Watery or Serous Ocular Discharge

When the ocular discharge is characterized as watery or serous, an initial goal of examination is to determine whether the discharge represents excessive lacrimation or inadequate tear drainage. Partial or complete obstruction of tear drainage results in epiphora and overflow of tears onto the face. Alternatively, excessive production of tears (hypersecretion) by the lacrimal glands results from irritation or inflammation of the surface structures of the eye, or from pain in or around the eye. Such hypersecretion may also result in tear overflow. Regardless of whether the discharge is caused by excessive lacrimation or epiphora, it may be exacerbated by exposure to environmental factors such as wind, cold, intense sunlight, airborne pollens, molds, and fine debris. Therefore, before concluding that ocular discharge is a result of epiphora per se, a complete ophthalmic examination is necessary to rule out inciting causes of hypersecretion.

At the start of the examination particular attention is first paid to determining whether ocular or periocular pain accompanies the ocular discharge. Blepharospasm and ventrally directed upper lid lashes often denote ocular discomfort. Conjunc­tival hyperemia, chemosis, miosis, and photophobia are often associated with uveitis. Glaucoma may manifest as corneal edema, limited pupillary light reflexes, and buphthalmia. Exophthalmia and nictitans prolapse often accompany inflammatory orbital disease. Common causes of surface ocular irritation and pain include abnormalities of the eyelids (e.g., entropion in foals, post-traumatic cicatricial ectropion, and facial nerve paralysis), cornea (ulcerative and nonulcerative keratitis), retained foreign bodies, conjunctiva (ulcerative, nonulcerative, parasitic, and neoplastic conjunctivitis), and exposure to irritating, noxious, or toxic substances (through airborne and direct contact routes). Although various causes of conjunctivitis may result in a proteinaceous serous discharge, corneal rupture with aqueous leakage can also present as a serous discharge without much evidence of pain. It is therefore important to exercise caution when opening the eyelids of a horse with serous discharge. Use of sedation and an auriculopalpebral nerve block before examination of the globe limits the risk for further injury to a fragile or compromised cornea and potential extrusion of intraocular contents if a rupture is present.

When serous ocular discharge is observed in eyes with clinically normal ocular and periocular tissues and no clinical signs of pain are evident, differential considerations include mild ocular surface irritation or allergic conjunctivitis from environmental allergens such as molds, hay, and dust. Not uncommonly, horses with serous ocular discharge are examined without a definitive cause being evident. Barring identification of a functional, structural, or conformational eyelid abnormality, compromised drainage or obstruction of tear flow should be considered next.