IOP

Question 1

IOP

Answer 1

IOP - fluid pressure inside eye

determined by tonometry

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

Question 2

patient explanation to measure IOP

Answer 2

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

Question 3

aqueous humor

Answer 3

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

Question 4

why measure IOP - decisions

Answer 4

tonometry: contributes to diagnosis of glaucoma

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

useful evaluation of tx/pt management

Question 5

Review: normal IOP?

Answer 5

upper limit of normal = 21 mmHg

(5-7% of population >21 mmHg)

4 mmHg or more of IOP asymmetry suggests pathology

Question 6

what influences IOP

Answer 6

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

Question 7

long term influences - genetics and fmaily history

Answer 7

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

Question 8

long term influences - age

Answer 8

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

Question 9

7 glaucoma-related traits

Answer 9

central corneal thickness (CCT)

IOPcc (mmHg)

IOPg (mmHg)

Corneal hysteresis (mmHg)

IOPp (mmHg)

Pulsatility of blood velocity (kH2)

Pulsatility of blood flow (H2U)

Question 10

Long term influences - gender

Answer 10

IOP of female is slightly higher than males

older females have greater increase in mean IOP with age

Question 11

Long term influences - race

Answer 11

mean IOP in blacks greater than that for whites

Question 12

long term influences - refractive error

Answer 12

increase in IOP with increasing axial length

increase in IOP with increasing myopia

Question 13

long term influences - seasonal variation

Answer 13

mean IOP highest in winter, lowest in summer

small variation

Question 14

short term influences - heart beat (ocular pulse)

Answer 14

influx of blood into eye with cardiac cycle

associated with arterial pulse

mean ocular pulse ~3 mmHg

Question 15

short term influences - breathing

Answer 15

due to change in systemic BP with respiration

~ 1-2 mmHg variation

Question 16

short term influences - diurnal variation

Answer 16

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

Question 17

short term influences - systemic conditions

Answer 17

increased IOP in:

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

Question 18

short term influences - sytemic conditions

Answer 18

decrease iop:

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

Question 19

short term influence s- intraocular conditions

Answer 19

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

decreased IOP: anterior uveitis, rhegmatogenous retinal detachment

Question 20

short term influences - postural variation

Answer 20

supine position increase IOP (0.3 - 6 mmHg)

Question 21

short term influence - total body inversion

Answer 21

increased IOP

5 min can 16 mmHg increase

Question 22

short term influences - lid/eye movement

Answer 22

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

Question 23

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

Answer 23

can raise iop variable amount

Question 24

short term influences - exertion

Answer 24

iop may increase or decrease depending on type of exertion

prolonged running/bicycling can produce decrease

straining can cause increase in IOP

Question 25

short term influences - food and drugs

Answer 25

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

Question 26

aqueous outflow

Answer 26

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

Question 27

short term influences - episcleral venous pressure

Answer 27

``` 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

Question 28

Types of tonometry

Answer 28

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

Question 29

Manometry: direct measurement of IOP

Answer 29

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

Question 30

applanation: flattening portion of cornea

Answer 30

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 ```

Question 31

applanation - complicating factors

Answer 31

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

Question 32

2 types of applanation

Answer 32

Constant force: force is constant so area is measured constant area: area is constant so forces is measured

Question 33

applanation - constant force

Answer 33

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

Question 34

applanation - constant area

Answer 34

prototype: goldmann applanation tonometer - gold standard for clinical measurement standard constant area diameter of 3.06 mm (force applied in gm x 10 = mmHg)

Question 35

goldmann tonometry drops

Answer 35

fluress: fluorescein and benoxinate flucaine = sodium fluorescein + proparacaine

Question 36

goldmann technique

Answer 36

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

Question 37

sources of error - Goldmann

Answer 37

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

Question 38

sources of error - goldmann (cont)

Answer 38

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

Question 39

Goldmann - disinfection

Answer 39

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

Question 40

other applanation tonometers

Answer 40

perkins applanation tonometer (hand-held goldmann type) mackay-marg tonometer tonopen non-contact tonometer (NCT)

Question 41

applanation: perkins tonometer

Answer 41

same biprism as goldmann can be used in H or V position

Question 42

applanation: mackay -marg tonometer

Answer 42

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

Question 43

applanation: mackay-marg tonometer technique

Answer 43

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

Question 44

applanation: cavitron biotronics tonometer

Answer 44

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

Question 45

applanation - palmscan proton tonometer

Answer 45

- 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

Question 46

applanation - tonopen

Answer 46

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

Question 47

applanation - non contact tonometer

Answer 47

puff of air produces force that flattens cornea flattening measure and converted to IOP

Question 48

tonometry indentation: measuring depth of indentation by constant force

Answer 48

schiotz tonometer: footplate - concave plunger needle/scale

Question 49

schiotz description

Answer 49

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

Question 50

schiotz basic concept - friedenwald nomogram

Answer 50

used to establish E based on 2 tonometer readings with different weights slope of line = E y-int = log Po

Question 51

schiotz technique

Answer 51

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

Question 52

schiotz - sources of error

Answer 52

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

Question 53

indentation tonometers (others)

Answer 53

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

Question 54

rebound tonometry

Answer 54

- 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

Question 55

Icare notes/assessment/reliability

Answer 55

-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)

Question 56

rebound tonometer

Answer 56

home version for patient to monitor

Question 57

dynamic contour tonometer

Answer 57

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

Question 58

dynamic contour (cont)

Answer 58

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

Question 59

dynamic contour (sources of error)

Answer 59

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

Question 60

Other tonometry

Answer 60

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

Question 61

comparison of tonometers

Answer 61

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