The ophthalmometer (keratometer) was invented by Hermann von Helmholtz in 1853. It empirically estimates, but does not directly measure, the central corneal power. It reads 2 points in the 2.8- to 4.0-mm zone. A simple vergence formula used in computing the corneal power in this region is then utilized to calculate the radius of curvature. Results are reported as radius of curvature in millimeters or refracting power in diopters.
The 2 basic keratometers are the Helmholtz type and the Javal-Schiøtz type. The former is more commonly used. The Helmholtz type is a 1-position device in which the image size is adjustable; the examiner aligns the “plus sign” and “minus sign” mires (Fig 2-12). The Javal-Schiøtz type is a 2-position instrument in which the object size is adjustable; the examiner aligns mires resembling a red square and a green staircase.
The manual keratometer has certain limitations. It measures only a small region of the cornea without providing information about the cornea central or peripheral to these points. Keratometry assumes that the cornea has a symmetric spherocylindrical shape with a major and minor axis separated by 90°. It also does not account for spherical aberration, and it is susceptible to focusing and misalignment errors. Finally, if the cornea is irregular, distortion of the mires reduces the accuracy of the measurement.
Despite these drawbacks, the manual keratometer provides accurate information for most patients. It is a valuable instrument in ophthalmology for measuring astigmatism. Clinically, corneal curvature data are used primarily for contact lens fitting, IOL calculations, and planning for corneal refractive surgery. However, it is not accurate for IOL power calculations in patients who have undergone previous corneal refractive surgery. It is also helpful for detecting irregular astigmatism, which is visible as distortion or irregularity in the appearance of the mires. Clinicians can also use the keratometer dynamically by comparing the measurements in primary gaze with those in upgaze. Steepening of the measurements in upgaze is an early sign of keratoconus.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.