Tonometry estimates intraocular pressure (IOP) indirectly from the force needed to deform the cornea
Applanation tonometers flatten a small area (e.g. Goldmann). Indentation tonometers push a plunger into the cornea (e.g. Schiøtz)
Normal IOP is about 10–21 mmHg (mean about 16 mmHg). It varies through the day by about 3–6 mmHg, often highest in the early morning

## Goldmann applanation

Imbert–Fick law: P = F ÷ A, for an ideal sphere with an infinitely thin, dry and flexible wall
The cornea is none of these. Its rigidity resists flattening, and the tear film's surface tension draws the prism towards it. At an applanation diameter of 3.06 mm the two cancel out
Worked example: the area is π × 1.53² = 7.35 mm². A force of 1 g (0.0098 N) over 7.35 mm² is 1,334 Pa = 10 mmHg. So the dial reading in grams × 10 gives the IOP in mmHg
The bi-prism splits the fluorescein-stained tear meniscus into two semicircles offset by 3.06 mm. When their inner edges just touch, the applanation diameter is 3.06 mm
Applanation displaces only about 0.5 µl, so it barely raises the IOP and is little affected by ocular rigidity
For more than 3D of corneal astigmatism, align the red mark on the prism holder with the minus cylinder axis, or average readings with the prism horizontal and vertical. Uncorrected, with-the-rule astigmatism reads low and against-the-rule reads high, by about 1 mmHg per 4D

## Sources of error

Central corneal thickness (CCT): thick corneas read high and thin corneas read low. Estimates vary from about 0.2 to 0.7 mmHg per 10 µm, so no single correction formula is reliable
Corneal oedema reads low (soft cornea) despite being thick. Scarring reads high
After myopic laser refractive surgery the cornea is thinner and weaker, so IOP reads low
Too much fluorescein (wide mires) reads high; too little reads low
Pressure on the globe, squeezing the lids, a tight collar or breath-holding reads high
Repeated measurements gradually reduce the IOP (tonographic effect)
Calibrate the tonometer regularly. Disinfect the prism or use disposable prisms (adenovirus, prions)

## Other tonometers

Tonometer	Principle	Notes
Perkins	Hand-held Goldmann applanation with a counterbalanced prism	Works in any position: supine patients, children, theatre
Tono-Pen	Mackay–Marg: a strain gauge in a small central plunger; readings averaged electronically	Portable; useful on scarred or irregular corneas and through bandage contact lenses
Rebound (e.g. iCare)	A light magnetised probe is fired at the cornea; it decelerates faster at higher IOP	No anaesthetic; good for children and screening; home self-tonometry versions
Non-contact (air-puff)	An air jet flattens the cornea; the moment of applanation is detected by maximum reflection of light from the flat cornea	No contact or anaesthetic; good for screening; may produce tear aerosol
Ocular Response Analyzer	An air-puff tonometer recording two applanations, inward and outward	Their difference is corneal hysteresis, a measure of viscoelastic damping. Low hysteresis is linked with glaucoma progression
Dynamic contour (Pascal)	A concave tip matches the cornea's shape; a pressure sensor in the tip measures IOP directly	Less affected by CCT; also gives the ocular pulse amplitude
Schiøtz	Indentation: a weighted plunger (5.5, 7.5, 10 or 15 g) indents the supine cornea; the scale reading converts to IOP with tables	Displaces a larger volume, so it depends on ocular rigidity

Low rigidity (high myopia, after scleral buckling or with intravitreal gas) reads falsely low on Schiøtz
High rigidity (hypermetropia, older age) reads falsely high

CCT matters for applanation (see Goldmann Tonometry). A thin cornea with a "normal" IOP can hide glaucoma; a thick cornea with a raised IOP may be ocular hypertension

Measure CCT with ultrasound or optical pachymetry when interpreting IOP (see Pentacam)
In children, rebound tonometry is often possible without anaesthetic; examination under anaesthesia lowers IOP with most agents (ketamine raises it slightly)
Digital palpation is only a rough guide, e.g. to detect a very high or very low IOP