Keratometry

Keratometry is 2% of the COA exam. The keratometer reports how steeply the front of the cornea curves, and those readings go straight into contact lens fitting and into the power calculation for the implant chosen at cataract surgery.

What it measures — the curvature of the central ~3 mm of the anterior cornea, expressed in diopters (or mm of radius); higher diopters = steeper cornea = shorter radius. Everything outside that small central zone is assumed rather than measured. Mapping the whole corneal surface takes corneal topography.

How it works — the instrument uses the tear-covered cornea as a curved mirror. It reflects lighted targets, the mires, off the cornea and measures the size of the reflected image: a steeper cornea makes a smaller image. A doubling system splits the image in two so the examiner lines up two copies that move together, and the eye's normal small movements stop mattering.

Converting units — keratometers convert radius to power with a standard formula: diopters = 337.5 ÷ radius in millimeters.

Most adult corneas read between about 40 and 48 D and average about 43–44 D. A reading well outside that range is not automatically wrong, but it is always worth repeating.

1. Focus the eyepiece — before the first patient, turn the eyepiece all the way toward plus so the crosshair blurs, then back until it just turns sharp, the same habit as at the lensometer. Skip it and your own focusing slips into every reading.

2. Seat the patient — chin in the rest, forehead against the band, instrument level with the eye. The patient looks into the instrument at the reflection of their own eye, which holds the measurement on the visual axis.

3. Center and focus — bring the mires onto the crosshair and focus until they are crisp.

4. Find the meridians — in the common one-position design, rotate the instrument until the plus signs line up; the axis scale then shows the first principal meridian. Turn one power drum until the plus signs overlap and the other until the minus signs overlap. Each drum gives the power of one principal meridian; the second lies 90° from the first.

5. Record — each eye, right first, as power at its meridian, for example 42.00 @ 180 / 44.00 @ 090, with a note on mire quality.

Timing — keratometry comes before drops, dye, and applanation, because the mires reflect off the tear film and those steps disturb it. Have the patient blink fully a few times just before measuring.

Lids — if a lid droops into the way, lift it with a finger braced on the bony orbital rim. Pressing on the globe bends the cornea and invents astigmatism.

The mires — focus and align the reflected rings, then read the two principal meridians. The difference between them is corneal astigmatism; meridians 90° apart make it regular (with-the-rule when the vertical meridian is steeper).

Worked example — 42.00 D @ 180 and 44.00 D @ 090. The vertical meridian has more diopters, so it is steeper and has the shorter radius. The cornea carries 2.00 D of with-the-rule astigmatism.

Irregular mires — distorted, unfocusable rings point to keratoconus, scarring, or a poor tear film; have the patient blink, then re-measure before concluding anything. If they stay distorted, record the best reading you can get with a note describing the mires, and tell the physician, who may order topography.

Cornea versus the whole eye — keratometry measures only the front of the cornea. The refraction measures the whole eye, so its cylinder need not match the difference between the K readings; the rest comes mostly from the crystalline lens.

Calibration — check the instrument against steel calibration balls of known curvature on the schedule the practice sets, and log the date, the result, and your initials. A keratometer that misreads a known ball reads every patient wrong. Take it out of service, report it, and let the physician decide which recent readings need repeating.

Corneas off the scale — a very steep cornea, as in advanced keratoconus, can run past the end of the drum. Placing a +1.25 D trial lens over the instrument aperture extends the steep end; a −1.00 D lens extends the flat end. The drum then no longer reads the true power, so convert the reading with the manufacturer's table and chart that an auxiliary lens was used.

Daily care — dust cover on when idle, optics cleaned only by the manufacturer's method, and the chin rest and forehead band disinfected between patients, with the disinfectant kept off the optics.

Contact lens fitting — K readings guide the starting base curve, especially for rigid lenses. Because a rigid lens neutralizes astigmatism on the front of the cornea through the tear layer beneath it, the K difference also helps the fitter predict how well a rigid lens will correct the eye.

Implant power for cataract surgery — K readings and axial length are the two essential measurements for the lens formula. An error of 1 D in the K readings produces roughly 1 D of refractive error after surgery. Measure both eyes; right and left readings usually agree within about 1 D, and a larger difference is re-measured before anyone uses the numbers.

Contact lens wearers — lenses temporarily reshape the cornea, rigid lenses most of all. Before readings that will be used for surgery, the patient leaves the lenses out for as long as the physician directs.

After LASIK or PRK — the central cornea has been reshaped, so standard K readings mis-estimate its true power and standard formulas give a wrong answer. Flag the surgical history prominently; the surgeon uses special methods.

Be ready to say what part of the cornea a keratometer measures, relate diopters to radius, name the calibration standard, work out the amount and type of astigmatism from two K readings, explain why keratometry comes before drops and applanation, recognize irregular mires, extend the range for a steep cornea, and list where K readings are used.