Why do I Need a Scan?

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What is A Scan Biometry?

An A Scan is a highly precise measurement of the eye used to help us choose the correct lens implant power before cataract surgery. Our office uses state of the art technology with the Zeiss IOL Master, a totally noncontact instrument. This A Scan device measures the length of the eye in addition to the shape of the surface of the eye, and the dimensions of other structures in the eye. It then runs calculations using multiple different formulas to help us predict which intraocular lens implant power will provide the best outcome for the patient.

How is an A Scan performed?

The patient places his/her chin on a chinrest and is asked to look at a small target. No anesthetic is needed as nothing touches the eye. The IOL Master, the instrument that performs the A Scan, is then focused on the eye and several measurements are taken. The IOL Master uses a process called interferometry, where a single light source is split into two wavelengths with mirrors, then directed into the eye and the pattern generated is used by a computer to provide measurements of the eye. This is a completely noninvasive test that poses no risk to the patient.

Will I feel anything during the A Scan?

No. This measurement does not touch the eye and is completely painless.

Is an A Scan required for a Premium Lens Implant?

When cataract surgery is performed, the natural lens in the eye is removed and replaced with a man-made intraocular lens implant (IOL). Regardless of the type of lens implant used, standard or Premium, measurements of the eye are needed before surgery. These measurements are highly precise, and combined with special computerized calculations, allow the surgeon to choose the lens implant power that is appropriate for each eye. Because each eye has a different size and shape, the power in the lens implant needed is different depending upon the many measurements of that eye. For this reason, an A Scan is always required prior to cataract surgery.

How reliable are the A Scan measurements?

The A Scan measurements are highly precise. The length of the eye can be measured with an accuracy of within 0.02mm. Nonetheless, if the patient’s eye is a little dry, or there is any inconsistency in the measurements taken at a particular visit, I will frequently bring patients back for an additional set of measurements to be absolutely sure of their accuracy. We do not bill the patient or insurance for additional measurements, but it allows me to know that I am providing the best result possible.

Patients who wear contact lenses often need to have several A Scan measurements. This is because contact lenses change the shape of the surface (the cornea) of the eye slightly. It takes some time for the cornea to “unmold” from the contact lens, so we always take several measurements of contact lens wearers to make sure that the cornea is fully unmolded before cataract surgery. This provides the best outcome. Patients who wear soft contacts usually take about a week without contact lens use to unmold, and rigid gas permeable contact lens wearers take about a month.

Is the A Scan covered by insurance?

Yes, an A Scan is always needed before cataract surgery, so it is covered by insurance. When we feel a repeat scan is needed, we do not bill the patient or the insurance.

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Tamara B. Wyse, M.D.

Tamara B. Wyse, M.D.Dr. Tamara Wyse is a board certified ophthalmologist with advanced fellowship degrees. She is a fellow of the American Academy of Ophthalmology and a clinical instructor on the faculties of both Northwestern University Medical Center and the University of Chicago.

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Northwestern Memorial Hospital The University of Chicago Medical Center Northwestern Memorial Hospital