Lens & Vitreous Flashcards
Describe the anatomy of the lens
Transparent, avascular, biconvex body
Lens vesicle is a hollow monolayer of cells develops into the lens where the anterior epithelium continually produces new cells (key in aging) whereas the posterior develops into the primary lens fibres
It sits in the hyaloid fossa posteriorly .
The lens itself is supported by fibres called zonules
Lens fibers make up the substance of the lens and are arranged in interdigitating layers. These fibers stretch from the equatorial region toward the anterior and posterior poles of the lens.
Describe the function of the lens
Supports the iris anteriorly
30-35% refractive power of eye
Accommodation (birds and primates > carnivores > herbivores)
Blocking UV light from the retina
What is the lens composition?
35% protein
Soluble proteins (crystallins) – 85% of protein content
Insoluble proteins (albuminoids)
65% water
Stable composition of the lens is vital to maintain transparency
If amount and type of proteins changes, cataract develops
How is nutrition provided to the fetal and adult lens?
Fetus
Tunica vasculosa lentis Anteriorly: pupillary membrane (no pupil in developing eye) Posteriorly: primary vitreous (hyaloid artery main blood vessel)
Adult
Aqueous humour oxygen and glucose enter mainly by diffusion
No blood vessels and no nerves
What happens to the lens as it ages?
Secondary lens fibres continue to form throughout life
Concentric growth (like bone)
New lens fibres form from anterior epithelium at the lens equator and wrap around embryonic nucleus
Older fibres denser and less transparent than younger fibres in cortex: nuclear sclerosis
Presbyopia = age-related reduced ability to focus (accommodate) on near objects
Describe the anatomy of the vitreous
Gel between lens & retina
Largest ocular structure Transparent Avascular & nerve free (lens & cornea are avascular)
99% water & 1% protein/cells
Describe the function of the vitreous
Shock absorber
Removes waste products
Maintains intraocular anatomy
What is the pathogenesis of Nuclear Sclerosis and Cataracts?
Nuclear Sclerosis:
Secondary lens fibres continue to form throughout life: from anterior epithelium at lens equator, and wrap around embryonic nucleus
Finite space within eye —> progressive compression of nucleus
Older fibres denser and less transparent than younger cortical fibres
Nucleus becomes SCLEROTIC (“hardened”)
Cataracts:
Any opacity of lens or its capsule
Part or all of the lens loses normal transparency and becomes WHITE
Change in normal protein ration within the lens
What are the main causes of Cataracts?
Hereditary
Senile
Metabolic (diabetes)
Trauma
Other intraocular disease
Exaplain how diabetes causes the onset of cataracts
Increased levels of glucose: hexokinase pathway overwhelmed and sorbitol produced instead
Sorbitol creates osmotic effect within lens – water absorbed
Lens fibres swell and turn white i.e. cataract develops
Occurs very quickly: diabetic animal can go blind in hours to days
Cataracts are very common in diabetic dogs
50% of dogs are blind 6 months after onset of diabetes
75% are blind at one year
Describe the appearance of Nuclear Sclerosis
Greyish blue haze
Describe the appearance of Cataracts
Variable, often white areas across the eye when examining, tapetum isn’t visible (no reflection)
Does Nuclear Sclerosis have a classification system?
No
Does Cataracts have a classification system?
Yes- complicated classification: age of onset, aetiology, extent & position
What effects does Nuclear Sclerosis have on vision?
Doesn’t affect vision significantly. Reduced accommodation
What effects does Cataracts have on vision?
Variable effects
What are the effects of Nuclear Sclerosis on ocular health?
None
What are the effects of Cataracts on ocular health?
May cause lens-induced uveitis
May cause lens-induced uveitis
Is surgery required for Nuclear Sclerosis?
No
Is surgery required for Cataracts?
Surgery can happen to fix the issue, but isnt always successful. Cataracts if remains can lead to uveitis
Explain how Distant direct ophthalmoscopy is conducted
0 dioptres setting
Dark room
Observe from arms length
Look at tapetal reflection
Explain the basic pathology of abnormal lens position (lens luxation)
Weakening of the zonules and the lens begins to luxate
Aphakic crescent is abnormal (shouldn’t see zonules in the normal animal)
Terriers are prone to luxation of the lens
Explain posterior lens luxation
The lens has fallen back into the vitreous this may lead to retinal detachment, the retinal detachment and leads to blindness
Explain anterior lens luxation
The lens has fallen forwards in the anterior chamber. This is dangerous as it disrupts the flow of aqueous humour by block the iridocorneal drainage angle.
A build up of this fluid causes an increase of pressure within the eye (glaucoma) and can be very painful
