Adjusting for Corneal Thickness
Why Corneal Thickness Matters for IOP Accuracy
Goldmann applanation tonometry was calibrated on an average corneal thickness of about 540 to 550 micrometers. Deviations from this norm alter the force needed to flatten the cornea and therefore skew measured pressure. Thick corneas resist indentation and can produce falsely high readings that may prompt unnecessary therapy. Thin corneas deform more easily, masking true pressure elevation and delaying treatment. Understanding this biomechanical bias is essential to set realistic target pressures and to avoid over or undertreating glaucoma suspects.
How Thickness Alters Tonometry Readings
Increased stromal thickness raises measured values because more force is required to applanate the cornea, while reduced thickness lowers readings below the true intraocular pressure. The effect is not perfectly linear and depends on corneal hysteresis, curvature, and hydration, but central corneal thickness remains a practical correction factor in everyday practice. Recognizing that a patient with 480 micrometers and a pressure of 18 mm Hg may be at higher risk than the number suggests can change your management plan. Conversely, a patient with 600 micrometers and pressures in the low twenties may not need immediate escalation if other risk markers are absent.
Pachymetry Methods and Best Practices
Ultrasound pachymetry provides quick point measurements by gently touching the cornea with a calibrated probe after topical anesthesia. Optical methods using OCT or Scheimpflug imaging offer non contact measurements and corneal maps that are useful after refractive surgery or when edema is suspected. For reliability, take multiple readings and average them, ensuring perpendicular probe placement and avoiding areas of epithelial disruption. Document the device used and the exact values so future comparisons remain meaningful.
Using Nomograms and Formulas Wisely
Adjustment charts such as Ehlers or Dresdner nomograms translate central corneal thickness into approximate pressure corrections. No single formula is universally accepted because corneal biomechanics vary, so treat these numbers as guidance rather than absolute truth. Apply corrections to contextualize risk, then integrate optic nerve evaluation, retinal nerve fiber layer data, and visual fields before changing therapy. When in doubt, set a lower target pressure for very thin corneas and confirm stability with structural and functional testing.
Clinical Decision Making and Follow Up
Incorporate pachymetry at baseline for all glaucoma suspects and ocular hypertensives, then repeat if corneal status changes due to edema, keratoconus, or refractive surgery. Adjust target pressures rather than simply adding or subtracting millimeters of mercury to a single reading. Reassess fields and OCT periodically to ensure that risk adjusted targets are preventing progression. Clear documentation of the corrected interpretation prevents future providers from misreading the raw tonometry value.
Documentation, Patient Education, and Communication
Record central corneal thickness for each eye, the correction factor applied, and the rationale for any target pressure changes. Explain to patients that their corneal thickness influences the pressure number and that treatment decisions consider more than a single reading. Provide written summaries so they understand why therapy may start or stop even if the raw pressure seems normal. Consistent messaging builds trust and supports adherence to follow up schedules and medication plans.