What is the correct method for measuring intraocular pressure (IOP)?

How to Measure Intraocular Pressure Correctly

Standard Measurement Technique

Goldmann applanation tonometry (GAT) remains the gold standard for measuring IOP in patients with normal corneas, performed at the slit lamp with topical anesthesia before gonioscopy. 1

Proper GAT Technique

• Perform measurement before gonioscopy to avoid artificially lowering IOP from lens pressure on the globe 1
• Apply topical anesthesia to the cornea 1
• Use a bright, narrow slit beam at the slit lamp 1
• Adjust the tonometer prism until the inner edges of the semicircular mires "just touch" 1
• Take measurements with the patient in a sitting position, as body position affects IOP readings 2

Critical Limitations of GAT

GAT measurements are significantly affected by corneal properties and can be highly inaccurate in diseased or surgically altered corneas. 1 The following factors introduce measurement artifacts:

• Central corneal thickness (CCT): Thin corneas (common in African Americans, mean 534 μm) underestimate true IOP, while thick corneas overestimate it 1
• Corneal hydration and edema 1
• Corneal curvature and astigmatism 1
• Irregular epithelial surface 1
• Stromal scarring 1
• Post-keratorefractive surgery (especially LASIK), where IOP may be significantly underestimated 1

Alternative Tonometry Methods for Abnormal Corneas

When corneal disease, edema, scarring, or post-surgical changes are present, alternative tonometry methods that are less influenced by corneal properties should be used instead of GAT. 1

Recommended Alternative Devices

Pneumotonometer:

• Uses a 5-mm fenestrated silicone tip that conforms to the cornea 1
• Generates 40 readings per second and measures ocular pulse amplitude 1
• Requires topical anesthesia 1
• Less influenced by corneal thickness than GAT 1

Dynamic Contour Tonometer (Pascal):

• Utilizes a concave piezoresistive sensor tip that causes corneal relaxation 1
• Samples IOP 100 times per second 1
• Calculates IOP independent of corneal properties 1
• Requires 6 seconds or 6 ocular pulse cycles for measurement 1
• Mounted to slit lamp, requires topical anesthesia 1

Ocular Response Analyzer:

• Uses bidirectional applanation with collimated air pulse 1
• Measures corneal biomechanical properties (hysteresis) 1
• Calculates "corneal-compensated" IOP 1
• Does not require topical anesthesia 1

Rebound Tonometry (iCare):

• Fires a magnetized plastic-tipped probe against the cornea 1
• Analyzes deceleration rate to calculate IOP 1
• Requires 6 measurements for accuracy 1
• Does not require topical anesthesia 1
• Consistently underestimates IOP by approximately 4.2 mmHg compared to GAT, which has significant clinical implications for glaucoma management 3

Non-contact Air-Puff Tonometry:

• Suitable for community screening 4
• Typically reads 2.72 mmHg higher than GAT on average 4
• Readings are higher than GAT in 74% of patients, especially when true IOP exceeds 24 mmHg 4

Special Clinical Situations

Corneal Edema During Acute Angle Closure

• Gonioscopic visualization may be impaired by corneal edema 1
• Apply topical glycerin to temporarily clear the cornea for better visualization 1
• Defer CCT measurement until after resolution of the acute attack 1

Penetrating Eye Injury

IOP measurement is absolutely contraindicated in penetrating eye injuries, as any pressure on the globe can cause extrusion of intraocular contents through the wound. 5

• Both contact and non-contact methods apply force that can disturb an open globe 5
• Assess for signs of hypotony or hypertony through visual inspection only 5
• Examine anterior chamber depth indirectly as a pressure indicator 5
• Defer all IOP measurement until after surgical repair 5
• Post-repair, use alternative tonometry if corneal irregularity exists 5

Key Clinical Pitfalls

Measurement Consistency

Use the same tonometry technique consistently at every visit to detect clinically meaningful IOP changes over time. 1 Switching between devices introduces variability that can mask true IOP elevation or falsely suggest progression.

CCT Considerations

• Measure CCT when evaluating glaucoma patients, as it affects IOP interpretation 1
• No validated nomogram exists to correct GAT readings for CCT 1
• Rely on optic nerve examination, imaging (OCT of nerve/RNFL), and visual field testing rather than attempting to "correct" IOP for CCT 1
• Thinner CCT is an independent risk factor for glaucoma beyond its effect on IOP measurement 1

Post-Keratorefractive Surgery

• GAT significantly underestimates true IOP after LASIK or other corneal refractive procedures 1
• Use pneumotonometry, dynamic contour tonometry, or ocular response analyzer in these patients 1

Device-Specific Awareness

• Handheld dynamic contour tonometer shows smaller intra- and inter-examiner variability than Perkins or Tono-Pen XL 6
• iCare consistently underestimates IOP across all pressure ranges and CCT values 3
• Air-puff tonometry overestimates compared to GAT, particularly at higher pressures 4