Transcript Goldmann Applanation Tonometry

Goldmann Applanation
Tonometry
Ted Barnett
E.M.B.
Sept. 2000
Introduction
• Applanation tonometry: measures IOP by
providing force which flattens the cornea.
• Variable force applanation tonometers
(Goldmann, Perkins, Draeger, MacKayMarg, and Tono-Pen and
Pneumatonometer): area of the cornea being
applanated held constant, variable for
applied.
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Sept. 2000
Principles
• based on Imbert-Fick law:
pressure within a sphere (P) is roughly equal to the
external force (f) needed to flatten a portion of the
sphere divided by the area (A) of trhe sphere
which is flattened: P = f / A
• applies to surfaces which are perfectly spherical,
dry, flexible, elastic and infinitely thin
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Principles (cont.)
• include force of cornea which pushes applanating
surface away from eye (N), subtract surface
tension of tear film toward the eye (M)
• since cornea has thickness, consider only
flattening of inner corneal area (A1)
• P = f / A1 + M - N
• when A1 = 7.35, M and N cancel out so:
• P = f / 7.35 mm2
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Principles (cont.)
• this internal area achieved when diameter of
external area of corneal applanation is 3.06mm
• at this external diameter, grams of force applied
multiplied by 10 is directly converted to mmHg
• measured pressure is 3% greater than IOP before
applanation (not corrected)
• minimal displacement (0.5ul) of fluid or increase
in IOP with applanation, thus unaffected by ocular
rigidity
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Technique of measurement
• plastic biprism which contacts cornea creates two
semicircles
• edge of corneal contact is visible after placing
fluorescein into tear film & viewing with cobalt
blue light
• manually rotate the dial calibrated in grams, force
is adjusted by changing the length of a spring
within the device.
• inner margins of semicircles touch when 3.06 mm
of cornea is applanated.
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Instructions to patient
• press head firmly against chin and forehead
rest.
• look straight ahead and fixate on a target
(e.g. examiners opposite ear)
• breathe normally, do not hold your breath
• blink immediately prior to measurement to
moisten cornea.
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Measurement (cont.)
• position patient’s head with forehead rest well
above eyebrows, allowing raising of eyebrows.
• anesthetic & fluorescein (0.25%) ,separately, or as
mixture (preserved) placed in inferior cul-de-sac.
• with maximal illumination of biprism the lamp is
moved toward the eye until the tip of biprism
contacts the apex of the cornea
• stop moving forward when limbus shines with
light, best observed with naked eye
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Measurement (cont.)
• After contact, semicircles visible through left (or
right) ocular. Center in field of view.
• Adjust vertically until semicircles equal in size.
• Tension dial adjusted so that inner edge of upper
and lower semicircles are aligned.
• Multiply dial reading (grams of force) by 10 to
obtain IOP (mmHg)
• Read at median over which arcs glide to control
for excursions due to ocular pulsations.
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Measurement (cont.)
• If slit-lamp moved too far toward patient the
pressure arm will push against a spring
which will press against the eye with a low
inoffensive force.
• Mires (flattened area) too large, moving dial
doen’t alter appearance.
• Solution: Draw back until regular pulsation
noted and appearance of mires normalizes.
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Measurement (cont.)
• Blue central area represents applanated cornea,
green semicircles are fluorescein-stained tears,
inner border of ring is demarcation between
flattened and non-flattened cornea.
• Without staining of tears, bright reflection from
air-cornea interface is seen; leads to
underestimation of IOP.
• Mires should be approximately 10% of circle
width.
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Errors in Measurement
• The fluorescein ring is too wide or too narrow:
• Too wide: occurs if prism not dried after cleaning
or lids touch prism. Overestimates IOP.
Solution: dry prism
• Too narrow: inadequte fluorescein concentration
may cause hypofluorescence. Underestimates
IOP. Solution: patient blinks or additional
fluorescein added.
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Errors (cont.)
• thin corneas produces underestimate
• thick cornea d/t increased collagen gives
overestimate, if d/t edema gives underestimate.
• inadequate vertical alignment of semicircles leads
to overstimate of IOP.
• distortion d/t irregular cornea influences accuracy,
less useful with corneal scarring.
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Errors (cont.)
• squeezing of eyelids, breath holding or
other Valsalva maneuvers, pressure on
globe, excessive EOM force applied to
restricted globe, vertical gaze, tight collars,
retreating patient, inaccurately calibrated
tonometer.
• repeated tonometry may induce decline in
estimated IOP.
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Error d/t corneal curvature
• increase of 1 mmHg for every 3D increase in
corneal power.
• more fluid displaced under steep cornea, increases
contribution of ocular rigidity in overestimating
IOP.
• the steeper the cornea, the more cornea must be
indented to produce standard area of contact.
• >3D astigmatism produces elliptical rather than
circular area
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Correction for astigmatism
• With semicircles displaced horizontally, IOP
underestimated by 1 mmHg for every 4D of
WTR astigmatism, vice versa for ATR
astigmatism.
• To minimize, prisms should be rotated so that axis
of least corneal curvature is opposite red line on
prism holder (i.e. align negative cylinder axis).
• Can average reading with vertical and horizontal
alignment of prism.
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Sterilization
• CDC recommendation (HIV, HSV, and
adenovirus): wipe tip clean and disinfect tip
only with bleach (1:10 dilution x 5”,
changed once daily).
• Alternative is 3% H2O2, changed at least
twice daily (affects tip less than bleach or
ETOH).
• Alternative #2: wiping tip with 70% ETOH
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Reliability
• Goldmann applanation is standard against
which others measured.
• Good accuracy in gas-filled eyes.
• Inter- and intraobserver variability (>30%
varied by 2-3 mmHg), due to subjective
nature of optical endpoint.
• Assume error of 2 mmHg.
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Calibration: Wessels & Oh (1990)
• Tested tonometers in ophthalmologists offices.
• 19% outside range of manufacturers specifications
(1mmHg of calibration), 4.5% > 2mmHg error.
• Annual recalibration in 86% of instruments.
• Practitioners who themselves performed
calibration had the most accurate instruments.
• Less than 15% knew how to perform calibration
check.
• Calibration here done 4 times/year
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Sept. 2000