His readings run higher than his true pressure. A thicker-than-normal cornea makes an applanation tonometer report an artificially high number. The average central cornea is about 550 micrometers, or 0.55 millimeters. At 615 micrometers his cornea is thick. So a reading of 24 mm Hg may not mean his eye pressure is truly 24. His real pressure is likely lower than the dial showed.
Applanation works by flattening a small circle of cornea. The instrument assumes an average cornea. Extra tissue resists flattening, so the dial must be pushed further to flatten it. That extra push is read as extra pressure. A thin cornea does the opposite and reads artificially low. That is why a thin cornea can hide real glaucoma. Thickness does not simply drop out of the math, and dilating the pupil does nothing to the reading. This is the whole reason pachymetry is done on glaucoma patients and glaucoma suspects. It lets the doctor read the pressure number correctly. A thick cornea can make a healthy eye look like it needs drops. A thin cornea can make a sick eye look safe. One number, corneal thickness, changes how the doctor reads every pressure that follows.
Carry two pairs into the exam room. Thick cornea, pressure reads too high. Thin cornea, pressure reads too low. Measure the center of the cornea and hold the probe straight on to the surface. Record the number in micrometers for each eye. The doctor reads it next to the pressure, never on its own. Knowing what corneal thickness does to a pressure reading is a key part of the COA supplemental testing content area.
Incorrect Answers
- A. This is the pattern of a thin cornea, not a thick one. His 615 micrometers sits well above the 550 micrometer average.
- C. Corneal thickness clearly shifts applanation readings in both directions. If it did not, no one would measure it in glaucoma suspects.
- D. Dilating drops open the pupil. They do not correct a tonometer, and pressure is normally checked before any drops go in.
Testing Pearls
- Average central corneal thickness is about 550 micrometers, and the edge is near 1.00 millimeter.
- Thick cornea gives a falsely high pressure; thin cornea gives a falsely low one.
- Pachymetry is ordered for glaucoma suspects and before refractive surgery.
- Ultrasonic pachymeters are used more often than optical ones, and they touch the cornea.
- Corneal swelling from a corneal dystrophy also thickens and clouds the cornea.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 10, Adjunctive Tests and Procedures.
