Goldmann Applanation Tonometry

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Goldmann Applanation Tonometry

• Ted Barnett

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Introduction

• Applanation tonometry measures IOP by providing
  force which flattens the cornea.
• Variable force applanation tonometers (Goldmann,
  Perkins, Draeger, MacKay-Marg, and Tono-Pen and
  Pneumatonometer) area of the cornea being
  applanated held constant, variable for applied.

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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 patients 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
  doent 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.
• gt3D 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
  (110 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 (gt30 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 gt 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