The Comparative Effect of Total Versus Partial Surgical Excision of Nictating Membrane on The Aqueous Tear Production and Ocular Surface Health in Donkeys (Equus Asinus)

The precise role of the nictating membrane is uncertain and inconsistently discussed in the literature, it is still up for debate. Currently, there have been no extensive investigations on the effect of nictating membrane excision on tear production in donkeys. Therefore, the current study aimed to evaluate the effects of total or partial surgical resection of the nictitating membrane on aquous tear production, pH, and corneal integrity via fluorescein dye and histological examination in donkeys. Donkeys were allocated into two equal groups. Group I (5 donkeys): donkeys were subjected to complete resection of the third eyelid in the left eye and the right one was a control. Group II (5 donkeys), donkeys were subjected to partial resection of the third eyelid in the left eye, and the right one was a control. All animals were observed for three months postoperative. The following criteria (ophthalmic examination, Schirmer tear test I (STT I), tear pH, and fluorescein staining test) were used for the evaluation of the eyes weekly for the first two months, then every two weeks for the last month. There were no significant variations in the mean values of STT I and pH of the total resection of the third eyelid (group Ι) in comparison with the partial resection of the third eyelid (group ΙΙ) (P > 0.05). In both eyes, there were no obvious clinical indicators of keratoconjunctivitis sicca (KCS). The histopathological examination of various regions such as eyelids, cornea, and conjunctiva in both groups compared to the control showed that all regions were perfectly normal. However, acinar atrophy, degeneration and periacinar fibrosis were visible in the glandular tissue of the third eyelid and cystic dilation and periductal fibrosis were visible in the nictitans gland’s duct system. In conclusion, neither complete nor partial resection of the third eyelid causes the development of KCS or alters the ocular surface integrity.