Diurnal and long-term variations in corneal thickness in the normal and alloxan-induced diabetic rabbit

The variation of central corneal thickness with age was measured weekly at 10am by ultrasound pachometry in 10 normal (5 to 19 weeks of age) and 10 alloxan-induced diabetic (9 to 19 weeks of age) rabbits. The diurnal variation of corneal thickness was also measured at 2 hour intervals for a period of 24 hours on both groups of rabbits at 19 weeks of age. A nonlinear increase in corneal thickness with age was noted in both groups, with the diabetic cornea becoming significantly thicker (p less than 0.05) than the normal control after 7 weeks of hyperglycemia. At 19 weeks of age, the diabetic rabbit cornea (379 +/- 18um) was 5.2% thicker than the normal control (360 +/- 10um). Both normal (26um) and diabetic (26um) rabbits demonstrated a similar sinusoidal diurnal variation of corneal thickness, with the cornea being thinnest in both groups near midnight. A negative exponential function derived using nonlinear regression analysis was found to adequately describe the age data, and could be used to provide a number of possibly useful clinical indices of corneal thickness variation with age. The data from this study demonstrate that 1) the variation of rabbit corneal thickness with age can be modelled using a negative exponential function, 2) alloxan-induced hyperglycemia is associated with an increase in rabbit corneal thickness, 3) alloxan-induced hyperglycemia does not appear to influence the diurnal variation of rabbit corneal thickness and 4) care should be taken in the interpretation of in vivo corneal thickness studies using normal rabbits of under 12 weeks of age (2kg body weight).