Chapter 58 - Surgery of the ocular surface Flashcards
What is the function of the cornea?
To allow light to enter the eye and contribute to the eye’s refractive power.
Figure 58-1. Regions of the conjunctiva. a, Fornix; b, palpebral conjunctiva; c, bulbar conjunctiva; d, bulbar conjunctiva of the third eyelid; e, palpebral conjunctiva of the third eyelid.
Figure 58-2. Blue-gray appearance of the iridocorneal drainage angle as seen through the normal equine cornea at the temporal and nasal limbus (arrows).
What type of collagen transition occurs at the bulbar limbus?
From uniformly organized collagen in the cornea to irregularly arranged collagen in the sclera.
Where is the bulbar limbus located?
At the junction between the cornea and the sclera.
Figure 58-3. Photomicrograph demonstrating the four layers of the equine cornea. a, Epithelium; b, stroma; c, Descemet’s membrane; d, endothelium (arrow) (hematoxylin and eosin, ×170).
What is Tenon’s capsule?
A thin fascial layer that envelops the globe beneath the bulbar conjunctiva.
What type of epithelium characterizes the conjunctiva?
Nonkeratinized stratified columnar to cuboidal epithelium.
Which part of the conjunctiva covers the inner surface of the eyelids?
The palpebral conjunctiva.
What are the two layers of the substantia propria in the conjunctiva?
The superficial adenoid layer and the deeper fibrous layer.
What role does the conjunctival goblet cells play?
They produce mucin, a key component of the tear film.
Which lymph nodes drain the medial conjunctival regions?
The mandibular lymph nodes.
What nerves innervate the bulbar conjunctiva?
The long ciliary branches of the ophthalmic division of the trigeminal nerve.
From which artery is the conjunctival blood supply derived?
The anterior ciliary arteries, branches of the ophthalmic artery.
What typical reaction does the conjunctiva show to injury?
Hyperemia and edema with possible swelling.
How does conjunctival epithelium heal after injury?
By mitotic proliferation and cellular migration over the substantia propria.
What is the primary function of the sclera?
To provide structural support and protection for the intraocular contents.
What happens to the mitotic rate of the conjunctival epithelium in response to central corneal injury?
It increases significantly.
Why does the sclera appear white?
Because its collagen fibers are irregularly arranged, scattering light.
What type of collagen predominates in the scleral stroma?
Type I and III collagen.
How does the sclera receive nutrients?
Through the vascularized episclera and the underlying choroid.
Which nerve supplies sensory innervation to the anterior sclera?
The long posterior ciliary nerves.
What healing process occurs after scleral trauma?
Fibroblasts aid in relatively rapid healing of apposed scleral incisions.
What structures meet at the limbus?
The epithelial cells of the cornea and conjunctiva.
What cells are located at the palisades of Vogt?
Limbal stem cells.
How many cell layers thick is the corneal epithelium?
Approximately 8 to 10 layers.
What type of collagen is most abundant in the corneal stroma?
Type I and type V collagen.
What is the shape of the cornea in an adult horse?
A horizontal ellipse.
How is corneal transparency maintained?
Through the lack of vasculature, uniform collagen arrangement, and endothelial pump mechanism.
What structure attaches the corneal epithelium to the stroma?
Hemidesmosomes and basement membrane-anchoring fibrils.
What are the main components of Descemet’s membrane?
An anterior banded layer and a posterior nonbanded layer.
What happens when corneal endothelial cell density decreases?
Fluid accumulates in the stroma, causing corneal edema.
What is the result of irregular collagen arrangement in the corneal stroma after injury?
The formation of a corneal opacity or scar.
How thick is Descemet’s membrane in adult horses?
Approximately 38 μm.
What is the primary function of the corneal endothelium?
To regulate corneal hydration by limiting fluid entry from the anterior chamber.
What prevents Descemet’s membrane from taking up fluorescein stain?
Its dense structure and composition as a basement membrane.
What pump mechanism does the corneal endothelium use?
The Na+/K+–ATPase pump.
What is the approximate central corneal thickness in an adult horse?
Between 0.77 and 0.89 mm.
What cell type is primarily responsible for synthesizing new collagen in corneal wounds?
Keratocytes.
How does the corneal stroma’s hydrophilic nature affect wound healing?
It relies on the intact epithelium and endothelial pump to maintain relative dehydration.
What feature distinguishes the posterior sclera’s innervation?
It is supplied by the short posterior ciliary nerves.
How does limbal stem cell recruitment affect corneal healing?
It aids in the reepithelialization of the cornea after injury.
How does the cornea receive nutrition despite being avascular?
Through the precorneal tear film and aqueous humor.
What effect does trauma to the corneal endothelium typically have on the cornea?
It results in stromal swelling and opacity due to fluid accumulation.
Where is corneal innervation density the greatest?
In the superficial corneal stroma, particularly in the central cornea.
Why are functional eyelids essential for corneal health?
They distribute the tear film, providing necessary nutrients and protection.
What nerve innervates the cornea, and what branch does it derive from?
The cornea is innervated by long ciliary nerves from the ophthalmic branch of the trigeminal nerve.
How does the cornea typically respond to injury?
With edema, inflammatory cell infiltration, neovascularization, and sometimes pigmentation or scarring.
What causes corneal edema?
Fluid accumulation from either the tear film (if epithelium is damaged) or aqueous humor (if endothelium is damaged).
How can chronic inflammation lead to corneal pigmentation?
Melanocytes migrate from limbal tissue into the basal corneal epithelium or stroma.
Which growth factors contribute to corneal neovascularization?
Vascular endothelial growth factor-A (VEGF-A) and fibroblast growth factor.
What is the main source of cells and growth factors for corneal wound healing?
the aqueous humor, tear film, and limbal vessels.
How quickly do basal epithelial cells migrate after corneal injury?
Within one hour of injury, cells begin moving towards the center of the lesion.
What role do matrix metalloproteinases (MMPs) play in wound healing?
They decrease cellular adhesion at wound edges, facilitating epithelial migration.
What happens if excess proteolytic activity occurs in the corneal stroma?
It can lead to corneal malacia (softening of the corneal tissue).
What is the role of transforming growth factor-β (TGF-β) in corneal healing?
It induces differentiation and infiltration of inflammatory cells into the defect.
How do small full-thickness corneal wounds seal?
With a fibrin clot that initiates epithelial and stromal healing.
What is the average healing time for a non-infected 7-mm corneal wound?
Approximately 11 days.
What can happen with large full-thickness corneal defects?
The iris may prolapse, leading to complications such as endophthalmitis.
What is neurogenic anterior uveitis, and what triggers it?
An inflammatory response due to corneal injury, triggered by prostaglandins and substance P release.
Which bacteria are commonly found on the normal equine ocular surface?
Gram-positive bacteria like Corynebacterium, Staphylococcus, and Bacillus.
What species of fungi are commonly isolated from equine eyes?
Aspergillus, Cladosporium, and Penicillium.
What infections commonly complicate keratomycosis?
Aspergillus and Fusarium species.
List three causes of infectious ocular surface diseases.
Bacterial, fungal, and viral infections.
How is corneal sensation tested during an ophthalmic examination?
By lightly touching the cornea with cotton to assess the corneal reflex.
Why is dim lighting used during an ocular examination?
To improve visualization of lesions with a bright focal light.
What is the purpose of an auriculopalpebral nerve block in ocular exams?
To reduce the strength of the blink reflex for easier examination.
Why might sedation be required during an ocular examination?
To restrain the horse and facilitate examination, especially if painful lesions are present.
What is fluorescein staining used for?
To detect corneal epithelial defects by binding to exposed corneal stroma.
How does the Seidel test work?
It detects aqueous leakage by applying fluorescein dye directly to a corneal wound.
What is rose bengal staining particularly useful for?
Detecting devitalized epithelial cells in conditions like equine herpesviral corneal disease.
When should culture and sensitivity testing be performed on ocular samples?
Before applying anesthetics or stains, as these can inhibit microorganism growth.
What is the purpose of cytology in ocular surface disease?
To identify inflammatory cells and infectious organisms.
What tool yields the best diagnostic samples in ocular cytology?
A cytobrush.
How are deep corneal ulcers typically sampled for cytology?
Carefully, using a cytobrush, Kimura spatula, or blunt scalpel end.
What type of stain can help identify fungal hyphae on cytology?
Gomori methenamine silver (GMS) or periodic acid–Schiff (PAS).
Which ocular examination technique should be performed before sedating the horse?
Preliminary evaluation of the eye, including assessing symmetry and evidence of discomfort.
How does incomplete lid closure affect corneal health?
It prevents normal tear distribution, which can harm corneal integrity.
What is the function of the palpebral reflex in equine eyes?
It helps ensure the eyelids close completely, distributing the tear film.
What effect can drugs like flunixin meglumine have on corneal neovascularization?
They can delay it but are often used for uveitis treatment and pain relief.
How does the iris respond in a collapsed anterior chamber due to corneal injury?
It can prolapse through the cornea, which complicates healing.
Figure 58-5. Cytobrush for cytology sampling (Microbrush).
Figure 58-6. (A) Kimura platinum spatula. (B) Technique for acquiring corneal samples for cytology and culture. The corneal surface is scraped with a Kimura platinum spatula.
(A) Kimura platinum spatula.
Figure 58-4. Superficial corneal ulcer stained positive with fluorescein.
Figure 58-9. Subpalpebral lavage kit (Mila International).
Figure 58-10. Subpalpebral lavage line placed superiorly with silicone tubing secured to the head with tape tabs.
