6.1 Goldmann Applanation Tonometry: Technique, Sources of Error & the Imbert-Fick Principle

The physics: Imbert-Fick and the 3.06 mm circle

The Imbert-Fick principle states that for an ideal, infinitely thin, dry, flexible sphere, the pressure inside equals the force applied to flatten a surface area divided by that area. A real cornea is none of those things: it has thickness and rigidity, which resist flattening, and a tear film whose surface tension pulls the prism down, assisting flattening.

Goldmann's insight was that at an applanated diameter of 3.06 mm — an area of about 7.35 mm² — those two opposing forces approximately cancel in an average cornea. The arithmetic then becomes convenient: 1 gram of force applied to that area corresponds to 10 mmHg, so the drum reading is multiplied by 10 to give pressure in millimetres of mercury. A drum reading of 1.8 is 18 mmHg.

Technique

1. Disinfect the prism per protocol and confirm it is dry before use.
2. Instil a topical anaesthetic with fluorescein (proparacaine or oxybuprocaine with sodium fluorescein, or an anaesthetic drop plus a fluorescein strip moistened with saline).
3. Set the slit lamp to the cobalt blue filter, the widest beam, highest illumination, and approach at approximately 60 degrees from the observation axis. Magnification is usually 10×.
4. Set the drum to about 1 (10 mmHg) before contact so the first touch is gentle.
5. Ask the patient to look straight ahead, keep both eyes open, and breathe normally. Hold the lids without pressing on the globe — press on the orbital rim, not the eye. A finger on the globe adds pressure and invalidates the reading.
6. Advance until the prism just touches the apex. Two semicircular mires appear.
7. Turn the drum until the inner edges of the two semicircles just touch, forming a shallow S. Do not overlap them and do not leave a gap.
8. Record the drum reading × 10, the eye, the time, and the method.

Reading the mires

The pulsation point is worth remembering: a normal ocular pulse moves the mires with the cardiac cycle, and the correct reading is the mid-position, not the peak.

Sources of error

Falsely high readings

• Pressing on the globe while holding the lids, or a tight collar, tie or breath-holding
• Excess fluorescein producing thick mires
• Squeezing the lids
• Thick central cornea, corneal oedema in some circumstances, or corneal scarring
• Valsalva manoeuvre, or the patient straining against the headrest
• Extraocular muscle restriction pulling on the globe in upgaze

Falsely low readings

• Insufficient fluorescein producing thin mires
• Thin central cornea, including after myopic LASIK or PRK
• Repeated applanation in quick succession (massage effect)
• Corneal oedema in other circumstances, or an irregular epithelium

Central corneal thickness. Readings are biased by thickness because the assumption of average rigidity fails. As a rule of thumb, a cornea substantially thicker than the roughly 545 micrometre average reads falsely high, and a thinner cornea reads falsely low. Published correction nomograms disagree with one another and none is universally accepted, so the current standard is to record the measured intraocular pressure and the measured central corneal thickness together and let the clinician interpret them — not to enter a "corrected" number in the chart as though it were measured.

This matters most in two populations: patients after myopic laser refractive surgery, whose thinned corneas mask genuinely raised pressure, and patients with thick corneas labelled ocular hypertensive who may not be.

High astigmatism

With corneal astigmatism above about 3 dioptres, the applanated area is elliptical rather than circular and the standard 3.06 mm assumption fails.

Correction: rotate the prism so the red mark on the prism holder aligns with the axis of the flattest meridian (the minus-cylinder axis). A common alternative convention is to set the red mark to the axis of the flat meridian read on keratometry. Where great accuracy is needed, measure at both 0 and 90 degrees and average the two readings.

Contraindications and infection control

Do not applanate when there is an epithelial defect, active infectious keratitis or conjunctivitis, or a suspected open globe. Use a non-contact method or defer.

Disinfection: the blueprint names cleaning and disinfecting tonometers as an explicit task. Follow the manufacturer's instruction; commonly the prism is wiped clean of debris, soaked in a validated disinfectant such as dilute sodium hypochlorite or 3% hydrogen peroxide for the specified contact time, rinsed thoroughly with water and dried, because residual disinfectant causes a toxic keratitis. Single-use disposable prisms and disposable tip covers eliminate the reprocessing question entirely and are standard where prion or adenovirus transmission is a concern. Inspect reusable prisms for chips and cracks and remove damaged prisms from service.