Evaluation of the effects of age and pituitary pars intermedia dysfunction on corneal sensitivity in horses

Abstract

To determine effects of age and pituitary pars intermedia dysfunction (PPID) on corneal sensitivity in horses. 20 adult horses allocated into 3 groups (PPID group, old [> 15 years old] horses with PPID [n = 5]; old group, old [> 15 years old] horses without PPID [9]; and young group, young [≤ 10 years old] horses without PPID [6]). All horses with PPID had hirsutism and abnormal fat deposition or laminitis; none of the old or young horses had hirsutism, abnormal fat deposition, or laminitis. A Cochet-Bonnet aesthesiometer was used to measure the corneal touch threshold (CTT) in both eyes of each horse. The nylon monofilament was applied at a maximum length of 60 mm to the central region of the cornea and length was decreased by 5-mm increments until a consistent blink response was elicited. Tear production was assessed in all eyes via the Shirmer tear test (STT). Mean ± SD CTT was significantly greater for young horses (47.50 ± 4.52 mm) than for horses in the old (28.06 ± 5.72 mm) and PPID (21.5 ± 3.37 mm) groups. Old horses had significantly higher CTT values than did horses with PPID. The STT values were within the reference range for all groups and did not differ significantly among groups. Corneal sensitivity decreased with both age and PPID. Because decreased corneal sensitivity is associated with impaired wound healing, increasing age and PPID may increase the risk for nonhealing or recurrent corneal ulcers in horses.