Postnatal development of corneal curvature and thickness in the cat

Objective  To evaluate the postnatal development of central corneal curvature and thickness in the domestic cat. Animals studied  Six Domestic Short-haired (DSH) kittens starting at 9 weeks of age and 6 adult cats. Procedures  Kittens were evaluated biweekly to monthly for a 12-month period, starting at age 9 weeks. Corneal development was monitored by hand-held keratometry and ultrasound biomicroscopy. Standard regression analysis using a nonlinear least squares method was used to generate a formula that would predict corneal curvature as a function of age. Results  Mean keratometry (K) values for the 9-week-old cats were 54.51 (±1.02) diopters (D) and these values steeply declined over the next 3 months to 44.95 (±0.90) D. Thereafter, K-values gradually decreased to reach a plateau by 12–15 months of age of 39.90 (±0.42) D. Because K-values still appeared to be slightly diminishing at this point, six other > 2-year-old cats were evaluated by keratometry and were found to have K-values of 38.99 (±0.81). Two to four diopters of astigmatism was common in young kittens whereas adult cats had a low mean degree of astigmatism (< 1 D). A formula that predicted keratometry values in diopters (K) as a function of age in weeks (w) was established as follows: K = 39.83 + 26.87 exp(−0.074 w). The central cornea increased in thickness primarily during the first 4 months of life with 9 week-old kittens having values of 0.379 (±0.012) mm; 16-week-old kittens, 0.548 (±0.021) mm and 67 week-old cats, 0.567 (±0.012) mm. Conclusions  The maturation process of the feline cornea proceeds over the first 1–2 years of life to attain an adult status that is characterized by a roughly spherical state of approximately 39 D corneal curvature, substantially flatter than the human cornea, and a central thickness similar to the human cornea. Research studies of the refractive or optical properties of the cornea in which cats are used as experimental animals should be conducted on animals greater than 18 months of age.