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D. B. Henson, T. Emuh; Measuring Patient Vigilance During Perimetry; The Pupillary Fatigue Index. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6195.
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© ARVO (1962-2015); The Authors (2016-present)
To report on a method for monitoring vigilance during a visual field test based upon an analysis of pupil size and unrest.
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.
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.
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.
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