8. Ophthalmology

CHAPTER 8. Ophthalmology

Wendy M. Townsend




EXAMINING THE EYE






I. Diagnostic tests




A. The Schirmer tear test I (STTI): Allows quantification of the production of the aqueous tear phase




1. Measures basal plus stimulated tear production


2. Must be performed before the application of any drops or topical anesthetic


3. Measure how far tears travel along the strip from the notch in 1 minute


4. Normal results can range from 15 to 25 mm/minute


5. Less than 10 mm/minute is suspicious of reduced tear production


6. Less than 5 mm/minute is diagnostic of dry eye


B. The Schirmer tear test II




1. Measures basal tear production


2. Performed after anesthetizing the cornea with proparacaine and drying the excess fluid from the conjunctival sac


3. Performed as for STTI


4. STTII reading ≈ 50% of STTI reading


C. Fluorescein staining




1. Administer from sterile, single-use impregnated paper strips (or single-use vials)


2. Moisten the strip with artificial tears or sterile saline and apply a small amount over the dorsal bulbar conjunctiva


3. Do not touch the cornea to avoid misleading stain deposition


4. Flush eye with sterile saline to prevent a false impression of stain uptake


5. In the presence of an epithelial defect, fluorescein stains the stroma bright green


D. Measurement of intraocular pressure (IOP)




1. Normal is 15 to 25 mm Hg


2. Digital assessment of IOP: Very unreliable and inaccurate


3. The Schiϕtz tonometer: Operates on the principle that the amount of indentation of a given area of the cornea is proportional to the IOP


4. Applanation tonometers measure the pressure required to flatten a given area of the corneal surface (this pressure is proportional to the IOP). The most commonly used applanation tonometer is the Tonopen


II. Basic ocular anatomy




A. Eyelids




1. Protect the globe


2. Distribute the tear film


3. Produce the lipid layer of the tears


4. Drainage of the tear film


B. Third eyelid (nictitating membrane, nictitans)




1. T-shaped cartilage skeleton


2. Nictitans gland produces part of the aqueous portion of tears


C. Tear film




1. Oily outer layer produced by meibomian glands


2. Aqueous layer (main portion) produced by lacrimal and nictitans glands


3. Inner mucoid layer produced by conjunctival goblet cells


D. Drainage of the tears




1. Lacrimal puncta at medial canthus of upper and lower eyelids


2. Canaliculi from puncta to lacrimal sac


3. Nasolacrimal duct empties into the nose at the nasal ostium


E. Globe: The wall of the globe itself consists of three main tunics (layers)




1. The outer fibrous wall (cornea and sclera)


2. A middle vascular coat: The uveal tract (iris, ciliary body, choroid)


3. An inner neuroepithelial layer (retina = retinal pigment epithelium + neurosensory retina)


DISEASES OF THE ORBIT






I. Anatomy of the orbit




A. Bony structure




1. Maxillary, frontal, lacrimal, zygomatic, sphenoid, and palatine bones


2. Herbivores have “closed” orbit with bone 360 degrees around orbital circumference


3. Dogs, cats, and others have “open” orbit with supraorbital ligament at lateral aspect


B. Extraocular muscles




1. Dorsal, medial, ventral, and lateral rectus muscles


2. Dorsal and ventral oblique muscles


3. Retractor bulbi muscles



DISORDERS OF THE EYELIDS






I. Structural or functional abnormalities




A. Agenesis (coloboma): A congenital defect in cats and horses




1. Cats: Upper temporal eyelid (lateral ⅓ to ⅔) fails to develop


2. Absence of eyelids results in secondary trichiasis and exposure keratoconjunctivitis


B. Ankyloblepharon: Adhesion of the eyelid margins to each other


C. Dermoid: A choristoma (normal tissue in an abnormal location)


D. Entropion: Inversion of the eyelid margin (usually results in secondary trichiasis)




1. Causes hairs to rub on cornea (secondary trichiasis)


2. Correct surgically with a modified Hotz-Celsus excision


E. Ectropion: Eversion of the eyelid margin


II. Eyelash abnormalities




A. Distichiasis: Cilia arise from meibomian gland openings


B. Districhiasis: More than one hair emanates from a meibomian gland opening


C. Ectopic cilia: Hair erupts through the palpebral conjunctiva


D. Trichiasis: Hair in normal location but misdirected toward the globe


III. Blepharitis: Inflammation of the eyelids




A. Bacterial: Most commonly Staphylococcus intermedius


B. Parasitic: Mites (e.g., Demodex or Sarcoptes in young dogs, Notoedres in cats, face flies and habronemiasis in horses)


IV. Neoplastic eyelid diseases




A. Canine




1. Benign


2. Meibomian gland adenoma most common


B. Feline




1. Malignant


2. Squamous cell carcinoma most common


C. Equine




1. Sarcoid: Locally invasive and fibroblastic neoplasm. Usually affect horses younger than 7 years of age


2. Squamous cell carcinoma: Predilection for horses with light circumocular pigmentation


D. Bovine: Squamous cell carcinoma, the most common eyelid neoplasm of cows


DISORDERS OF THE CONJUNCTIVA






I. Anatomy and physiology




A. The conjunctiva is a mucous membrane that lines the posterior aspects of the eyelids, the nictitating membrane, and the sclera before terminating at the limbus (corneoscleral junction)


B. Functions of the conjunctiva




1. Protective covering of the eye and adnexa


2. A source of the mucus portion of the tear film (via goblet cells)


3. First line of defense against potential pathogens (via conjunctival associated lymphoid tissue)


II. Congenital abnormalities: Dermoid (a choristoma) normal elements of skin in an abnormal location




A. Aberrant dermis affecting conjunctiva and/or cornea (sometimes also eyelids)


B. Appears as an elevated, often pigmented mass, usually bearing hairs


C. Surgical excision is curative


III. Neonatal disease. Neonatal conjunctivitis (ophthalmia neonatorum)




A. Infection before lid separation; often bacterial in puppies and viral or chlamydial in kittens


B. Treatment: Separate lids, flush with sterile saline, apply broad-spectrum antibiotic ointment four times daily in dogs, tetracycline in kittens


C. Sequelae: Symblepharon (conjunctival adhesion to adjacent tissues), corneal perforation, fibrosis, blindness



DISORDERS OF THE NICTITATING MEMBRANE




DISEASES OF THE CORNEA






I. Anatomy




A. Layers of the cornea




1. Tear film


2. Epithelium


3. Basement membrane


4. Stroma


5. Descemet membrane: Does not take up fluorescein stain


6. Endothelium


B. Corneal response to insult




1. Edema: Loss of epithelial or endothelial layers lead to edema. Fluid accumulation


2. Vacularization: Normal cornea does not have blood vessels


3. Pigmentation: It can arise from chronic corneal irritation


C. Keratitis: Inflammation of the cornea


II. Keratoconjunctivitis sicca (KCS; dry eye)




A. Inflammation of the cornea and conjunctiva as a result of low aqueous component of the tear film


B. Causes




1. Immune-mediated destruction of the tear-producing glands: Most common cause (usually bilateral)


2. Congenital: Gland aplasia (unilateral or bilateral)


3. Iatrogenic: Excision of nictitans gland (unilateral or bilateral) plus later loss of lacrimal gland function


4. Drug induced: Sulfa drugs, etogesic (usually bilateral)


5. Trauma: Often follows prolpase or proptosis of the globe (usually unilateral)


6. Neurogenic: Interference with nerve supply to glands (usually unilateral)


7. Following infection: Distemper in dogs, herpesvirus in cats (unilateral or bilateral)


C. Clinical signs of KCS




1. Ocular irritation or pain


2. Conjunctivitis


3. Ocular discharge


4. Keratitis


D. Diagnosis of KCS




1. Clinical signs (see above)


2. Schirmer tear test readings


E. Treatment of KCS




1. Reduce immune destruction of tear producing glands: Cyclosporine


2. Stimulate tear production: Oral pilocarpine


3. Parotid duct transposition: Moving the parotid duct so that it opens into the lower conjunctival fornix


III. Ulcerative keratitis




A. Investigation of corneal ulcer




1. Fluorescein stain: Show presence of ulcer and demonstrate extent


2. Judge depth of ulcer. Note: Descemet’s membrane does not stain with fluorescein


3. Swab from edge of ulcer crater for culture (bacterial, or fungal in some cases)


4. Smear from edge of ulcer for cytology



C. Midstromal corneal ulcer




1. Potentially more serious


2. Watch carefully


3. Perform culture and cytology


4. Antibiotics should be given frequently if ulcer is infected (e.g., initially every 2 hours)


5. 1% atropine to dilate the pupil as needed but not more than 4 times daily


D. Deep ulcers or descemetoceles




1. Perforation can occur, so treat as an emergency


2. Usually require surgical repair




a. Conjunctival pedicle flap is preferred technique. It provides a blood supply to the defect for healing and physical protection to seal the defect


b. If surrounding cornea is healthy and not infected or melting, a corneoscleral transposition can be performed

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Apr 6, 2017 | Posted by in GENERAL | Comments Off on 8. Ophthalmology

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