13 - lens
describe why the embryology of the lens leads to lens fibers being unrecognized as self by the immune system
the lens fibers develop entirely within the confines of the lens capsule, which is basement membrane of the epithelial cells
because lens fibers never have contact with the immune system, self-tolerance never develops for them
list factors that contribute to transparency of the lens
avascularity
lack of pigmentation
no keratinization
precise arrangement
minimal organelles
precise protein ratios
low hydration
precise regulation of hydration level
what is the normal function of the lens
focus light rays onto the retina → optimal vision and image processing
what is a cataract? what is nuclear sclerosis? how are the two differentiated?
cataract
increased opacity of the lens
occurs when osmotic effects break lens fibers, insoluble proteins outnumber soluble proteins
leads to vision loss
nuclear sclerosis
highly compacted nuclear lens fibers
blue-grey opalescent appearance to center of lens
usually bilaterally symmetric
differentiation
cataract is opaque → light is disorganized or blocked, prevents view of retina, tapetal reflection is lost in mature stages, may be unilateral or bilateral
nuclear sclerosis is transparent, retina is visible with ophthalmoscope, tapetal reflection is maintained, usually bilaterally symmetric
describe cataracts by stage of development
incipient → earliest and smallest visible cataract. <10% of lens is opaque
immature → >10% of lens is opaque. tapetal reflection may be seen and some retinal detail observable
mature → solid, opaque, white marble. no tapetal reflection or retinal detail seen. PLRs remain normal as long as retina is normal
hypermature → lens starting to shrink and reabsorb. capsule is wrinkled with ragged edges
morgagnian → nucleus liquefies, may sink to bottom of lens
list causes of cataract formation
3 types: hereditary, senile, ocular manifestation of systemic disease
genetic
diabetic
secondary to Progressive retinal atrophy
secondary to uveitis
traumatic, electric shock
nutritional
radiation
secondary to iris-to-lens persistent pupillary membranes
secondary to hyaloid artery/tunica vasculosa lentis
how are PLRs affected by mature cataracts
they’re not, as long as the retina is undamaged
why lens-induced uveitis occurs, how it is treated, and why it must be treated chronically
cataract causes inflammation due to leakage of lens proteins
tx: topical anti-inflammatories
chronic uveitis can lead to secondary glaucoma, retinal detachment, posterior synechiae and keratitis
what is the pathogenesis of diabetic cataract formation
elevated blood glucose → elevated aqueous glucose → elevated lens glucose → normal anaerobic glycolysis (by aldose reductase) is overwhelmed → metabolism shunted to sorbitol pathway → increased osmolality → fluid sucked into lens fibers → swell and break
identify the only treatment option for restoring vision to patients blinded by cataracts
surgical removal of cataract
what breed of dog is genetically predisposed to primary lens luxation? what is the genetic defect that leads to primary lens luxation
Jack Russell terriers & related breeds
know the differences between primary and secondary lens luxation
primary
due to genetic defect → zonular dysgenesis → instability in lens position
secondary
loss of zonules caused by intraocular disease → uveitis, claucoma, neoplasm, trauma
know the difference between lens subluxation and lens luxation
subluxation
lens is only partially dislocated
some zonules remain attached to lens
diagnose via presence of aphakic crescent
luxation
lens is completely dislocated from its normal position
all zonular attachments lost
almost always preceded by subluxation
can be anterior or posterior (posterior less painful)
know the definition of aphakic crescent
space between lens equator and pupillary margin