6.1 Tonometry & pachymetry

Measuring Intraocular Pressure

Tonometry is the measurement of intraocular pressure (IOP), the fluid pressure inside the eye produced by the balance between aqueous humor production and its outflow. Normal IOP ranges from 10 to 21 mmHg, and elevated pressure is the single most important modifiable risk factor for glaucoma. As a COT you will perform tonometry many times each day, so mastering the principles, instruments, and error sources is essential for accurate, repeatable numbers that the physician can trust.

Goldmann Applanation Tonometry (GAT)

Goldmann applanation tonometry is the gold standard. It mounts on the slit lamp and works on the Imbert-Fick principle, which states that the pressure inside a sphere equals the force needed to flatten a portion of its surface divided by the flattened area (P = F/A). Goldmann chose a flattening diameter of 3.06 mm because at exactly that diameter the cornea's structural rigidity and the tear film's surface tension cancel each other out, so the measured force corresponds directly to the true IOP.

Procedure: instill a topical anesthetic (such as proparacaine) combined with fluorescein dye, then view the cornea through the cobalt-blue filter. The doubling prism splits the illuminated tear meniscus into two glowing green semicircles called mires. Turn the calibrated dial until the inner edges of the two mires just touch, forming a smooth S-shape as they pulse gently. Read the number on the dial at that endpoint and multiply it by 10 to obtain the IOP in mmHg — a dial reading of 1.6 equals 16 mmHg.

Common errors to avoid:

• Too much fluorescein produces thick mires and a falsely high reading; too little dye gives thin mires and a falsely low reading.
• Pressure on the globe from the eyelids, the patient squeezing, or the examiner holding the lids can raise the reading artificially.
• Corneal astigmatism greater than about 3 diopters distorts the mires into unequal shapes. Rotate the prism so the red reference mark aligns with the negative cylinder axis, or average two readings taken 90° apart.
• Repeated applications deplete the tear film and can abrade the epithelium, so limit re-checks.

Other Tonometry Methods

• Non-contact (air-puff) tonometry flattens the cornea with a calibrated pulse of air. It needs no anesthetic or dye, which makes it useful for screening and for staff who do not perform contact tonometry, but it is more affected by corneal properties and is less accurate at higher pressures.
• iCare rebound tonometry bounces a tiny disposable probe off the cornea and measures its deceleration. It requires no anesthetic, is very fast, and works well in children and anxious patients.
• The Tono-Pen is a handheld electronic applanation device that averages several taps and reports a mean with a reliability percentage. It needs anesthetic and a disposable latex-free tip cover, and it excels at the bedside, in wheelchairs, and on scarred or irregular corneas.

Pachymetry and Corneal Thickness

Pachymetry measures central corneal thickness (CCT), normally about 540–560 µm. CCT matters because applanation tonometry assumes an average thickness, so any deviation biases the reading:

• A thin cornea underestimates the true IOP — the number looks reassuringly low while the real pressure may be higher.
• A thick cornea overestimates the true IOP — the number looks alarmingly high while the real pressure may be normal.

This is why CCT is a critical part of evaluating ocular hypertension and glaucoma risk. The Ocular Hypertension Treatment Study established thin CCT as an independent predictor of progression to glaucoma. Ultrasound pachymetry uses a small contact probe (with anesthetic) held perpendicular to the central cornea, while optical and OCT-based pachymetry are non-contact. Always record the value so the physician can interpret the IOP in context; many clinics document both the raw IOP and a thickness-adjusted estimate.

Practical Technique and Safety

Seat the patient at the slit lamp with the forehead firmly against the band and the chin in the rest, and instruct them to look straight ahead, breathe normally, and avoid squeezing or holding their breath, all of which transiently raise IOP. Set the cobalt-blue illumination at low-to-medium magnification with the beam angled about 60°. Advance the prism until it just contacts the cornea; over-indenting flattens too much and inflates the reading, while an incomplete touch reads low. Take the measurement quickly to avoid drying the cornea, and if the patient blinks, re-wet with a fresh drop rather than pressing harder.

Calibration and disinfection protect both accuracy and safety. Check GAT calibration periodically with the calibration bar, especially if readings seem to drift or the physician questions a value. Between patients, disinfect a reusable prism exactly per manufacturer instructions — for example a dilute sodium hypochlorite or hydrogen peroxide soak followed by a water rinse and thorough drying — or use single-use disposable tips to prevent transmission of adenovirus and other pathogens. Never touch the tip surface that contacts the eye, and always record which eye was measured and the time of day, since IOP has diurnal variation.

Tonometry Methods at a Glance