Tear Tests for Diagnosis of Ocular Disorders in Animals
The precorneal tear film has three layers. The outer layer is composed of lipid from the meibomian glands, the middle aqueous layer is from the lacrimal gland and the third eyelid gland, and the inner layer of mucus that attaches the tear film to the cornea is from the conjunctival goblet cells.
The Schirmer tear test measures the aqueous portion of the tear film produced by the lacrimal and third eyelid glands. Values are reported in millimeters of wetting per minute. This should be measured before any drops are applied to the eye. A Schirmer I test is used without topical anesthetic and measures baseline and reflex tearing. A Schirmer II test, which uses topical anesthetic and stimulation of the nasal mucosa with a cotton-tipped applicator, measures reflex tearing (this is used more in humans than in animals).
The phenol red test uses a cotton thread soaked with phenol red (phenolsulfonphthalein), a pH indicator. The color changes from yellow to red in response to contact with tears (alkaline). It requires placement for only 15 seconds compared with 60 seconds for the Schirmer tear test.
Ocular Stains for Diagnosis of Ocular Disorders in Animals
Fluorescein sodium stain is a hydrophilic dye used to evaluate tear film stability (tear film breakup time), integrity of the corneal epithelium (ulcers), corneal integrity (Seidel test), nasolacrimal duct patency (Jones test), and intraocular angiography. Fluorescence is intense under cobalt blue (450–500 nm) or ultraviolet light. Any microbial cultures should be taken before fluorescein staining. Loss of the lipophilic corneal epithelium results in binding by the hydrophilic stroma. Lack of binding centrally may indicate absence of stroma because Descemet membrane is lipophilic.
A Seidel test using concentrated fluorescein can be used to check corneal integrity. Under close inspection with magnification and cobalt blue or ultraviolet light, a leak in the cornea shows up as a rivulet of clear or light green aqueous running down the orange or dark green stained corneal tear film. After application of a drop to the eye it normally takes 5–15 minutes to pass through the nasolacrimal duct to the nares. In brachycephalic patients, the dye may not appear at the nares because the nasolacrimal duct may drain into the caudal nasopharynx.
Rose bengal is a stain used to identify damage to the conjunctival and corneal epithelial cells. It will also stain healthy cells. It is used in cats to look for dendritic ulcers due to feline herpesvirus. It can sting on application.
Lissamine green is a green organic dye used to stain dead and degenerate conjunctival cells in the diagnosis of keratoconjunctivitis sicca (dry eye).
Autonomic Nervous System Agents for Diagnosis of Ocular Disorders in Animals
The iris and ciliary body muscles are innervated by the autonomic nervous system. The effect of the sympathetic stimulation is the fight or flight response and results in pupillary dilation (ie, mydriasis). The opposite result occurs with the parasympathetic system (ie, miosis). The ciliary muscle is used in accommodation (focusing with the lens). Where there is corneal damage or intraocular inflammation, there is a response that results in ciliary muscle spasm, miosis, and intraocular pain. There is also often damage to the blood aqueous barrier, which is seen as aqueous flare.
Mydriatics
Mydriatics are agents used to dilate the pupil. Mydriatics are divided into two basic categories based on their mechanism of action: sympathomimetics and parasympatholytics. Sympathomimetics (agonist) stimulate the dilatory muscle of the iris; in contrast, parasympatholytics (antagonist) cause paralysis of the iris and ciliary body musculature (iridocycloplegia).
Sympathomimetics
Phenylephrine (2.5% and 10%) is an alpha-adrenergic sympathomimetic used as an adjunct for pupillary dilation and local vasoconstriction and hemostasis. It acts as an agonist on the iris dilator muscle and also mimics the absent neurotransmitter in the diagnosis of third-order Horner syndrome. Frequent use can cause an increase in heart rate and blood pressure.
Parasympatholytics
Tropicamide is a parasympatholytic that inhibits the action of acetylcholine on the smooth muscle of the iris and ciliary muscle. This results in iris dilation (due to the dilator muscle not being opposed) and cycloplegia. Tropicamide has a short onset of effect (15–20 minutes) and duration (3–5 hours). It is predominately a mydriatic used for routine dilation to assist ocular examination of the lens and fundus. It has very little effect on the ciliary muscle (cycloplegia). It is used for routine dilation and examination of the lens and fundus.
Atropine 1%, another parasympatholytic, also causes mydriasis but is a more effective cycloplegic. Time to onset is 15–30 minutes, but the effect can last 24 hours to 7 days or longer. Its use is primarily therapeutic to control miosis, stabilize the blood aqueous barrier, and relieve painful reflex ciliary muscle spasm secondary to corneal disease and anterior uveitis. It should not be used for routine diagnostic dilation to examine the lens and fundus. Atropine should be used with care. In small animals, it can be toxic because of systemic absorption, and, in horses, frequent daily use has been associated with signs of colic.
Miotics
Pupillary constriction (miosis) can be achieved by use of a parasympathomimetic (agonist) or a sympatholytic (antagonist).
Parasympathomimetics (cholinergics) mimic the action of acetylcholine on the muscarinic receptors of the iris and ciliary body muscle. This causes miosis and increased conventional aqueous humor outflow from contraction of the ciliary body muscle and opening of the iridocorneal angle for treatment of primary glaucoma. The most common drug is pilocarpine 1%– 2%, applied every 8–12 hours. Time of onset is 15 minutes, with maximal effect in 4 hours. Duration of action can last up to 24 hours, but for maximal efficacy, application every 8–12 hours is best.
An alternative is a topical anticholinesterase inhibitor, demecarium bromide 0.125%–0.25%, every 12–24 hours. Parasympathomimetics cause miosis, and the constant contraction of the ciliary muscle can be painful (brow ache in humans). Parasympathomimetics also increase vascular permeability, which can reactivate latent iritis and intensify any concurrent anterior uveitis. Their use will exacerbate problems in the case of an anterior lens luxation or subluxation. Dilute pilocarpine (0.1%–0.2%) can be used in treatment of neurogenic keratoconjunctivitis sicca and management of pupil constriction in internal ophthalmoplegia. Oral pilocarpine has been used in treatment of keratoconjunctivitis sicca but can cause toxicity and, in some cases, death.
For More Information
Also see pet health content regarding drugs used to treat eye disorders.
