What is axial length and why is it important?

Axial length is the physical length of the eye from the front corneal surface to the retina, measured in millimeters. The average adult axial length is about 23.5 mm. It is the most important factor in calculating the correct IOL power for cataract surgery. A 1 mm error in axial length produces approximately a 2.50 diopter error in the postoperative refraction, meaning the patient would need glasses or a lens exchange to correct the residual error.

What is the difference between contact and immersion A-scan?

Contact A-scan places the probe directly on the anesthetized cornea, which can compress the cornea by 0.1-0.3 mm and slightly underestimate axial length. Immersion A-scan uses a fluid-filled shell between the probe and the eye, eliminating corneal contact entirely. Immersion technique is more accurate and preferred, especially in short, highly hyperopic eyes where small measurement errors have the greatest impact on IOL power.

What does a valid A-scan waveform look like?

A valid A-scan waveform shows a series of characteristic spikes from anterior to posterior: an initial spike from the anterior cornea, intermediate spikes from the posterior cornea and anterior lens, a large spike from the posterior lens surface, a tall and sharp retinal spike (the most critical landmark), and a lower scleral spike just behind. The retinal spike must be tall, sharp, and perpendicular -- a slanted or absent retinal spike indicates the probe is off-axis and the measurement is invalid.

Why is excessive probe pressure a problem in A-scan biometry?

Pressing the probe firmly against the cornea indents the eye, compressing the cornea and shortening the measured axial length by 0.1-0.3 mm. This error causes the calculated IOL power to be too weak (because the formula thinks the eye is shorter than it actually is), resulting in a patient who is left hyperopic (farsighted) after cataract surgery. A light, steady probe touch -- just enough to maintain corneal contact -- prevents this error.

When is A-scan preferred over optical biometry?

Optical biometry (IOLMaster, Lenstar) is preferred when possible because it is non-contact, faster, and more precise. A-scan ultrasound is used when optical biometry cannot obtain reliable readings -- most commonly with very dense cataracts that block the instrument's infrared laser (the cataract is too opaque for the light to pass through and reflect accurately). A-scan can penetrate opaque media because ultrasound, unlike light, passes through cataracts.

A-Scan Ultrasonography for the CPOA Exam

What Is A-Scan Ultrasonography?

A-scan ultrasonography (amplitude scan) is a one-dimensional ultrasound technique used primarily to measure the axial length of the eye -- the distance from the front surface of the cornea to the retina (or more precisely, to the vitreoretinal interface). Axial length is the most important measurement in calculating the correct intraocular lens (IOL) power before cataract surgery.

While optical biometry (such as IOLMaster or Lenstar) has largely replaced contact A-scan in many practices, A-scan remains essential when optical biometry cannot penetrate a dense cataract or when the patient cannot maintain fixation. The CPOA may be trained to perform A-scan or to assist the doctor during the procedure.

Why Axial Length Matters

The eye's axial length is the primary determinant of refractive error:

A longer eye (axial length >24 mm) is myopic (nearsighted).
A shorter eye (axial length <22 mm) is hyperopic (farsighted).
The average adult axial length is approximately 23.5 mm.

Before cataract surgery, the axial length (combined with corneal curvature from keratometry) is entered into an IOL power formula to calculate the correct implant power. A 1 mm error in axial length measurement produces approximately a 2.50 D error in the postoperative refraction -- a significant miscalculation that may require spectacle correction or lens exchange.

💡 Clinical Tip: Precise axial length measurement is one of the most important technical contributions the CPOA makes to patient outcomes in cataract surgery. A measurement error that could have been avoided results in a patient who needs glasses after a procedure they expected to correct their vision.

Contact vs. Immersion A-Scan

Contact A-Scan

The ultrasound probe is placed directly on the anesthetized cornea. The probe compresses the cornea slightly, which can cause the axial length to be underestimated (shorter reading than true length). This is the most common error in contact A-scan and is reduced by using minimal probe pressure and verifying that the corneal compression echo is appropriate.

Immersion A-Scan

A fluid-filled scleral shell (immersion shell) is placed between the probe and the eye, so the probe does not touch the cornea. This eliminates corneal compression and gives more accurate readings. Immersion technique is preferred for accuracy, especially in short eyes (high hyperopia) where small errors matter most.

CPOA Role in A-Scan Biometry

Patient Preparation

Instill topical anesthetic in the eye to be measured (contact A-scan).
Have the patient sit or recline comfortably.
Enter patient data into the A-scan machine (name, date of birth, test eye, desired postoperative refraction target).
Inform the patient: "We are going to measure the length of your eye using a small probe/sound waves. You will feel gentle pressure on your eye."

During the Measurement

Ask the patient to look straight ahead (or at the probe's internal fixation light).
The probe is applied gently to the center of the cornea -- perpendicular, with minimal pressure.
The A-scan display shows a waveform with characteristic spikes: anterior cornea, posterior cornea/anterior lens, posterior lens, retinal spike.
A valid measurement has a high, sharp retinal spike followed by a lower scleral spike.
Take at least 5-10 readings and the instrument averages them. Most machines flag low-quality measurements automatically.

Documenting Results

Record axial length in mm (typically to 2 decimal places), anterior chamber depth, and lens thickness.
Document for both eyes even if only one eye is being operated on (for comparison).

⚠️ Common Mistake: Pressing the probe too firmly against the cornea. Excessive probe pressure compresses the cornea and shortens the measured axial length by 0.1-0.3 mm, causing the IOL power to be calculated too weak. The postoperative result is a hyperopic (far-sighted) surprise. Always use the minimum pressure required to maintain contact -- a light, steady touch.

Identifying a Valid Waveform

On the A-scan display, a valid reading shows:

Initial spike: Anterior cornea surface
Small intermediate spikes: Posterior cornea and anterior lens surface
Large lens spike: Posterior lens surface
High retinal spike: The key landmark -- should be tall, sharp, and perpendicular
Low scleral spike: Just behind the retinal spike

A slanted, low, or absent retinal spike suggests the probe is off-axis and the measurement is invalid.

Key Takeaways

A-scan ultrasound measures axial length -- the critical input for IOL power calculation before cataract surgery.
Average axial length is 23.5 mm. A 1 mm error produces a ~2.50 D postoperative refractive error.
Immersion A-scan is more accurate than contact because it eliminates corneal compression artifact.
Contact probe must be applied with minimal pressure to avoid shortening the measured axial length.
A valid waveform shows a tall, sharp, perpendicular retinal spike.
Take 5-10 readings and average; flag low-quality measurements for the doctor.