Tonometry in Ophthalmology

What is Tonography?

The procedure of recording measurements (as of intraocular pressure) with a tonometer. It is detected by C value 0.22-0.28 ul/min/mmHg.

What is tonometry?

Tonometry is a procedure which is used to measure the intraocular pressure (IOP) or the pressure inside the eye. 

Indentation tonometry

• Schiotz tonometry: Friedenwald nomogram is used to record the value
• Reading depends on scleral rigidity and therefore it is less reliable.
• Scleral rigidity is less in children, myopic patients.
• Scleral rigidity is more in hypermetropic patients.

Also read: Pediatric Image-Based Questions for NEET PG

Applanation Tonometry

• Applanation tonometer is based on Imbert Fick Law, P= Force/Area
• Applanation tonometer is divided into
• Variable force Applanation tonometer (Area is fixed and the area is 3.06 mm diameter) - E.g., Goldman applanation tonometer.
• Fixed force Applanation tonometer

Goldmann Applanation Tonometer

Here the periphery of the cornea is not affected. Fluorescein dye is applied before placing the applanation tonometer. Two semicircles are seen when the cornea is touched

• Reading is measured when the inner margin of two semicircles touches each other.
• IOP measurement depends on Central Corneal Thickness (CCT).
  • Thick Cornea → Overestimate IOP
  • Thin Cornea → Underestimate IOP
  • 10 microns change in CCT means 0.7mmHg of IOP change.

Variable Force Tonometers

1. Goldmann applanation tonometer→ Gold standard
2. Perkins (hand held version of goldmann)

3. Draeger (measures Wide range of IOP)

Draeger Tonometer

• Draeger tonometer is similar to Perkins
• It has a different set of prisms
• It operates with a motor

4. Pulsair (non-contact tonometer)

5. Airpuff (non-contact tonometer)
6. Grolmanns (non-contact tonometer)
7. Tonopen (based on both indentation and applanation used in infants)

8. Mac kay marg (based on both indentation and applanation)
9. Rebound tonometer used for self measurement.
10. Transpalpebral Tonometer

Also read: Ophthalmology Image-based Questions for NEET PG Preparation

Types of Transpalpebral Tonometer

Digital

• Soft
• Firm (Normal)
• Hard

Diaton

Diaton is a reliable tonometer for clinical purposes.

Proview

Transpalpebral spring compressed device

Fixed Force Tonometer

MaklaKov tonometer

It has a fixed force. When the tonometer first touches & flattens the cornea, the ink is transferred to the cornea. The tonometer was then printed on a piece of paper.

Barraquer Tonometer

Manually held on the corneal. Fluid ring visible at scope

Latest Tonometer

Contact

Pascal DCT (Dynamic Contour Tonometry). Reading does not depend on Central corneal Thickness. This is the choice of instrument in Refractive surgery and post-lasik patients.

Non-contact

Ocular response analyser (ORA)

It measures the biomechanical property of the cornea when taking the difference between when

the cornea applanates in and when it bends out.  If the difference is smaller the risk of glaucoma is higher. Low value means, more risk of progression of glaucoma

Also read: NEET PG 2025: High-Yield Topics for Ophthalmology

Important One-liners 

• The facility of aqueous flow (ease of aqueous flow through outflow system) is measured by tonography. It is depicted by the C value, which is 0.22- 0.28 μl/min/mmHg.
• The most common complication of steroids in the eye is glaucoma.
• Steroid induced glaucoma is primarily due to increased secretion of GAG.
• Antiglaucoma drug; Netarsudil- increases the contractile property of trabecular meshwork and increases the trabecular outflow.
• Tonometer of choice in infants is Tonopen > perkins
• Tonometer of choice in scarred and irregular cornea is Tonopen > Mac kay marg
• Tonopen is used in patients with bandage contact lenses.
• Rebound tonometer is used for self-measurement of IOP.
• Transpalpebral tonometer is used in keratoprosthesis

IOP Changes 

• Normal IOP - 10-21 mmHg
• Normal diurnal variation is 5 mmHg.
• If diurnal variation is 5 -8 mmHg - glaucoma suspect.
• If diurnal variation is >/= 8 mmHg - glaucoma.
• IOP is more in the morning because of cortisol levels in the body.

Frequently Asked Questions

Q1. Applanation tonometer is based on?

Ans. Imbert Fick Law, P= Force/Area.

Q2. Rebound tonometer is used for?

Ans. self-measurement of IOP.

Q3. Applanation tonometer is divided into?

Ans. Variable force Applanation tonometer (Area is fixed and the area is 3.06 mm diameter) - E.g.,

Goldman applanation tonometer. Fixed force Applanation tonometer

Q4. Cup disk ratio is?

Ans. > 0.5 (Normal C/D ratio </=0.3)

Q5. What is Buphthalmos?

Ans. more pressure in the eyeball causes expansion of the eyeball called Buphthalmos. Seen in

True Congenital Glaucoma and Infantile Glaucoma

Also read: Important Clinical &amp; Non-Clinical Questions in Ophthalmology

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