What is the proper technique to measure intraocular pressure using a tonometer?

Proper Technique for Measuring Intraocular Pressure with a Tonometer

Use Goldmann applanation tonometry (GAT) as your standard technique for measuring IOP in patients with normal corneas, but switch to alternative methods when dealing with diseased, surgically altered, or abnormal corneas where GAT becomes unreliable. 1

Standard Technique: Goldmann Applanation Tonometry

For patients with healthy, normal corneas:

• Apply topical anesthesia to the cornea
• Mount the device to a slit lamp
• Position the patient comfortably at the slit lamp with chin and forehead properly placed
• Use a bright, narrow beam that illuminates the measurement area
• Look for the "just touching" inside edges of the semicircular mires through the Goldmann applanation prism tip - this is the subjective endpoint
• Take the measurement when the mires are properly aligned

Critical caveat: GAT becomes highly inaccurate when corneal properties are altered by disease, surgery, or abnormal anatomy. Factors that compromise GAT accuracy include: 1

• Altered corneal thickness or hydration
• Corneal curvature changes or astigmatism
• Irregular corneal epithelial surface
• Corneal stromal scarring

Alternative Techniques for Abnormal Corneas

When corneal abnormalities exist, strongly consider these alternative methods: 1, 2

Rebound Tonometry (Preferred for Convenience)

• No topical anesthesia required - major advantage
• Uses a magnetized plastic-tipped metal probe fired rapidly against the cornea (0.25 m/sec)
• Requires 6 measurements for accuracy
• Software analyzes deceleration rate and contact time (~0.05 seconds)
• More influenced by corneal properties than some alternatives, but may still estimate "true IOP" more accurately than GAT in diseased corneas 1

Dynamic Contour Tonometry (Most Cornea-Independent)

• Calculates IOP independent of corneal properties - key advantage
• Requires topical anesthesia and slit lamp mounting
• Uses piezoresistive sensor sampling IOP 100 times/second
• Concave tip causes corneal relaxation to minimize corneal influence
• Requires 6 seconds or 6 ocular pulse cycles for measurement 1, 2

Pneumotonometer

• Requires topical anesthesia
• Uses pneumatic sensor with 5-mm fenestrated silicone tip that conforms to cornea
• Generates 40 readings/second
• Also measures ocular pulse amplitude 1

Ocular Response Analyzer

• No topical anesthesia required
• Uses collimated air pulse causing bi-directional corneal applanation
• Measures corneal biomechanical properties (hysteresis)
• Calculates "corneal-compensated" and GAT-equivalent IOP 1

Non-Goldmann Applanation Tonometer

• Requires topical anesthesia
• Uses 1-mm micro-strain gauge transducer
• Measures 500 samples/second, averages 8-10 readings
• Outer ring flattens adjacent cornea to reduce its influence 1

Critical Clinical Principle

Use the same technique consistently from visit to visit - this is essential for detecting clinically meaningful IOP elevations over time. 1, 2 Early detection of elevated IOP allows timely initiation of IOP-lowering therapy before irreversible optic nerve damage occurs, which directly impacts patient morbidity and quality of life.

Common pitfall: Relying solely on GAT in diseased or post-surgical corneas frequently results in undetected secondary IOP elevation, leading to preventable optic nerve damage. 1