Intro to IOP and Tonometry

Question 1

what does tonometry measure

Answer 1

indirect estimation of IOP at one point in time by measuring resistance of eye to deformation by applying force to anterior surface

Question 2

when should tonometry not be used

Answer 2

1. red eye of infectious origin (exception is herpetic eye disease)
2. severely traumatized cornea
3. open globe
4. hyphema
5. eyes w/ keratoprosthesis (artificial cornea)

Question 3

what are the structures involved in aqueous humor dynamics

Answer 3

1. limbus
2. main route of aq humor outflow
3. ciliary body (site of aq humor production and other outflow route of unconventional or uveoscleral outflow)
4. iris and lens

Question 4

what dynamic factors does IOP depend on

Answer 4

1. rate of aq humor production
2. circulation of aq humor
3. outflow of aq humor
4. volume of vit humor
   5 elasticity of cornea/scerla (ex. ocular rigidity)

Question 5

flow of aq humor against resistance generates ____ necessary for shape and optical properties of eye

Answer 5

IOP

Question 6

what is corneal hysteresis

Answer 6

the difference btwn inward and outward applanation pressures

• difference in first/repeated applanation measures
• indication of biomechanical properties of cornea

Question 7

what is ocular hypotension
what is ocular hypotony
what can you get hypotony from? (2) what are examples

Answer 7

oculular hypotension: IOP below normal range
ocular hypotony: IOP < ~5mmHg
hypotony from aq. loss
-surgery, trauma, RD
hypotony from decreased production of aq:
-infl, medications, proliferative vitreworetinopathy

Question 8

what is ocular hypertension

Answer 8

IOP above normal range (>21mmHg) w/ no detectable changes in vf or damage to optic nerve

Question 9

who is at risk for ocular hypertenstion

Answer 9

african americans
age 40+
\+family history
high myopia 
diabetes

Question 10

what is glaucoma

Answer 10

group of ocular diseases w/ various cuases that ultiately are associated w/ a progressive optic neuropathy leading to loss of vision function
-atrophy of the optic nerve and loss of retinal ganglion cells and their axons

Question 11

what kind or glaucoma patients are usually asymptomatic

Answer 11

POAG

Question 12

what are some signs of glauc

Answer 12

• loss of retinal rim-consequent enlargement of C/D ratio
• elevated IOP
• thin corneas
• thinning/damage of NFL
• drepeatable vf loss
• peripapillary atrophy, flame hem

Question 13

what are the 2 methods of tonometry

Answer 13

1. indentation
   - indentation of cornea
2. applanation
   - flattening of cornea

Question 14

how is IOP determined in goldman applanation tonometry

Answer 14

IOP determined by amount of force needed to flatten a small corneal surface

Question 15

what is the imbert-fick principle (goldman is based on this)

Answer 15

for dry thin walled sphere, presure inside is equal to the force necessary to flatten the surface divided by the area flattened
P=F/A
-but the cornea is aspheric, wet, and not perfectly flexible, and not infinetely thin soooooo
P+S(surface tension) = F*Ai(inner area cornea) + B (force required to bend cornea)

Question 16

the 2 beam-splitting prisms w/in the applanating unit optically convert circular area of corneal contact into ____

Answer 16

semicircles

Question 17

the prisms of the GAT are adjusted so the ______ of semicicles overlap when 3.06mm of cornea is applanated

Answer 17

inner margins of semicircles

Question 18

what is the idea thickness of the mires

Answer 18

1/10 diameter of semicircles

Question 19

what happens if the tonometry mires are too thick

what happens if they are too thin

Answer 19

too thick: IOP will be overestimated-blow some fluid from tips/lids (decrease fluroesine)

too thin: IOP will be underestimated: add more fluorescein

Question 20

what do you do if the mires are overly pulsating

Answer 20

apply very slightly more pressure, just enough to stop it

Question 21

if the space w/in (inside) semi-circles are splotchy and indistinct, what is happening

Answer 21

too much pressure is being applied

-pull back slightly on joystick

Question 22

what irregularly shaped corneas, what would you use to take pressure

Answer 22

use tonopen or NCT

bc irreg shaped corneas will distort mires and have inaccurate measures

Question 23

if there is high astig how do you do tonometry

Answer 23

if >3D of astig: align markings on tonometer tip to match minus cyl axis of astig

Question 24

thicker corneas result in _____ IOP
thinner corneas result in ____ IOP
what is the exception

Answer 24

ticker=falsely high IOP
thinner=falsely low IOP

exception: thick corneas due to edema are easier to indent due to high water content and result in falsely low IOP

Question 25

repeated tonometry measurements push aq out of ant chamber, resulting in what

Answer 25

falsely low IOP

wait 10 sec

Question 26

tonometry should always be performed ____ other tests that apply force to the eye

Answer 26

before

Question 27

artificially high IOP can be a result of…

Answer 27

• pt anxiety
• pressing globe when holding eyelids open
• too tight shirt collar