lecture # 19
DESCRIPTION
Lecture # 19. Eye diseases of cornea, lens and vitreous 4 /9/13. Animal wikis. Great! Some of my favorites Writing: manatees, hummingbirds Link to eye design: barn owls, panda. Wiki homework. Be thinking about your wiki final project topic Email it to me by end of Thursday - PowerPoint PPT PresentationTRANSCRIPT
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Lecture #19
Eye diseases of cornea, lens and vitreous4/9/13
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Animal wikis
• Great!• Some of my favorites
Writing: manatees, hummingbirdsLink to eye design: barn owls, panda
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Wiki homework
• Be thinking about your wiki final project topicEmail it to me by end of Thursday
• It is fine if your topic evolves as you gather informationMay want to focus it down if find lots infoMay need to expand if not so much
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Anterior eye disease
• CorneaDystrophiesRefractive errors
• LensCataracts
• Vitreous Glaucoma
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Function of cornea
• Performs ≈70% of focusing
• Protects eye from outside world
• No blood supplyCleaned and nourished by tears and aqueous humour
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Corneal disease• Conjunctiva
Mucous membrane lining eyelid and sclera Contains tiny blood vessels
• Pink eye - conjunctivitisInfection by either bacteria or virus
• Corneal infectionsBacterial or fungal invasion into corneal layers
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Dry eye
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Tears
• Basal tearsConstantly produced to nourish and moisten eyeMixture of aqueous and oily secretions
• Reflex tearsMade in response to irritation or emotionMore watery
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What are tears?
• Tears are made of three layersOily, lipid layer - keeps aqueous layer from evaporatingAqueous layer - keeps eye moistMucin layer - helps aqueous layer spread
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Meibomian gland produces lipid part
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Lacrimal glands produce aqueous partTears drain to naso-lacrimal sac
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Goblet cells produce mucus
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Tears then need to drain
Tears then drain out through holes in eyelid
If drain too quickly, eyes become dry
Plug these holes
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Dry eye
• If meibomian glands get blocked, there will not be enough lipids and tears will evaporate too quickly
• To unclog glandsHeat treatmentsDoxycyclineNutritional supplements
• May be other reasons not enough lipids
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Dry eye
• If there is not enough aqueous part of tearsUse artificial tearsPlug up drainage holes so stay on eye longer
• May also be problems with mucin layer which wets the eye and helps aqueous layer to spreadNot sure how to improve it
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Cornea has 5 layers
1. Epithelium10% of thicknessBlocks foreign matterAbsorbs O2 and nutrients
from tearsEpithelia cells grow and
are anchored to basement membrane
Many tiny neurons - very sensitive to pain
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Cornea has 5 layers
2. Bowman’s layerStrong layer of fibers composed of collagen
If injured it forms scar tissue
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Cornea has 5 layers3. Stroma
Comprises 90% of cornea thickness
Composed mostly of collagen (16%) and water (78%)
Gives cornea shape and transparencyUpper part of stroma repairs itself but lower part does not
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Cornea has 5 layers4. Descemet’s membrane
Thin but strong protective layer
Made of collagen (different from stroma)
Made by endothelium
Can regenerate after injury
Descemet’s membrane
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Cornea has 5 layers5. Endothelium
Extremely thin
Fluid slowly leaks from inside eye into stromaEndothelium pumps it back out so stroma doesn’t get cloudy!!
Endothelium does not regenerate - if damaged, need corneal transplant
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Corneal dystrophies
• Over 20 kindsDystrophy - abnormal developmentInheritedAffect both eyes equallyBegin in one of 5 layers and spread to others
Layers become cloudy - so can’t see
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Keratoconus• Thinning of middle of
cornea (stroma) causes cornea to change shape- cone like
• Most common corneal dystrophyAffects 1:2000
• Inherited or from wearing hard contacts or eye injuryUsually stabilizes and correct with glasses / contacts
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Lattice dystrophy
• Build up of amyloid (protein) deposits in upper to middle stroma
• Create a lattice which worsens and makes cornea cloudy
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Fuchs dystrophy
• Endothelial layer deteriorates
Can’t pump out aqueous humour so cornea swells
Vision becomes blurry
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Treatments for corneal dystrophies
• Corneal transplants Match by blood type 20% rejection rate
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Treatment for corneal scars
• Phototherapeutic keratectemy Laser ablationRemove scarred or damaged tissue
Use UV excimer laser under computer control
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Refractive error
• If cornea has wrong curvature, image on retina is out of focus
Myopia - image focused in front of retina : 25% of people
Hyperopia - image focused behind retina
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Refractive error• Astigmatism
Cornea is more curved in one direction than the other (like spoon or football)
Multiple focal lengths so multiple images
Always blurry
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Treatments for refractive errors - reshaping the cornea
• RK - Radial keratotomy• PTK - Phototherapeutic keratectemy• LASEK - Laser assisted sub-epithelial
keritectomy • LASIK - Laser Assisted In Situ Keratomileusis
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Radial keratotomy• Modify cornea shape
by cutting slits• Developed in Russia in
1970s• Unpredictable healing• Vision may change
through day or over time
• Not recommended
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Treatment for refractive errors• Phototherapeutic
keratectomy Can also be used to reshape cornea - correct myopia
Remove epithelial layer and reshape upper part of cornea
Epithelial layer regenerates
Keratectomy - remove part of cornea
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LASEK surgery• Laser assisted sub-
epithelial keratectomy• Cut and peel back
epithelial layer• Re-shape upper
stroma just below epithelium with laser
• Replace epithelial layer
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LASIK refractive surgery
• Laser Assisted In Situ KeratomileusisCut a flap in cornea with blade or laser (this cuts more than just epithelium)Laser vaporizes stroma to reshape itFlap is folded back though doesn’t seal
• Epi-LASIK cuts thinner flap so does reseal
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What happens during LASIK surgery
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Reshaping of cornea
• Near sighted
• Far sighted
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Comparisons suggest LASEK and LASIK produce equivalent results
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Some reasons NOT to do LASIK• You may not be suited for procedure:
Eye diseaseThin corneasUnstable vision
• Vision may get worseUnstable cornea
• No long term data• LASIK corneal flap may be deep in cornea
These tissues do not regenerateFlap is permanent
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Possible complications - starbursts
LASIKdisaster.com
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Possible complications - halos
LASIKdisaster.com
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Ghosting
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Near sighted problems - PRK
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Far sighted problems
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Possible problems
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NEI - cataracts
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Lens
• LensTransparent so light is efficiently transmitted
High index so light is focused onto the retina
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Lens composition• Composed of water
and lens crystallins (90% of protein)
• Crystallins made once and then stored in lens for rest of life
• Must remain soluble to be transparentEye lens fiber cells filled
with crystallins
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Crystallins• α-crystallins
Related to heat shock proteins
• β and γ crystallins
γ crystallins are symmetric
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Other proteins can be co-opted to form part of lens
Many are active metabolic enzymes elsewhere in body!!!
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Recruitment of proteins
• Recruited to lens by changing gene expression• May be result of gene duplication followed by
new expression• Proteins selected which highly stable
Contribute to index of refractionInsensitive to UV damage
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Crystallin structure
• Crystallins are present from birth • Processes which damage protein are bad
Oxidation, deamidation, cleavageResult in protein unfolding
• Normally α crystallins are chaperones keeping other proteins folded
• As lens proteins unfold, α crystallins used upUnfolded proteins form precipitatesLoss of lens transparency
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Cataracts• Clouding of lens• Typically occurs with
age• 50% of people > 80
have cataracts• Cataracts affect 5.5
million people in US
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Cateract symptoms• Blurry vision• Poor night vision• Problems with glare
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Cataracts
• Congenital
• Age related
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Age related cataract prevention
• Decrease sun exposure• Increase antioxidants• Stop smoking• Get eye exam
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Treatment #1• Cut small incision (3
mm)• Remove front of lens to
expose cataract• Use ultrasound to
fragment cataract• Remove fragments
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Treatment #2
• Replacement lensMade of plasticBlocks UV
Is flexible so can attach to eye focusing musclesFocus near and far!
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Treatment #2• Introduce
replacement lens into lens capsule
• May only replace part of lens
• Can improve spectral transmission (more blue)
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Glaucoma
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Glaucoma
• Variety of diseases that result in loss of retinal ganglion cells
• Loss begins in periphery• 50% of people have glaucoma and don’t
realize it
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Fluid flow at front of eyeAqueous humor is generated by ciliary body and flows into anterior chamber to nourish eye
Flows out where cornea and iris meet Iridocorneal angle
Trabecular and uveoscleral drainage Spongy tissues
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Fluid flow at front of eye
If fluid does not drain: Pressure in eye builds up
This damages retinal ganglion cells and vision is lost
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Measuring eye pressure• Applanation tonometry
Measure applied pressure necessary to deflect cornea
• Noncontact tonometryMeasure air pressure needed to deflect cornea
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Caveats
• High intraoccular pressure (IOP) is highest risk factor but:Majority of people with high IOP do not get glaucomaOptic nerve damage can occur even without high pressure
-Low tension or normal tension glaucoma
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Risk factors
• Affects 70 million people
• Age2 % over age 40; 7% over age 80Over age 40 - African Americans 5x more likelyOver age 60 - Mexican Americans more likely
• Family history of glaucomaThough not Mendelian trait
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Symptoms
Gradual loss of peripheral visionCan be slow loss over years No painDifficult to notice effects
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Kinds of glaucoma
• Open angle glaucomaFluid seems to keep flowing
• Developmental glaucomaAnterior portion of eye doesn’t develop correctly
• Pigmentary glaucomaIris pigment epithelium atrophies and pigment clogs drainage of fluid
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Open angle glaucoma
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Genetics
• 9 loci identified so far that initiate primary open angle glaucomaExplain only small % of cases
• Two genes which cause early onset glaucomaMyocilin (3% of cases)Optineurin
• Not obvious how genes cause the diseaseExpressed in both retinal ganglion cells and trabecular meshworkMay cause problems if protein misfolding
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Treatments
• Eye drops or pillsDecrease fluid production or increase drainage
• Laser trabeculoplastyLaser widens holes in drainage meshwork
• Conventional surgery Create new exit pathways
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Open fluid flow in meshwork or sclera
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Use of marijuana to treat glaucoma
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Next time
• Gene therapyHow do you replace a faulty gene?