VN November 2022

Vetnuus | November 2022 34 Regulars I Ophthalmology Column This study showed no difference in the incidence of corneal injury between groups receiving a lubricant and those having lubricant with having their eyelids taped closed. Based on findings from other studies, a few things seem to be important. • Operations over 2hrs may further increase the incidence of corneal injury. • In lateral recumbency, the down-facing eye in people may have a higher incidence of corneal damage. It has not been confirmed in dogs. • Dogs that underwent CSF sampling or MRI were not more likely to develop corneal erosion than patients that did not. • Under GA, the dog’s eyes will rotate ventromedially, exposing the dorsolateral aspect of the cornea. The lesions found in the present study were located in the axial cornea, which suggests that corneal injury in canine patients might occur during the immediate pre-or post- operative period when the eye is in a central position. Therefore, one might argue that corneal protection during the times immediately prior to induction of GA and during recovery from GA might be particularly important. The concluding consensus is that using a topical lubricant is considered the standard of care for any veterinary patient undergoing GA. [Ref: A prospective, masked, randomized, controlled superiority study comparing the incidence of corneal injury following general anaesthesia in dogs with two methods of corneal protection, Veterinary Ophthalmology. 2022;25:291–296.] 2] Serum used for melting corneal ulcers: Corneal ulceration is a common clinical problem in domestic animal species. Corneal ulcers that affect only the epithelium often heal with minimal clinical intervention [these do not require serum as a treatment]. In contrast, ulcers that involve the corneal stroma may require substantially more treatment. Complications resulting from the destruction of stromal collagen can cause rapid progression of an ulcer to a descemetocele or perforated cornea. Regardless of the inciting cause, the progression of ulcers affecting the corneal stroma is mediated by collagenases that break down stromal collagen. Normally there is a fine homeostatic balance between matrix metalloproteases [MMP] and serine proteases and the endogenous enzyme inhibitors, and any disturbance of this can result in uncontrolled collagen degradation. Degradation of corneal collagen, or keratomalacia [melting], is caused by an increase in collagenases from various endogenous (corneal epithelial cells, leukocytes, and tear film) and exogenous (bacterial or fungal) sources. In human medicine, serum has been described as an effective inhibitor of corneal collagenase. Its inhibitory effect is attributed to α-2 macroglobulin, which forms tight complexes with collagenases and inhibits their activity. Serum has several advantages when compared with other anticollagenases, as it is well tolerated when topically administered in the eyes and can be readily obtained from healthy donors. Thus, serum is frequently used as an anticollagenase for the treatment of corneal ulcers in both human and veterinary medicine. It appears that cat serum is more effective than dog and horse serum. One does not have to collect blood from the specific patient. Horse serum can be used in dogs and cats and vice versa. It has been shown that storage of serum for 180 days at –20° or –80°C did not significantly affect the ability to reduce proteinase activity. Storage of equine serum at –18°, 4°, and 23°C for 7 days did not alter its ability to inactivate MMPs in equine tears over a 7-day period. The general consensus is that once collected or defrosted from a frozen source, serum should be refrigerated and stable for at least 8-14 days. Contamination at the container nozzle or cap always remains a potential problem, so careful application without the nozzle making contact with the patient remains critical. When managing melting ulcers, serum should initially be applied every 30-60 minutes. The frequency may be reduced if clinical improvement is evident. Other anti-metalloproteases include EDTA, which chelates zinc or calcium, which are required as cofactors for MMPs. Tetracyclines chelate zinc and calcium, inhibit endogenous antitrypsin degradation, and may inhibit leukocyte migration. This is why oral doxycycline can be used as it is excreted in the dogs’ tears. [Ref: 1] Evaluation of species differences and the effects of storage duration and temperature on the anticollagenase efficacy of canine, feline, and equine serum on in vitro corneal degradation AJVR • Vol 76 • No. 11 • November 2015 2] Tear film concentrations of doxycycline following oral administration in ophthalmologically normal dogs JAVMA • Vol 249 • No. 5 • September 1, 2016 3] In vitro evaluation of the inhibitory effect of canine serum, canine fresh frozen plasma, freeze–thaw-cycled plasma, and SolcoserylTM