A-Scan for eye is a very important diagnositc tool used in ophthalmology. It is used for lens power calculation before cataract surgery and is performed in all patients undergoing this surgery 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 for the calculation of intraocular lens power. A scan full form is Amplitude scan.
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, measurements of the eye are needed before surgery. Here, most patients ask us which is the best lens and more can be read about that.
These measurements are highly precise. When combined with special computerized calculations, these allow the surgeon to choose the power of 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 eye is always required prior to 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 also 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 their lens choices that they commonly use. Each lens has what is known as an A contstant. This A constant is entered in that formula and based on the A constants that have been entered in the machine the report shows various lens options and their powers. In this report you will see calculations for Monofocal lenses of different companies. The top right is for Technis IOL. This same chart is used for the Eyhance lens and the Technis multifocal lens and Synergy Lens.
There are a few types of A scan for eye available. You must remember the measuring methods are different but the measurements that are taken are the same.
The contact method
This 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. To avoid any discomfort to the patient, anesthetic eye drops are instilled in the patients eyes before the procedure.
In this method anesthetic drops are instilled in the eye and an immersion chamber is placed on the eye. This chamber is fill with fluid and the same ultrasound probe wihch was used n the contact method is placed in the fluid. 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 technique and the immersion method. The video you see below is of an optical biometry. This is supposed to provide most accurate eye length measurement. Optical biometry may not be able to take the necessary measurements in very dense cataracts or in subcapsular cataracts. Sometimes if optical biometry cannot take the necessary measurements then one may have to perform the contact or immersion methods of A scan of eye.
There are various companies that make these machines and most of the popular ones are very good.
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. Most common being measuring the axial length of the eye ball. Sometimes in very dense cataracts the retina cannot be examined. It is very important to perform a detailed retina examination before a cataract surgery. Your ophthalmologist wants to rule out whether the retina is normal or is there a retinal detachment. The axial length measurement gives us a clue to the position of the retina. If the axial lenght is short it may indicate a retinal detachment. Also if the measurement spikes that we get while doing the scan are not of a good quality we know that something is not right in the eye and these conclusions can be arrived upon inpiste of not being able to see the retina.
Conditions like a posterior staphyloma are also picked up on an A scan if the retina cannot be examined. Here there is a buldge in the posterior or back portion of the eye. More often this buldge is at the posterior pole of the eye. This buldge increases the length of the eyeball and this increased axial length is then measured.
Usually a good 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. These additional tests can be performed 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 along with the findings of the scan.
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 is used to touch 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.