PAM Test Before Cataract Surgery

Eye doctor consulting with older man

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Cataracts cause vision loss in many people. In advanced cases of cataracts, surgical removal of the cloudy lens is the best treatment. Before you commit to cataract surgery, however, it's best to know how much of your vision loss can be contributed to cataracts.

If your eye is otherwise healthy, your visual acuity should improve to normal levels once the cataract is removed. However, some patients with cataracts may also have another eye disease that can make it hard to determine if removing the cataract will improve vision.

The approximate level of vision gained can be estimated by using a PAM test, or a potential acuity meter test. A PAM test is not much more complicated than reading a typical eye chart. What makes a PAM test unique is that instead of having you look out at an eye chart on a wall, an eye chart is projected directly into your eye and onto your retina with a certain light similar to a laser, that attempts to bypass the cataract itself. Because the test bypasses the cataract, vision measured is an estimate of how much your vision may improve after cataract surgery is performed.

The PAM test is particularly useful for patients with other eye diseases such as macular degeneration. Although cataract surgery is safe, risks of surgery do exist. If removing the cataract will not significantly improve vision, the surgeon and patient may decide together that it is not worth the risk of surgery, or at least wait until a later date to see if the cataract worsens.

The PAM test can also be used for other conditions where the clarity of the light pathway is of concern. Doctors may use it to assess best corrected vision for large refractive errors, corneal or vitreous opacities, blood in the eye, posterior capsular opacities, and asteroid hyalosis. 

What Happens During a PAM Test?

A PAM test is not much more difficult than reading the typical eye chart. The lights of the examination room will be dimmed. The potential acuity meter is mounted on a biomicroscope called a slit lamp. The magnification on the slit lamp is set to the lowest magnification. Your pupil will most likely be dilated. Next, the operator will input the approximate prescription of your glasses into the machine. You will then see a light and then an eye chart. The letters are red, or different than a typical eye chart. Any head movement should be minimized. You will then be asked to read the smallest line of letters you can see. The test may be repeated a few times to verify the results. The test takes 5 to 10 minutes per eye. 

PAM Test Accuracy

Most studies show that the potential acuity meter predicted visual acuity correctly to within two lines in approximately 80-90 percent of patients. Other studies show that PAM testing actually tends to underestimate the best vision people receive so results are usually better than the PAM test predicted. In very severe cataracts, the results are not as accurate. It is important to point out that the results of a PAM test do not necessarily prevent patients from having cataract surgery. However, it is more important to help the surgeon choose the proper intraocular lens implant. For example, if a PAM test shows consistently visual acuity may not be 20/20, the surgeon is less likely to help the patient choose a multifocal intraocular lens implant because of extra costs that the patient may have to pay.

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