PURPOSE: To report dry eye patients’ functional visual acuity, which was measured after sustained eye opening for 10–20 seconds, as a simulation of visual function of daily acts of gazing, which is defined as looking at an object with involuntary blink suppression.

METHODS: Interventional clinical nonrandomized comparative trial. We measured ordinary best-corrected visual acuity and functional visual acuity in non-Sjögren’s syndrome (non-SS, N = 10) and Sjögren’s syndrome (SS, N = 12) patients and in normal controls (N = 8), prospectively. Surface regularity index (SRI) of corneal topography was also measured under routine circumstances and after sustained eye opening. Blink rates while gazing were measured during reading in another 28 dry eye patients and during driving in another 8 normal controls.

RESULTS: Functional visual acuity did not change (1.27–1.16) in normal controls, but decreased significantly from 1.18–0.336 in non-SS patients (P = .0007) and from 1.15–0.228 (P < .00001) in SS patients. SRI after sustained eye opening increased in non-SS (P = .032) and SS patients (P = .0007), but not in the normal controls. Blink rates during reading (P < .001) and driving (P = .012) were significantly decreased from baseline blink rates.

CONCLUSIONS: This study shows that the visual function of dry eye patients becomes abnormal with ocular surface irregularity when the eye is kept open for 10–20 seconds. Our data indicate impaired visual function in dry eye patients while gazing. Functional visual acuity may be important in daily activities.