Tonometry: measuring intraocular pressure
- Examination
Tonometry measures the pressure inside the eye. Normal is about 10 to 21 mmHg; a high pressure is a risk for glaucoma, and a very high one is an emergency.
See it in 3D: Cornea · Ciliary body · Iris
What it is
Intraocular pressure depends on the balance between aqueous humour production and drainage. Tonometers estimate it from the force needed to flatten or indent the cornea. Goldmann applanation at the slit lamp is the reference method. Rebound tonometers need no anaesthetic and suit children and screening. Air-puff tonometers are common in optometry. Finger palpation detects only a very hard or very soft eye.
Normal values and scales
- Normal intraocular pressure: about 10 to 21 mmHg, with a mean near 15 to 16 mmHg.
- Pressure varies through the day and is often higher in the morning.
- Thick corneas overestimate the pressure and thin corneas (including after laser refractive surgery) underestimate it.
- In acute angle closure the pressure can exceed 40 to 50 mmHg.
- Low pressure after trauma suggests an open globe or a ciliary body injury.
How it is done
- Explain the test; for applanation, instil an anaesthetic drop with fluorescein.
- Goldmann: with the cobalt blue light, bring the probe to the centre of the cornea and turn the dial until the two green semicircles just touch.
- Read the dial and record the pressure, the time of day and the method.
- Rebound or hand-held devices: hold the probe perpendicular to the central cornea and take the average of several readings.
- Do not press on the eye when an open globe is suspected; clean or use disposable tips between patients.
How to interpret it
- Many people with pressure above 21 mmHg never develop glaucoma (ocular hypertension), and some with normal pressure do (normal-tension glaucoma).
- Central corneal thickness (pachymetry) is measured to interpret a borderline pressure.
- Corticosteroids, by any route, can raise the pressure in susceptible people within weeks.
Next step / Initial management
- Pressure above 21 mmHg or a difference between the eyes: ophthalmology for disc, field and corneal thickness assessment.
- Very high pressure with pain, a red eye or halos: emergency (acute angle closure pathway).
- Patients on long-term steroids: periodic pressure checks.
- Suspected open globe: no tonometry; shield and refer.
Test yourself
Think of the answer, then tap the question to see it.
What is the normal range of intraocular pressure?
About 10 to 21 mmHg.
How does a thin cornea after laser refractive surgery affect applanation readings?
It makes them falsely low, so true pressure is underestimated.
When is tonometry contraindicated?
When an open globe is suspected, because pressure on the eye can extrude its contents.
Elsewhere in DurgiPedia
Connections
See it in 3D3
The structures of Durgical's 3D anatomy where it is found.
In Pharmacology2
Educational content; not a clinical guideline and no substitute for assessment by a health professional. For sudden visual loss, severe eye pain, a chemical burn or an eye injury, go to an emergency department at once.
Sources: Tonometry (StatPearls Publishing (NCBI Bookshelf), 2023); Applanation Tonometry (StatPearls Publishing (NCBI Bookshelf), 2023); Intraocular Pressure (StatPearls Publishing (NCBI Bookshelf), 2024).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.