The gap between the two K readings gives the amount of corneal astigmatism. Keratometry measures the front surface curve of the cornea in two meridians. A meridian is one line drawn across the cornea, like a clock hand. This cornea is 43.50 diopters in its flattest meridian and 45.75 diopters in its steepest meridian. Subtract the smaller from the larger. The cornea has about 2.25 diopters of corneal astigmatism.
Astigmatism means the cornea is not curved the same in every direction. It is shaped more like the back of a spoon than a ball. Keratometry gives an objective, measured number for that shape. It reports the meridian of maximum curve and the meridian of minimum curve. It also reports the axis of each meridian in degrees. Here the steep meridian sits at 90 degrees and the flat one at 180 degrees. A diopter is the unit of lens power, so a K reading is a power, not a distance. Keratometry cannot give eye length, corneal thickness, or a cell count. Many offices use an automated keratometer, often built into an automated refractor. The manual instrument is also called an ophthalmometer, and both report the same two powers and axes.
Read every K reading as two powers with two axes. The difference between the two powers is the corneal astigmatism. Those numbers guide contact lens fitting and are required before refractive surgery. Taking and recording accurate K readings is a core skill in the biometry content area of the COA exam.
Incorrect Answers
- B. Axial length is the front-to-back length of the eye, from cornea to retina. It is measured with A-scan ultrasonography and reported in millimeters, not with a keratometer.
- C. Central corneal thickness comes from pachymetry, which is reported in micrometers rather than diopters. A keratometer never touches the eye or measures depth.
- D. Endothelial cells are counted from photographs taken by specular microscopy, which magnifies the innermost corneal cell layer. Cell counts are numbers of cells, not diopters of power.
Testing Pearls
- Keratometry measures the front corneal curve in two meridians and reports the axis of each.
- The difference between the two K powers is the amount of corneal astigmatism.
- K readings are recorded in diopters, for example 43.50 x 180 / 45.75 x 90.
- K readings are used for contact lens fitting, intraocular lens power, and refractive surgery planning.
- A keratometer is also called an ophthalmometer; automated versions are common in clinic.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 5, Optics and Refractive States of the Eye.
