Diagnostic Approach to Ocular Discharge

Chapter 148

Diagnostic Approach to Ocular Discharge

Noelle T. McNabb

Numerous conditions affecting the eye and adnexal structures cause ocular discharge, and development of discharge warrants a comprehensive ophthalmic examination. Information regarding the character and volume of ocular discharge, including duration of onset, recurrence, age of the horse, and response to prior treatment, is useful diagnostically. It is also important to determine whether the discharge is accompanied by clinical signs of ocular irritation, pain, or swelling and whether the horse is systemically ill or visually impaired. Examination of the equine eye and ocular adnexa is facilitated by sedation, application of topical anesthetic, and regional eyelid motor and nerve blocks. With a thorough diagnostic evaluation, inciting ophthalmic disorders are most consistently identified, and a prompt treatment plan can be developed, which is of particular concern when the disorder may threaten vision. To enable the best understanding of the conditions that can result in ocular discharge, an overview of basic anatomy and function of the lacrimal system is essential.

Anatomy of the Lacrimal System

Composition of Normal Tear Film

The tear film is trilayered, with the components produced by the meibomian glands (outer oily layer), the lacrimal and nictitans glands (aqueous layer), and the conjunctival goblet cells (inner mucin layer). The mucin layer coats the corneal epithelium, which provides an optically smooth and large surface area, and binds the aqueous layer of the tear film to the epithelium. This mucus layer also contains protective immunoglobulins and lysozymes, and aids in trapping bacteria and debris. The middle aqueous layer of the tear film accounts for the greatest part of the tear film volume. It is the primary layer for corneal and conjunctival lubrication, adds nutrition to the cornea, and contains vitamin A, proteinases, proteinase inhibitors, growth factors, and cytokines that affect corneal health. It mechanically flushes away debris, bacteria, and metabolic waste products (CO2 and lactic acid) from the ocular surfaces. The thin outermost lipid layer retards aqueous tear evaporation. The ocular surface and tear film also contain a community of commensal and transient microbial flora. The commensal population is primarily composed of gram-positive bacterial and fungal organisms, which under certain conditions become opportunistic pathogens.

Distribution of Tear Film

The tear film is a dynamic structure, and every blink redistributes the tears and restructures this corneal extracellular matrix. The upper eyelid provides most of the corneal protection and excursion across the ocular surface and is therefore of greatest importance in the distribution of tear film. Blinking propels the tears across the ocular surfaces toward the medial canthus and ventral-medial conjunctival fornix. The ventral fornix collects the tears (which form the lacrimal lake), and the apposition between the cornea and the lower eyelid prevents the normal tear volume from spilling onto the face. The nictitans does not play a major role in tear distribution in the horse.

Drainage of Tear Film

Tears are primarily eliminated from the ocular surface by drainage through the upper and lower eyelid (lacrimal) puncta and, to a lesser extent, by evaporation. The eyelid punctal openings are 2 to 3 mm in diameter, are slitlike to oval shaped, and lie within the palpebral conjunctiva 8 to 10 mm superior and 5 to 7 mm ventral to the medial canthus. Secretions travel through the upper and lower canaliculi, which join to create the nasolacrimal sac. Distally, the duct extends from the lacrimal sac (within the lacrimal bone) into the maxilla and terminates in the nasal vestibule. The intraosseous portion of the nasolacrimal duct is enclosed in a canal that extends through both the lacrimal bone and the bony lamina of the medial wall of the maxilla. This segment of the duct lies dorsal to an imaginary line that extends from the medial canthus to the infraorbital foramen. Should trephination for sinus or dental disease occur dorsal to this demarcation, this intraosseous segment of nasolacrimal duct is susceptible to damage or laceration. The duct opens at the nasal puncta (slitlike to oval and 3 to 5 mm in diameter) at the mucocutaneous junction of the ventral-lateral aspect of the nasal vestibule.

Characterization of Ocular Discharge

Numerous ophthalmic disorders can manifest as ocular discharge, and the character of the discharge can be diag­nostically useful. A discharge can develop when any component of the trilaminar tear film is produced in excess or is deficient, when the tear film is inadequately dispersed, or when there is partial or complete obstruction of tear drainage. The abnormal ocular discharge may be watery (resulting from hyperlacrimation or impaired tear drainage), mucoid (resulting from tears deficient in volume), purulent or mucopurulent (resulting from tears containing inflammatory cells, infectious agents, devitalized epithelial cells, or collagen), or hemorrhagic. The type of discharge may also change over time. Therefore the eye and ocular adnexa should be carefully and methodically examined for overall health and function when ocular discharge is observed (see Chapter 140).

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.

Serous Ocular Discharge in Foals

Newborn foals may develop entropion, which is inward rolling of the eyelid margins. It can be a primary condition or may arise secondary to dehydration (reducing orbital fat volume), emaciation, recumbency (as in septic or otherwise compromised foals), and microphthalmia. Entropion can obstruct tear drainage through the eyelid puncta and result in epiphora, but it also incites excessive lacrimation secondary to rubbing of the eyelid hairs on the cornea and conjunctiva. Placement of two to three temporary tacking (vertical mattress) sutures (in 3-0 to 4-0 monofilament) in the affected lid for 2 to 4 weeks is typically effective.

Tear-flow obstruction in foals or yearlings is otherwise most often caused by congenital atresia or dysgenesis of the distal segment of the nasolacrimal system. Specifically, it involves either atresia of the nasal meatus puncta (imperforate nasal puncta) or dysgenesis of the distal segment of the nasolacrimal duct. Foals may be affected unilaterally (most common) or bilaterally. Only mild epiphora is typically evident until the foal is 3 to 4 months of age, which is likely related to a lower volume of tear production in foals. Infrequently, tear obstruction is the result of imperforate eyelid puncta (atretic or ectopic lacrimal puncta). With that condition, direct examination of the nasal vestibule confirms the absence of a puncta, and a fluctuant swelling may be seen ventromedially, representing the dilated distal portion of the nasolacrimal duct. Over several months, the pooled tears and mucoid secretions favor microbial overgrowth and inflammation of the lacrimal sac and nasolacrimal duct, resulting in dacryocystitis. Thick mucopurulent discharge at the medial canthus with reflux exudation on manipulation of the medial eyelid is typical. The globe and conjunctiva are not usually involved, unless chronic dacryocystitis has incited severe blepharoconjunctivitis.

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Jul 8, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Diagnostic Approach to Ocular Discharge

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