IOP Flashcards
IOP
IOP - fluid pressure inside eye
determined by tonometry
important in glaucoma – routine screening when the pressure is too high for the eye
patient explanation to measure IOP
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
aqueous humor
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
why measure IOP - decisions
tonometry: contributes to diagnosis of glaucoma
usefule in making decision to treat patients: IOP > 30 usually treated
useful evaluation of tx/pt management
Review: normal IOP?
upper limit of normal = 21 mmHg
(5-7% of population >21 mmHg)
4 mmHg or more of IOP asymmetry suggests pathology
what influences IOP
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
long term influences - genetics and fmaily history
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
long term influences - age
IOP increase with age, mostly accounted for increase in BP with age
7 glaucoma-related traits
central corneal thickness (CCT)
IOPcc (mmHg)
IOPg (mmHg)
Corneal hysteresis (mmHg)
IOPp (mmHg)
Pulsatility of blood velocity (kH2)
Pulsatility of blood flow (H2U)
Long term influences - gender
IOP of female is slightly higher than males
older females have greater increase in mean IOP with age
Long term influences - race
mean IOP in blacks greater than that for whites
long term influences - refractive error
increase in IOP with increasing axial length
increase in IOP with increasing myopia
long term influences - seasonal variation
mean IOP highest in winter, lowest in summer
small variation
short term influences - heart beat (ocular pulse)
influx of blood into eye with cardiac cycle
associated with arterial pulse
mean ocular pulse ~3 mmHg
short term influences - breathing
due to change in systemic BP with respiration
~ 1-2 mmHg variation
short term influences - diurnal variation
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
short term influences - systemic conditions
increased IOP in:
- systemic htn
- diabetes
- obesity
- inc. pulse rate
- hypothyroidism
- hypoosmotic state
- acromegaly
- hyperthermia
- inc. Hb
- certain meds
short term influences - sytemic conditions
decrease iop:
- acute hypoglycemia
- hyperthyroidism
- myotonic dystrophy
- ipsilateral cartoid artery disease
- hyperosmotic states
- horner’s syndrome
- HIV infections
- pregnancy
- certain meds
short term influence s- intraocular conditions
increased IOP: ocular trauma, neovascularization, lens (phacolytic)
decreased IOP: anterior uveitis, rhegmatogenous retinal detachment
short term influences - postural variation
supine position increase IOP (0.3 - 6 mmHg)
short term influence - total body inversion
increased IOP
5 min can 16 mmHg increase
short term influences - lid/eye movement
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
short term influence - pressure on globe can raise IOP by variable amounts
can raise iop variable amount
short term influences - exertion
iop may increase or decrease depending on type of exertion
prolonged running/bicycling can produce decrease
straining can cause increase in IOP
short term influences - food and drugs
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
aqueous outflow
2 major outflow
1) conventional (75%): TM -> schlemm’s canal ->episclerial veins
2) nonconventional (25%) uveoscleral (suprachoroidal) pathway
short term influences - episcleral venous pressure
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
Types of tonometry
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
Manometry: direct measurement of IOP
- Direct measurement of IOP
- Method: eye is cannulated, cannula connected to water column
- clinically impractical (done when dead)
applanation: flattening portion of cornea
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
applanation - complicating factors
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
2 types of applanation
Constant force: force is constant so area is measured
constant area: area is constant so forces is measured
applanation - constant force
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
applanation - constant area
prototype: goldmann applanation tonometer
- gold standard for clinical measurement
standard constant area diameter of 3.06 mm (force applied in gm x 10 = mmHg)
goldmann tonometry drops
fluress: fluorescein and benoxinate
flucaine = sodium fluorescein + proparacaine
goldmann technique
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
sources of error - Goldmann
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
sources of error - goldmann (cont)
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
Goldmann - disinfection
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
other applanation tonometers
perkins applanation tonometer (hand-held goldmann type)
mackay-marg tonometer
tonopen
non-contact tonometer (NCT)
applanation: perkins tonometer
same biprism as goldmann
can be used in H or V position
applanation: mackay -marg tonometer
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
applanation: mackay-marg tonometer technique
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
applanation: cavitron biotronics tonometer
replaced mackay-marg in 1979
less than 27 mmHg -> underestimates IOP
more than 27 mmHg -> overestimates IOP
applanation - palmscan proton tonometer
- 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
applanation - tonopen
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
applanation - non contact tonometer
puff of air produces force that flattens cornea
flattening measure and converted to IOP
tonometry indentation: measuring depth of indentation by constant force
schiotz tonometer:
footplate - concave
plunger
needle/scale
schiotz description
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
schiotz basic concept - friedenwald nomogram
used to establish E based on 2 tonometer readings with different weights
slope of line = E
y-int = log Po
schiotz technique
- anesthetize cornea
- place pt in supine position
- have pt fixate overhead target
- separate eyelids
- rest footplate on cornea
- average ocular pulsations
- note cale reading
schiotz - sources of error
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
indentation tonometers (others)
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
rebound tonometry
- 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
Icare notes/assessment/reliability
-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)
rebound tonometer
home version for patient to monitor
dynamic contour tonometer
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
dynamic contour (cont)
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
dynamic contour (sources of error)
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
Other tonometry
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
comparison of tonometers
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
