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Optometry Clinical Skills 3 > IOP > Flashcards

IOP Flashcards

1
Q

IOP

A

IOP - fluid pressure inside eye

determined by tonometry

important in glaucoma – routine screening when the pressure is too high for the eye

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2
Q

patient explanation to measure IOP

A

fluid is being made and fluid is being drained in the eye everyday. In glaucoma there is either too much fluid made or drainage is blocked. Extra fluid causes pressure within eye that presses on retina/optic nerve and damages it

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3
Q

aqueous humor

A

secreted by ciliary epithelium lining the ciliary processes.

enters posterior chamber - flows around the lens - through the pupil into AC - then through the trabecular meshwork to Schlemm’s canal to episcleral veins

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4
Q

why measure IOP - decisions

A

tonometry: contributes to diagnosis of glaucoma

usefule in making decision to treat patients: IOP > 30 usually treated

useful evaluation of tx/pt management

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5
Q

Review: normal IOP?

A

upper limit of normal = 21 mmHg

(5-7% of population >21 mmHg)

4 mmHg or more of IOP asymmetry suggests pathology

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6
Q

what influences IOP

A

long-term influences: genetics and family history, age, gender, races, refractive error, seasonal variation

short term influences: heartbeat, breathing, diurnal variation, systemic conditions, intraocular conditions, postural variation, eyelid movement, doctor technique, exertion, episcleral venous pressure

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7
Q

long term influences - genetics and fmaily history

A

polygenic, multifactorial role

phenotype determined by greater than 1, and possibly many genes together with environmental factors, heritability estimates for IOP range from 0.29 - 0.36

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8
Q

long term influences - age

A

IOP increase with age, mostly accounted for increase in BP with age

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9
Q

7 glaucoma-related traits

A

central corneal thickness (CCT)

IOPcc (mmHg)

IOPg (mmHg)

Corneal hysteresis (mmHg)

IOPp (mmHg)

Pulsatility of blood velocity (kH2)

Pulsatility of blood flow (H2U)

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10
Q

Long term influences - gender

A

IOP of female is slightly higher than males

older females have greater increase in mean IOP with age

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11
Q

Long term influences - race

A

mean IOP in blacks greater than that for whites

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12
Q

long term influences - refractive error

A

increase in IOP with increasing axial length

increase in IOP with increasing myopia

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13
Q

long term influences - seasonal variation

A

mean IOP highest in winter, lowest in summer

small variation

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14
Q

short term influences - heart beat (ocular pulse)

A

influx of blood into eye with cardiac cycle

associated with arterial pulse

mean ocular pulse ~3 mmHg

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15
Q

short term influences - breathing

A

due to change in systemic BP with respiration

~ 1-2 mmHg variation

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16
Q

short term influences - diurnal variation

A

mean in normals: ~ 5mmHg

diurnal variation > 10 mmHg -> pathology

glaucoma patients have mean 13 mmHg (can be > 30 mm Hg)

peak classically morning

24 hr rhythm IOP in NTG

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17
Q

short term influences - systemic conditions

A

increased IOP in:

  • systemic htn
  • diabetes
  • obesity
  • inc. pulse rate
  • hypothyroidism
  • hypoosmotic state
  • acromegaly
  • hyperthermia
  • inc. Hb
  • certain meds
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18
Q

short term influences - sytemic conditions

A

decrease iop:

  • acute hypoglycemia
  • hyperthyroidism
  • myotonic dystrophy
  • ipsilateral cartoid artery disease
  • hyperosmotic states
  • horner’s syndrome
  • HIV infections
  • pregnancy
  • certain meds
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19
Q

short term influence s- intraocular conditions

A

increased IOP: ocular trauma, neovascularization, lens (phacolytic)

decreased IOP: anterior uveitis, rhegmatogenous retinal detachment

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20
Q

short term influences - postural variation

A

supine position increase IOP (0.3 - 6 mmHg)

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21
Q

short term influence - total body inversion

A

increased IOP

5 min can 16 mmHg increase

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22
Q

short term influences - lid/eye movement

A

voluntary blink can 10 mmHg increase

hard lid squeece - 50 mmHg increase

voluntary lid fissure widening - 2 mmHg increase

horizontal gaze can produce slight increase

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23
Q

short term influence - pressure on globe can raise IOP by variable amounts

A

can raise iop variable amount

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24
Q

short term influences - exertion

A

iop may increase or decrease depending on type of exertion

prolonged running/bicycling can produce decrease

straining can cause increase in IOP

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25
Q

short term influences - food and drugs

A

alcohol can lower IOP

caffeine can produce slight transient rise in iop

marijuana will decrease IOP

excess water intake will increase IOP

becoming dehydrated will decrease IOP

drinking large quantitties of fluid in a short time will raise IOP

26
Q

aqueous outflow

A

2 major outflow

1) conventional (75%): TM -> schlemm’s canal ->episclerial veins
2) nonconventional (25%) uveoscleral (suprachoroidal) pathway

27
Q

short term influences - episcleral venous pressure

A 
IOP = F/C + Pe
F = aqueous flow rate
C = outflow facility
R = resistance to aqueous outflow
Pe = episcleral venous pressure

IOP depend directly on Pe

Pe can be increased by : tight collar, increased central venous pressure, straining,
valsalva increase IOP via Pe

28
Q

Types of tonometry

A

1) manometry - direct measurement
2) applanation - flattening portion of the cornea
3) indentation - measuring depth of indentation by constant force
4) rebound - measure deceleration speed that correlates to IOP
5) dynamic contour - direct measure
6) other forms

29
Q

Manometry: direct measurement of IOP

A
  • Direct measurement of IOP
  • Method: eye is cannulated, cannula connected to water column
  • clinically impractical (done when dead)
30
Q

applanation: flattening portion of cornea

A

flattening or applanating portion of cornea

based on imbert-fick law: W = pt x A

w = external force against sphere
Pt = pressure in sphere
A = area flattened by external force
31
Q

applanation - complicating factors

A

validity of law that sphere is: perfectly spherical, dry, perfectly flexible, infinitely thin

cornea fails all criteria: aspherical, moisture creates surface tension, lack of flexibility, cct = 0.54 mm

modified imbert-fick law:
W + S = Pt Al + B
W = P when Al = 7.35

32
Q

2 types of applanation

A

Constant force: force is constant so area is measured

constant area: area is constant so forces is measured

33
Q

applanation - constant force

A

prototype - maklakov tonometer

  • flat bottom weights handled by loosely fitting guide
  • dry paste spread on bottom of weight
  • weight lowered onto anesthetized cornea
  • remove weight and apply bottome to special paper
  • measure diameter of disturbed area

not corrected for: surface tension, ocular rgidity, tear encroachment on disturbed area

volume displacement requires conversion table

34
Q

applanation - constant area

A

prototype: goldmann applanation tonometer
- gold standard for clinical measurement

standard constant area diameter of 3.06 mm (force applied in gm x 10 = mmHg)

35
Q

goldmann tonometry drops

A

fluress: fluorescein and benoxinate

flucaine = sodium fluorescein + proparacaine

36
Q

goldmann technique

A

anesthetized cornea

instill NaFl

rotate prism

dim room lights

check for staining

set mag 16x

introduce blue cobalt filter

swing slit lamp beam to ~60 degrees

open slit beam fully

rotate force drum to reasonable IOP (10 mmHg)

blink, then beyond ear

advance slit lamp

center biprism in front of cornea

blue central area = flattened cornea
green arcs = stained tear fluid around edge

adjust force on drumand measure criterion:
inner edge of 2 semicircles must line up, mean = point where inner edges touch

37
Q

sources of error - Goldmann

A

semicircles (mires):

  • if too wide = falsely high IOP (suggest lacrimation, wipe biprism dry and repeat)
  • if too thin = falsely low IOP (spread fluorescein/ apply drop of fluress)
  • improper vertical alignment = falesly high IOP

corneal variables:
-thin cornea = falsely low IOP
- thick cornea = falsely high IOP (if due to stromal thickness/ if edema = low)
- inc. corneal curvature = inc. iop
-marked corneal astig: WTR -> underestimate
ATR -> overestimate
min. error by rotating biprism to pt’s cyl axis IF astig > 3D

prolonged contact/repeated measurements:
-decrease in IOP due to inc. lid fissure widening, tonographic effect, can also cause injury to cornea

38
Q

sources of error - goldmann (cont)

A

eyelash caught b/n cornea and biprism:
- sig. increase IOP

increased venous pressure:

  • increased IOP
  • tight clothing

pressure on patient’s globe by examiner

  • touching globe increase IOP
  • attempted lid closure will raise IOP
39
Q

Goldmann - disinfection

A

Important necause HIV and Hepatitis virus

soak biprism for 10 min in bleach (diluted 1:10) or 3% H2O2
do not soak tip in alcohol

40
Q

other applanation tonometers

A

perkins applanation tonometer (hand-held goldmann type)

mackay-marg tonometer

tonopen

non-contact tonometer (NCT)

41
Q

applanation: perkins tonometer

A

same biprism as goldmann

can be used in H or V position

42
Q

applanation: mackay -marg tonometer

A

force measured = required to keep end of plunger flush with surrounding sleeve

effect of corneal rigidity is transferred to sleeve so plunger only reads IOP

1.5 mm dia plunger mounted to stiff spring

plunger protudes 10 um flat sleeve

force required to keep plate flush with sleeve is electronically monitored and recorded

43
Q

applanation: mackay-marg tonometer technique

A

touch tip cornea and advance

tracing reflects force required to keep plunger flush with sleeve

1st tracing rises plunger supports IOP and corneal bending force over its face

tracing crests when applanation area reaches 1.5 mm dia

tracing falls (as corneal bending force transferred to sleeve)

initial trough occurs when cornea dia. of 6 mm is flattened (sleeve bearing all of corneal bending force, IOP = from baseline to bottom of 1st trough)

tracing rises again (due to artificial elevation of IOP)

when withdrawn, pattern reversed

readings instantaneous, so average readings

applanation dia. when corneal bending force transferred to sleeve is large

44
Q

applanation: cavitron biotronics tonometer

A

replaced mackay-marg in 1979
less than 27 mmHg -> underestimates IOP
more than 27 mmHg -> overestimates IOP

45
Q

applanation - palmscan proton tonometer

A
  • hand-held applanation
  • pneumo-tonometer
  • generates small flow of air
  • can measure IOP in any orientation
  • measures on scared or imperfect corneas
  • independent of corneal curvature
46
Q

applanation - tonopen

A

uses mackay-marg principle

1.02 mm dia central plunger extends 5 um beyond 3.22 mm diameter footplate

voltage change digitized, stored and analyzed

improper waveforms rejected

acceptable waveforms stored

mean iop displayed in mmHg

useful for: infants, young children, wheelchair bound, head tremors, nystagmus, bedside, eyelid swelling, irregular astig, irregular corneas, corneal edema, corneal scarring

overestimate IOP with low IOP
understimate IOP with high IOP

if number is less than 80% repeat measurement

47
Q

applanation - non contact tonometer

A

puff of air produces force that flattens cornea

flattening measure and converted to IOP

48
Q

tonometry indentation: measuring depth of indentation by constant force

A

schiotz tonometer:
footplate - concave
plunger
needle/scale

49
Q

schiotz description

A

at 0: plunger protrudes 1/20 mm beyond footplate

a 5.5 gm weight permanetly fixed to plungle and weight can be increased

  • pt looks up, start with 5.5 gm weight read scale, add second weight for reading
  • weight of tonometer the actual IOP to higher level

IOP with tonometer in position = actual IOP + scleral rigidity E

-concept: volume displacement change in pressure from p0 to pt is expression of resistance eye offers to displacement of voume of fluid

50
Q

schiotz basic concept - friedenwald nomogram

A

used to establish E based on 2 tonometer readings with different weights

slope of line = E
y-int = log Po

51
Q

schiotz technique

A
  • anesthetize cornea
  • place pt in supine position
  • have pt fixate overhead target
  • separate eyelids
  • rest footplate on cornea
  • average ocular pulsations
  • note cale reading
52
Q

schiotz - sources of error

A

assumption all eyes respond same indentation is not true

ocular rigidity: conversion tables based on average E, high E yields falsely high IOP, low E yields fasely low IOP

blood volume alteration

corneal influences: steeper/thicker cornea can falsely high IOP

53
Q

indentation tonometers (others)

A

pneumatic tonometer

  • 4.4 mm dia footplate indents cornea
  • gas builds up enough P escape through port in center of footprint
  • low IOP = overestimated
  • high IOP = underestimated
54
Q

rebound tonometry

A
  • measure deceleration speed correlated to IOP

icare tonometer: small, portable 8.8 oz

  • electromagnetic device records rate of deceleration on rebound
  • bounces magnetized probe off cornea, detects deceleration of probe by eye
  • deceleration is more rapid if IOP high
  • deceleration slower of IOP low
  • device tends to overestimate IOP compared to goldmann
55
Q

Icare notes/assessment/reliability

A

-be perpendicular to cornea
-lateral groove should be horizontal
- need to be 4-8mm from cornea
-reliability:
P solid = most reliable

p bottom = next reliable

p middle = p flashes and line in middle

p top = least reliable

error messages: e o2 = instrument too far
e o4 = instrument slanted downward (probe too fast)

56
Q

rebound tonometer

A

home version for patient to monitor

57
Q

dynamic contour tonometer

A

may reduce errors associated with CCT and with biomechanical property changes induced by laser corneal refractive surgery

contoured cylindrical tip: concave surface with radius of 10.5 mm, contact surface of ca. 7 mm, miniaturized piezoreistive pressure sensor 1.7 mm built flush into contact surface, detects stain due to applied presssure

58
Q

dynamic contour (cont)

A

curvature of tip approximate shape cornea assumes if pressure within and outside eyeball were identical and if no forces were thus acting perpendicularly on the cornea

no tangential or net bending forces are acting within area of cornea touching tip

tonometer tip rests on cornea with constant appositional force of 1 g

tip must rest on at least 3 sec

ocular pulse amplitude also measured (opa = difference between min and max of values of pulsatile IOP wave contour)

DCT is self calibrating
and independent of CCT

1 = optimal, 2-3 = acceptable, 4-5 = unacceptable

59
Q

dynamic contour (sources of error)

A

DCT tip not corrected centered on eye

pt has extremely flat cornea adheres to entire tip surface

sensorcap not properly mounted

data quality poor (q>3) due: poor patient cooperation, too short a time on cornea, low OPA (<1 mmHg)

excessive lacrimation will result in low DCT readings and no real oscillation

60
Q

Other tonometry

A

1) proview phosphene tonometer: based on principle that pressure applied to sclera can produce self-perceptible visual phenomenon

when phosphene produced, pressure is read from scale on side of device

poor agreement with GAT

61
Q

comparison of tonometers

A

most precise = manometry

most accurate in eyes with regular cornea = goldmann (generally accepted standard)

perkins applanation tonometer: compares favorably with goldmann, accurate in H or V position, useful with infants and children

schiotz tonometer: read lower than goldmann consistently

mackay-marg type tonometer: highly significant correlation with goldmann, reads systemically higher than mean of the two

nct: reliable within normal iop ranges, disadvantages - reliability decrease in higher iop range, reliability limited by abnormal cornea or poor fixation
advantages: eliminates potential hazards of contact, can be reliably used by paraprofessionals, useful in mass screening

pneumatic tonometer: close correlation with goldmann, statistically higher readings than goldmann

rebound tonometer: tend to overestimate IOP compared to GAT, no anesthetic useful in mass screening

dynamic contour tonometer: mean DCT measurements higher than mean GAT, DCT closer to true intracameral pressure, DCT measurements statistically independent of corneal thickness

mackay-marg type tonometer: most accurate on scarred or edematous cornea/ irregular cornea

Optometry Clinical Skills 3 (8 decks)

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