Tonometry

Tonometry is 7% of the exam. It covers measuring intraocular pressure (IOP) by applanation, rebound, and non-contact methods, and knowing which readings can be trusted. The assistant measures and records; the physician interprets.

What IOP is — the fluid pressure inside the eye, measured in millimeters of mercury (mmHg). It keeps the eye firm and holds its shape.

Where the pressure comes from — the ciliary body makes aqueous humor, the clear fluid that fills the front of the eye and nourishes the cornea and lens. Aqueous drains mainly through the trabecular meshwork into Schlemm's canal, and a smaller share leaves by the uveoscleral route (see the General Medical Knowledge guide). As in a sink with the tap running, the level holds steady while the drain keeps up and rises when drainage falls behind. Glaucoma drops lower pressure by slowing production, improving drainage, or both; the drug classes are in the Pharmacology guide.

Normal range — roughly 10–21 mmHg, with diurnal variation of a few mmHg; highest typically in the morning. Population averages sit around 15–16 mmHg.

Glaucoma is judged by the nerve — glaucoma is optic nerve damage, usually with a matching pattern of field loss (see the Visual Field Testing guide). High pressure is its main risk factor and the one that treatment can lower, but the physician diagnoses glaucoma by examining the nerve (often with OCT imaging) and testing the field, never from a pressure alone.

IOP is a risk factor, not a verdict — both ocular hypertension (a high pressure with a healthy optic nerve) and normal-tension glaucoma (nerve damage at pressures inside the normal range) exist. Asymmetry of more than 2–3 mmHg between eyes is worth flagging even when both are 'normal.' The number goes to the physician — never to the patient as a diagnosis.

Central corneal thickness — thick corneas over-read, thin corneas under-read; pachymetry (measuring corneal thickness) contextualizes every pressure. A thin cornea is also a glaucoma risk factor on its own. Chart the pressure as measured; the physician weighs the thickness (see the Supplemental Testing guide).

Pressure moves through the day — a normal afternoon reading can miss a higher morning pressure, so the physician sometimes orders checks at several times of day.

Goldmann applanation tonometry (GAT) mounts on the slit lamp. Its tip, a small clear split prism (the biprism), flattens (applanates) a tiny area of the central cornea, and the force needed, set on a drum, gives the pressure. Other tonometers are checked against it. GAT is the reference standard: anesthetic + fluorescein (combination drop or moistened strip), cobalt blue light, ×10 on the drum.

Where it fits — applanation comes after acuity and pupils, and after keratometry, optical biometry, and tear-film tests, which its anesthetic and dye would disturb. Pressure is usually checked before dilating drops; follow the office routine.

1. Prepare — explain the test and confirm drug allergies before any drop goes in. Contact lenses come out first. A lens distorts the reading, and fluorescein stains a soft lens. For applanation, the anesthetic goes in before or with the fluorescein, so the dye and the probe are comfortable and the patient does not squeeze (anesthetics are covered in the Pharmacology guide). Warn that the drop stings briefly.

2. Setup — forehead firmly on the band, chin on the rest, straight-ahead gaze. Loosen tight collars; coach normal breathing (Valsalva and breath-holding raise pressure). If lids need help, brace fingers on the orbital bone — never press the globe. Ask for both eyes open wide, then select the cobalt blue filter and swing the tonometer into place.

3. The touch — probe meets central cornea only; no lids, no lashes. Advance slowly until the tip just meets the cornea and two green semicircles, the mires, appear. Center them so the upper and lower halves look the same size; unequal halves mean the tip is off center.

4. The endpoint — turn the drum until the INNER edges of the two fluorescein semicircles just touch. Overlapping mires = too much force; separated mires = too little. The mires pulse slightly with the heartbeat; set the drum so the inner edges touch midway through each pulse.

5. Read the drum — it is marked in grams of force, so multiply by 10 for mmHg: a drum reading of 1.6 is 16 mmHg. Measure the right eye first, then the left.

6. Afterward — warn patients not to rub the numb eye for 15–20 minutes, because a numb cornea can be scratched without the patient feeling it.

Repeats — rapid-fire repeated applanation massages aqueous out and drifts the reading down. Take one good measurement.

Disinfection is the non-negotiable — tonometer tips touch tears, the classic vector for epidemic keratoconjunctivitis. Reusable Goldmann tips get a timed disinfectant soak, rinse, and dry per CDC/manufacturer protocol — or the clinic uses disposable tips. A dry wipe disinfects nothing. Follow the office protocol: wipe tears and debris off the tip first, since wiping cleans but does not disinfect, then soak it for the product's full contact time, rinse well (leftover disinfectant can burn a numb cornea), dry it, and never autoclave it (see the Microbiology and Equipment Maintenance and Repair guides).

Mire troubleshooting — thick, wide mires (too much fluorescein) over-read; thin, faint mires under-read; ragged mires in dry eye need fresh blinks and a prompt re-measure; oval mires in high astigmatism (about 3 D or more) need the biprism rotated per its markings or two readings 90° apart, averaged.

Calibration — the tonometer itself is checked on the practice's schedule with the manufacturer's check bar, and one that fails comes out of service, with the physician told (see the Equipment Maintenance and Repair guide).

Readings from different devices are not interchangeable.

Non-contact (air puff) — nothing touches the eye, no drops; ideal for screening; confirm meaningful readings by applanation. Warn the patient about the puff and its sound so they do not flinch, have them open wide and watch the target, and take several readings per eye, which are averaged. Squeezing and breath-holding raise this reading too.

Rebound (such as iCare) — a tiny bouncing probe, no anesthetic; the pediatric and home-monitoring workhorse. Hold the instrument level, aim the probe straight at the center of the cornea, and use a new single-use probe for every patient.

Handheld electronic applanation — a pen-shaped, small-tip electronic applanation tonometer, such as the Tono-Pen. It needs anesthetic, averages several readings with a confidence percentage, and is the tool for scarred and irregular corneas that ruin Goldmann mires. A fresh disposable cover goes on the tip for each patient; touch the central cornea lightly, and repeat the set rather than charting it when the confidence figure is poor.

Schiotz — weighted-plunger indentation on a supine patient; historic, but still an exam answer.

Safety — defer contact tonometry and ask the physician when there's a suspected open globe, fresh abrasion, or surface infection. A suspected open globe gets no pressure measurement of any kind, air puff and rebound included, until the physician directs. Pressure on an open eye can push its contents out, and touching an infected or scratched cornea worsens it. A patient with suspected adenovirus infection (EKC) also gets the physician's go-ahead first, with a disposable tip where possible (see the Microbiology guide).

Recording — record the TIME with every reading, plus the method and each eye ("Ta 16/18 @ 2:15 pm"). Note when glaucoma patients last took their drops — a treated 16 and a missed-dose 16 mean different things.

What cannot wait — a very high pressure, such as one in the 50s, in a red, painful eye, especially with halos, headache, or nausea, fits acute angle-closure glaucoma: the iris has suddenly blocked the drainage angle, and vision can be lost within hours. Tell the physician at once, while the patient is still in the chair. Hold any dilating drops until the physician has seen the patient. Any alarmingly high reading reaches the physician before the patient leaves.

Be ready to give the normal range, explain why one pressure does not diagnose glaucoma, tell ocular hypertension from normal-tension glaucoma, state how corneal thickness biases applanation, run Goldmann from drops to endpoint, convert a drum reading to mmHg, name what raises or lowers a reading, match each tonometer to its use, say when tonometry is deferred, disinfect a tip, and recognize the angle-closure emergency.