lens Flashcards
lens anatomy
derived from surface ectoderm
clear, colorless biconvex structure
refraction and accomodation
lens capsule, anterior epithelium, lens fiers, zonules
components of lens
lens capsule-selective permeability, barrier function
anterior epithelium-mitotically active throughout life, source of lens fibers
cortex-outermost, younger portion of lens
nucleus-innermost, oldest portionof lens
equator-periphery of lens, zonular insertion
Lens fibers
dervied from anterior epithelium
elongate and lose organelles-U shape
make up the cortex and nucleus
orderly arrangement
continuous growth
Y suture
not actual structures but where the lens ribers meet
upright Y anteriorly, inverted Y posteriorly
how does a lens age
lens increasingly compacted over time
newest fibers form in outer portion of lens
older fibers compact into nuclear–>nuclear sclerosis
zonules
originate from ciliary body and insert onto lens capsule at equator
factors contributing to lens transparency
barrier function of lens capsule
relative dehydration
lack of blood vessels
lack of pigment
loss of organelles from lens fibers
orderly arrangement of lens fibers
lens nutrition
aqueous humor-glucose
anaerobic glycolysis-hexokinase converts ot lactic acid
small amount of aerobic glycolysis
examination of lens
must dilate
retroillumination
slit beam
lens opacification
heterogenous bright white appearance of purkinje 2
purkinje 3 not visible due to density of lens opacity
slit beam
disruptionof homogenous beam anywhere between images 2 and 3 indicates lenticular lesion
absence of images indicates absence of structure
Nuclear sclerosis
density of nucleas increases with age due to continuous growth of lens fibers
newest fibers formed in outer cortex older fibers compacted into nucleus
circular central lentcular clouding
does not block light-can see tapetal reflection clearly
used transillumination after pupillary dilation
Cataracts
opacity within the lens
interferes with light transmission
opacity may be extremely small and cause no visual disturbance or may occupy the entire lens and cause blindness
causes of cataract
anything that alters lens nutrition, energy metabolism, protein metabolism, osmotic balance, physical structure
inherited-#1 in dogs, Uveitis #1 cats, horses
degenerative
Metabolic disease
nutritional
trauma
toxins
glaucoma
radiation
classification of cataract
based on size/stage of development-incipient, incomplete/immature, complete/mature, resorbing/hypermature
positions within lens-capsular, subcapsular, cortical, nuclear, equatorial
age of onset-congenital, juvenile, senile
etiology
incipient cataract
occupies <10-15% of the lens
usually no detectable visual compromise
common in olders dogs
incomplete/immature cataract
larger than incipient
tapetal reflection still visible
may or may not be associated with visual compromise
complete/mature cataract
occupies all of the lens
tapetal reflection not visible
results in blindness
resorbing/hypermature cataract
leakage of lens material through capsule into anterior chamber
loss of lens contents causes capsule to wrinkle
also see mineralization
occurs with chronicity
morgagnian cataract
extensive resoprtion of lens cortex
cortex liquefies, allowing harder nucleus to sink
degenerative/senile cataracts
degeneration of lens associated with aging
by 9.4 yeras, 50% of dogs have some degree of cataract
progression is very slow
Diabetes mellitus cataracts
always bilateral
occurs in most diabetic dogs, rarely in cats
increased aqueous glucose saturates hexokinase in anaerobic glycolysis
excess glucose metabolized by aldose reductase in sorbitol pathway
build up of sortibol in lens creates osmotic gradient
associated with more dramatic uveitis
nutritional cataracts
milk replacer-arginine deficiency, bilateral nuclear cataracts, usually triangular in shape, may become less visible over time as nucleas compacts
hypocalcemia-multiple incipient cataracts
inherited cataracts cataracts
purebred dogs predisposed
usually recssive inheritance
other causes of cataracts excluded
most common etiology in dogs
uveitis cataracts
disrupts nutrition to lens
inflammatory material and synechiae can also decrease lens transparency
most common cause of cataracts in cats and horses
glaucoma cataracts
impaired lens nutrition
trauma cataracts
blunt trauma via uveitis
penetrating trauma causes disruption of fiber arrangement, fluid influx, inflammation
sequelae to cataract
vision impairment/blindness
lens-induced uveitis and complications of uveitis
glaucoma, lens luxation, retinal detachment
phacolytic uveitis
slow leakage of lens protein
most obvious with hypermature/resorbing cataract
chronic, low-grade inflammation
phacoclastic uveitis
sudden release of lens protein
secondary to penetrating ocular trauma, lens rupture
rapid progression, severe inflammation
requires rapid, aggressive intervention
tx of cataracts
referral to ophthalmologist
monitoring
anti-inflammatory medications
phacoemulsification and intraocular lens implanation
to maximize surigcal success for cataracts
perform surgery early in disease process
preop control of uveitis
intensive postop anti-inflammatory meds
diligent post op reevaluation
lens subluxaiton
partial dislocation of lens s a result of degeneration of some but not all zonules
slight shifting of lens position
luxation
degeneration of all zonules resulting in complete dislocation of lens
lens freely moves around within eye
lens subluxation and luxation clinical signs
anterior lens luxation results in acute evidence of ocular pain
subluxation and posterior luxation may be asymptomatic
blepharospasm
epiphora
episcleral congestion
cloudy apperance to eye-corneal edema
cataract
lens subluxation and luxation examination findings
aqueous flare
IOP may be elevated
vitreous in anterior chamber
tilted purkinje image 2
less distance between purkinje 1 & 2
more distance between purkinje 1 & 2
absence of purkinje 2
buphthalmos
visualization of equator
iridonesis
phacondonesis
aphakic crescent
causes of lens (sub)luxation
primary lens subluxation and luxation
secondary lens-uveitis, glaucoma, age-related degeneration, severe ocular trauma
consequences of (sub)luxation
may occur acutely or in long term
glaucoma
uveitis
retinal detachment
refferal
management of lens subluxation if shifting is small
micromotions of loose lens can induce uveitis
topical anti-inflammatory medications
continued monitoring for progression
miotic therapy
management of lens subluxation-larger aphakic crescent or visible iridodonesis or phacodonesis
phacoemulsification and intraocular lens implanation
intracapsular lens extraction
urgent referral
50% dogs visual and glaucoma free 1 year post op
enucleation
170-180 degrees corneal incision made along dorsal limbus
cornea is elevated and lens extracted
latanoprost
encourage trapping of lens in vitreous cavity
concurrent anti-inflammatory therapy
