Lens & Vitreous Flashcards

1
Q

Describe the anatomy of the lens

A

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.

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2
Q

Describe the function of the lens

A

Supports the iris anteriorly

30-35% refractive power of eye

Accommodation (birds and primates > carnivores > herbivores)

Blocking UV light from the retina

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3
Q

What is the lens composition?

A

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

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4
Q

How is nutrition provided to the fetal and adult lens?

A

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

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5
Q

What happens to the lens as it ages?

A

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

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6
Q

Describe the anatomy of the vitreous

A

Gel between lens & retina

Largest ocular structure

Transparent

Avascular & nerve free
	(lens & cornea are avascular)

99% water & 1% protein/cells

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7
Q

Describe the function of the vitreous

A

Shock absorber

Removes waste products

Maintains intraocular anatomy

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8
Q

What is the pathogenesis of Nuclear Sclerosis and Cataracts?

A

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

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9
Q

What are the main causes of Cataracts?

A

Hereditary
Senile
Metabolic (diabetes)
Trauma
Other intraocular disease

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10
Q

Exaplain how diabetes causes the onset of cataracts

A

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
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11
Q

Describe the appearance of Nuclear Sclerosis

A

Greyish blue haze

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12
Q

Describe the appearance of Cataracts

A

Variable, often white areas across the eye when examining, tapetum isn’t visible (no reflection)

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13
Q

Does Nuclear Sclerosis have a classification system?

A

No

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14
Q

Does Cataracts have a classification system?

A

Yes- complicated classification: age of onset, aetiology, extent & position

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15
Q

What effects does Nuclear Sclerosis have on vision?

A

Doesn’t affect vision significantly. Reduced accommodation

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16
Q

What effects does Cataracts have on vision?

A

Variable effects

17
Q

What are the effects of Nuclear Sclerosis on ocular health?

A

None

18
Q

What are the effects of Cataracts on ocular health?

A

May cause lens-induced uveitis

19
Q

May cause lens-induced uveitis
Is surgery required for Nuclear Sclerosis?

A

No

20
Q

Is surgery required for Cataracts?

A

Surgery can happen to fix the issue, but isnt always successful. Cataracts if remains can lead to uveitis

21
Q

Explain how Distant direct ophthalmoscopy is conducted

A

0 dioptres setting
Dark room
Observe from arms length
Look at tapetal reflection

22
Q

Explain the basic pathology of abnormal lens position (lens luxation)

A

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

23
Q

Explain posterior lens luxation

A

The lens has fallen back into the vitreous this may lead to retinal detachment, the retinal detachment and leads to blindness

24
Q

Explain anterior lens luxation

A

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