Effects of aging on corneal biomechanical properties and their impact on 24-hour measurement of intraocular pressure

PURPOSE: To study the effects of aging on corneal biomechanical properties and their impact on 24-hour measurement of intraocular pressure (IOP). DESIGN: Experimental study. METHODS: Fifteen older volunteers with healthy eyes (age range, 50 to 80 years) were housed for one day in a sleep laboratory with a 16-hour diurnal or wake period and an eight-hour nocturnal or sleep period. Every two hours, sitting corneal hysteresis, corneal resistance factor, and IOP were measured. Central corneal thickness (CCT) was measured using an ultrasound pachymeter. Data were compared with previous observations in 15 healthy younger volunteers (age range, 20 to 25 years). RESULTS: Variations in 24-hour corneal hysteresis and corneal resistance factor were not significant in the older subjects, but there were time-dependent variations in CCT and IOP. The nocturnal CCT was thicker than the diurnal CCT, but the IOP difference between the diurnal and nocturnal periods was not significant. Cosine-fits of CCT and IOP showed synchronized 24-hour rhythms. The phase timing of CCT rhythm appeared significantly earlier than the phase timing of IOP rhythm. Compared with younger subjects, older subjects had a lower mean 24-hour corneal hysteresis and corneal resistance factor, but not a lower CCT. Phase timings of 24-hour rhythms of CCT and IOP were significantly delayed by aging. CONCLUSIONS: Aging may lower corneal hysteresis and corneal resistance factor, but neither parameter shows a significant 24-hour variation. Aging may not change CCT significantly, but can shift its 24-hour rhythm. The 24-hour IOP pattern in this group of older subjects is not an artifact resulting from a variation in corneal hysteresis, corneal resistance factor, or CCT.