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Tonometry

Description

This test estimates the fluid pressure within the eye, or intraocular pressure (IOP), with a special instrument (tonometer) that exerts gentle pressure on the cornea (the transparent disc in front of the iris and pupil of the eye). Tonometry is included as part of a full eye examination. Several techniques may be used.

Applanation (Goldmann) tonometry, considered the most accurate method, measures the amount of force necessary to flatten a specific area of the cornea. The higher the IOP, the greater the force required to flatten the cornea.

Schiotz (or indentation) tonometry uses a preset amount of weight to gently press a plunger into the cornea, making a slight indentation. The amount of corneal indentation, which is proportional to IOP, is measured.

Noncontact (or air-puff) tonometry determines IOP by blowing a small puff of air toward the eye and recording the air rebounding from the corneal surface. It is not as accurate as the other tonometry methods.

Pneumotonometry uses a special device that is applied to the side of the eye. The amount of pressure required to flatten the tip indicates pressure inside the eye.

Purpose of the Test

To measure IOP.

To aid in the diagnosis of glaucomas, vision-threatening disorders marked by an elevation in IOP that can gradually (or suddenly) destroy the optic nerve.

To detect low IOP, which can be caused by injury, inflammation, detachment of the retina, or poor blood supply to the eye.

Who Performs It

An ophthalmologist, an optometrist, or a specially trained nurse or technician.

Special Concerns

If you have an irregularly shaped or deformed cornea, a special tonometer must be used.

Before the Test

If you wear contact lenses, remove them before the test.

Loosen or remove any restricting clothing around the neck, such as a neck tie.

What You Experience

Applanation tonometry:

The examiner will administer topical anesthetic eye drops, as well as eye drops containing a special dye called fluorescein.

You will sit in a chair in front of a slit-lamp microscope, an instrument that permits the examiner to view both the front and inside of the eye. Your head will be positioned comfortably against a chin rest and padded forehead bar.

You will be asked to direct your eyes to the bottom of the examiner’s ear.

The tonometer, which is mounted on the slit lamp, is moved in front of the eye so that the tip touches the cornea. The examiner adjusts the amount of force applied until the device flattens the central cornea by a standard amount. This reading is an indication of IOP.

The test is repeated on the other eye.

The procedure takes one to two minutes.

Schiotz tonometry:

The examiner will administer topical anesthetic eye drops.

You will lie on your back on an examination table and will be instructed to look upward.

The examiner will gently retract the skin around the eye to help hold the eyelids open.

The Schiotz tonometer is a hand-held device consisting of a weight, a calibrated scale, a plunger, and a concave (curved) footplate that rests snugly on the cornea. The examiner will carefully lower the device into place, and a preset amount of weight is used to press the plunger gently into the cornea and make a slight indentation.

Corneal resistance, which is proportional to IOP, deflects the plunger upward.

The test is repeated on the other eye.

The procedure takes one to two minutes.

Noncontact tonometry:

The noncontact tonometer, as its name implies, does not come into contact with the eye so a topical anesthetic is unnecessary.

A small puff of air is blown against the cornea. A pressure sensor in the tonometer records the amount of air rebounding off the corneal surface.

The test is repeated on the other eye.

The procedure takes one to two minutes.

Pneumotonometry:

You are asked to look straight ahead.

A special tonometer directs a stream of air into the sensing tip. The amount of air pressure required to flatten the tip indicates IOP.

The instrument prints a tracing on a piece of graph paper.

Risks and Complications

All of these methods are painless and pose virtually no risk to the cornea. Any minor corneal scratches by the tonometer usually heal within 24 hours.

After the Test

If anesthetic eye drops were applied, do not rub your eyes for at least 30 minutes or until the numb sensation wears off to avoid injuring your cornea.

If you wear contact lenses, do not reinsert them for at least two hours after the test.

You may resume your normal activities.

Results

IOP is measured in millimeters of mercury (mm Hg). Normally, IOP falls between 10 and 20 mm Hg. Higher levels may indicate glaucoma or ocular hypertension (elevated IOP that has not yet caused optic nerve damage).

If IOP is normal, no further testing is needed.

If IOP is elevated, an ophthalmologist must perform other tests to confirm a diagnosis of glaucoma, including visual inspection of the optic nerve with a magnifying device called an ophthalmoscope and visual field testing, or perimetry.

If IOP is low, further testing will be needed to determine the cause of the low pressure.

 

From The Johns Hopkins Consumer Guide to Medical Tests. You can order this book now on our secure server.

 

 

 


 


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2005
WHITE PAPERS
Vision

The vision White Paper from The Johns Hopkins White Papers series is an annual, in-depth report written by Hopkins physicians.


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Johns Hopkins Consumer Guide to Medical Tests
Look up the latest information on a wide variety of preventive screening and diagnostic tests in The Johns Hopkins Consumer Guide to Medical Tests.

 

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