Abstract
Purpose: :
To report on a method for monitoring vigilance during a visual field test based upon an analysis of pupil size and unrest.
Methods: :
Pupil diameter (PD) was recorded at 60Hz using a CRS VET system (CRS, Cambridge, UK) from 12 patients attending the glaucoma outpatient clinics at Manchester Royal Eye Hospital. Patients were instructed to fixate a central target and to press a response button when they saw a stimulus that was randomly presented 5 degrees either to the left or right of fixation. A repetitive up/down bracketing strategy was used (1dB steps, 2s inter stimulus interval) at each test location to track threshold sensitivity for a continuous period of 10 minutes. Stimuli were presented on a monitor controlled with a CRS Visage system. PD was continuously compared to that at the beginning of the session, using a 1s moving window, to provide the PD ratio, the range of PD was compared to that at the beginning of the session, using a 10s moving window, to provide PD unrest ratio. These 2 ratios were combined to provide a continuous measure of the Pupillary Fatigue Index (PFI), PFI = PD unrest ratio/PD ratio, which compensated for baseline pupil characteristics whilst being sensitive to changes in pupil size and unrest.
Results: :
All patients showed periods of pupil constriction (typically in the order of 1mm) and pupillary fatigue waves (oscillations with amplitude of ~0.5mm and ~6s period) during the 10min session although the extent and duration of these signs of sleepiness showed wide inter-patient variability. The PFI was >2 for an average of 16% (range 3-67%) of the test period. The period of high PFI (>2) increased with test duration (average 3% in first 2.5min vs 33% in last 2.5min). The probability of a stimulus being seen was dependent upon PFI. At a suprathreshold increments of 1 and 2dB the probability of being seen decreased from 83 to 49% for presentations where the PFI<1.25 and PFI>=2.
Conclusions: :
The vigilance of a patient can be monitored during a visual field type test with the PFI. Vigilance deteriorates during a visual field type test by an amount that is patient dependent. Loss of vigilance is associated with a drop in sensitivity and will increase the variability of visual field results.
Keywords: perimetry • pupil • clinical research methodology