How to Manage Excessive Tearing in the Horse

The primary function of the lacrimal system is to produce and drain the tear film from the surface of the eye. The tear film is composed of three integrated layers: the aqueous, mucus, and lipid layers. The aqueous layer is produced by the orbital lacrimal gland and gland of the third eyelid and secreted onto the ocular surface through ductules that traverse the conjunctiva. The mucus layer is produced primarily by conjunctival goblet cells and secreted directly onto the ocular surface. The lipid layer is produced by the meibomian glands present at the eyelid margin and is also secreted directly onto the ocular surface. Tear film plays an essential role in corneal health; it provides oxygen and nutrients to the corneal epithelium while removing debris and waste products. Tear film also keeps the ocular surface lubricated and plays an important part in the optics of the eye, allowing light to pass uninterrupted from the external environment into the eye. Immunologic properties of the tear film are important for ocular surface defense and health. The old adage, “no foot, no horse,” is certainly true; its ocular equivalent is “no tear film, no eye”! The nasolacrimal drainage system is responsible for removing the tear film from the ocular surface. The structures involved in the nasolacrimal drainage system include the ocular (proximal) puncta, the canaliculi, the lacrimal sac, the nasolacrimal duct, and the nasal (distal) puncta (Fig. 1). The ocular puncta are located in the medial canthus, with one punctum located just inside the eyelid margin of the upper eyelid and one punctum located just inside the eyelid margin of the lower eyelid. The ocular puncta are difficult to visualize without magnification. Normal tear film flow through the ocular puncta is facilitated by blinking and by the apposition between the eyelid and the ocular surface. Each punctum is drained by a canaliculus that connects to a common lacrimal sac. This lacrimal sac is drained by the nasolacrimal duct, which runs through the skull and empties through the nasal puncta, located on the ventral floor of the nasal passage (Fig. 2). The nasolacrimal system exists in a balance between tear production and tear drainage by the nasolacrimal drainage system. Normal horses have an ocular surface tear volume of approximately 230 L and a tear flow rate of 33.62 L/min, with