A-Scan for the eye is an essential diagnostic tool used in ophthalmology. We use this scan for lens power calculation before cataract surgery. All patients undergoing cataract surgery get this scan at Eye Solutions in Mumbai. A-scan ultrasound biometry, commonly referred to as an A-scan, is a diagnostic test used by ophthalmologists. It provides data on the eye length to calculate intraocular lens power. A-scan complete form is an Amplitude scan.
A-scan for both eyes costs Rs 1200 at Eye Solutions. It takes only a few minutes, and patients don't experience discomfort.
When cataract surgery is performed, the natural lens in the eye is removed and replaced with an artificial lens (IOL). Regardless of the type of lens used, Standard IOLs or Premium IOLs, eye measurements are needed before surgery. Here, most patients ask us which is the best lens, and you can read more about that.
These measurements are exact. When combined with particular computerized calculations, these allow the surgeon to choose the power of the intraocular lens, which is appropriate for each eye. Because each eye has a different size and shape, the power in the lens needed is different for each eye. For this reason, an A-Scan for the eye is required before cataract surgery.
These days cataract surgery is not only a surgery to get rid of the cataract. Over the years, it has gradually become like a refractive surgery. In other words, apart from removing the cataract, ophthalmologists also want to leave the patients glasses-free. Thus, ophthalmologists need to calculate the lens power with a high level of accuracy. An A-scan in Mumbai is available with most ophthalmologists and at Eye Solutions.
Different machines can throw out different kinds of A-scan reports. However, what these machines calculate remains the same. Here is a copy of an A-scan report. The section on the top shows the various measurements.
This information is then fed into a formula to calculate the lens power. Every hospital has its lens choices that they commonly use. Each lens has what is known as an A constant, which is entered in that formula. The report shows various lens options and their powers based on the A constants entered. This report shows calculations for different Monofocal lenses; the top left is for Technis IOL. We use the same chart for the Eyhance lens, Technis multifocal lens, and Synergy Lens.
There are a few types of A-scan for the eye available. It would help if you remembered the measuring methods are different, but the measurements taken are the same.
The contact method
The contact method is also known as the applanation method. Here an ultrasound probe comes in contact with your cornea. This contact should be very light, and we will then get accurate measurements. We instil anaesthetic eye drops in the patients' eyes before the procedure.
In this method, after instilling anaesthetic drops in the eye, an immersion chamber is placed on the eye. We fill this chamber with fluid and use the same ultrasound probe used in the contact method. The idea is that because the probe does not touch the eye, the measurements are more accurate.
Here, the technology used is different from the contact and immersion methods. The video you see below is of optical biometry, which provides the most accurate eye length measurement. Optical biometry may not take the necessary measurements in very dense cataracts or subcapsular cataracts. Sometimes, if optical biometry cannot take the required measurements, one may have to perform the contact or immersion methods of A scan of the eye.
Various companies make these machines, and the most popular ones are excellent. Zeiss, Quantel Medical and Tomey are the more popular A scan of eye machines.
Sometimes the A-scan eye test is used for other purposes, too—the most common being measuring the axial length of the eyeball. Periodically in very dense cataracts, your cataract specialist cannot examine the retina. It is essential to perform a detailed retina examination before cataract surgery. Your ophthalmologist wants to rule out whether the retina is normal or detached. The axial length measurement gives us a clue to the position of the retina. If the axial length is short, it may indicate a retinal detachment. Also, if the measurement spikes that we get while doing the scan are not of good quality, we know that something is not right in the eye, and these conclusions can be arrived upon despite not being able to see the retina.
Conditions like a posterior staphyloma are also picked up on an A-scan if your cataract specialist cannot examine the retina. There is a bulge in the rear or back portion of the eye. More often, this bulge is at the posterior pole of the eye. This bulge increases the length of the eyeball, and this increased axial length is then measured.
Usually, an excellent A-Scan machine or what is known as a Biometer is enough. However, there can be certain situations where we may perform an additional test. Your doctor can perform these further tests if your cornea is highly curved or has astigmatism. These tests could be Corneal Topography or Aberrometry. These tests measure the corneal surface and corneal thickness—the results of these tests aid in determining the lens power.
No. All you have to do is place your chin on the chin-rest of the instrument, and a picture is taken, which takes less than 10 seconds per eye. Sometimes the eye is numbed with eye drops, and the probe touches the eyes. Because of the numbing eye drops, one does not feel anything.
The entire procedure takes not more than 5 minutes. Watch the video below to understand how quick it is.