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Ocular Physiology #2 > Corneal and Sclera part 2 > Flashcards

Corneal and Sclera part 2 Flashcards

1
Q

what are 2 things that prevent the corneal stroma from swelling

A
  1. the barrier function of the endothelium

2. the pump function of the endothelium

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3
Q

what percentage is the water in the corneal stroma maintained at

A

78%

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4
Q

which contributes more to prevent corneal stroma swelling: the epithelium or endothelium border

A

the epithelium

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5
Q

what 4 things occur to the corneal stroma is there is edema

A

displace the fibrils, increase scatter, halos, and bad vision

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6
Q

when the endothelium is disrupted, what is the rate that the cornea swells at

A

127 microns per hour

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7
Q

what happens when the cornea swells

A

movement of fluid and solutes from the aqueous humor into the stroma, through the incomplete barrier of the intact cell layer, it reaches the max amount and stops

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8
Q

if the metabolic pump is disrupted, what is the rate the cornea swells at

A

33 microns per hour

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9
Q

is it normal to have leakage of fluid into the stroma

A

yes it is a vital function because the cornea is avascular

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10
Q

what does the fluid that leaks into the cornea provide

A

nutrients (glucose and amino acids)

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11
Q

what is the maintenance of corneal thickness and water content dependent on

A

temperature (metabolically energy-dependent process)

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12
Q

what happens if the cornea is cooled

A

it swells

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13
Q

what happens if the cornea has a normal temperature

A

the cornea has normal thickness

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14
Q

what is the phenomenon called where temperature affects corneal swelling

A

temperature reversal

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15
Q

how much water is moved by the endothelium from stroma to aqueous humor per hour

A

6-8 ml

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16
Q

where is the Na+K+ATPase pump located in the corneal endothelium

A

in the basolateral membrane

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17
Q

how many Na+K+ATPase pumps are there per cell

A

1.5 x 10^6

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18
Q

what inhibits the N+K+ATPase pumps in the corneal endothelium

A

Ouabain

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19
Q

how does Ouabain inhibit the pumps in corneal endothelium

A

it stops sodium transport, causes corneal swelling, prevents temperature reversal and eliminates the transendothelial potential difference

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20
Q

how is the N+K+ATPase pump affected in corneas with Guttata

A

increase in pump densities per cell and greater capacity for the pump to counteract the leak

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21
Q

what are 2 ocular functions of the Na+K+ATPase pump

A
  1. control of corneal hydration

2. production of aqueous humor

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22
Q

why does the corneal stroma readily take up water

A

to maintain corneal hydration

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23
Q

what happens when the sodium ions are transported into channels between endothelial cells

A

it creates an osmotic pressure and water follows

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24
Q

how is the N+K+ATPase pump affected in corneas with inflamed edematous

A

decreased pump site density despite the increased permeability

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25
Q

what 3 things should an ideal intraocular irrigating solution contain

A
  1. energy source- glucose
  2. adequate buffer- bicarbonate
  3. substrate- calcium, glutathione
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26
Q

what aids the sodium ions in the right direction after being pumped

A

the density of sodium ions in Descemet’s membrane

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27
Q

what happens in poorly buffered intraocular irrigating solutions

A

may expose the cornea to pH extremes and cause corneal edema

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28
Q

what does normal saline lack

A

the components of the aqueous humor

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29
Q

what is the buffers job in a solution

A

to maintain the pH

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30
Q

what happens with 0.9% NaCl irrigating solution

A

causes cornea swelling (loss of pump and barrier)

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31
Q

what is an ideal intraocular irrigating solution

A

Glutathione bicarbonate ringers solution GBR

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32
Q

what happens with 0.9% saline and lactated Ringer’s solution

A

the cornea is unable to resist the pH change (buffering capacity of the aqueous humor)

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33
Q

what happens when a large defect occurs as a result of surgical insult or a decompensation episode in keratoconus

A

a more extensive cell migration occurs

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34
Q

what is keratoplasty

A

a corneal transplant

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35
Q

what are 2 things in the endothelium that are needed for proper corneal function

A
  1. adequate endothelial cell density

2. endothelial cells of uniform size and shape

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36
Q

what happens to the cells after a keratoplasty

A

migration of endothelial cells over the wound edge to the periphery, development of tight junctions, development of pumps

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37
Q

what is polymegathism

A

a change in cell size

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38
Q

what is pleomorphism

A

a change in cell shape

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39
Q

what causes both polymegathism and pleomorphism to increase

A

age

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40
Q

what is CV (coefficient of variation)

A

the standard deviation of mean cell area

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41
Q

what is normal endothelium CV

A

about 0.25

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42
Q

what if CV > 0.25 in the endothelium

A

the cell size is variable and is polymegathism

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43
Q

a healthy cornea has what percentage of hexagonal cells

A

70-80%

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44
Q

what would indicate endothelial stress and pleomorphism

A

a decrease in hexagons with an increase in cells with more than or fewer than 6 sides

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45
Q

what is the range of myopia that LASIK has no significant effect on endothelial cell density or percent of hexagonal cells

A

2.25 to 14.5 D

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46
Q

what does the residual stroma post-LASIK need to be in order to maintain and protect the corneal endothelial structure and barrier function

A

200 microns

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47
Q

how many years does it take for the CV of cell sizes to improve after LASIK if the patient previously wore CL

A

3 years

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48
Q

what part of the cornea does LASIK affect

A

corneal stroma not endothelium (about 12-15 microns per diopter)

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49
Q

will cataract surgery cause endothelial cell loss in the cornea

A

yes - study saw mean loss of 8.5% after 12 months

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50
Q

what two diseases can cause a decrease in cell density

A

type 1 diabetes and glaucoma

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51
Q

what would cause corneal endothelium to remodel

A

stress (KCN) and excessive glucose (diabetes type 2)

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52
Q

what happens to the endothelial corneal cells in keratoconus (KCN) and Type 2 diabetes

A

morphology changes, no decrease in cell density, increase of cell size and decrease of hexagonal cells

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53
Q

what happens to the corneal endothelium in glaucoma suspects with elevated IOP (ocular hypertension)

A

after 40 y/o there is a decrease in endothelial cell density

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54
Q

what happens to endothelial cells in long-term use of rigid polymethylmethacrylate lenses and daily/extended wear CL’s

A

polymegathism

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55
Q

why would you need to take special care when doing cataract surgery on those with glaucoma or diabetes

A

they have a decreased endothelial cell density

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56
Q

what is responsible for the change in endothelial morphology in CL’s

A

CL induced hypoxia- CLUE syndrome (contact lens use endotheliopathy)

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57
Q

does the cornea have nerves

A

yes- richly supplied with sensory nerves

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58
Q

what is the source of innervation for the cornea

A

ophthalmic division of the Trigeminal nerve (CN5) via anterior ciliary nerves

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59
Q

which parts of the cornea are not innervated

A

Descemet’s membrane and endothelium

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60
Q

what type of nerve classification are most corneal receptors

A

nociceptors

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61
Q

what are nociceptors

A

stimulation results in the perception of pain

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62
Q

why are corneal abrasions, ulcers and bullous keratopathy extremely painful conditions

A

the cornea has nociceptors which have the lowest threshold for mechanical stimulation

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63
Q

what 3 conditions result in corneal sensory denervation

A

stroke, diabetic retinopathy, and herpes simplex

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64
Q

what does sensory denervation cause in the cornea

A

high incidence of epithelial erosions and neurotrophic ulcers

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65
Q

what would result from the loss of foreign body sensations on the cornea

A

mechanical cornea damage

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66
Q

what happens to corneal sensitivity after LASIK

A

causes a decrease in corneal and conjunctival sensitivity up to 16 months

67
Q

do both soft and rigid gas-perm CL reduce corneal sensitivity

A

yes- or you would be aware of the CL on the eye

68
Q

corneal metabolism depends on oxygen from what 3 things

A

atmosphere, aqueous humor, and limbal vasculature

69
Q

what is the largest source of oxygen for the cornea

A

atmosphere

70
Q

what is the normal amount of oxygen in aqueous humor compared to tears

A

40mm Hg in aqueous humor and 155mm Hg in tears

71
Q

how much more oxygen does the epithelium consume compared to the stroma

A

10 times more

72
Q

when the eye is closed, how is oxygen delivered to the cornea

A

by the vascularized superior palpebral conjunctiva

73
Q

what are the percentages of oxygen delivered to the cornea for the eyelid open and eyelid closed

A

21% for open and 8% closed

74
Q

what happens to oxygen tension following cataract surgery

A

it may increase in the aqueous humor as a result of decreased oxygen metabolism by the crystalline lens

75
Q

why would the cornea have a greater tolerance for hypoxic stress following cataract surgery

A

the oxygen in the aqueous supplements that in the tears

76
Q

what delivers the nutrients to the cornea

A

the aqueous humor via the ciliary body (lesser amounts by the tears and limbal vessels)

77
Q

how does the cornea receive glucose

A

from the glycogen stores in the corneal epithelium

78
Q

why does wearing CL cause edema

A

decreases available oxygen to the corneal epithelium, resulting in corneal edema (20% increase in corneal thickness)

79
Q

how much oxygen reserve do hard CL use in 8 hours of wear

A

80%

80
Q

what is the glucose from the aqueous humor or glycogen stores converted to

A

pyruvate by the Embden-Meyerhof pathway (glycolysis)

81
Q

what is the product from the conversion of glucose to pyruvate

A

two molecules of adenosine triphosphate (ATP) per glucose molecule

82
Q

what increases under hypoxic conditions that causes corneal edema

A

increase in lactate production (pyruvate is converted by lactate dehyrogenase to lactate)

83
Q

what are 3 symptoms of epithelial edema

A

halo and rainbow formation, increased glare sensitivity (light scattering) and decreased contrast sensitivity

84
Q

which direction does stromal edema manifest

A

posterior direction (because the anterior surface of the cornea is structurally fixed by Bowmans layer and the anterior stroma)

85
Q

what causes vertical striae in the cornea

A

buckling of the stroma and Descemet’s membrane

86
Q

what percentage of edema do vertical striae become visible

A

8%

87
Q

how does increased amounts of lactate lead to corneal edema

A

the lactate diffuses from the epithelium into the stroma osmotically, inducing epithelial and stromal edema

88
Q

at what oxygen level can corneal deturgescent (clarity) be maintained

A

as low as 25mm Hg

89
Q

what is a disadvantage of wearing a hard CL versus a soft lens

A

impermeable to oxygen (soft has oxygen permeability)

90
Q

what is an advantage of having a small diameter of a hard CL

A

good movement needed for tear exchange

91
Q

what is the underlying mechanism of polymegathism

A

certain byproducts of long term hypoxia and stress (12HETE) has the ability to inhibit the Na+K+ATPase of the endothelial metabolic pump

92
Q

what is a disadvantage of have a large diameter of a soft CL

A

moves minimally on the eye- bad for tear exchange

93
Q

what does CL wear stimulate in polymegathism

A

the arachadonic acid cascade in corneal epithelium from the lenses and the protein they absorb from water

94
Q

what causes volume-regulated stress of the cornea

A

long term CL (10 years) and diabetic patients

95
Q

extended wear CL have been associated with what 3 things

A

decreased rate of mitosis (rubs on the stem cell layer), reduced oxygen uptake and glucose utilization, and smaller numbers of intercellular desmosomes

96
Q

what condition can occur due to the compromised epithelial barrier in EW CL

A

increased likelihood of ulcerative microbial keratitis

97
Q

why is the epithelial barrier compromised in extended wear CL

A

there is reduced metabolic activity

98
Q

what does a lack of vitamin A cause

A

Xerophthalmia (occurs late in the progression of a vitamin A deficiency)

99
Q

what happens with Xerophthalmia

A

keratinization of the epithelium (becomes opaque)

100
Q

what are the symptoms of Xerophthalmia

A

keratinization (opaque), decreased mucin production, dry spots on cornea, night blindness (prior to xerophthalmia)

101
Q

what requires vitamin A

A

mucin production

102
Q

what is the volume of a normal adult tear film

A

7-9 microliters

103
Q

what is the maximum amount of fluid that the cul-de-sac can contain

A

20-30 microliters

104
Q

what is the amount of the average drop of topical medication

A

50 microliters

105
Q

what would cause tear dilution or a change in tear pH levels

A

reflex tearing caused by irritating or hypertonic solutions

106
Q

what might decrease the bioavalability of drugs

A

increased protein concentration in tears bathing inflamed or infected eyes

107
Q

what is the initial barrier for drug penetration

A

the epithelium (tight junctions)

108
Q

how is the epithelium a barrier for drugs

A

they limit the absorption of hydrophilic, ionized substances and favor penetration of lipid-soluble hydrophobic compounds

109
Q

what does the loss of corneal epithelium greatly enhance when instilling drugs

A

penetration of hydrophilic water-soluble pharmacologic agents (such as gentamicin)

110
Q

in clinic, how can you disrupt the corneal epithelium to make drug delivery easier

A

anesthetic causes a few spots of epithelial damage

111
Q

how is the corneal stroma a drug barrier

A

the hydrophilic nature results in a barrier to lipid based drugs

112
Q

how is the corneal endothelium a drug barrier

A

drug penetration is determined by the molecular size

113
Q

how can you increase drug penetration

A

by increasing duration of the contact of the drug with the ocular surface

114
Q

what are examples of mechanical means to increase drug penetraion

A

press on lacrimal sac, use viscous drops, suspensions or ointments

115
Q

why would you want to block a beta blocker from entering the body

A

because it will lower the heart rate (press on lacrimal sac to prevent)

116
Q

what are examples of slow release delivery systems to increase drug penetration

A

contact lenses, porcine collagen corneal shields and punctal plugs

117
Q

what are 4 types of preservatives used in eye drops

A

BAK (benzalkonium chloride), chlorhexidine digluconate, polyquarternium-1, and thimerosal

118
Q

what is the most common type of drug preservative

A

BAK (benzalkonium chloride)

119
Q

which preservative has been discontinued because of allergic reactions

A

Thimerosal

120
Q

what is the antibacterial action of BAK based on

A

the detergent property of the compound

121
Q

what does the detergent property of BAK do to bacterial cells

A

breaks down bacterial walls (or other walls) that it is exposed to. It also damages epithelium and endothelium cell walls

122
Q

what happens when one drop of 0.01% of BAK is used

A

it increases the permeability of the cornea to fluorescein

123
Q

what type of healing does BAK inhibit

A

epithelial wound healing

124
Q

which patients have an increased fluorescein permeability secondary to the compromised epithelial barrier

A

severe dry eye patients

125
Q

what is the down side of using EDTA to counteract BAK

A

it chelates calcium, which is required for maintenance of tight junctions

126
Q

what is ethylenediaminetetraacetic acid (EDTA)

A

used to stabilize BAK-containing formulations

127
Q

should ophthalmic products contain EDTA

A

no, formulations with EDTA should be avoided by patients with dry eyes (EDTA may increase corneal permeability)

128
Q

what is the sclera made of

A

connective tissue containing fibroblasts, collagen, and proteoglycans

129
Q

how much of the sclera is water

A

70%

130
Q

how much of the sclera is dry weight

A

30%

131
Q

what percentage of the dry weight of the sclera is collagen fibers

A

75%

132
Q

the sclera is essentially avascular, what are the 2 sources for the exceptions

A

superficial vessels of the episclera and intrascleral vascular plexus

133
Q

what type of collagen is in the sclera

A

type 1: resists tension

134
Q

what is located at the posterior sclera

A

scleral canal and the lamina cribrosa

135
Q

which arteries, veins, and nerves go through sites of perforation at the posterior sclera

A

long and short posterior ciliary arteries, short ciliary veins, ciliary nerves and the vortex veins

136
Q

where do the tendons of the recti muscles insert

A

into the superficial sclera collagen

137
Q

how much of the total globe is the sclera

A

95%

138
Q

where does the scleral thickness decrease

A

towards the equator and right before the recti muscles insert

139
Q

where is the sclera the thickest

A

near the optic nerve

140
Q

what is the scleral fiber in diameter

A

25-230nm

141
Q

how are the scleral fibers arranged

A

randomly

142
Q

what is the diameter of the corneal fibers

A

25nm

143
Q

what are the 3 distinct fibers in the sclera

A

elastic, elaunin, and oxytalan

144
Q

why would infants have a bluish tint in their sclera

A

the sclera is still developing and the choroid is visible

145
Q

what does form vision deprivation result in

A

axial elongation of the ocular globe causing myopia

146
Q

how do changes in the rate of proteoglycan synthesis cause myopia

A

it accumulates in the sclera causing axial elongation

147
Q

does the stroma increase or decrease in tissue density as we age

A

increase

148
Q

what 4 things happen to the sclera as age increases

A

progressive degeneration of collagen and elastic fibers, loss of GAGs, scleral dehydration, and accumulation of lipids and calcium salts

149
Q

what happens to the prostaglandins as it enters the eye

A

the drug gets into the eye in a readily absorbed form and changes the structure into the active compound and get absorbed in the sclera through the ciliary base

150
Q

what color does the sclera turn as we age

A

yellow and it thins

151
Q

is the sclera more or less rigid as we age

A

more rigid- decreased scleral elasticity

152
Q

is the sclera innervated

A

no but is allows passage of nerves

153
Q

what is the primary mechanism of drug permeation across the sclera

A

passive solute diffusion through an aqueous pathway

154
Q

what are 3 factors that control diffusion rates of drugs across the sclera

A

tissue hydration, tissue thickness, and size and volume fraction of proteoglycans present

155
Q

what is the pH level the sclera can swell to

A

near a pH of 4

156
Q

what 3 things do not alter scleral permeability

A

age, cryotherapy and diode laser treatments

157
Q

does surgical thinning increase or decrease scleral permeability

A

increases

158
Q

does scleral permeability increase or decrease with increased molecular weight and radius

A

decreases

159
Q

what IOP can decrease sclera permeability to small molecules by half

A

15-60 mmHg

160
Q

if the sclera is exposed to various prostaglandins in organ culture, is there an increase or decrease in pemeability

A

increase

161
Q

what causes an increase in expression of matrix metalloproteinases

A

when the sclera is exposed to various prostaglandins

162
Q

if you have a large molecule, how can transscleral delivery be improved

A

taking advantage of thinner regions of tissue, increasing scleral hydration, and transient modification of sclera ECM

Ocular Physiology #2 (7 decks)

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