posterior segment Flashcards
structures in posterior segment
vitreous humor
retina
optic nerve
choroid-tapetum
posterior sclera
vitreous development
primary vitreous-hyaloid artery, nourishment during embryogenesis, regression start d45 gestation, complete by postnatal d14
secondary vitreous: adult vitreous body
tertiary vitreous: lens zonules
vitreous composition
water (98%) hyaluronic acid, collagen
semi-solid gel
transparent and colorless
functions of vitreous
globe structure and growth
metabolism
refraction
retinal development
two layers of neuroectoderm apposed during invagination of optic cup
outer layer–>retinal pigmented epithelium
inner layer–>neurosensory retina
potential space between RPE and NS retina
retinal pigmented epithelium
outermost retinal layer
contains melanin-except dorsal half, allow visualizationof tapetum, lesser amounts in color dilute animals
functions: metabolism, phototransduction
vascular supplied by choroid
retinal blood vessels
within inner aspect of NS retina
nourish inner retina only
short posterior ciliary arteries-arise around optic disc
retinal vascular patterns
Holangiotic
paurangiotic
merangiotic
anangiotic
holangiotic
dog, cat, cow
vessels across most of retinal surface
most common pattern
paurangiotic
horse
vessels extend only a short distance around optic disc
merangiotic
rabbit
vessels extending medially and laterally only
anangiotic
birds, reptiles
no retinal vessels
pecten, conus papillaris
Mueller cells
structural and metabolic roles
nerve fiber layer
axons of ganglion cells
form optic nerve
photoreceptors
absorb photons, convert to electrical impulses
rods, cones
cells bodies in outer nuclear layer
outer segments interdigitate with RPE cells
rods
scotopic vision
motion
cones
photopic vision
color
high-resolution vision
area centralis
higher concentration of cones
photoreceptor
dorsolateral to optic disc
fovae in some species
light transmission
light entering eye first absorbed by PR
transmitted though layers to NFL
from NFL to optic nerve
impulse along optic nerve to optic chiasm
optic tract to lateral geniculate nucleus, pretectal nuclei
optic nerve
axons from ganglion cell layer mature towards optic stack
converge at optic disc, axons bundle together to form optic nerve
continue posteriorly: optic chiasm and optic tracts
transmission of impulses from eye to CNS
location of optic nerve: dogs and cats
near tapetal/nontapetal junction
location of optic nerve: horses
within nontapetal fundus
choroid includes:
blood vessels
tapetum
melanocytes
connective tissue
choroidal blood vessels
supply outer retina
larger vessels: parallel to retina, osmotic gradient pulling fluid out of RPE
choriocapillaris: perpendicular to retina, supplies outer retina, “Stars of Winslow”-end of capillaries
tapetum
inner choroid
superior half of globe
triangular
reflectivity and color depend on composition
increases available light
absent in diurnal, most primates, most birds and reptiles, pigs
sclera
outer fibrous tunic: collagen, fibrocytes, melanocytes, cartilage and bone in some spp
insertion of EOM
lamina cribrosa-sieve like optic nerve exit
subalbinotic: variation of normal
color dilute animals
decreased melanin in RPE
allows visualization of choroidal vessels
tapetum may be absent
neuro-ophthalmic exam
testing intergrity of visual pathways
Menase response
dazzle
PLR
menace response
CN II, VII, cortex, cerebellum
some vision
dazzle
CN II, subcortical visual pathways
does not test vision
pupillary light reflex
direct and consensual
CN II, III
does not test vision
fundic examination
use indirect ophthalmoscope or direct fundoscopy, PanOptic
ultrasound
when fundus is not visible
retinal detachment
masses
vitreous debris
electroretinogram (ERG)
retinal function-PR
electrical potential generated when photons strike PR
indications: assessment of retinal function when fundus is not visble or is normal in appearance
blurry images
entire image should be in focus at same time
if not, objects in different planes-depressed lesion, raised lesion, retinal detachments
optic nerve changes
shape: coloboma, growths (neoplasia, excessive myelin)
color: red (bleeding), dark (atrophy)
borders: should be distinct, indistinct indicate optic neuritis
size: small (micropapilla, atrophy), large (optic neuritis)
vascular changes
tortuosity
engorgement
hemorrhage
vessel attenuation
lack of vessels across nerve head (dogs)
tapetal changes
hyperreflectivity
hyporeflectivity
nontapetal changes
depigmentation
infiltrates
hemorrhage
Persistent hyaloid artery
failure of regression of hyaloid artery
complete or partial
patent or nonpatent
may be associated with cataract
rarely requires tx
vitreous degeneration
breakdown of gel structure
vitreous liquefies
may prolaspe anteriorly-could plug drainage channel
may predispose to retinal detachment
normal aging change
post-inflammatory
no tx needed
asteroid hyalosis
calcium and phospholipid precipitates in vitreous “floaters”
normal aging change
no treatment needed
vitritis
inflammation of the vitreous
extension of chorioretinitis
RBC, WBC, proteinaceous debris
very slow turnover
micropapilla & optic nerve hypoplasia
congenitally small optic nerve
uni or bi lateral
+/- blindness
no tx
hypoplasia-A, normal-B
retinal dysplasia
abnormal differentiation of retinal layers
nonprogressive
may now be predisposed to retinal detachments, visual deficits and blindness
acquire: Feline panleukopenia, k9 Herpesvirs, BVD
retinal dysplasia types
folds-liniear, hyporeflective or white, raised
geographic-horseshoe shape, dorsal renital venule
complete: retinal detachment, blind
retinal dysplasia tx
none-few folds, no behavioral signs of vision loss
periodic monitoring-more extensive retinal area affects, looking for retinal detachment
Collie eye anomaly (CEA)
abnormal mesodermal differentiation
bilateral
polygenic
lesions: Choroidal hypoplasia-temporal to optic disc, posterior polar coloboma, retinal detachment, hyphema
What does “go normal” mean in regards to Collie eye anomaly?
as dogs age, their RPE melanin increases which can mask underlying hypoplasia
screening for CEA should be performed by 8 weeks of age
tx for collie eye anomaly
usually none-possible sx for retinal detachment
breeding programs to minimize DEA within population
taurine deficiency
feline central retinal degeneration
normally-high taurine concentrations in photoreceptors and heart muscle
PR degeneration-tapetal hyperreflectivity, begins in area centralis (superior and temporal to optic disc), enlarges into a ellipse extending nasally, eventually diffuse
concurrent dilated cardiomyopathy
taurine deficiency dx and tx
confirm with blood taurine levels
taurine supplementation-will stop progression but not reverse changes, will reverse cardiomyopathy
progressive retinal atrophy (PRA)
inherited photoreceptor degeneration
bilateral, symmetrical
progressive vision loss over months
early onset or later onset
typical hx of patient with PRA
purebred dog/cat, young adult
visual deficits first noted at night or dim light
slowly day viison also compromised
abnormalities occur and worsen over months
PE findings for PRA
resting mydriasis
slow to absent PLR
meance may be absent
retinal vascular attenuation
tapetal hyperreflectivity
dark optic disc
severeity of examination findings correlates stage of disease
Tx for PRA
none
due to inherited nature, avoid breeding affected or carrier animals
sequela of PRA
cataracts
almost all dogs
may occur years after PRA dx
progressive
lens-induced uveitis must be managed to avoid painful complications-glaucoma, lens luxation
Sudden acquired retinal degeneration syndrome (SARDS)
dogs
bilateraly symmetrical, idiopathic, acute retinal degeneration
predisposed: middle age, FS, overweight, more common in winter
Typical hx for SARDS
sudden blindness (within 1 day)
cushing’s-like signs
PU,PD, PP, blood work ofte shows increased ALP, lymphopenia, increased chol
SARDS examination findings
resting mydriasis
normal or slow PLR
menace absent
normal funduscopic exam
confirm blindness with electroretinogram-no waveform
tx for SARDS
no proven effective therapy
