The normal cornea is about 1.00 millimeter thick at its outer edge. Corneal thickness is not the same everywhere. The center is the thinnest part, at about 0.55 mm, or 550 micrometers. The tissue then thickens steadily toward the edge. So the edge measures close to twice the central value. A micrometer is one thousandth of a millimeter and is written µm.
Pachymetry, sometimes spelled pachometry, is the measurement of corneal thickness. Knowing the normal pattern keeps the technician from calling a normal edge reading abnormal. It also explains why the probe must be centered on the cornea when a central value is wanted. An off-center probe drifts toward thicker tissue and reads too high. Ultrasonic pachymeters, which use reflected sound waves, are the type used most often. Optical pachymeters attach to the slit lamp and measure between the epithelium and the endothelium, the outer and innermost cell layers. Some ultrasonic probes are wired to a tabletop unit, and others are pen-like and portable.
Carry two numbers into the exam room: 0.55 mm at the center and 1.00 mm at the edge. Thickening in a swollen cornea, as in a corneal dystrophy, shows up clearly against those values. Report the reading in the unit the doctor uses, and say where on the cornea you measured. Knowing normal corneal thickness and how to record it is part of the biometry content area of the COA exam.
Incorrect Answers
- A. About 0.25 mm is less than half the central thickness. No part of a healthy adult cornea measures that thin, and such a reading suggests probe or machine error.
- B. About 0.40 mm would mean the cornea thins toward the edge. The normal cornea does the opposite, growing thicker as the probe moves outward.
- C. About 0.55 mm is the central value given in the question. Thickness is not uniform, so the same number cannot be the expected edge value.
Testing Pearls
- Normal cornea: about 550 µm (0.55 mm) centrally and about 1.00 mm peripherally.
- 1 micrometer is one thousandth of a millimeter, so 550 µm equals 0.55 mm.
- Ultrasonic pachymeters use reflected sound and are used more often than optical ones.
- Center the probe: an off-center reading lands on thicker tissue and reads high.
- Pachymetry is ordered for corneal dystrophies, before surgery, and for glaucoma suspects.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 10, Adjunctive Tests and Procedures.
