Measuring Intraocular Pressure Accurately

Standard Measurement Technique

The American Academy of Ophthalmology recommends Goldmann applanation tonometry (GAT) as the gold standard for measuring IOP in patients with normal corneas, performed at the slit lamp with topical anesthesia before gonioscopy 1, 2
Measurement should be performed before gonioscopy to avoid artificially lowering IOP from lens pressure on the globe 1, 2
Topical anesthesia should be applied to the cornea before measurement 3, 4
A bright, narrow slit beam at the slit lamp should be used for measurement 1
The tonometer prism should be adjusted until the inner edges of the semicircular mires "just touch" 3, 4

GAT measurements are significantly affected by corneal properties and can be highly inaccurate in diseased or surgically altered corneas 3, 4
Central corneal thickness (CCT) affects GAT measurements, with thin corneas underestimating true IOP and thick corneas overestimating it 5, 6
Corneal hydration and edema, corneal curvature and astigmatism, irregular epithelial surface, and stromal scarring can also introduce measurement artifacts 3, 4
Post-keratorefractive surgery, IOP may be significantly underestimated by GAT 5, 6

The American Academy of Ophthalmology recommends alternative tonometry methods, such as pneumotonometer, dynamic contour tonometer (Pascal), ocular response analyzer, and rebound tonometry (iCare), for patients with corneal disease, edema, scarring, or post-surgical changes 3, 4, 7
Pneumotonometer uses a 5-mm fenestrated silicone tip that conforms to the cornea and generates 40 readings per second, measuring ocular pulse amplitude 3, 4, 7
Dynamic contour tonometer (Pascal) utilizes a concave piezoresistive sensor tip that causes corneal relaxation and samples IOP 100 times per second, calculating IOP independent of corneal properties 3, 4, 7
Ocular response analyzer uses bidirectional applanation with collimated air pulse and measures corneal biomechanical properties (hysteresis), calculating "corneal-compensated" IOP 3, 4, 7
Rebound tonometry (iCare) fires a magnetized plastic-tipped probe against the cornea and analyzes deceleration rate to calculate IOP, requiring 6 measurements for accuracy 4, 7

In cases of corneal edema during acute angle closure, gonioscopic visualization may be impaired, and topical glycerin can be applied to temporarily clear the cornea for better visualization 1, 2
IOP measurement is absolutely contraindicated in penetrating eye injuries, as any pressure on the globe can cause extrusion of intraocular contents through the wound 8
In post-repair cases, alternative tonometry should be used if corneal irregularity exists 8

Consistency in tonometry technique is crucial to detect clinically meaningful IOP changes over time, and switching between devices introduces variability that can mask true IOP elevation or falsely suggest progression 3, 4, 7
CCT should be measured when evaluating glaucoma patients, as it affects IOP interpretation, and no validated nomogram exists to correct GAT readings for CCT 5, 6
Rely on optic nerve examination, imaging (OCT of nerve/RNFL), and visual field testing rather than attempting to "correct" IOP for CCT 5, 6
Thinner CCT is an independent risk factor for glaucoma beyond its effect on IOP measurement 5, 6
GAT significantly underestimates true IOP after LASIK or other corneal refractive procedures, and alternative tonometry methods should be used in these patients 5, 6

Thick corneas (>570 μm) cause Goldmann applanation tonometry to overestimate true IOP, while thin corneas (<555 μm) cause it to underestimate true IOP, according to the American Academy of Ophthalmology 9, 10, 11
The World Glaucoma Association explicitly recommends against using correction factors to adjust IOP values in individual patients, due to the lack of a generally accepted correction formula 9, 10, 12

Instead of correcting IOP for CCT, use CCT as an independent risk stratification tool, as recommended by the American Academy of Ophthalmology 9, 10, 11
Risk of developing POAG was greater in eyes with CCT <555 μm compared to CCT ≥588 μm, according to the Ocular Hypertension Treatment Study (OHTS) and the European Glaucoma Prevention Study (EGPS) 10, 11

The relationship between thin CCT and glaucoma progression shows mixed evidence, with the Early Manifest Glaucoma Trial (Level I evidence, 255 patients) showing that thin CCT is a risk factor for progression in patients with baseline IOP ≥21 mmHg 9, 10
Corneal hysteresis appears to provide additional independent risk information beyond CCT alone, according to the American Academy of Ophthalmology 9, 10