Tonometry Methods & Artifacts

Goldmann Applanation Tonometry

Goldmann applanation tonometry is still the clinical gold standard. Mounted on a slit lamp, it works from the Imbert-Fick law: for an ideal, thin-walled sphere, the pressure inside equals the external force needed to flatten its surface divided by the flattened area. Fluorescein in the tear film glows under cobalt blue light, producing semicircular mires whose overlap marks the applanation endpoint. The device applanates a fixed 3.06 mm corneal diameter because at that diameter the outward force from corneal rigidity balances the inward pull of tear film surface tension. The two effects cancel, so the applied force reflects true IOP directly. That calculation assumes a standard central corneal thickness of about 520 µm.

Alternative Tonometry Devices

When Goldmann isn't practical, or the cornea won't give it a reliable reading, several other devices fill the gap:

• Perkins tonometer: a portable, battery-powered handheld version of Goldmann, useful under anesthesia or for patients who can't sit at a slit lamp.
• Tono-Pen: a portable handheld applanation device useful when no slit lamp is available or the cornea is irregular.
• Pneumatonometer: a gas-filled sensor with a flexible silicone diaphragm that conforms to irregular, scarred, or edematous corneas where Goldmann is unreliable. IOP is derived from the air pressure needed to keep the diaphragm flat.
• Schiotz tonometer: an older indentation device. A weighted plunger sinks into the cornea, and the depth of indentation converts to an estimated IOP via a scale.
• Rebound (iCare) tonometry: a lightweight magnetized probe briefly touches the cornea and bounces back, without requiring topical anesthetic, which makes it especially useful in children.
• Non-contact (air-puff) tonometry: a calibrated air pulse flattens the cornea with no instrument contact at all; an optical sensor detects maximal flattening.
• Dynamic contour tonometry (Pascal): uses a contour-matched sensor tip and direct piezoresistive pressure sensing rather than applanation, which makes it largely independent of corneal thickness, curvature, and rigidity. It was designed specifically to sidestep the CCT artifact that affects Goldmann.

Corneal Artifacts

Because Goldmann assumes a 520 µm cornea, any real deviation from that thickness skews the reading in a predictable direction. A thicker-than-average cornea resists applanation more and produces a falsely elevated reading; a thinner cornea, for instance one that has had LASIK, deforms too easily and produces a falsely low reading. Pachymetry measures central corneal thickness directly and helps with interpreting a reading that looks off. The ocular response analyzer goes a step further, using bidirectional air-puff applanation to measure corneal hysteresis: the viscoelastic damping of corneal tissue, calculated from the pressure difference between the inward and outward applanation events. Hysteresis is thought to correlate independently with glaucoma progression risk.