Ophthalmology Exam

Limitations: Single STT-1 <5 mm/min diagnostic for KCS, but sequential testing more reliable; environmental factors (stress, time of day) affect values

Intraocular Pressure (IOP) - Tonometry

Normal Ranges (Applanation Tonometry)

• Dogs: 12-22 mmHg (mean 15-18)
• Cats: 12-20 mmHg (mean 15-17); typically 2-3 mmHg lower than dogs
• Rabbits: 12-22 mmHg

Glaucoma Thresholds

• IOP >25-30 mmHg warrants investigation for glaucoma

• 35 mmHg with clinical signs (pain, mydriasis, corneal edema, lens subluxation) = acute glaucoma emergency

• Baseline IOP varies by individual; serial measurements more useful than single value

Rebound Tonometry: Non-contact; convenient but 1-3 mmHg higher than applanation; acceptable for screening

Technique: Applanation (gold standard); requires anesthesia/topical blocks in fractious animals; avoid if corneal ulcer suspected (contraindicated)

Fluorescein Staining

Technique: 0.5-1% sterile fluorescein strip or solution; examine under cobalt-blue light (Wood's lamp, slit lamp)

• Positive stain (green): Indicates loss of corneal epithelium (ulceration, erosion, foreign body)
• Negative stain (punctate dark spots in epithelium): Desquamated epithelial cells, superficial keratitis, KCS changes

Differential Diagnosis by Staining Pattern

• Central ulcer: Melting ulcer, deep stromal involvement; risk of perforation
• Dendritic pattern: Suspected herpesvirus (feline, equine)
• Punctate staining: KCS, superficial keratitis, early ulceration

Precautions: Do not apply pressure; avoid excessive fluorescein (causes false staining); irrigate eyes post-exam to prevent dye absorption

Common Conditions by Species

Dogs

1. Keratoconjunctivitis Sicca (KCS)

   • Prevalence: 1-5%; breeds: Cocker spaniels, Bulldogs, West Highland terriers
   • Diagnosis: STT-1 <15 mm/min (ideally <10); history of chronic conjunctivitis, xerosis
   • Treatment: Artificial tears (TID-QID minimum), cyclosporine (0.5% BID-TID; onset 4-6 weeks), tacrolimus ophthalmic, pilocarpine (rarely used)

2. Glaucoma

   • Acute: IOP >35 mmHg, mydriasis, pain (blepharospasm, photophobia), corneal edema, red eye
   • Chronic: Gradual blindness, buphthalmos (large eye), cupped optic disc
   • Breeds predisposed: Beagles, Cocker spaniels (primary); secondary in uveitis, anterior lens luxation

3. Cataracts

   • Breed-related (hereditary): Poodles, Labrador retrievers, Miniature schnauzers
   • Diagnosis: Lens opacity on direct/indirect ophthalmoscopy; shadows in pupil
   • Treatment: Surgical (phacoemulsification) for vision-threatening; medical treatment not effective

4. Corneal Ulcers

   • Classification: Superficial vs. deep vs. perforating
   • Fluorescein positive: Indicates epithelial loss
   • Melting ulcer: Stromal collagenase activity; risk of rapid progression/perforation; requires intensive care

5. Entropion

   • Inversion of eyelid margin; irritates cornea
   • Breeds: Labrador retrievers, Chow Chows, Basset hounds, Rottweilers
   • Treatment: Surgical correction (blepharoplasty)

Cats

1. Herpes Simplex Keratitis (Feline Herpesvirus-1, FHV-1)

   • Presentation: Conjunctivitis, keratitis, dendritic ulcers (fluorescein-positive)
   • Diagnosis: PCR, viral culture (less sensitive); clinical presentation in endemic regions
   • Treatment: Antiviral ointment (idoxuridine, vidarabine 5×/day), L-lysine (250-500 mg BID oral)

2. Keratoconjunctivitis Sicca

   • Less common than in dogs; associated with FIV, Chlamydia infection
   • Diagnosis: STT-1 <10 mm/min
   • Treatment: Same as canine (cyclosporine, artificial tears)

3. Glaucoma

   • Secondary to uveitis common; primary glaucoma less common than dogs
   • Associated conditions: Anterior uveitis, inflammatory disease, lymphoma
   • IOP threshold: >20 mmHg warrants investigation; >25-30 mmHg with signs = acute glaucoma

4. Cataracts

   • Less common hereditary form than in dogs; diabetes-related common (rapid onset)
   • Diagnosis: Lens opacity
   • Treatment: Surgical (phacoemulsification) if vision desired; medical management less effective

5. Anterior Uveitis

   • Often secondary to systemic disease (FIV, FeLV, toxoplasmosis, fungal)
   • Signs: Pain (blepharospasm, photophobia), corneal edema, iris color change, hyphema (blood in AC)
   • Topical: Atropine (dilates pupil, improves comfort); dexamethasone/betamethasone QID (caution: avoid if corneal ulcer)

Exotic Species

• Rabbits: Cataracts common; KCS rare; dacryocystitis (blocked tear duct) frequent; need specialized anesthesia for tonometry
• Reptiles: Spectacle (brille, transparent covering); retained spectacle common; no eyelids in snakes

Breed Predispositions

Workflow

1. Examine external eye (lids, lashes, conjunctiva, sclera)
2. Perform Schirmer tear test (STT-1) prior to fluorescein (dye interferes)
3. Apply fluorescein; examine corneal surface under light
4. Tonometry (applanation if ulcer ruled out)
5. Direct and indirect ophthalmoscopy (dilate with tropicamide 1% if indicated)
6. Assess accommodation, pupillary light reflex, consensual response

Key Species Differences

• Dogs: Higher baseline IOP (15-18 mmHg); STT values 15-25 mm/min; breed-specific screening available (OFA, CERF)
• Cats: Lower tear production (10-20 mm/min); IOP slightly lower (15-17 mmHg); FHV-1 endemic; secondary uveitis common
• Exotics: Specialized anesthesia required; reference ranges differ significantly

Limitations

• Single IOP measurement insufficient for glaucoma diagnosis; serial measurements, fundic assessment essential
• Fluorescein staining does not assess depth of ulceration (ultrasound or clinical assessment needed)
• Some breed predispositions lack sufficient statistical data; breeding decisions should use multi-generational pedigree assessment
• Ophthalmic examination in conscious fractious animals may be incomplete; sedation/anesthesia may be required
• Referral to board-certified ophthalmologist recommended for glaucoma, anterior uveitis, corneal melting, and surgical candidates