Tear film and ocular surface diseases in cats and dogs. Part I. Notes on pathophysiology

Dry eye or keratoconjunctivitis sicca is a widely recognized and commonly diagnosed disease in small animals. It has been historically defined as tear production deficiency; however, the pathogenesis of this disease is much more complex than previously recognized. In human ophthalmology ocular surface diseases are recognized as a disturbance of the so called "Lacrimal Function Unit", an integrated system comprising lacrimal glands, ocular surfaces and lids, and the sensory and motor nerves that connect them. The aim of this paper is to review the past and recent human and veterinary literature on the topic, with special focus on the complex interactions between the anatomical, dynamic and physiologic components of the lacrimal function unit.