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.
Why is cytology recommended for corneal ulcers and abscesses?
It provides immediate data on inflammation and presence of infectious organisms.
What is the main difference between fluorescein and rose bengal staining?
Fluorescein stains stromal defects, while rose bengal stains devitalized cells.
what type of bacteria is Corynebacterium?
Normally found healty ocular surface
Positive
Rod
what type of bacteria is Staphylococcus?
Normally found healty ocular surface
Positive
Cocci
what type of bacteria is Streptomyces?
Normally found healty ocular surface
Positive
Rod
what type of bacteria is Streptococcus?
Normally found healty ocular surface
Positive
Cocci
what type of bacteria is Acinetobacter?
Normally found healty ocular surface
Negative
Coccobacilli
what type of bacteria is Pseudomonas aeruginosa?
Found in ocular disease
Negative
Rod
what type of bacteria is E. Coli?
Found in ocular disease
Negative
Rod
what type of bacteria is Moraxella?
Normally found healty ocular surface
Negative
Coccobacilli
what type of bacteria is Enterobacter spp.
Found in ocular disease
Negative
Rod
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.
What is the main advantage of topical application of ophthalmic drugs?
It provides a high concentration at the site of action, especially for ocular surface conditions.
Which factors influence the retention of topical ophthalmic medications?
Precorneal factors like lacrimation and nasolacrimal drainage, and drug formulation characteristics.
Why might ophthalmic ointments be chosen over solutions for equine eyes?
They have prolonged retention time and ease of administration.
Why is it recommended to wait 5 minutes between applications of different solutions?
To avoid diluting the previously applied drug.
What precaution should be taken if the anterior chamber is penetrated?
Use an ophthalmic solution instead of ointment.
How can frequent medication administration be facilitated in horses?
By placing a subpalpebral lavage line.
Why should surgical scrubs with detergents not be used near the eye?
They may cause epithelial loss and ulceration.
Which drugs are commonly combined in triple-antibiotic formulations?
Neomycin, polymyxin B, and bacitracin or gramicidin.
Why are aminoglycoside antibiotics used in combination for ocular infections?
To expand the spectrum of activity, especially against gram-positive organisms.
What is a preferred monotherapy for bacterial ocular infections?
Fluoroquinolones, such as Ofloxacin.
How can antifungal drug penetration be improved for deeper corneal infections?
By debriding the corneal epithelium or adding dimethyl sulfoxide (DMSO) to the formulation.
What caution should be given to owners when using chloramphenicol?
It can cause aplastic anemia in humans; gloves are recommended.
Why are topical antifungals often used prophylactically in horses?
Horses are prone to fungal keratitis, especially with corneal injuries.
What is the only commercially available topical ophthalmic antifungal?
Natamycin ophthalmic suspension
What are the risks of using topical corticosteroids in corneal ulcerations?
They delay wound healing and can potentiate infections.
Which NSAID solutions are commonly used topically for ocular inflammation?
Flurbiprofen 0.03% and diclofenac 0.1%.
What are the two commonly used topical anesthetics for the eye?
Proparacaine 0.5% and tetracaine 0.5%.
How long does the anesthetic effect of proparacaine and tetracaine typically last?
About 25 to 30 minutes.
Why is continuous use of topical anesthetics for therapeutic purposes discouraged?
They cause epithelial toxicity and destabilize the tear film.
When should ocular cultures ideally be collected in relation to anesthetic application?
Before instilling the anesthetic due to its antimicrobial effects.
Why is ketamine usually avoided for surgeries involving the anterior chamber?
It increases intraocular pressure.
How is central positioning of the cornea achieved during surgery?
By using neuromuscular blocking agents like atracurium with positive pressure ventilation.
what magnification tools are commonly used for corneal surgery?
Operating microscopes or head loupes with 2.5× or 4× magnification.
Which forceps are recommended for handling conjunctival tissue?
Colibri and Bishop-Harmon forceps.
What type of scissors are suggested for conjunctival incisions?
Fine-tipped blunt tenotomy scissors like the Stevens or Westcott.
Which instruments are specifically useful for suture-tying in corneal surgeries?
McPherson tying forceps.
What are the potential complications if corneal drying occurs during anesthesia?
Postoperative corneal ulceration.
Which disposable cautery units are typically used in equine eye surgeries?
Fine-tipped disposable cautery units.
What is the main purpose of using viscoelastic agents during anterior chamber surgeries?
To maintain intraocular pressure and prevent collapse of the anterior chamber.
What type of suture material is preferred for corneal surgery?
Absorbable 910 polyglactin.
Why is absorbable 910 polyglactin preferred for ocular surfaces?
It’s well-tolerated, reduces trauma risk, and provides secure knot handling.
Which suture pattern is best for bulbar conjunctiva apposition?
Simple-continuous pattern.
What needle type is recommended for 6-0 absorbable sutures in conjunctival surgery?
Reverse cutting or spatulated needle.
For scleral defects, which suture pattern is used?
Simple-interrupted pattern.
At what depth should corneal sutures be placed in full-thickness defects?
Approximately 80% of the corneal thickness.
What risk arises if corneal sutures are placed too superficially?
The posterior wound can gape, risking integrity and increasing scarring.
What complication occurs if corneal sutures are full-thickness?
It can cause anterior uveitis and allow aqueous humor leakage.
What suture pattern is used in penetrating keratoplasty?
Simple-interrupted pattern.
Why is a spatulated needle preferred for corneal suturing?
It minimizes corneal collagen damage by creating a circular path.
At what angle should the needle enter the corneal surface?
Close to 90 degrees.
Why is equal entry and exit distance important in corneal suturing?
It ensures symmetrical suture placement for better wound stability.
Which test checks for aqueous humor leakage after corneal repair?
Seidel test with fluorescein dye.
How should the patient’s head be positioned for ocular surgery?
Laterally with eyelids as horizontal as possible.
What solution is used to prepare the ocular adnexa for surgery?
1:50 povidone-iodine solution.
Why should cotton swabs be avoided for hemostasis in ocular surgery?
They shed fibers and are abrasive to the corneal epithelium.
Which tool is preferred for hemostasis on the ocular surface?
Sterile cellulose ocular sticks.
What is used to achieve additional hemostasis if needed?
2.5% phenylephrine or 0.1 mg/mL epinephrine on a cotton-tipped applicator.
Which agents help maintain anterior chamber depth if it’s compromised?
Sodium hyaluronate 2% or hydroxypropyl methylcellulose 2%.
What complication can arise from viscoelastic agents postoperatively?
Ocular hypertension.
Why is a protective eye-cup mask used postoperatively?
To prevent self-trauma to the surgical site.
What is conjunctivectomy?
Resection of a region of conjunctiva.
Why might general anesthesia be required for conjunctivectomy?
For excision of larger conjunctival or corneoconjunctival lesions.
What is the primary goal of performing a conjunctival biopsy?
To obtain tissue for histopathological examination.
What type of suture is used for conjunctival defects larger than 5 mm?
6-0 polyglactin 910.
Why is suture placement avoided in palpebral conjunctival defects?
To prevent irritation and ulceration of the cornea.
What suture type is commonly used for bulbar conjunctival lacerations?
Simple-continuous 6-0 absorbable suture.
What diagnostic procedure confirms scleral laceration extent?
A complete ophthalmic examination.
hich suture material is suggested for scleral defect closure?
5-0 absorbable suture, like polyglactin 910.
What indicates that the anterior chamber might be leaking aqueous humor?
Shallowing of chamber depth and decreased globe turgidity.
What agents are used to manage anterior uveitis post-surgery?
Topical atropine and systemic NSAIDs.
How is a lateral canthotomy performed?
By making a 5-10 mm incision extending laterally from the canthus.
What suture pattern is recommended to close a lateral canthotomy?
Simple-continuous for subcutaneous layer, simple-interrupted for skin.
What is the function of figure-of-eight sutures in lid margin repair?
To secure apposition without causing corneal trauma.
When is a simple-interrupted pattern chosen over simple-continuous?
For areas requiring precise and isolated tension, like scleral defects.
Why is phenylephrine applied directly in conjunctivectomy?
For localized hemostasis control.
What is the preferred tool for a superficial keratectomy incision?
Tenotomy scissors.
Why are conjunctival defects typically left to heal by second intention?
The conjunctiva heals rapidly without needing primary closure.
What is the typical thickness of the cornea?
0.7 mm.
Why is general anesthesia recommended for a superficial keratectomy?
To ensure safety and precision during the procedure.
What are two key tools used to perform the initial incision in superficial keratectomy?
A No. 64 or No. 69 Beaver blade and Corneal Colibri forceps.
What is the purpose of a superficial keratectomy?
To remove the corneal epithelium and anterior stroma to a specific dept
Why is a lamellar plane important in a superficial keratectomy?
To maintain a level and uniform corneal depth during dissection.
What is a Martinez corneal dissector used for?
It’s a lamellar separator used to undermine the lesion.
How is the remaining corneal bed inspected after stromal dissection?
Carefully, to confirm if additional tissue removal is needed.
How long does epithelialization take post-superficial keratectomy?
1 to 2 weeks.
What adjunctive therapy is sometimes used for limbal-based SCC removal?
Beta irradiation, cryotherapy, or mitomycin C.
What additional procedures can aid healing if adjunctive therapy is used?
A conjunctival hood graft or amniotic membrane graft.
List a common complication of superficial keratectomy.
Scarring, infection, or corneal perforation.
What is the goal of adjunctive therapies for ocular surface tumors?
To reduce recurrence after tumor removal.
When is a corneal graft indicated?
If dissection is deeper than 50% stromal depth.
What depth of tissue does beta irradiation penetrate?
1 to 2 mm.
Name a radioactive isotope used in beta irradiation therapy.
Strontium-90 (90Sr).
What is the maximum safe dose for beta irradiation therapy?
500 Gy.
What is the reported recurrence rate for beta irradiation combined with superficial keratectomy in equine corneal SCC?
17%.
Why is cryotherapy less ideal for corneal lesions?
It can damage the corneal endothelium at temperatures over 25°C
What temperature range is used for cryotherapy in limbal lesions?
−20°C to −40°C.
What complication is associated with high doses of beta irradiation?
Permanent corneal endothelial damage.
What type of cryotherapy delivery device is most preferred?
Contact CO2 or nitrous oxide units with small probe tips.
List a complication of cryotherapy.
Chemosis, subconjunctival hemorrhage, or transitory uveitis.
What is the recurrence rate for CO2 laser ablation in equine ocular tumors?
14%.
Describe a visual sign that indicates effective CO2 laser ablation.
White discoloration at the treated site.
What should be avoided during CO2 laser treatment?
Excessive laser energy and duration.
What is mitomycin C’s role in adjunctive therapy?
It acts as an antineoplastic and antifibrotic agent
What concentration of mitomycin C is typically used?
0.04%.
How is mitomycin C applied post-surgery?
Topically, QID for 7 days with a 7-day interval.
What is the primary action of mitomycin C?
Inhibiting DNA synthesis.
For which lesions is intratumoral cisplatin used?
Conjunctival lesions.
What complication can mitomycin C cause due to its toxicity?
Stromal corneal ulceration or conjunctival necrosis.
What complication can result from beta irradiation?
Progressive corneal edema.
What treatment combines with CO2 laser to treat surface ocular neoplasia in horses?
Superficial keratectomy.
What type of tissue forms as a minor complication of CO2 laser treatment?
Granulation tissue.
What specific temperature limit is crucial to avoid endothelial damage in cryotherapy?
Greater than 25°C.
How does mitomycin C prevent tumor recurrence?
By inhibiting the DNA of residual cancer cells.
Which adjunctive therapy is most likely to cause bullous keratopathy?
Mitomycin C.
What determines the depth of incision during superficial keratectomy?
The depth required to remove abnormal tissue effectively.
