Refractive Media of the Eye Flashcards
(109 cards)
1
Q
What is Snell’s Law?
A
Snell’s law: for a given pair of media, the ratio of the sines of angle of incidence and angle of refraction is equal to the refractive index of the second medium with regard to the first (n21) which is equal to the ratio of the refractive indices
2
Q
What is refraction?
A
The change in direction of light as it passes from one medium to another, of different optical density’
3
Q
How big is the cornea?
A
Anterior cornea forms 1/6th of the eyeball it’s roughly 10.6mm vertically x 11.5mm horizontally so not a perfect semi-circle, slight elliptical shape.
Thinnest in the center and as you travel towards the periphery it becomes bigger; 0.5ml in centre and in periphery it’s around 0.7-0.8ml
The cornea is the main structure responsible for the refraction of light entering the eye. It separates the air, with a refractive index of 1.00, from the aqueous humor, with a refractive index of 1.33.
4
Q
How thick is the cornea?
A
Thinnest in the center and as you travel towards the periphery it becomes bigger; 0.5ml in centre and in periphery it’s around 0.7-0.8ml
5
Q
How many layers are there to the cornea (front to back)?
A
1) Epithelium
2) Bowman’s layer (membrane)
3) Stroma (Substantia Propria)
4) Descemet’s membrane
5) Endothelium.
6
Q
What’s the Stroma also known as?
A
Substantia Propria
7
Q
Can you see the layers of the cornea macroscopically?
A
No - In a healthy cornea you shouldn’t be able to see these layers macroscopically but only under slit-lamp or microscope
8
Q
How many layers are in the Epithelium of the cornea?
A
5 Stratified layers
(10 around the limbus)
9
Q
What are the Epithelium layers of the cornea made up of?
A
Superficial Cells (2 or 3)
Wings Cells (2 or 3)
Basal Cells (1)
Basement Membrane
10
Q
Where does the Epithelium of the Cornea become thicker?
A
Epithelium gets thicker towards the periphery as you go towards the limbus (periphery of the cornea). At limbus epithelium becomes continuous with the conjunctiva; more obvious in non-white individuals as it can cause pigmentation (melanocytes) around the limbus.
11
Q
As we go down the layers of the Epithelium of the Cornea, what do the layers become?
A
More columnar
12
Q
How many layers of the Epithelium are there normally? What about around the Limbus?
A
5 normally, 10 around the Limbus
13
Q
How big is the Epithelial layer of the Cornea?
A
Around 50 - 60 micrometers wide
14
Q
What are superficial cells in the Cornea?
A
Layer of the Epithelium in the Cornea, they’re flat and linked by desmosomes which keep them tightly locked together. They also have ridges (microvilli & microplicae) that can extend into the tear film, this is to stop the tear film from evaporating by holding onto them in order to get all that the cornea needs from the tears to get it’s nutrition. It also gets oxygen from the tear film.
The superficial cells are flattened, nucleated, nonkeratinized (anywhere you need tougher skin it’s keratinized like nails; these cells aren’t as flexible but we need the cornea to be less tough = nonkerztinized), squamous cells, and the deepest cells are columnar.
15
Q
What are desmosomes?
A
Keep cells tightly locked together so fluid cannot pass
16
Q
Why does the Epithelium of the Cornea have ridged desmosomes?
A
To stop the tear film evaporating before they can get oxygen and nutrients from the tears - they hold on to the tear film
17
Q
What are Wing Cells?
A
Middle layer of cells in the Epithelium of the Cornea and contain gap junctions that allows for cells to communicate
18
Q
What is are Basal cells?
A
Bottom layer of the Epithelium of the Cornea that are columnar and rest on a basement membrane
19
Q
How does the Epithelium repair?
A
Epithelial cells at the limbus have a high turnover and so they move the cells near the limbus over to the damaged areas (via folds) to replenish surface epithelium cells to an extent (but cannot regenerate completely depending on damage levels).
20
Q
What’s the Limbus also known as?
A
Corneoscleral Junction
21
Q
What are the ridges on superficial cells known as?
A
Microvilli and Microplicae
22
Q
How do wing cells permit free intercellular communication?
A
The lateral borders of the cells show many interdigitations, and the presence of numerous gap junctions permits free intercellular communication in this zone.
23
Q
How does the basal plasma membrane attach to the basement membrane?
A
Through Hemidesmosomes
24
Q
What runs between the epithelial cells of the Cornea?
A
Naked nerve endings of sensory nerve fibres that are sensitive to pain.
25
How often do epithelial cells of the Cornea replenish?
It has been estimated that a complete turnover of corneal surface epithelial cells takes place every 7 days. New cells are formed by mitotic division in the limbal basal cell layer.
The stratified squamous epithelium covering the anterior surface of the cornea is capable of rapid regeneration after an abrasion.
26
Where is Bowman's layer?
Immediately beneath the basement membrane of the corneal epithelium
27
How thick is Bowman's Layer of the Cornea?
8 - 12 micromillimeters (um)
28
What are some features of Bowman's Layer of the Cornea?
Acellular
8-12 micrometers in thickness
Made of collagen fibrils
Ends abruptly at the limbus
Merges with the Stroma at its base
29
How is are the collagen fibrils organised in Bowman's Layer of the Cornea?
Microscopy reveals its collagen to be finer and more randomly arranged than that in the substantia propria (Stroma)
30
If Bowman's Layer is damaged in the Cornea, what are the cells replaced by?
Fibrous Tissue
31
What is Bowman's structure between?
Below the Epithelium and above the Stroma (Substantia Propria; merges with it at its base)
32
What are some important facts about the Stroma?
Organized in layers (lamellae) perpendicular to one and other
Forms around 90% of the corneal thickness
Transparent, fibrous and compact
33
What makes up the Stroma (Substantia Propria)?
Lamella (sheet) that are organised in 90 degree layers to the layer above it with roughly 250 layers of collagen fibrils
34
What makes the Stroma transparent?
The fact the layers are in 90 degree layers and that the space between them is completely uniform.
Avascular
Smooth epithelium
Regular stromal arrangement
Relatively ‘dry’ structure
35
What can happen to the Stroma layers if they are impacted?
If the spaces between these lamella are impacted for some reason then vision becomes clouded so a person will report hazy or blurry vision i.e. a corneal foreign body deep in the cornea results in inflammation and thus fluid in these areas.
36
What happens to the lamella layers of the Stroma in Glaucoma?
The fluid will push apart the lamella layers. In something like Glaucoma, the aqueous will push the layers closer and closer together. In order to stop cornea becoming over or under hydrated in a healthy eye, the cornea has processes stopping fluid entering the stroma through a pump system. If damaged will be replaced with fibrous tissue and thus scar tissue; will affect vision if significant.
37
What are some key features of the Descemet's Layer of the Cornea?
Lies on the posterior aspect of the stroma
Forms the basement membrane of the endothelium
Strong and uniform
If incised, curls into the AC and comes away from the stroma and endothelium
Small peripheral projections into the AC – Hassall Henle Bodies (increase in number over time)
38
What does the Descemet form?
The basement membrane of the ENDOthelium
39
When is the only time the Descemet's layer damaged?
Only time it’s really damaged is when needing to go through this layer for surgery like cataract surgery; If we incise it in surgery it will stick itself back down but will scar over at the entry point. Can find it under a slit-lamp as homogenous If cut through this layer it curls up towards the aqueous inside the anterior chamber (so moves away from the Stroma and Endothelium).
40
How is the Descemet's layer of the Cornea organised?
Ina hexagonal pattern of fibrils.
41
What happens to the Descemet's layer as you get older?
It protrudes into the anterior chamber (AC)
42
What does the Descemet's layer merge with?
The Trabecular Meshwork and forms part of it BUT it looks like it terminates at the limbus
43
Where is the Descemet's layer of the Cornea?
Posterior surface of the substantia propria (stroma) and is the basement membrane of the ENDOthelium.
44
How thick is Descemet's layer of the Cornea?
Strong and homogenous and measures 10 - 11um.
Sharply defined from the stroma and thicker than the endothelium.
45
What's the anterior border ring of the trabecular network know as?
Line of Schwalbe
46
What is the basement membrane of the endothelial cells?
Descemet's Membrane
47
What are some key facts about the Endothelium layer of the Cornea?
Single layer of flattened cells
Contain lots of mitochondria, ER and golgi apparatus = active role in synthesis
Transport fluid from AC to cornea via active transport
Polygonal shape
48
What does it mean for the Endothelium to have a lot of mitochondria?
Active transport going on and so needs ATP to do it. Endoplasmic reticulum help make the endothelium.
49
What's the role of the Endothelium of the Cornea?
Transports fluid as they need hydration, nutrients and oxygen; the deeper cells get this from the aqueous humour.
50
How does the Endothelium of the Cornea control fluid intake?
Have Desmosomes to tightly pack together the cells to stop fluid in but they have a sodium-potassium ATP pump to (Sodium out, potassium in). When sodium leaves a cell, water naturally follows it so this is how the endothelium allows water into and out of the cell; pumps more water out than water in to keep it relatively dehydrated. This keeps the stroma dehydrated.
51
How many layers is the Endothelium?
A single layer of flattened cells polygonal in shape
52
What are some features of the cell cytoplasm in Endothelium?
The cell cytoplasm contains numerous mitochondria, a prominent endoplasmic reticulum, and a Golgi apparatus (to get things out of the cell), indicating that the endothelium plays an active role in the synthesis and transport of fluid.
53
What does the blood supply and drainage in the Cornea look like?
Ciliary arteries stop at the limbus, they shouldn’t go past the limbus. Cornea is avascular and doesn’t have lymphatic drainage. No blood supply so don’t need lymphatic drainage.
54
How does the cornea get oxygen?
Capillaries from the anterior ciliary arteries end at the edge of the cornea provide oxygen to the peripheral cornea
Central cornea receives oxygen from the air via diffusion through the tear film
= The central part of the cornea receives oxygen indirectly from the air via oxygen dissolved in the tear film, whereas the peripheral part receives oxygen by diffusion from the anterior ciliary blood vessels.
55
What is the nerve supply to the cornea?
These nerve fibers are derived from the ophthalmic division of the trigeminal nerve, mainly through the long ciliary nerves.
Via the long ciliary nerve > annular plexus in the sclera > become unmyelinated in the stroma and form the subepithelial plexus > intraepithelial plexus in Bowmans layer
Long ciliary V1 of Cranial Nerve 5 (I.e. Ophthalmic Nerve of the Trigeminal Nerve) from the Nasociliary nerve
Penetrate the sclera behind limbus to form a plexus which then penetrate the stroma in a radial shape (myelinated at this point). Once going through the stroma it is now unmyelinated i.e. naked, and move forward through stroma to epithelium. This makes them incredibly sensitive (being unmyelinated) hence why it hurts a lot having eyelash on the cornea
56
What factors reduce transparency of the cornea?
- Significantly increased IOP (intraocular pressure)
- Oedema due to any cause
- Fibrosis after trauma
- Inflammation and infection
With advancing years, the cornea becomes less translucent and dustlike opacities, due to condensation in the stroma, may occur in the deeper parts of the stroma. Bowman's and Descemet's membranes also increase in thickness.
57
What maintain the normal tissue fluid content of the corneal stroma?
The corneal epithelium and the corneal endothelium
58
What layers of the cornea can result in oedema of the stroma?
The junctional complexes of the epithelial cells prevent passage of tear fluid into the cornea or loss of tissue fluid into the tear fluid. The endothelium limits the uptake of aqueous humour by way of both its barrier function and its active transport mechanism. Trauma to either of these layers will result in oedema of the stroma. Death of the surface cells causes oedema, but the density of Bowman's layer tends to inhibit spread into the deeper stroma.
59
How can acute glaucoma cause corneal oedema?
Acute glaucoma can produce corneal oedema by opening up the gaps between the endothelial cells. Because of the high intraocular pressure the aqueous fluid is pushed through the corneal stroma, where it accumulates beneath and among the epithelial cells.
60
How is the cornea innervated?
Cornea is innervated via the long ciliary nerve (which comes off the nasociliary division of V1)
61
By 60 what's our ability to change out accommodation in PD?
Presbyopia – unable to accommodate. 40 can change power by 8-10D but at 60yo it’s an ability to change by 1-2D.
62
What's the n of the anterior and posterior surfaces of the lens?
Have anterior and posterior surfaces of the lens, these aren’t significantly different (1.35 vs. 1.4 in the middle and then posterior again is 1.36)
63
How big is the lens?
Lens – born lens is smaller (6.5mm) than adult (10mm) in length.
64
What's the total dioptric power of the eye?
58D
65
What part of the lens do we do surgery on?
We tear off the anterior capsule for surgery. Capsule surrounds the entire lens.
66
Where are epithelial cells located on the lens?
Anterior surface of the lens
67
What is a lens bow?
Where the nucelli line in a baby in utero
68
Does the nucleus of the lens move?
No
Starts in utero but continues throughout life. Anterior cells migrate towards edges of equator and once reaching long circumference at the equator they turn and start to get longer. Once they do this they compact/push into the middle of the lens. This is what give the cells structure. As you get older they become more compact in the middle. The oldest part of the lens are the oldest as the cells have moved outwards, turned, and then moved to the centre. Deeper in lens is the nucleus, the newer part (bits surrounding it) are the cortex (no specific boundary to where this becomes the nucleus, it’s just whatever is around the nucleus as they’re migrating at this point, when it gets to the point of them not moving this is the nucleus).
69
Is the anterior or posterior surface more or less convex?
Anterior surface is less convex than it's posterior surface
70
What are the anterior and posterior poles of the lens?
The center points on its anterior and posterior surfaces are referred to as the anterior and posterior poles, respectively; a line joining the poles forms the axis of the lens; the marginal circumference of the lens is called the equator.
71
How thick is the lens in adults?
4mm thick
72
What is the equator of the lens encircled by?
The equator of the lens is encircled by the ciliary processes and lies 0.5 mm from them. The lens, which has considerable flexibility, is kept in position by the suspensory ligaments.
73
What is the capsule of the lens?
Elastic membrane that envelops the entire lens that's thickest closest to the equator
74
What are the anterior and posterior capsules of the lens in contact with?
Inner surface of the anterior capsule is in contact with the epithelium and inner surface of the posterior capsule is in contact with the lens fibres
75
What is the function of the capsule of the lens?
Primary function =
Change the shape of the lens
Secondary function =
Acts as a barrier to large particles. Will allow small compounds in via diffusion
76
Where is the epilthelium of the lens found?
Anterior surface of the lens and as they migrate to the equator they elongate to form columnar cells and at the equator they become lens fibers
77
What is the function of the epithelium of the lens?
Epithelium has 2 functions depending on where the cells are – more central cells are involved in the transport of substances from the AqH to the lens interior, and secretion of capsular material. Cells at the equator and responsible for contributing to the mass of the lens
78
Where are the earliest formed fibres?
Centre of the lens (lens nucleus) = oldest cells
Later fibres (newest) form the lens cortex
79
Where do zonules (suspensory fibres) fibers arise and run toward?
The zonule fibers arise from the epithelium of the ciliary processes and run toward the equator of the lens. The fibers fuse to form about 140 bundles.The larger bundles are straight and reach the lens capsule in front of the lens. Together they form the anterior zonular sheet.
The equatorial region, or circumference, of the lens is attached to the ciliary processes of the ciliary body by the zonule, as previously noted. The pull of the radiating fibers of the zonule tends to keep the elastic lens flattened, permitting the eye to focus on distant objects.
80
How does accommodation to near objects occur?
To accommodate the eye for close objects, the ciliary muscle contracts.
The meridional fibers pull the choroid and ciliary body forward, and the circular fibers, acting as a sphincter, move the ciliary body inward.
This relieves the tension on the radiating fibers of the zonule. This process allows the elastic lens to assume a more nearly globular shape.
At the same time, the sphincter pupillae muscle contracts, so that the pupil becomes smaller and only the light rays going through the thickest, central part of the lens impinge on the retina.
81
What happens to the lens as we age?
No cells are lost from the lens and lens fibres continually migrate to the centre but this centre becomes less pliable and more dense
Capsule thickens with age compact nucleus sclerosis occurs as the lens becomes more dense and happens to everyone (not the same as a cataract, people with Down Syndrome will get a cataract and need an intraocular lens)
Central portion becomes sclerosed and yellowish
Eventual reducation in VA
82
What are some facts about the vitreous?
Fills the eyeball behind the lens and occupies around 4/5th of the eyeball
Lies between the lens and the retina
Has a small element of refraction and helps keep the neural retina pressed up against the non-pigemented layer
Anteriorly it has a saucer shaped depression called the hyaloid fossa, which accommodates the shape of the lens
Liquid gel with a more dense cortex and liquid center
Colorless and 98% water. The rest is a mix of hyaluronic acid, amino acids, proteins, salts and ascorbic acid
Refraction 1.33
83
What is the Hyaloid Fossa?
Anteriorly the lens has a saucer shaped depression called the hyaloid fossa, which accommodates the shape of the lens
84
What's the Hyaloid Canal?
A narrow channel runs forward from the optic disc to the posterior pole of the lens = hyaloid canal which carried the hyaloid fossa when the eye developed in utero
Used to have an artery running here in utero but 6-8wks before birth this artery disappears.
85
What does the vitreous contain?
Contains a network of fine collagen fibrils which form a scaffolding, which are present more in the cortex than the centre
As you age your vitreous becomes more liquid than gel, and the process expands outwards from the centre
86
Where is the vitreous more runny?
Vitreous is more runny ( less viscous) in the middle than on the outside which is more gel-like. Should be colourless in a healthy eye. 98% water, has some salts, electrolytes etcs
87
Where is the vitreous most strongly adhered?
As the retina moves forward, it ends at the Ora Serrata (Vitreous base) which is also the strongest place for the vitreous to be adhered. So if someone has a vitreous detachment (from the retina) it’s more likely posteriorly. It’s also strongly adhered at the optic disc but not as much as the Ora Serrata.
88
What is the Ora Serrata?
Vitreous base - strongest place for the vitreous to be adhered
89
What is Vitreous Synchysis?
Synchysis – normally the vitreous is gel like outside and runny inside vitreous, but can get pockets of where the gel becomes more watery so the vitreous collapses into these pockets and, because it’s contracting into this, it pull on the retina (as attached to this) which causes flashes and floaters as it causes the photoreceptors on the retina to stimulate. Floaters – light hits these blobs of liquid which casts a shadow on the retina.
90
Can you live without vitreous?
Yes
91
What is retinal detachment?
Retinal detachment occurs when the layers of the retina separate – specifically the neurosensory retina from the retinal pigment epithelium
92
What are the three types of retinal detachment?
Rhegmatogenous,
Tractional, and
Exudative.
93
What is Rhegmatogenous Retinal Detachment?
Neurosensory lifted away from RPE. Water/fluid between the layers. Blindness in certain visual fields depending on where it detaches. Shafers sign on slit lamp
94
What is Tractional Retinal Detachment?
Vitreous membranes pull on the retina which separates it from the epithelial layer beneath. There are no tears or holes formed. This is more common in patients with diabetic retinopathy. Caused when collagen fibrils adhere to the retina (proliferate) and causes syneresis and as they contract it pulls the retina off of the eye. Treatment is the same as in Rhegma via air gas or retinal plexy (stick it back)
95
What is Exudative Retinal Detachment?
An underlying retinal disease leads to the build-up of exudative fluid underneath the retina (e.g. exudative tumours, inflammation and posterior scleritis). No retinal tear, may have choroidal tumour so get fluid build up in choroid that pushes on the retina. Different treatment via oncology which is likely to be eye removal
96
What often precedes retinal detachment?
Posterior Vitreous Detachment
97
What are the risk factors of retinal detachment?
- Age >40
- Male
- Myopia (near-sightedness)
- Family history of retinal detachment
- Previous ocular surgery
- Trauma
- Retinal detachment in the contralateral eye
- Diabetic retinopathy (a condition in people with diabetes that affects blood vessels in the retina)
- Certain other eye diseases, including retinoschisis (when the retina separates into 2 layers) or lattice degeneration (thinning of the retina)
98
What does a retinal detachment look like on an ophthalmoscopy?
Shows as retinal detachment and see bright yellow-white light line on the retina (so not nice and pink as we’d be looking at the RPE)
99
What are the symptoms of retinal detachment?
- Painless loss of vision
- Flashing lights and floaters
“Cobwebs” in the peripheral vision
Shadow or grey curtain moving across the field of vision* floaters for the first time or more floaters than you had before. The new floaters are usually noticed suddenly. There may be lots of new floaters or commonly a large cobweb-like floater Caused from syneresis from the bundled collagen fibrils * flashes of light in your vision (photopsia), usually like brief ‘streaks’ of light in your side (peripheral) vision in the temporal visual field.. When your vitreous pulls away from your retina, your retina reacts to this stimulation by sending a signal to your brain. Your brain processes this signal as a small, short flash of light.
- Blurred vision
100
What will the slit lamp show of the anterior vitreous in a retinal detachment?
Slit lamp examination may reveal a ‘Tobacco dust’ appearance of the anterior vitreous: a result of pigment cells migrating through a tear in the retina (also known as Shafer’s sign)
101
What clinical findings are there of retinal detachment?
- Poor visual acuity
- Relative afferent pupillary defect
- Altered red reflex (grey or folded appearance)
- Reduced visual acuity (if the macula is involved)
- Visual field defects
- A sheet of sensory retina billowing towards the centre of the globe
102
What's a Vitrectomy?
The most common form of retinal detachment treatment; vitreous and subretinal fluid is drained to allow the retina to lie flat against the epithelium
103
What's Pneumatic Retinopexy?
Suitable in straightforward cases of retinal detachment – a gas is injected into the vitreous that flattens and pushes the retina back onto the epithelium and then laser is used to hold the retina in place and to stop the vitreous from filling the area again
104
What's a Scleral Buckle?
Cryotherapy (freeze therapy) or laser photocoagulation to create a scar around the retinal break. A scleral buckle is sutured around the eye to indent the retina and close the break. Silicone around the eye to close a tear but changes eye shape, used more in younger patients as they can tolerate this better.
105
What are the two causes of posterior vitreous detachment?
- Synchysis – becomes water-like from being gel-like
- Syneresis – where collagen fibrils tighten (instead of cobweb-like they become more ball-like and as they’re doing this pull on the vitreous and thus on the retina)
106
What symptoms does PVD cause?
* floaters for the first time or more floaters than you had before. The new floaters are usually noticed suddenly. There may be lots of new floaters or commonly a large cobweb-like floater Caused from syneresis from the bundled collagen fibrils
* flashes of light in your vision (photopsia), usually like brief ‘streaks’ of light in your side (peripheral) vision in the temporal visual field.. When your vitreous pulls away from your retina, your retina reacts to this stimulation by sending a signal to your brain. Your brain processes this signal as a small, short flash of light.
* blurred vision
Asymptomatic often until the detachment gets closer to the optic disc
107
How can PVD lead to a retinal tear?
PVD can lead to a retinal tear. This is because the vitreous may be more firmly attached in certain places to the retina. As your vitreous moves away from your retina in PVD, it can pull on your retina, causing it to tear. The risk of this happening tends to be highest when the symptoms of PVD first begin. Once the vitreous has fully detached from the retina, the risk of developing a tear from PVD goes back down. Retinal tears need to be lasered as if the vitreous also develops a hole the vitreous can go into the retinal tear and damage the retina = have a vitrectomy but even this has a risk of pulling on optic nerve and the ora serrata
108
What happens in a vitreous haemorrhage?
In a vitreous haemorrhage the choroid splits and blood goes into the vitreous through a hole in the vitreous.
109
What is the Substantia Proprio?
Stroma
