Articles Education Glaucoma Optometrist

TONOMETRY: The Basis for Diagnosing Glaucoma and Evaluating Therapeutic Response


AJAY SHAHI , OPTOMETRIST

Intraocular pressure (IOP):

The pressure created by the continual renewal of fluids within the eye. The normal range of IOP is 10 to 21 mmofHg.

Mechanism of IOP

It consists of:
a. Formation of aqueous humor
b. Drainage of aqueous humor

Intraocular Pressure can be measured by:

  1. Manometry
  2. Tonometry
  1. Manometry: Only direct measure of IOP

USES

  • Used for continuous measurements of IOP
  • Used in experiment, research work on animal eyes

Pitfalls

  • Not practical for Human beings
  • Needs General Anesthesia
  • Introduction of needle produces breakdown of blood aqueous barrier and release of prostaglandins which alter IOP

TONOMETRY

An indirect method of IOP measurement

Three basic types of Tonometer:

  1. Indentation
  2. Applanation
  3. Non-Contact

TONOMETRY / PALPATION METHOD

Intraocular Pressure (IOP) can be estimated by the response of the eye to pressure applied by finger pulp.

PROCEDURE
Patient looks down Index finger of both hands are used One finger (F2) is kept stationary which feels the fluctuation produced by indentation of globe by other finger (F1)

  • Indents Easily = Low IOP
  • Firm to Touch = Normal IOP
  • Hard to Touch = High IOP

Note: Not Accurate, Rough estimation of IOP especially in uncooperative patients or in absence of instruments.


The concept of digital palpation tonometry. A trained examiner uses both index finger to gauge the IOP by palpating the sclera. In the simulation experiment, forces from 2 sensors are measured at a given displacement as function of IOP. 

Source: https://www.researchgate.net

OCULAR RIGIDITY

Measurement of distensibility or resistance to deformation of ocular coats

  • Importance in indentation tonometer
  • Increase in ocular rigidity – increase IOP
  • Decrease in ocular rigidity – decrease in IOP

Corneal Rigidity

  • The ability of corneal tissue to resist deformation
  • Important in Applanation Tonometers
  • Provided by collagen lamellae – 90% of corneal thickness
  • Theoretically, average corneal rigidity (taken as 520 μm in GAT)

Classification:

Applanation Tonometer Classification

INDENTATION TONOMETRY

  • Based on fundamental fact that plunger will indent a soft eye more than hard eye.
  • Currently in Use – Schiotz
  • Was devised in 1905 and continued to refine it through 1927

BASIC CONCEPT & THEORY OF INDENTATION

  • As soon as tonometer is placed on cornea different forces come into play
  • W – Weight of tonometer
  • A – Area
  • Vc – Volume displaced after indentation
  • T – Tensile force, set up in an outer coat of eye at everywhere tangentially to the corneal surface
  • So additional force T to original baseline IOP
  • Resting intraocular pressure (PO) which is artificially raised to the new value (P1)
  • Thus the scale reading of tonometer actually measures the artificially raised IOP
  • So the additional force T to original baseline IOP
  • Resting Intraocular Pressure (Po) which is artificially raised to a new value (P1)
  • Thus, the scale reading of tonometer actually measures the artificially raised IOP
Fig: Schotiz Tonometer, Source CEHJ

Read also: How to measure IOP by Schiotz Tonometer

MERITS
• Easy to use
• Simplicity
• Low cost price

DEMERITS
• Gives false reading when used in eyes with abnormal scleral
rigidity
• False low levels of IOP with low scleral rigidity seen in high
myopes and following ocular surgeries.
• Difficult to perform in uncooperative patients Children

ERRORS (INDENTATION TONOMETER)

 Errors inherent in the instrument
• Due to difference in weight, size, shape and curvature of footplate

 Errors due to contraction of EOMs – Tends to increase IOP
 Errors due to Accommodation
• Pt. looking at Tonometer – Accommodation comes into play
• Contraction of ciliary muscles increases the facility of aqueous
outflow by pulling on trabeculae
• Thus causes some lowering of IOP
 Errors due to ocular rigidity
Errors due to variation in corneal curvature
Steeper or Thicker cornea will cause greater displacement of fluid
Causes falsely high IOP readings
Errors may arise in cases:
Microphthalmos
• Buphthalmos
• High Myopia
• Corneal Scars
 Moses Effect
At low scale reading, the cornea may mould into space between Plunger and Hole
Pushing the Plunger up and leading to a falsely high-pressure reading

APPLANATION TONOMETRY

  • The concept – introduced by GOLDMANN (1954)
  • Based in IMBERT FICK LAW
  • States that the pressure inside an ideal sphere (P) is equal to force (F)required to flatten (A)
  • P = F/A (Froce/Area)

GOLDMANN APPLANATION TONOMETER (GAT)

Most standard and Accurate Tonometer
• Named after Austrian-Swiss ophthalmologist Hans Goldmann (1899–1991) in 1950
• Consists of a double prism mounted on a slit lamp
• The prism applanates the cornea in an area of 3.06 mm diameter


Measures the force required to applanate the cornea over a circular area of diameter 3.06 mm
Applanates an area of diameter 3.06 mm for 3 reasons
• Amount of fluid displacement is negligible (Approx. 0.5 micro litre)
• Surface tension force and the force required to counteract the corneal rigidity act opposite to each other
• Tonometer force becomes equal to the force in mmHg

STERILIZATION

• CDC recommendation (HIV, HSV and Adenovirus): wipe tip clean and disinfect tip only with bleach (1:10 dilution x 5”, changed once daily).

• The alternative is 3% H2O2, changed at least twice daily (affects tip less than bleach or ETOH – ethanol or ethyl alcohol).

• Alternative #2: wiping tip with 70% ETOH

Read : How to measure IOP with Goldmann Applanation Tonometer

POTENTIAL ERRORS

Patients related
• Thin cornea
• Thick cornea
• Astigmatism
• Corneal edema

Technical
T out of calibration
• Repeated Tonometry
• Pressing on eyelids or globe
• Squeezing of the eyelids
• Observer bias (expectations and even numbers)

  • REFERENCES:
  • Kanski’s Clinical Ophthalmology
  • https://accessemergencymedicine.mhmedical.com/Content.aspx?bookId=683&sectionId=45343803
  • https://bjo.bmj.com/content/101/4/1
  • https://www.medscape.org/viewarticle/877495_transcript
  • https://www.semanticscholar.org/paper/Tonometry-through-the-ages.-KniestedtPunjabi/36b52134b74b219a50e6255f510777ba7d51caf5/figure/1
  • https://www.slideshare.net/siddarthgautam16/different-types-of-tonometry

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