The equine cornea is provided with sensory innervation by the long ciliary nerves, which are branches of the ophthalmic division of the trigeminal nerve.1,2 The superficial portion of the cornea receives more sensory innervation with pain receptors than the middle and deeper portions.2 Corneal sensitivity has been evaluated in a number of species by use of a CTT. Corneal sensitivity varies with a number of factors, including species, area of the cornea, and skull type. It has been reported that the cornea of humans is considered the most sensitive of those species studied, followed by the cornea of cats, rabbits, and dogs.3–8 Corneal sensitivity has been measured in horses and guinea pigs as well, with horses having relatively high corneal sensitivity3,9 (although direct comparison is difficult because results are variably reported in different measurement units [grams per square millimeter or centimeter] and by use of different aesthesiometer filament strengths) and guinea pigs having relatively low corneal sensitivity, comparable to that of brachycephalic cats.10 The central portion of the cornea is more sensitive than the peripheral portion in dogs,8 cats,4 horses,3,9 and guinea pigs,10 although differences are not always significant. For cats4 and dogs,7 having a brachycephalic skull type is associated with having lower corneal sensitivity.
Corneal sensitivity was measured by use of the CTT in previous equine studies with the Cochet-Bonnet aesthesiometer. Brooks et al9 found that the mean CTT for adult horses, sick foals, and healthy foals was 4.82 ± 0.87 cm, 3.21 ± 0.24 cm, and 5.01 ± 0.61 cm, respectively, suggesting that healthy foals had the most sensitive corneas, whereas sick foals had the most insensitive corneas and highest CTT.
Topical ophthalmic anesthetics are routinely used to facilitate measurement of intraocular pressure, collection of corneal and conjunctival scrapings for cytologic examination, and removal of ocular foreign bodies in many species. In horses, a topical anesthetic is used in conjunction with a local anesthetic to place a subpalpebral lavage catheter. The most widely used topical ophthalmic anesthetic agent is 0.5% proparacaine hydrochloride. Tetracaine, another topical ophthalmic anesthetic, is more likely to cause pain upon administration, conjunctival irritation, and chemosis.11 Recent studies by Herring et al12 and Binder and Herring13 have determined the duration of effect of proparacaine in dogs and cats; however, the duration of effect in horses is unknown. In dogs, a significantly greater anesthetic effect resulted after instillation of 2 drops of proparacaine, compared with 1 drop. Also, the maximal anesthetic effect lasted for 15 minutes with 1 drop and for 25 minutes with 2 drops.12 In cats, the maximal anesthetic effect lasted only 5 minutes after instillation of 1 drop of proparacaine, as measured by use of the Cochet-Bonnet aesthesiometer.13 The latter duration is considerably shorter than that of dogs.
The Cochet-Bonnet aesthesiometer, a commercially available aesthesiometer, has been used routinely to determine corneal sensitivity in many species and to identify the duration of effect of proparacaine in cats and dogs by measuring the CTT.3,4,6–10 The CTT corresponds to the length of nylon filament that induces a blink reflex upon contact with the cornea. The Cochet-Bonnet aesthesiometer contains an adjustable-length nylon filament, which is applied to the cornea. The length of the nylon filament directly corresponds to the amount of pressure being applied to the corneal surface at a particular time. When the CTT for a particular horse is reached, the corneal reflexive response is elicited via blinking, retraction of the globe, and prolapse of the nictitating membrane (ie, the third eyelid). Therefore, a higher CTT will correspond with a shorter filament length because of increased pressure and decreased corneal sensitivity, whereas a lower CTT will correspond with a longer filament length from decreased applied pressure and therefore increased corneal sensitivity.3 This instrument has also been safely and effectively used to measure corneal sensitivity in foals and adult horses.9 The purpose of the study reported here was to use the Cochet-Bonnet aesthesiometer to measure the onset and duration of effect of 0.5% proparacaine hydrochloride in the eyes of healthy adult horses. We hypothesized that the duration of corneal anesthesia induced with 0.5% proparacaine hydrochloride in horses would be shorter than that of dogs and cats because of the increased sensitivity of the equine cornea, compared with corneal sensitivity in cats and dogs.
Corneal touch threshold
SAS, version 9.1.3, SAS Institute Inc, Cary, NC.
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Binder DR, Herring IP. Duration of corneal anesthesia following topical administration of 0.5% proparacaine hydrochloride solution in clinically normal cats. Am J Vet Res 2006;67:1780–1782.
Beech J, Zappala RA, Smith G, et al. Schirmer tear test results in normal horses and ponies: effect of age, season, environment, sex, time of day and placement of strips. Vet Ophthalmol 2003;6:251–254.