What is a Keratometer?
The keratometer is an instrument utilized to measure the curvature of the cornea. The prefix Kerato- means ‘cornea’, and the suffix -metry means ‘to measure’. The keratometer is used to diagnose and manage corneal conditions such as keratoconus, astigmatism, or dry eye. It is also used prior to cataract surgery and to prescribe specialty contact lenses.
Key Takeaways
- The keratometer is an instrument that measures the corneal curvature and power.
- The keratometer is used for diagnosing eye conditions like keratoconus and astigmatism, for prescribing specialty contact lenses, or before cataract surgery for lens measurements.
- The average adult keratometry (K) reading is 43.00 to 44.00, and values higher than 47.00 may indicate keratoconus
Understanding Keratometer
The keratometer is an instrument that measures the cornea’s curvature. The cornea is the clear dome-shaped covering over the front of the eye that focuses light onto the retina. A keratometer projects a circle of light onto the cornea. If the circle is spherical and completely round, there is no astigmatism. If the circle is distorted or oval-shaped, the cornea has astigmatism. Astigmatism means that the corneal curvature is not the same in all directions. The keratometer allows an eye doctor to measure the curvature of the cornea, and convert that into power (diopters) of astigmatism. Measuring the corneal astigmatism and power is important when diagnosing certain eye conditions like keratoconus.
There are two different types of keratometers: manual and automatic keratometers. The manual keratometer is advantageous in that it allows for a direct visualization of the tear film reflection. However, a manual keratometer is rarely used in the clinic. Most doctors do not have manual keratometers because they take up space, are expensive and take training and time to get good results. On the other hand, the autorefractor keratometer is a machine that automatically measures the corneal curvature while having the patient look at an image like a hot air balloon. The autorefractor keratometer is easier and faster to perform, and tends to be the preferred method of keratometry in the modern day.
The autorefractor keratometer is used by having the patient place their chin onto the chinrest of the machine and look into the machine. The patient will see a target such as a hot air balloon at the end of the road. While the patient fixates on the target, the machine automatically calculates the curvature measurements of the eye, as well as an approximate prescription.
Principle Of Keratometry
The surface of the cornea reflects light like a convex mirror. The theory of the keratometer is that it measures the curvature of the cornea by measuring the image produced by the reflection of the cornea. A keratometer essentially uses the optical principles of a small reflecting telescope.
Clinical Indications of Keratometer
There are various clinical indications for using a keratometer:
- Astigmatism evaluation
Keratometers help in assessing the corneal astigmatic error. - Contact lens fitting
Keratometers help evaluate the corneal surface to create a perfectly fitting contact lens, especially for hard gas permeable contact lenses. - Corneal shape
Keratometers help monitor the shape of corneas with keratoconus and determine if the keratoconus is progressing. - Contact lens flexure
Keratometers assess the gas permeable lens flexure. - IOL (intraocular lens) power estimation
Keratometers help measure corneal curvature before cataract surgery, in order to calculate the power of the intraocular lens (IOL) that is placed in the eye during cataract surgery.
Keratometry Use For Keratoconus
Keratoconus is a progressive eye disease where the cornea becomes shaped like a cone. Keratoconus causes thinning, steepening, and bulging of the cornea. This disease tends to affect the eyes starting in puberty, and continues progressing until the thirties. It is associated with a history of eye rubbing and atopy (asthma, allergies, eczema, or atopic dermatitis). If keratoconus progresses to an advanced stage, it can cause permanently reduced vision that cannot be corrected with glasses. One sign of keratoconus that can be detected using a keratometer is corneal steepening. In keratoconus, the corneas become steeper and the keratometry reading is thus higher. The average keratometry value for a normal eye is 43.00 to 44.00, whereas eyes with keratoconus have keratometry values of 47.00 and higher.
Drawbacks Of Keratometer
There are no risks associated with doing keratometry. The keratometer tends to be very accurate in most cases and helps to guide clinical decision making. However, like all technical instruments, it may have some pitfalls.
- The keratometer measures a small area of the cornea. It only measures the central 3-4 millimeters of the cornea, while the whole cornea is 12 millimeters in diameter.
- The keratometer does not have 100% accuracy, especially if measurements are taken when blinking excessively, moving the head, or not focusing on the target.
- The keratometer cannot accurately measure corneas with irregular astigmatism, when principal meridians are not 90 degrees apart. However, the majority of people with astigmatism have “regular” astigmatism.
