Lens Diseases Flashcards
What are lens fibers? What is the function of the lens? How is it able to do this?
elongations of lens epithelial cells suspended by zonules from ciliary processes
refraction of light into the retina
avascular - gets nutrition from aqueous humor produced by the choroid plexus
What structures are present in the lens?
- anterior and posterior capsules
- lens fibers
- nucleus
- cortex
- suture lines
- zonules - 360 degrees around lens, keeps in place and maintains ability to change shape of lens to see at different lengths (near = flattened)
What are the 2 major compositions of the lens?
- 65% water
- 35% protein
What 2 types of proteins are found in the lens?
- soluble crystallins
- insoluble albuminoids
immune privileged - if capsule barriers are broken down, immune cells are able to react to proteins
How does the lens differ with age?
nucleus grows outward over time
What are lens suture lines?
area where lens fibers meet, most obvious with cataracts (Y-shaped)
Lens nucleus vs. cortex:
What is nuclear sclerosis?
normal change in the lens (8-9 y/o) where the fibers become denser with age, often mistaken for cataracts
(increased density causes light rays to be reflected back only at the nucleus (pearl), leaving the cortex clear)
How do dogs with nuclear sclerosis present?
bilaterally symmetric —> aging change!
able to still perform a fundic exam —> not dense enough
What is a cataract? Most common causes in dogs? Cats? Other etiologies?
opacity of the lens
DOG = genetic*, DM, uveitis
CAT = uveitits
- congenital/developmental
- metabolic - hypocalcemia
- nutritional
- toxic
- retinal degeneration
- old age
- radiation
- trauma
What are the 3 ways of defining cataracts?
- STAGE - incipient, immature, mature, hypermature
- LOCATION - cortical, equatorial, capsular, nuclear
- TYPE - congenital, hereditary, diabetic, age
What is the top cause of cataracts in dogs?
genetic —> these dogs should not be bred, can occur at any stage of life
- Poodles!
What are congenital cataracts? What 3 other developmental abnormalities are associated?
cataracts the begin during fetal life and are present at birth
- persistent pupillary membrane
- microphthalmia
- persistent hyaloid artery - blood within lens fibers
What is cause of diabetic cataracts?
excess glucose in aqueous humor overwhelms the normal hexokinase pathway, where aldose reductase causes sorbitol buildup —> sorbitol is osmotically active and draws water into the lens (swells and breaks down fibers)
- 100% of dogs with DM will eventually develop cataracts
- doesn’t happen to the same extent in cats (low aldose reductase in adult cats)
How does retinal degeneration cause cataracts? How can this etiology be differentiated from others?
oxidation factors are produced when the retina is degenerating, leading to cataract formation and decreased lens nutrition
other causes will NOT cause mydriasis or decreased PLR as the retina is still intact and able to process light
What stages of cataracts are likely to be caused by lens-induced uveitis? How does this happen?
mature and hypermature —> quickly progressing = DM, younger dogs
lens capsule damage causes leakage of lens proteins, inciting inflammation in the anterior chamber
What are the 2 types of lens-induced uveitis?
- phacolytic - slow leakage
- phacoclastic - direct exposure to lens material
How is lens-induced uveitis treated? What can happen in severe cases?
topical prednisolone or NSAID +/- oral NSAID
can cause glaucoma or retinal detachment, requiring surgery
Lens-induced uveitis:
hazy dots = WBCs on back of cornea
What are the 4 stages of cataract progression?
- incipient - less than 15% lens affected; small and focal
- immature - 15-90% of lens affected, tapetal reflection seen
- mature - 100% lens affected, tapetal reflection missing (completely white)
- hypermature - lens resorption, wrinkled lens capsule, sparkly
At what stage is surgical treatment of cataracts most effective?
immature
Incipient cataract:
- ventral focal area (~15%)
- rest of lens is clear
- likely no vision problems at this point
Incipient cataract:
- ventral opacity
- ~15% affected
Immature cataract:
- whole lens is cloudy except for a ventral strip
- tapetum present
- up to 90% of lens affected
best time for surgery!
Mature cataract:
- suture lines more prominent
- entirely opaque
- no tapetum
Hypermature cataract:
- sparkly appearance
- lens resorption
- lens wrinkling
Hypermature cataract:
- wrinkled lens
- lens resorption
How do most patients present with cataracts? What is important for a proper ophthalmic exam?
- lens opacity noted (owners may be seeing nuclear sclerosis!)
- decreased vision
- signs of uveitis - redness, squinting, blepharospasm
dilate the pupils to properly examine the lens
How can a tapetal reflection be achieved to differentiate stages of cataracts?
retroillumination with a transilluminator —> highlights opacities and makes cataract easier to see
How are cataracts diagnosed? What 2 diagnostics must be performed prior to cataract surgery?
ophthalmic exam and characteristics of each type of cataract (differentiate from nuclear sclerosis!)
- electroretinogram - flatline will indicate retinal degeneration
- ocular ultrasound - can observe retinal function to determine if sight is possible following sx (good for immature and older cataracts where fundic exams are not possible)
What is the standard cataract surgery performed? What must also be included post-op?
phacoemulsification with ultrasound energy and placement of an intraocular lens
control of further uveitis and DM
What are 4 aspects of a proper candidate for phacoemulsification?
- good GA candidate
- good ERG
- normal ocular ultrasound
- no secondary issues - glaucoma, uveitis, retinal detachment
What is phacoemulsification?
Piezoelectric handpiece that converts electrical energy to mechanical enery to emulsify the lens via oscillation creating cavitation bubbles that break up lens material
What is a pseudophakia?
artificial replacement lens placed in capsular bag following phacoemulsification —> refracts light and restores normal vision
What are 4 possible complications associated with cataracts and cataract surgery?
- KCS - common after surgery, especially on diabetics
- anterior uveitis
- glaucoma
- retinal detachment
What are common causes of traumatic lens capsule rupture? What is the main concern associated?
- cat claws
- corneal foreign bodies
- may be able to see lens material in anterior chamber
phacoclastic uveitis - severe lens induced uveitis due to exposure to lens fibers —> aggressive medical management +/- phacoemulsification if larger than 1.5 mm
What are 3 common sequelae of lens capsule ruptures?
- cataract formation
- severe phacoclastic uveitis
- glaucoma, infection, loss of eye
What causes lens instability? What are the 2 types?
weakened lens zonules
- partial - subluxation
- complete - luxation
What position is lens subluxation found in? What are some causes?
only partial zonular tear = lens still behind iris (may jiggle with eye movement)
- cataract
- glaucoma (buphthalmos)
What is the hallmark sign of lens subluxation?
aphakic crescent - area where the edge of the lens can be seen forming a crescent +/- aqueous humor mixing vitreous at the edge (haziness)
What are 2 additional signs of lens subluxation?
- iridodonesis - iris moves independently of the rest of the eye
- phacodonesis - lens moves freely within the eye
usually the lens supports the iris, but it is unable when loose
What is lens luxation? What position can the lens be found in?
all zonules are torn, allowing the lens to move freely
anterior in anterior chamber (emergency!) or posterior in vitreous cavity
What are the 2 types of lens subluxation?
- PRIMARY = no other ocular disease, more common in terriers, Australian shepherds, and blue heelers
- SECONDARY = cataracts (dogs, cats), glaucoma (cats, horses)
Lens luxation:
cannot see pupil = anterior
Lens luxation:
lens anterior to iris and pupil
What are 2 options for lens luxation treatment?
- transcorneal reduction - push lens posterior to iris and trap with medication that constricts the pupil
- intracapsular extraction - entire lens (+ capsule) removed
What are the 3 most common complications associated with intracapsular extractions?
- retinal detachment
- glaucoma
- aphakia (no lens)
