuation and false negative rates as possible indicators of the presence of a small moving scotoma even in the presence of normal thresholds.