March 25, 2008
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Immersion A-scan ultrasound worth the extra effort

Technique eliminates corneal compression and allows measurement through the use of a scleral shell.

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You have found the perfect patient for your first premium presbyopia-correcting IOL: She has the proper expectations, she is easygoing, has minimal astigmatism, is a lifelong hyperope and currently suffers from poor vision and glare from her cataract. But because of her dense posterior subcapsular cataract, your optical coherence interferometry machine is unable to measure her axial length. You will need to use your A-scan ultrasound machine to accurately measure the axial length to ensure the precision of your IOL calculations.

The ultrasound A-scan works by projecting a sound wave through the eye and then measuring reflections. A reflection happens at the junction of two different densities, such as the interface between the posterior lens capsule and the vitreous. These echoes are shown as tall spikes of a high amplitude on the display, hence the name amplitude-scan, which is often abbreviated as A-scan.

The five primary amplitude spikes

Uday Devgan, MD, FACS
Uday Devgan

There are five primary amplitude peaks seen on an ocular A-scan: the cornea, the anterior lens, the posterior lens, the retina and the sclera (Figure 1). The gain of the ultrasound machine should be set to emphasize these peaks while avoiding over-saturation that would cause less resolution and the smearing of the retina and scleral spikes together. A high-quality scan should have these five spikes clearly defined and of approximately the same amplitude.

When looking at the retina spike, make sure that it is at a 90° angle, perpendicular and rising straight up without a step, notch or curve at the base. If the probe is off-axis, the retina will be measured at a spot other than the central macula, often resulting in a shorter than true axial length and a postoperative myopic surprise.

For our axial length measurements, the ultrasound gates will measure the distance from the cornea to the retina at the center of the macula. This number is then used for our IOL calculations, along with the patient’s keratometry.

Measuring the correct axial length is critical for determination of IOL power, and it can be a significant source of error if you are not careful. For every 1-mm error in axial length measurement, there will be about a 3 D refractive surprise in an average eye. For hyperopic eyes, the error will be even greater. The most common error seen in applanation A-scan measurements is corneal compression, which leads to a falsely shorter axial length, a higher IOL power and a postoperative myopic surprise.

Figure 1: The five primary amplitude spikes in A-scan ultrasound
The five primary amplitude spikes in A-scan ultrasound: cornea, anterior lens, posterior lens, retina and sclera.
Image: Devgan U
Figure 2: Technique of performing immersion A-scan ultrasound using the Accutome A-Scan Plus Technique of performing immersion A-scan ultrasound using the Accutome A-Scan Plus and a scleral shell. Notice the patient is parallel to the floor and is fixating on the ceiling target, which results in a high-quality A-scan.
Image: O’Connor J, Accutome

Immersion technique

To eliminate corneal compression, immersion A-scan can be performed. Instead of directly touching the cornea and risking compression such as with applanation, immersion allows measurement through the use of a scleral shell. The patient is reclined, the eye is anesthetized with topical agents and the scleral shell is gently applied. The ultrasound probe is fixated in the shell, which is then filled with balanced salt solution. The saline serves to couple the sound waves of the probe to the eye with no risk of corneal compression.

Although the use of immersion eliminates many of the potential errors, the A-scan is still technician dependent, so care must be taken to standardize the technique for your practice. Patients should be reclined so that their head is parallel to the ground and they are using their other eye to fixate on a ceiling-mounted target. Technicians should monitor the display for tall, even spikes and reproducibility of the measurement during repeat testing of the same eye. Extra care should be taken in eyes that are highly myopic because there is often a posterior staphyloma that leads to an overestimation of the axial length and resultant postoperative hyperopia for the patient. In eyes with deep anterior chambers, there may be misplacement of the ultrasound gates that would result in an erroneous measurement.

Double checking the math

You should always look at the measurements of both eyes together. Remember that significant anisometropia is unusual in the general population, so if the axial length between eyes differs by 0.3 mm or more, think twice. If the patient has a history of anisometropia in her old glasses, then differing axial lengths may be expected and accurate.

It is equally important to track your results to increase accuracy with your specific keratometer and your specific A-scan ultrasound. This will allow you to hone your results and deliver more accurate postoperative outcomes to your patients. When you see the patient the day after surgery and she successfully reads the 20/20 line, the sense of satisfaction for both you and the patient will be worth the extra effort of performing immersion A-scan.

For more information:

  • Uday Devgan, MD, FACS, is in private practice at Devgan Eye Surgery in Los Angeles, Beverly Hills, and Newport Beach, California. Dr. Devgan is Chief of Ophthalmology at Olive View UCLA Medical Center and an Associate Clinical Professor at the Jules Stein Eye Institute at the UCLA School of Medicine. Dr. Devgan can be reached at 11600 Wilshire Blvd., Suite 200, Los Angeles, CA 90025; 800-337-1969; fax: 310-388-3028; e-mail: devgan@gmail.com; Web site: www.DevganEye.com. Dr.Devgan is a consultant to Abbott Medical Optics and Bausch & Lomb, and is a stockholder in Alcon Laboratories and formerly in Advanced Medical Optics. He has no direct financial interest in the products discussed in this article, nor is he a paid consultant for any companies mentioned.