How a Tonometer is used to Measure Eye Pressure

Pressure in the eye is called Intraocular Pressure (IOP). Tonometer is the instrument that is used to determine IOP. The presence of any elevated IOP can be dangerous because people with varying degree of IOP elevation may develop certain damage to the optic nerve. The optic nerve collects all the visual information from the retina and transmits the information to the brain where signals are interpreted as vision.

When changes occur in the optic nerve leading to a decrease in peripheral vision due to the loss of the nerve tissues; then the patient can be diagnosed to be suffering from Glaucoma. Glaucoma occurs when the normal fluid in front of the eye chamber, the aqueous humor, is blocked from leaving the eye during the normal aqueous turn over process. Due to this IOP, is elevated in the eye which results into certain damage to the optic nerve.

We have several types of Tonometers that are available for measuring IOP. The most accurate Tonometer measures the force needed to flatten a certain area of the cornea.

The types of Tonometers are:

1. Indentation tonometer
2. Applanation or Goldmann tonometer
3. The Airpuff non-contact tonometer

Indentation (Schlotz) Tonometer

It consists of:

• Handle for holding the instrument in vertical position on the cornea.
• A foot plate which rests on the cornea.
• Plunger which moves inside the foot plate.
• Bent lever whose short lever rests on the plunger and the long arm acts as a pointer needle.

The depth to which the plunger indents the cornea is indicated by the movement of the pointer needle on the scale. Lower IOP will have low indentation. The eye has to be anaesthetized while using tonometer. The weight of 5 gm, 7.5 gm, and 10 gm can be used in the tonometer depending on the eye pressure.

Applanation or Goldmann Tonometer

This is the most popular and accurate Tonometer. Applanation means the flattening of cornea by the application of force. Higher IOP will require more force for achieving a certain flattening. The Tonometer consists of a double prism mounted on a slit lamp. The prism is made to applanate (flatten) the cornea of the patient’s eye. The cornea is anesthetized before carrying out any measurement. The cornea and biprism are illuminated with a cobalt blue light from the slit lamp. The Applanation force against the cornea is adjusted until the inner edges of the two semicircles just touch and form a horizontal S shape as shown below:

Related: Use of Ophthalmoscope in Eye Examination

The Airpuff Non-contact Tonometer

This is a very safe technology since there is, ‘’no touch’’. It uses Airpuff to flatten the cornea. It is simple and easy method to measure IOP. The procedure does not require numbing eye drops to anaesthetize the eyes.

You can also check: Ophthalmology Resources