Intraocular pressure measurement

Intraocular pressure reflects the balance between aqueous humour production and outflow, principally through the drainage angle. Tonometry estimates that pressure from the eye's response to a mechanical interaction; it does not directly measure optic-nerve health. A raised value may occur without established glaucomatous damage, while glaucoma can develop at values that are not above a population reference range. This distinction matters whenever an apparently reassuring number is used to dismiss suspicious field loss or a damaged-looking nerve.

Goldmann applanation tonometry measures the force needed to flatten a defined area of the cornea. At the slit lamp, fluorescein and topical anaesthetic allow a prism to contact the central cornea while the examiner identifies a standard optical endpoint. NICE specifies Goldmann-type applanation for glaucoma case finding and slit-lamp-mounted Goldmann applanation as part of diagnostic assessment. A handheld applanation device can be useful when positioning is difficult, but the method and conditions still need recording. A numerical result from one instrument should not automatically be treated as interchangeable with every other instrument.

Non-contact tonometry uses an air pulse, while rebound tonometers use the behaviour of a small probe briefly contacting the cornea. Rebound measurement often needs no anaesthetic and can support selected clinical or home monitoring arrangements after training. These approaches have practical advantages, but readings can differ with corneal properties and technique. A screening air-puff result should lead to an appropriate clinical assessment rather than a stand-alone glaucoma diagnosis or referral decision. Device quality indicators help identify unreliable readings, but a favourable indicator cannot remove all clinical sources of error.

Decide whether measurement is safe before preparing the drops. Ask about trauma, corneal disease, recent surgery, medicines and previous glaucoma treatment. Inspect the eye for signs that contact or pressure might be harmful. Suspected open-globe injury is a contraindication to tonometry, including an air-puff attempt; protect the eye and seek urgent specialist care instead. An ulcerated or markedly abnormal cornea and some postoperative circumstances need expert advice about the safest method and the interpretability of any result. Do not palpate an injured eye as a substitute for an unavailable instrument.

For Goldmann measurement, explain the brief contact and position the patient comfortably at the slit lamp with stable forehead and chin support. Check the instrument's calibration status and use a compatible clean prism or appropriate single-use tip. Administer the selected anaesthetic and fluorescein correctly. Ask the patient to breathe normally, look ahead and avoid squeezing the lids. Advance the prism gently to the central cornea under observation, obtain the two fluorescent semicircles and adjust to the endpoint where their inner borders meet. Withdraw before reading and documenting the final result according to the device's scale and instructions.

Reliable technique includes noticing when the image is unsuitable. Poor alignment, an irregular tear film, excess fluorescein, corneal scarring or lid squeezing can affect the result. Supporting a lid must not put force through the globe. If a reading is implausible or discordant with the examination, correct an identifiable technical problem and repeat within an appropriate plan. Do not repeatedly applanate a painful or damaged surface just to obtain a preferred number. Significant astigmatism and abnormal corneal anatomy may need specific technique or an alternative device chosen by an experienced examiner.

Corneal thickness and biomechanics influence applanation, so pressure belongs alongside central corneal thickness and the rest of a glaucoma assessment. A thick or thin cornea should prompt thoughtful interpretation rather than an invented universal correction formula. Prior corneal refractive surgery can further complicate comparison with previous readings. Pressure also varies over time and can be affected by treatment timing and adherence. Record these circumstances when following an established patient; a change between visits may reflect disease, treatment, measurement conditions or a combination.

For an adult without acute symptoms undergoing glaucoma case finding, NICE recommends considering a repeat pressure or field assessment before referral when appropriate. Refer for optic-nerve damage, a consistent glaucomatous field defect or pressure of at least 24 mmHg on Goldmann-type applanation, with results accompanying the referral. The threshold is a case-finding criterion, not an acute angle-closure definition and not a guarantee that 23 mmHg is harmless. Urgent clinical circumstances bypass repetition on another occasion. The College of Optometrists lists pressure of 40 mmHg or more as requiring emergency assessment within twenty-four hours independently of cause; symptoms may require attention sooner.

Measurement ends with communication and follow-up. Explain whether the value is reassuring in context, needs confirmation or requires specialist assessment, and distinguish suspicion from an established diagnosis. In glaucoma monitoring, the treating service sets a target and review interval from disease severity, progression and treatment tolerance. Patients using prescribed drops should continue their existing plan unless an appropriate clinician changes it. A single lower reading should not prompt unsupervised treatment cessation, and a higher reading needs assessment of symptoms, adherence, technique and the intended target rather than automatic self-escalation.