1.3-1.4 The Opaque Eye Flashcards
What is the term for having no lens in the eye?
aphakia (can also be used for negative space, e.g., the aphakic crescent in an eye with posterior lens luxatation)
Where in the eye does the lens sit?
posterior to the iris and aqueous capsule, and anterior to the vitreous capsule
the hyaloid fossa (AKA patellar fossa) is a depression on the anterior surface of the vitreous body in which lines the crystalline lens
The lens originates from what, embryologically?
ectoderm (as does the corneal epithelium and eyelid)
- remember: the ectoderm is the outermost germ layer in animals; it gives rise to the skin, nervous system, and sense organs
What are clinical signs of lens disease?
- decreased transparency (cataracts)
- phacodonesis (wobbly lens)
- iridodonesis (trembling iris: a sign of lens subluxation)
- aphakic crescent (lens subluxation)
What is the difference between lens luxation and subluxation?
the lens is considered completely luxated when it lies outside of the hyaloid fossa
- free floating in the vitreous
- anterior to the iris
- within the iris
if still within the hyaloid fossa but partially displaced, it is considered subluxated
What posterior chamber diseases cause eye opacity?
- vitreal liquifaction
- persistent hyaloid artery
- PHPV: persistant hyperplastic primary vitreous (persistant embryological vasculature in vitreous; usually unilateral and non hereditary - random)
- vitreal degeneration
- vitreal hemorrhage
- vitritis
- asteroid hyalosis
- synchysis scintillans
What anterior chamber diseases can cause eye opacity?
uveitis is the most common cause (aqueous flare, hyphema, hypopyon)
How is aqueous humor created?
formed by the ciliary body via carbonic anhydrase (limiting enzyme)
- enters anterior chamber
- convnetional drainage via iridocorneal angle
- unconventional drainage via iris, ciliary body, vitreous
What is glaucoma?
A overarching pathology denoting inadequate drainage of aqueous humor from the anteroir chamber, leading to increased IOP and subsequent optic nerve disfunction
What are the most important diagnostic tests for glaucoma?
- tonometry (via tonovet or tonopen)
- gonioscopy: uses gonioscope and slit lamp to check the iridocorneal drainage angle
List some symptoms of glaucoma.
- **Haab’s stria**: (PATHOGNOMONIC) horizontal breaks in the descemet membrane due to stretching of the globe under increased IOP; descemet's membrane is less elastic than the corneal stroma
- corneal **edema**
- episcleral congestion
- conjunctival hyperemia
- **epiphora** associated with **mydriasis** (epiphora alone is too non-specific)
- aqueous flare
How is glaucoma managed?
- medical: reduce the production or increase the outflow of aqueous humor
- surgical: laser, enucleation, shunt
medical management can be frustrating and last for years, requiring constant IOP checks
What is vitreous liquefaction / degeneration?
age-related redistribution of the fibrillar components of the vitreous body
- can lead to vitreal opacites
What is synchysis scintillans?
a rare degenerative ocular condition characterized by the accumulation of cholesterol crystals in the vitreous humor of the eye
small, highly refractive opacities in the posterior chamber of the eye that freely move in a gravity-dependent manner, giving a snow globe-like effect (may be in anterior chamber as well)
- often no obvious clinical signs
What is asteroid hyalosis?
degenerative eye condition marked by a buildup of calcium and lipids in the vitreous humor
- caused by genetics and diabetes
What are the layers of the cornea?
- precorneal tear film
- epithelium (6-7 cell layers)
- basement membrane
- corneal stroma: collagen fibrils and water (most of the cornea)
- descemets membrane (1 cell thick; lipophilic)
- corneal endothelium
no pigment or vaculature; relatively dehydrated to retain opacity
What diagnostic tests should you perform in the instance of corneal opacity?
- schirmer tear test (to indicate irritation or KCS)
- fluorescein stain for ulcer
- seidel test for perforating coneal injuries
- cytology (+ culture and sensitivity)
How long does it take for the cornea to heal?
- epithelium: basal cell turnover in 7 days (small defects in as little as 24hrs)
- vascularization for healing occurs from limbus (corneal scleral junction): lesions at the center of cornea take longer to heal
- stroma: keratocytes proliferate in weeks to months, will leave scar
- endothelium: very limited regenerative abilities
List the corneal diseases.
- corneal ulcer
-
corneal sequestrum (cats)
Part of cornea has died off and is rejected -
acute bullous keratopathy (cat)
Intense corneal Bullae form with no associated inflammation (multiple causes) -
keratitis
- chronic superficial keratitis, “pannus” (dogs)
- eosinophilic keratitis (cats and horses version of pannus)
- immune mediated keratitis (FHV, leishmania)
- pigmentary keratitis (pugs)
What causes corneal edema?
a breach of the corneal epithelium (ulcer) or endothelium (uveitis, glaucoma, chronic anterior lens luxation)
- epithelium loss can result in a 200% increase in corneal thickness
- endothelium loss can result in a 500% increase in corneal thickness
What can cause corneal ulcers?
- mechanical trauma (FB, trichiasis, ectopic cilia, etc.)
- lid conformation (entropion, ectropion)
- KCS (dry eye)
- degenerative processes: (e.g., calciumn mineralization associated with HAC and DM)
What are the grades of corneal ulcer?
- Grade 1: epithelial defect
- Grade 2: persistent epithelial defect
- Grade 3: non-pregressive anterior stromal ulcer
- Grade 4: deep, progressive stromal ulcer (descemetocele)
- Grade 5: corneal perforation, iris prolapse
What is SCCED?
sponaneous chronic corneal epithelial defect
- chronic, indolent, non-healing ulcers with high rate of occurance
- common in boxers, welsh corgis, staffies
- predisposed by unusual corneal anatomy/healing
- diagnosed by exclusion; when sampling for cytology, note whether or not a membrane peels off with the brush (indicator for SCCED)
- this is the ONLY ulcer that should be debrided due to the abnormal membrane
What is progressive corneal ulceration?
deep stromal ulcers usually due to microbial invasion
- bacteria/fungus/neutrophils can activate proteinases and collagenasis causing progressive, “melting” ulcer
- axon reflex: corneal nerve endings release substance P which triggers prostaglandin replease and cause secondary uveitis
How are corneal ulcers treated medically?
- antibiotics
- mydriatic cycloplegics (atropine - reduces ciliary spasms)
- anti-collagenases
- anti-inflammatories
- immunosuppression
How are corneal ulcers treated surgically?
- conjunctival graft (provides blood supply for healing)
- corneo-conjunctival transposition
- corneal grafting
reserve third eyelid flaps for special diseases (proptosis)
