An A-scan is an ultrasound test that measures the length of your eye from front to back. The “A” stands for amplitude, referring to the spike pattern the sound waves create as they bounce off structures inside the eye. It’s most commonly performed before cataract surgery to help determine the right power for an artificial lens implant, but it also plays a growing role in tracking nearsightedness in children.
How the Scan Works
A small probe emits a thin beam of sound waves at a frequency of about 10 MHz. As the beam travels through the eye, it hits each internal structure and bounces back. The probe detects those returning echoes and converts them into a series of vertical spikes on a screen, with each spike representing a different tissue boundary. The height of each spike tells the technician how strong the echo was, and the spacing between spikes reveals the distances between structures.
The sound waves pass through four distinct zones inside the eye, each with its own known speed of travel: the cornea, the anterior chamber (the fluid-filled space behind the cornea), the lens, and the vitreous cavity (the gel-filled space that makes up most of the eyeball). By combining echo timing with those known speeds, the device calculates precise distances in millimeters. The most important measurement is axial length, the total front-to-back distance of the eye. In a healthy adult, that number typically falls between 22 and 25 mm.
Why It Matters for Cataract Surgery
When a clouded natural lens is removed during cataract surgery, it’s replaced with an artificial intraocular lens (IOL). Choosing the right IOL power depends heavily on axial length. A difference of just 1 mm in eye length can shift the needed lens power by 2.5 to 3.0 diopters, enough to leave someone with noticeably blurry vision after surgery. That’s why accurate measurement is critical.
Surgeons plug the A-scan’s axial length reading into a formula along with corneal curvature and anterior chamber depth to calculate the ideal IOL power. Several formulas exist, but they all rely on the same core inputs. Getting those inputs right is one of the biggest factors in whether a patient ends up with clear vision after the procedure or needs glasses to correct a residual prescription error.
Contact vs. Immersion Technique
There are two ways to perform an A-scan. In the contact method, the ultrasound probe is placed directly against the surface of your eye after numbing drops are applied. This is the more common approach and works well in most cases, but the probe can slightly compress the cornea, which may shorten the measurement by a small amount.
The immersion method avoids this issue. Instead of touching the eye directly, the probe is suspended in a small cup of saline solution that sits over the eye. Because nothing presses against the cornea, measurements tend to be slightly more consistent. A study comparing the two techniques in children found average lens prediction errors of 1.11 diopters with contact A-scan and 1.03 diopters with immersion, a difference that wasn’t statistically significant. In practice, both methods produce reliable results, though immersion is sometimes preferred when maximum precision matters or when the patient is a child under general anesthesia.
Tracking Nearsightedness in Children
Beyond cataract surgery, A-scan biometry has become an increasingly valuable tool for monitoring myopia (nearsightedness) in children. Axial length is the single most important anatomical factor in whether someone becomes nearsighted. Eyes that grow too long focus light in front of the retina instead of on it, producing blurry distance vision. The eyeball typically stops growing around age 16 to 18, so the window for intervention is during childhood.
Clinicians now use biometry measurements taken at six-month intervals to track how quickly a child’s eye is elongating. Research has shown that the rate of axial length change over the first six months strongly predicts how much a child’s prescription will worsen over the following year. One study found that each additional 0.1 mm of growth in the first monitoring period carried the highest predictive weight for identifying children at risk of progressive myopia. That kind of data helps eye care providers decide whether to start treatments like special contact lenses or eye drops designed to slow eye growth.
Special Circumstances That Affect Accuracy
Certain eye conditions can throw off an A-scan reading. If the eye has been filled with silicone oil after retinal surgery, the sound waves travel more slowly through the oil than through the eye’s natural gel. The result is a falsely long measurement. To correct for this, clinicians apply a conversion factor of 0.71 to translate the measured length into the true axial length.
In eyes where the natural lens has already been removed but not yet replaced (a state called aphakia), the scan looks different. Instead of the usual pair of spikes representing the front and back surfaces of the lens, a single spike appears from the remaining capsule. The immersion technique is generally preferred in these cases to avoid compression artifacts that could further reduce accuracy.
How It Differs From a B-Scan
You may hear A-scan and B-scan mentioned together, since both use ultrasound to examine the eye. The key difference is what they show. An A-scan produces a one-dimensional line of spikes representing distances along a single axis. A B-scan produces a two-dimensional cross-sectional image, similar to the ultrasound images used in pregnancy. The B-scan probe is physically larger because it contains a motor that sweeps the ultrasound beam back and forth to build that image.
The two tests serve different purposes. An A-scan answers the question “how long is this eye?” A B-scan answers “what does the inside of this eye look like?” B-scans are especially useful when cataracts or other conditions block the doctor’s view of the retina, allowing them to check for detachments, tumors, or bleeding that can’t be seen with a standard eye exam. In many cataract evaluations, both scans are performed together.