Third Eyelid, Cornea, Lacrimal System

First section is all third eyelid stuff

What is the function of the third eyelid?

Protection of cornea

Distribution of tear film

Contains the nicitans gland which contributes about 30-40% of the aqueous component of the tear film

What are some diseases of the third eyelid?

Nictitans gland prolapse

Scrolling of the third eyelid cartilage

Prolapse can be due to neurological disease

What does scrolling of the cartilage mean?

Where the T shaped cartilage becomes abnormally curved

The mechanism of this is unknown

What is cherry eye and what causes it?

Prolapsed nictitans gland

Poor anchoring of gland to orbital periosteum

Inflammation of the cland causing a loss of anchoring ligament

Ok moving on to cornea

What are the layers of the cornea from outer to inner

Tear film

Epithelium

Stroma

Descemet’s membrane

Endothelium

What is the function of the cornea?

Maintain globe structure

Transparency for vision

Here’s a list of the corneal diseases

Keratoconjunctivitis sicca

Infectious keratitis

Eosinophilic keratitis

Other non-ulcerative keratitis

Sequestrum

Lipid keratopathy

Primary and secondary edema

Neoplasia

What is corneal ulceration?

Loss of epithelium (I think it has to be the full thickness of the epithelium)

Can also involve stroma and descemet’s membrane

What are the four phases of corneal healing?

Latent

Migration

Proliferation

Cell substrate attachment

What happens in the latent phase?

No attempt to heal ulcer yet

Fibronectin polymerases onto wound bed - provisional anchoring extracellular matrix

Apoptosis of cells damaged by wound stimulus

Increased metabolic activity and cell structural changes in leading cells behind apoptozed ones

Reduction in adhesion of basal cells to basement membrane

What happens in the migration phase?

Migration of entire epithelial sheet over the wound bed

There is no cell division at the wound bed, they are just sliding in to fill it

Leading edge cells flatted out over wound area and develop filopodia (like antennae)

Cells at the periphery of the cornea undergo mitotic division while the wound bed is migrated over

What happens during the proliferation phase?

Basement membrane remodeling

Transformation of basal cells into the layers of stratified epithelium and eventually squamous cells

What happens during the attachment phase?

New hemidesmosome attachments formed. This process can take up to a year to form a complete new attachment

Describe the process of stromal healing

Keratinocytes in stromal collagen transform into fibroblasts

Epithelial compromise → swelling of stroma, apoptosis of keratinocytes/fibroblasts, invasion of inflammatory cells

Inflammatory cells recruited from conjunctival vessels to injury to protect from infection but can cause tissue damage

Remaining fibroblasts migrate to wound and deposit fibrotic material

Number of transformed keratocytes decline once wound is filled

Slow remodeling phase over weeks to years to reoganize collagen and restore transparency

Outcome can be: regeneration of normal structure, scarring, stromal melting, neovascularization

What are the clinical signs of an ulcer?

Pain!

• Blepharospasm, ocular discharge

• Reflux uveitis - neurological spasm of the iris and ciliary body

Conjunctival hyperemia

Focal corneal edema (loss of epithelium allows ingress of water)

Cornea neovascularization

What causes ulcers?

Most commonly it is trauma

Poor corneal health

• External irritation bc of all those eyelid/eyelash issues from last lecture

• Tear film: reduced quantity, quality, blink, etc

Corneal pathology

• Lipidosis

• Corneal edema

• Superficial chronic corneal epithelial deficit (SCCED)

Infectious disease (feline herpesvirus)

How common is spontaneous chronic corneal epithelial deficit?

Pretty common in boxers, corgis, frenchies, etc

What is SCCED?

Condition affecting the epithelium that causes a non-healing ulcer

A smooth flat sheet forms on the surface so cells can’t grip to slide across and heal

You end up with extra staining around the ulcer bc the cells are still being made but cannot fill in the gap

What pathogens are often involved with loss of stroma in deeper ulcerations?

Pseudomonas and Streptococcus spp

They make colleganases 

What would you see on cytology of a melting ulcer?

Neutrophils

Epithelial cells

Bacteria (maybe)

What animals get corneal sequestrum?

Felines, rarely equine

What is it and what causes it?

It is an area of necrotic cornea

Often underlying FHV is a cause

Looks black bc black pigment absorbed from feline tear film

What breeds are predisposed to lipid keratopathies?

Cavaliers, boxers, huskies

What is it?

Benign usually non-progressive accumulation of lipid in corneal fibroblasts (cholesterol crystals)

Can be a response to corneal insult or due to systemic lipid abnormalities

What causes chronic superficial keratitis? What are the signs on the eyes?

UV induced

Neovascularization

Pigmentation

Fibrosis

Which animals get eosinophilic keratitis?

Feline, equine

Signs? Causes?

FHV association 

Eosinophils in dense white plaques

Neovascularization

What causes corneal edema?

Endothelial cell loss (→ decreased pumps which the cornea needs to push water out)

Glaucoma: forces fluid into the cornea

Uveitis: inflammation of the endothelial cells

Ulceration: loss of waterproof barrier

Limbal disease

What are inclusion cysts?

Epithelial cells implanted into stroma

They grow and shed

Leaves creamy white fluid in center of epithelial cell lined cyst

What dis 

Neoplasia on cornea. it rare

Okayyyyy moving on to lacrimal system

What is the lacrimal system in charge of?

Tear production and drainage

What are the components of the tear film? What makes each component

Mucus: made by goblet cells in conjunctiva

Aqueous: lacrimal gland in upper eyelid, nictitans gland in third eyelid

Lipid: meibomium glands on eyelid margin

What are the functions of the tear film?

Lubrication

Immune modulation

Removal of debris

Nutrition to the cornea

What causes lacrimal gland insufficiency?

Congenital (in curly coat cavaliers)

Drug induced (sulphonamides)

Immune mediated

Neurogenic

Iatrogenic

Systemic dz

What is immune mediated keratoconjunctivitis sicca (KCS)?

When the body recognizes self antigens in the lacrimal gland

Starts a T cell response against the antigens

→ Reduction in tear film quantity and quality

What are the clinical signs of KCS?

Blepharospasm

Mucoid (body making mucus to try to protect the eye)/mucopurulent (if secondary infection) discharge

Conjunctivitis

Keratitis

Ulceration

May see dry nares but that is more common with neurogenic dry eye

What are some causes of problems draining tears?

Imperforate ducts (at either end)

External compression: cysts, trauma, dental dz (rabbit), parasites (onchocerca)