Keratometry: what to expect

• Keratometry is an eye test that measures the curvature of the cornea, the transparent outermost layer of the eye.
• Keratometry is mainly used to measure the extent and direction of astigmatism, which is important for determining the eye’s prescription during routine examination.
• Data from keratometry can also be used to guide contact lens fitting, diagnose corneal conditions, and plan eye surgeries like LASIK.

The cornea is the clear, outermost dome at the front of the eye and plays a key role in focusing light onto the retina so that we can see clearly. For good vision, the cornea needs to be smooth, evenly shaped, and transparent. Keratometry is a non-invasive eye test that measures the curvature (shape) of the cornea1 , to better understand the focusing power of the eye and its prescription – astigmatism in particular.

The main purpose of keratometry is to assess the shape of the cornea and diagnose astigmatism. Astigmatism is a very common type of refractive error (focusing error) that usually occurs because of an irregularly shaped cornea – imagine the surface of a rugby ball, compared to the spherical surface of a tennis ball. An irregular cornea causes light passing through the eyeball to be processed in an uneven manner, which leads to blurred or distorted vision at all distances.

Measuring astigmatism is important when determining the eye’s prescription for glasses or contact lenses, as correcting blur from astigmatism can result in significant improvements to vision and comfort. Assessing the shape of the cornea is essential when fitting contact lenses, to help ensure the lenses are compatible with the eye’s shape, and sit comfortably on the eye.

Keratometry measurements can be occasionally be used to diagnose eye conditions that affect the shape of the cornea, such as keratoconus. In some cases, keratometry data is also used as part of calculations for intraocular lens (IOL) power in patients who require cataract surgery, and evaluate a patient’s suitability for vision correction surgery.

Keratometry is a quick, safe, and painless test that is often performed as part of a standard eye examination.

Prior to the test, you will be seated in front of a specialised instrument known as a keratometer. Your chin and forehead will rest on the machine’s supports, to keep the head steady during the testing process. Your eye care professional will ask you to look to focus on a series of images inside the keratometer for a few seconds while the measurements are obtained.

The test does not involve any physical contact, and no eye drops are required. Patients of all ages find it comfortable and easy to have keratometry performed.

Keratometry measurements are sometimes called ‘K-readings’. These are measurements that describe how curved the cornea is, how it bends light, and the direction of the cornea’s curvature.

For example: 45.00D @ 180 / 46.50D @ 090

Keratometry readings simply indicate how curved the cornea is along a particular direction. In this example, “45.00D @ 180” means that the cornea bends light at a strength of around 45 units called diopters (D), along a direction of 180 degrees (horizontally). If the cornea has astigmatism, there will be two sets of readings – one for the steepest part of the cornea, and another for the flattest part. In this example, the second reading is “46.50D @ 090”.

These numbers may seem complex and unfamiliar, but there is no need to worry about interpreting them yourself. Your optometrist or eye doctor will interpret the readings and use them to guide the right treatment decisions.

Keratometry is a standard part of a routine eye examination and provides a reliable basic measure of astigmatism and corneal shape. This information is useful in various ways, including:

• Assessing the extent and direction of astigmatism
• Determining the prescription for glasses and/or contact lenses
• Determining the correct fit for contact lenses
• Planning eye surgeries such as cataract surgery and LASIK
• Identifying signs of an abnormally-shaped cornea such as in keratoconus

When a more comprehensive assessment of the cornea is required, your optometrist may recommend an additional test called corneal topography. Corneal topography produces a full, comprehensive map of the cornea, allowing for more precise assessment and management in situations such as:

• Fitting orthokeratology (OK) lenses
• When a corneal condition is suspected
• Monitoring changes in the eye over time
• Before certain eye treatments or procedures