Cornea Flashcards
1
Q
Major concepts about the cornea
A
- part of the fibrous tunic
- transparent
- tough
- highly innervated
- avascular
- major refractive component of the eye
2
Q
Cornea is the anterior _____ of the eye
A
1/6th
3
Q
Which has steeper curvature, cornea or sclera?
A
Cornea
4
Q
What does the merger of the cornea and sclera form?
A
External scleral sulcus
5
Q
Where does vasculature abruptly end in eye
A
Corneoscleral limbus
6
Q
Anterior view of cornea shape
A
Oval
7
Q
Diameter of horizontal anterior cornea
A
11.7-12.6mm
8
Q
Diameter of vertical anterior corneal
A
10.6-11.7mm
9
Q
What is the shape of the cornea posteriorly
A
Perfect sphere
11.7
10
Q
Radius of curvature of the posterior cornea
A
6.5mm
11
Q
Radius of curvature of the anterior cornea
A
7.8mm
12
Q
Overall power of cornea
A
Ranges from +43.1D to +48.8D
13
Q
Index of refraction of cornea
A
1.376
14
Q
Unequal curvature of the cornea that prevents light from being focused to a single point on the retina
A
Astigmatism
15
Q
Flat and steep meridians are 90 degrees apart
A
Regular astigmatism
16
Q
Steeper in vertical meridian, like a football laying on its side
A
With the rule astigmatism
17
Q
Steeper curvature in the horizontal meridian, like a football standing on its tip
A
Against the rule astigmatism
18
Q
Steeper curvature in oblique axis
A
Oblique astigmatism
19
Q
Steepest and flattest meridians are not 90 degrees apart
A
Irregular astigmatism
20
Q
Unequal curvature of the lens
A
Lenticular astigmatism
21
Q
Average CCT
A
0.52mm
22
Q
Average peripheral corneal thickness
A
0.70mm
23
Q
Thicker corneas and IOP reading
A
Higher than normal IOP reading
24
Q
Thinner corneas and IOP reading
A
Lower IOP reading
25
Measures central corneal thickness using ultrasound
Pachymeter
26
When might you use a pachymeter on a patient
Suspect glaucoma but getting normal IOP reading, or when someone looks normal but gets higher IOP readings
27
Fascia Bulbi (Tenon's Capsule) strongly adheres to episclera and conjunctiva. Forms so key of eye
Bulbar limbus
28
Juncture of cornea and sclera
Corneoscleral limbus
29
Conjunctiva extends 1mm beyond corneoscleral limbus. Arcades stop here
Conjunctival limbus
30
Central 4mm of the cornea
Optical zone
31
What is the cornea predominantly made of
Extracellular material
32
How many layers to the anterior epithelium
4 layers
33
How many layers to the posterior epithelium
1
34
What is the bulk of the cornea
Stroma
35
Layers of cornea from front to back
- tear film
- anterior epithelium
- bowmans layer
- stroma
- descemet's membrane
- endothelium
- aqueous humor
36
How thick is tear film
7 microns
37
Volume of tear film
6.2 +/- 2 microliters
38
What is contained in the tear film
Electrolytes, metabolites, proteins, and lipids
39
What are the 3 layers of the tear film from anterior to posterior
Lipid layer
Aqueous layer
Mucous layer
40
Secreted by meibomian glands in the palpebrae and carbuncle. Prevents evaporation of aqueous layer
Lipid layer
41
Secreted by the lacrimal gland
Aqueous layer
42
Secreted by goblet cells in the conj. Helps aqueous stick
Mucous layer
43
How do the oily and the mucous layer help the aqueous layer
The oily layer prevents the aqueous layer from evaporating, while the mucous layer decreases surface tension and provides a smooth, wetable surface for the aqueous layer
44
What helps join the mucous layer of the tear film
Glycocalyx
45
Overflowing of tears
Epiphora
46
Why do people get epiphora
Overproduction of tears because lipid layers is absent and causes evaporation
47
What helps people who have problems with the lipid layer
Hot compress to help meibomian glands express lipid layer
48
What is the corneal epithelium composed of
Stratified squamous epithelium
49
How thick is the corneal epithelium
50-60 microns
50
What is the corneal epithelium continuous with
Bulbar conj
51
What layer of the cornea contains melanocytes and langerhans cells?
Peripheral part of the corneal epithelium
52
What does the corneal epithelium rest on
Basement membrane, allows for migration of basal cells
53
How many layers of cells is the corneal epithelium composed of
5-7, non keratinized, non secretory.
54
What are the cells from superficial to deep of the corneal epithelium
Apical
Wing
Basal
55
Apical layer of the corneal epithelium
1-2 layers of flattened, squamous cells near the surface
56
What are the wing cells of the corneal epithelium
2-3 layers
| Attach to each other by desmosomes and gap junctions
57
Basal cells of the corneal epithelium
A single layer of columnar cells
| -are the only cell of the corneal epithelium that undergo mitotic divsion.
58
What is the basal surface of the basal cells of the corneal epithelium attached to?
Held to the underlying bowmans layer by hemidesmosomes
59
What do the microvilli and microplicae on the surface of the corneal epithelium do?
Pick up O2 from the air that is dissolved in tears, assist in maintaining mucous layer for adherence of aqueous layer.
60
Loss of the microvilli and microplicae on the corneal epithelium
Can occur following regular use of CL, leading to transient dry eye symptoms until new cells reach the surface and develop microvilli
61
Where does the mucin layer of the tear film attach to the cornea
Microplicae on the corneal epithelium
62
What kind of junctions between the surfaces of apical cells in the corneal epithelium?
Tight junctions. Represent a semipermeable barrier of the corneal epithelium. Zone occludens take 1 hour to replace
This forms barrier, nutrients have to go through the cell
63
How many wing cells centrally (corneal epithelium)?
2-3 layers
64
How many wings cells peripherally (corneal epithelium)?
4-5
65
Shape of wing cells in corneal epithelium
Posterior surface is concave and hte anteiror surface is convex
66
How are wing cells attached to each other
Desmosomes, also have gap junctions
67
How does the shape of the wing cells change as they move towards surface
Beceom progressively flatter due to pressure from eyelids
68
How do basal cells attach to wing cells
Desmosomes
69
How do basal cells communicate with each other
Gap junctions
70
How are basal cells attached to one another
Desmosomes
71
This formed the complex responsible for the firm attachment of the epithelium to the underlying stroma
The hemidesmosomes and its associated anchoring fibrils
72
What happens to the anchoring filaments that are attached to the basal cells of the epithelium during DM?
BM becomes thick, anchoring filaments cant hold on well and cause BM issues, sloughing issues
73
Are there free nerve endings in the basal layer of the corneal epithelium?
Yes
74
Herpes simplex keratitis
- herpes hangs out in the DRG
| - forms this dendritic structure because of gap junctions in epithelium via nerve
75
Contact solutions and cornea
Preservatives in contact lens solutions can be toxic to the cornea
76
A crutch on the eyes cornea, usually painful, but not severe. Commonly caused in sports by a scratch from the fingernail
Corneal abrasion
77
Symptoms of corneal abrasion
Pain
Feeling that something is in the eye
Tearing
Discomfort when blinking
78
First reaction for people when they get a corneal abrasion?
Rub their eye
79
What to do for corneal abrasion
Rinse with clean water or saline, do not rub
80
Treatment for corneal abrasion
See eye care prof
Aggressive lubrication
Antibiotic ointment or drops
81
How deep does a corneal abrasion go?
Surface epithelium scratch only!
82
What is used to assess the size and depth of he corneal abrasion?
Fluorescein dye
83
Punctuate epitheliopathy
Lower 1/3rd of corneal stains with fluoroscein. Sleeping with eyes ope, looking down when reading
84
Peripheral margin of cornea. Stem cell depot. Basal cells start here, move down and out
Palisades of Vogt
85
What are some examples of corneal epithelial dystrophies
Meesmans dystrophy
Subepithelial mucinous dystrophy
Gelatinous drop-like dystrophy
86
Is bowmans layer a basement membrnae?
No
87
Role of bowmans layer
Believed to provide structural support
88
Make up of Bowmans layer
- Randomly oriented collagen fibers with no keratocytes (acellular)
- 8-12 microns thick
- contains channels for passage of nerve fibers
- cannot be stripped away from the stroma as a continuous sheet
89
Can bowmans layer regenerate?
No, it is formed prenatally. If damaged it is replaced by thickened epithelium
90
Where does the bowmans layer end
At limbus
91
Why is bowmans layer no longer called bowmans membrane?
Because it does not stain with periodic acid-Schiff.
PAS negative
92
Corneal dystrophy of bowmans layer type 1
Confined to bowmans
93
What is 90% of corneal thickness?
Stroma
94
How thick is stroma
450-500 microns
95
Make up of stroma
- Collagen organized into layers (lamellae).
- GAGs
- keratocytes between lamella
96
What do GAGs do in stroma
Pull aqueous humor into cornea
97
What do keratocytes do in stroma
Maintina ground substance and collagen. Active in inflammation
98
Flats cells, reside between stromal lamellae, and communicate via gap junctions
Keratocytes
99
Light scattering and hte stromal lamella
1% of light gets scattered.
Spacing is 1/2 wavelnegth of visible light
damage/swelling causes more light scatter, appears opaque or white
100
What is thinning of the stroma called
Keratocconus
101
Keratoconus
Thinning of stroma. Causes irregular astigmatism, typically bilateral. Misshapen because the stroma cant resist IOP
102
Munson's sign
In advanced keratoconus the corneal protrusion may cause angulation of the lower lid on down gaze
103
Difference between corneal abrasion and corneal ulcer
Abrasion is in epithelium and ulcer is in the stroma.
104
How can you get herpes in the stroma as well as the epithelium?
Because herpes rides on the nerves and the nerve fibers pass through anteiror stroma
105
Pseudomonas keratitis
- CL wearers get it
- penetrates though BM
- corneal melt
- goes through the epithelium and the stroma
106
Corneal hydrops
Descemets membrane rupture
107
Types of corneal stromal dystrophies
- lattice dystrophy type I
- granular dystrophy type II
- schnyder's dystrophy type I
- congenital stromal dystrophy
108
Arcus senilis
Lipid deposits in stroma. Usually around limbus
109
What does it mean if the arcus senilis is monocular?
Carotid artery blockage on the side with the eye that does not have arcus senilis.
110
Basement membrane of the corneal endothelium
Descemet's membrane
111
Descemet's membrane and age
Thickens throughout life from 3 microns to 15 mictrons
112
What happens if decemet's membrane is damaged?
Regenerates
113
Anterior banded zone of descemets membrane
Laid down before birth
114
Posterior unbranded zone of descemet's membrane
Laid down after birth
115
How does descemet's membrane terminate?
Abruptly as Schwalbe's line
116
Wilson's disease
Liver disease, lenticulate nucleus of brain
-produces Kayser-Fleisher Ring (copper deposits in Descemet's membrane)
117
Hassall-Henle bodies
Occur in peripheral cornea and is a normal occurrence with age
118
Corneal guttata
Occur in central cornea and indicate abnormal function of endothelium. Indicate endothelial dystrophy
119
Difference between Hassall-henle bodies and Guttata?
Hassan-henle is in peripheral cornea and Normal with age
Guttata in central corneal and indicate endothelial dystrophy
120
Thickness of corneal endothelium
4-6 microns thick
121
How many cell layers is corneal endothelium
1
122
Where does the apex of the corneal endothelium face
Anterior chamber
123
Where does the basal portion of the corneal endothelium face?
Secreted Descemet's membrane
124
How are the cells of the corneal endothelium joined?
Macula occludens (leaky barrier), form gap junctions with neighboring cells
125
What do the cells of the corneal endothelium do?
Pump aqueous humor out of the cornea to maintain hydration (pump leak system)
126
Aqueous and the corneal endothelium
- aqueous leaks in from anterior chamber
| - cells pump aqueous humor out of the cornea to maintain hydration (pump leak system)
127
Minimum number of cells in corneal endothelium to maintain normal function
400-700 cells/mm2
128
Loss of cells in corneal endothelium
Can cause changes in size (polymegathism) and shape (pleiomorphism)
129
No pump action in the corneal endothelium results in what
Stromal edema
130
Do corneal endothelial cells regenerat?
No
131
Breakdown of pump leak system in corneal endothelium
As corneal endothelial cells become stressed, they drop out. Remaining cells expand in size and shape to compensate. Less pump available to counterbalance leaks of aqueous humor into corneal stroma, causing chronic endothelial edema
132
Types of corneal endothelial dystrophy
- Fuch's endothelial dystrophy
- posterior polymorphous dystrophy
- congenital hereditary endothelial dystrophy
133
Vasculature of the cornea
None
134
How does cornea get nutrients
Derived from the tear film and aqueous humor, and vasculature at peripheral cornea.
135
What is the vasculature at the peripheral cornea
Superficial pericorneal plexus (conj)
| Deep pericorneal plexus (episclera)
136
Lymphatic of cornea
None
137
How many anterior ciliary artery are there?
7
138
What do anterior ciliary arteries give rise to
Anterior and posterior conj arteries
139
Ciliary flush
If it is a more red color, its is the superficial pericorneal plexus of the cornea
If it is more pink, it is the deeper pericorneal plexus involving the iris
140
Corneal neovascularization
Poor CL wearer
| Eye becomes hypoxia, blood vessels grow forward in center of cornea
141
Drain to muscular veins in the rectus muscles
Anteiror ciliary veins
142
What is the most highly innervated structure in the body
Cornea
143
Corneal sensitivity
300-600 times that of the skin
144
Sensory innervation of cornea
Conveyed via the long posterior ciliary nerves
145
How does the long posterior ciliary nerve course
In the subarachnoid space and forms an annular plexus of smaller branches within the region of the corneoscleral limbus
146
Small radial branches extend into the middle and anteiror stroma and form a ____________ just below bowmans layer
Subepithalial plexus
147
Where are the nerves of the cornea located
Anterior stroma, bowmans, and epithelium
148
Photorefrctive keratectomy
-corneal epithelium is debriefed with surgical tool
-ablate anterior stroma
-bandage contact lens placed over cornea
-re-epitheliztion
=painful for first 24 hours
149
Laser in situ keratomileusis (LASIK)
- flap created and folded back
- anterior stroma ablated using laser
- flap replaced
150
Radial incisions cut into stroma to flatten the cornea
Radial keratotomy
151
Scleral contact lenses
Large diameter RGP CL that vaults the cornea and rests on the bulbar conj and underlying sclera
152
Who could benefit from scleral CL?
- irregular astigmatism
- penetrating keratoplasty
- keratoconus
- pellucid marginal degeneration
- high refractive error
- dry eye
- autoimmune disease
- graft versus host disease
- aphakia
153
How do scleral CL work
Liquid reservoir (non preserved saline solution) neutralizes irregular corneal surface e
154
Vault height in scleral lens
The proper sag height or vault is important to ensure teat lens reservoir adequately covers the entire cornea
155
How do you assess vaulting of scleral lens
Fluorescein is mixed with saline to assess it and the clearance of scleral contact lens over the corneal surface and limbus
156
Tear reservoir of scleral contact lenses
Must bathe the cornea from apex of the cornea to the limbus
