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What is the lens power test (biometry) before cataract surgery?

When the cloudy natural lens is removed in cataract surgery, the artificial lens that replaces it must have the right power for your eye, just as spectacles must have the right number. The lens power test, also called biometry or the A-scan test, is how that power is chosen. It is one of the most important steps before surgery. For an overview of cataract care, see cataract surgery in Vasco, Goa.

Why does the new lens need a power?

Your natural lens does a large part of the eye's focusing. Once it is removed, the artificial lens must take over exactly. Eyes differ: a shorter eye tends to be long-sighted and needs a stronger lens; a longer eye tends to be short-sighted and needs a weaker one. Even a small error in the lens power shows up as a spectacle power after surgery. So each eye is measured on its own.

What does the test measure?

• The length of the eye (axial length), from the front of the cornea to the retina at the back. At ReNew Vision this is measured with A-scan biometry, which uses ultrasound, much like a scan elsewhere in the body.
• The curve of the cornea (keratometry), the clear front window of the eye, which also bends light.
• Sometimes other measurements, such as the depth of the front part of the eye.

These numbers go into a formula that gives the lens power. The surgeon also decides, with you, what the lens should aim for: usually clear distance vision, sometimes near. For a toric lens, the amount and direction of astigmatism are also taken into account. Read choosing a lens for cataract surgery.

How is the test done?

1. You sit comfortably, and numbing drops are put in the eye.
2. For the curve of the cornea, you look at a target light in a machine while readings are taken. Nothing touches the eye.
3. For the length of the eye, a small ultrasound probe is placed gently on the front of the numbed eye for a few seconds while you look straight ahead. Several readings are taken to make sure they agree.
4. Both eyes are usually measured, so the two can be compared.

The test takes a few minutes. Because of the numbing drops, most people feel only a light touch. Do not rub your eyes for about half an hour afterwards, until the numbness wears off.

How should I prepare?

• Contact lenses: if you wear them, stop wearing them before the test for as long as advised, usually at least a week for soft lenses and longer, often several weeks, for hard (rigid) lenses, because lenses change the shape of the cornea for a while. Ask us how long for your type of lens.
• Past eye surgery: tell us if you have had LASIK or any other laser or surgery for spectacle power, or any other eye surgery. Bring old records if you have them, because the formula is adjusted for eyes that have had LASIK.
• Old spectacle prescriptions: bring them if you have them; past powers can help the surgeon.
• Dry eye: a very dry eye can affect the readings. If you have dry eye, you may be given drops to use before the test.
• Avoid eye make-up on the day.

How accurate is the lens power test?

Today's measurements and formulas let the surgeon choose the lens power closely for most eyes, but no measurement is perfect. Many people still have a small spectacle power after surgery, which is corrected with spectacles. Very long or very short eyes, and eyes that have had LASIK, are harder to predict. That is one reason new spectacles are checked a few weeks after surgery, and why the surgeon may use the first eye's result when planning the second; see the gap between surgery on both eyes.

What other tests are needed before cataract surgery?

• A complete eye check-up: vision, spectacle power, eye pressure, the lens and cornea under a microscope, and the retina after dilating drops. See eye check-up in Vasco.
• Blood sugar and blood pressure checks, especially if you have diabetes or high BP.
• A fitness note from your physician, if your general health needs it.

The tests are part of what decides the overall cost; read what decides the cost of cataract surgery.

The lens power test is done at ReNew Vision Eye Hospital, Vaddem, Vasco da Gama, after the cataract check-up. The eye OPD is open Monday to Saturday, 4 pm to 7 pm. Call or WhatsApp 072489 88039.

Questions patients ask

What is an A-scan test for cataract?

It is an ultrasound measurement of the length of the eye. Together with the curve of the cornea, it is used to work out the power of the artificial lens before cataract surgery.

How long does the biometry test take?

The measurements take a few minutes. If it is done on the same day as a full eye check-up with dilating drops, the whole visit takes longer.

Is the lens power test painful?

Numbing drops are used, so most people feel only a light touch of the probe for a few seconds. Do not rub your eyes for about half an hour afterwards.

Should I stop wearing contact lenses before biometry?

Yes, usually for at least a week for soft lenses and longer, often several weeks, for hard (rigid) lenses, because contact lenses change the shape of the cornea for a while. Ask us how long to stop for your type of lens.

Can I drive after the lens power test?

The numbing drops alone do not stop you seeing, but if your pupils are dilated for an eye check-up on the same day, your vision will be blurred for a few hours. Then do not drive; come with someone.

This page is general eye-health information, not personal medical advice. Your eye doctor advises you after examining your eyes.