2. Diseases of the cornea- Keratitis, ulceration, perforation.

What are the five layers of the cornea?

1. Epithelium

2. Bowman's layer (not present in dogs and cats)

3. Stroma

4. Descemet's membrane

5. Endothelium

   

What is the function of the corneal epithelium?

Outermost protective barrier against pathogens, foreign particles, and environmental insults

What is the function of the corneal stroma?

Contributes to corneal transparency due to its highly organised collagen fibres and contains keratocytes

What is the function of Descemet's membrane?

Structural support and regulation of nutrient and waste exchange between the cornea and aqueous humour

What is the function of the corneal endothelium?

Maintains corneal transparency by regulating hydration and fluid balance

What is the limbus?

The transition zone between the cornea and sclera

What is the significance of the limbus?

1. Richly vascularised (supplying nutrients to the avascular cornea)

2. Contains stem cells for epithelial regeneration

3. Dense network of nerves, lymphatics, and immune cells

What is the uvea?

The middle layer of the eye (vascular tunic) consisting of the iris, ciliary body, and choroid

What are the functions of the uvea?

1. Regulation of intraocular pressure

2. Control of pupil size

3. Accommodation for near-vision

4. Nourishment of the retina

5. Absorption of excess light

What is uveitis?

Inflammation of the uveal tract

Which structures in the eye are avascular?

Cornea, lens, and vitreous humour

What is keratitis?

Inflammation of the cornea, with or without ulceration

How is keratitis classified?

By aetiology, depth, appearance, and whether it is ulcerative or non-ulcerative

What are some bacterial causes of keratitis?

1. Staphylococcus aureus

2. Staphylococcus epidermidis

3. Streptococcus pneumoniae

4. Pseudomonas aeruginosa

5. Chlamydia

6. Enterobacteriaceae

What are some viral causes of keratitis?

1. Canine herpesvirus 1 (CHV-1)

2. Feline herpesvirus 1 (FHV-1)

What are the general clinical signs of viral keratitis?

Punctate, dendritic or geographical corneal ulcers. They remain superficial if ulcerated.

Non-ulcerative appear as a circumferential ring of superficial corneal vascularisation with epithelial & subepithelial leukocyte infiltrate in the peripheral cornea.

What are the most common fungal isolates in canine mycotic keratitis?

Aspergillus spp. and Candida spp.

What are the clinical signs of mycotic keratitis?

Ulcerative or non-ulcerative lesions. Candida infections often present as raised, yellowish-white, or greyish-white plaques or ulcerated lesions. Aspergillus infections are usually ulcerative with extensive stromal inflammation and melting

What are examples of non-ulcerative/superficial keratitis?

1. Pannus/chronic superficial keratitis

2. Pigmentary keratitis

3. Deep keratitis

What is pannus/chronic superficial keratitis/non-ulcerative keratitis?

A progressive, immune-mediated inflammatory condition affecting the cornea in dogs

Which breeds are predisposed to pannus?

German Shepherds, Greyhounds, and Border Collies

What are the clinical signs of pannus?

Greyish-pink film on the eye progressing to corneal opacity due to cellular infiltration and vascularisation

How is pannus treated?

Lifelong corticosteroids

What is pigmentary keratitis?

Deposition of pigment on the cornea secondary to chronic corneal irritation in dogs

Which breeds are predisposed to pigmentary keratitis?

Small brachycephalic dogs

What are the clinical signs of pigmentary keratitis?

Brown to black pigmentation on the cornea (starting at the limbus), corneal vascularisation, opacity, ulceration/erosions (in severe cases), ocular irritation, redness, tearing, squinting, and potential vision impairment/blindness if the central cornea is affected

What is deep keratitis?

Keratitis caused by canine adenovirus-1 infection/vaccination

What are the clinical signs of deep keratitis?

Uveitis, corneal opacity ("blue eye"), and severe eye pain often disproportionate to clinical findings

How is deep keratitis diagnosed?

Thick rime of fluorescein stain uptake

How is keratitis treated?

1. Preventive ATB. Dexpanthenol gel (promote wetness). CSS (immune-mediated only if no ulceration is present), cyclosporin (immune-mediated), anti microbial/fungal etc- based on aetiology.

2. Grid keratotomy- needle scratch grid lines onto cornea, allowing healthy cell to grow → promote healing

3. Diamond burr debridement

What is ulcerative keratitis?

A break in the corneal epithelium exposing the underlying stroma

What are some causes of ulcerative keratitis?

Eyelid abnormalities, foreign bodies, trauma, and keratoconjunctivitis sicca (KCS)

What are the clinical signs of ulcerative keratitis?

Hyperlacrimation, blepharospasm, photophobia, conjunctival hyperaemia, corneal oedema, and potentially miosis with aqueous flare

How is ulcerative keratitis diagnosed?

Clinical signs and fluorescein stain retention by the corneal stroma

What are the differential diagnoses for ulcerative keratitis?

Trauma, entropion, KCS, FHV-1 infection, pannus, and Boxer ulcer

How is ulcerative keratitis treated?

1. Addressing the underlying cause

2. Topical antibiotics (gentamicin for G+, chloramphenicol for G-)

3. EDTA (protease inhibitor), topical atropine

4. Corticosteroids are contraindicated!

5. Pedicle grafts or corneal transplantations for deep ulcers

6. Grid keratotomy may be performed

How are corneal ulcers classified by depth?

Superficial, stromal, descemetoceles, and perforations

What is a spontaneous chronic corneal epithelial defect/indolent ulcer/Boxer ulcer?

A superficial corneal ulcer with a non-adherent/peeled corneal edges. Recurrent uveitis may be present.

What is the aetiology of indolent ulcers?

Congenital, occurring spontaneously without trauma.

What is the pathogenesis of indolent ulcers?

A defect in the corneal layers, specifically separation of the stroma from the corneal epithelium.

What are the clinical signs of indolent ulcers?

Blepharospasm, hyperlacrimation, conjunctival hyperaemia, eye discharge, ocular pain, corneal cloudiness, and sometimes visible defects or white spots on the cornea.

How are indolent ulcers diagnosed?

Clinical signs, slit-lamp examination, fluorescein dye staining, and cultures.

What are the differential diagnoses for indolent ulcers?

Traumatic ulcer, immune-mediated keratitis, infectious ulcer, and KCS.

How are indolent ulcers treated?

Removal of abnormal epithelium and stroma (corneal debridement) with a cotton tip and application of topical antibiotics. Surgical procedures like superficial keratectomy, conjunctival grafting, or temporary tarsorrhaphy may be necessary.

What is a melting ulcer?

Liquefaction of the corneal stroma by proteases produced by bacteria (e.g., Pseudomonas spp.), fungi, or inflammatory cells.

What are the causes of melting ulcers?

Infected ulcers, direct bacterial action (e.g., Pseudomonas, haemolytic Streptococcus), and chemical injury, especially alkali burns.

What are the clinical signs of melting ulcers?

Severe eye pain, redness, tearing, blurred vision, photophobia, eye discharge, and a white to yellow spot on the cornea that may have a mushy or gelatinous texture.

How are melting ulcers diagnosed?

Visualisation, slit-lamp biomicroscopy, fluorescein staining, and corneal swab culture.

How are melting ulcers treated?

1. Topical and/or systemic antibiotics/antifungals

2. Topical anti-proteases (EDTA)

3. Lubricating eye drops

4. Antiviral drugs

5. Enucleation may be necessary in severe cases.

What are stromal abscesses?

A pocket of infection of corneal tissue that usually manifests as a light grey or soft pink haze in its early stages

What is the aetiology of stromal abscesses?

1. Worsening of ulcers (rate of generation and protection mechanisms are unbalanced).

2. Can be due to eyelid abnormalities (ectopic cilia, entropion, irritants, trauma, immune-mediated, infection, dystrophy, precorneal tear film defects

What are the clinical signs of stromal abscesses?

Similar to ulcer but with the presence of yellow or white stromal infiltrate

How is a stromal abscess diagnosed?

Clinical signs

What is the treatment for stromal abscesses?

• Topical: chloramphenicol, natamycin, atropine

• Systemic: NSAIDs, trimethoprim, itraconazole

• Surgery: debride, pedicle flap on penetrating keratoplasty

What is a descemetocele?

A deep corneal lesion where the epithelium and stroma are completely destroyed, leaving only Descemet's membrane and endothelium.

What is the significance of Descemet's membrane in descemetoceles?

It is a tough, elastic membrane (but only 3-12 μm thick) that forms the final barrier against perforation.

What happens if a descemetocele ruptures?

A full-thickness corneal perforation occurs, aqueous humour is lost, and iris prolapse may occur.

What can occur after rupture of Descemet's membrane?

Contamination of the anterior chamber, which may lead to endophthalmitis and a much poorer prognosis for saving the eye and vision.

What is the aetiology of descetoceles and perforation?

Progression of deep corneal ulcers or trauma

How are descemetoceles and perforations diagnosed?

Full eye examination. Fluorescein → Seidel test for presence of aqueous humour leakage from globe

How are descemetoceles and perforations treated?

Conjunctival grafts and often surgery (suturing for small injuries).

What is an example of a neoplasm affecting the cornea?

Dermoid cyst

What is a dermoid cyst?

A congenital neoplasia (choristoma) where tissue collects under the skin, often near the lateral canthus

Which breeds have a genetic predisposition to dermoids?

German Shepherds, Dalmatians, and St. Bernards

What are the clinical signs of a dermoid cyst?

Visible/palpable mass on the eyelid edge, usually near the lateral canthus, typically non-tender. May cause mild irritation or discomfort. Rarely, it may rupture causing acute inflammation and redness

How are dermoid cysts diagnosed?

Clinical signs, inspection, and biopsy

How are dermoid cysts treated?

Surgical removal of abnormal tissue, including hair follicles, followed by lid margin closure.

Blepharoplasty may be needed for insufficient lid margin length

Sliding or grafting procedures may also be used for closure.

What is an example of a congenital defect affecting the cornea?

Persistent pupillary membrane

What is persistent pupillary membrane (PPM)?

A congenital defect involving remnants of foetal tissue in the pupil area. It can be iris-to-iris, iris-to-lens, or iris-to-cornea.

What are some surgical procedures for the cornea?

1. Superficial keratectomy

2. Keratoplasty/corneal grafting

3. Conjunctival flap

4. Corneoscleral transpositioning

What is superficial keratectomy?

Excision of a portion of the corneal epithelium and stroma.

What is keratoplasty/corneal grafting?

Surgical transplantation of corneal tissue.

What is a conjunctival flap?

A tissue covering used for deep or slow-healing corneal lesions.

What is corneoscleral transpositioning?

A corneal transplant using adjacent healthy corneal tissue and conjunctival tissue to cover the ulcer and provide structural support.

Why does the eye have an unpaired number of muscles?

Everything is paired except musculus retractor bulbi (unpaired)

What is an autoimmune cause for keratitis?

Pannus

Which breeds is keratoconjunctivitis sicca more common in?

Cocker spaniel and chihuahua

What methods can be used for diagnosis of KCS?

1. Schirmer Tear Test (under 8mm)

2. Fluorescein test