Why is axial length so important for cataract surgery?

Axial length is the primary determinant of IOL power. Even small errors in this measurement lead to significant post-surgical refractive surprises. A 0.1 mm error in axial length produces approximately a 0.25 diopter error in the final IOL calculation. Patients who undergo premium IOL implantation for spectacle independence are particularly sensitive to measurement errors.

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

In contact A-scan, the probe rests directly on the anesthetized cornea, which can cause slight corneal compression and produce a falsely short axial length. Immersion A-scan uses a saline-filled shell that keeps the probe from touching the cornea, eliminating compression artifact and providing more accurate readings. Immersion is generally considered the more accurate technique.

What does the A-scan waveform look like, and what are the important spikes?

The A-scan displays a series of vertical spikes on a horizontal axis. The key spikes represent the cornea, anterior lens, posterior lens, retina, sclera, and orbital tissue. For axial length measurement, the distance from the initial corneal spike to the retinal spike is the primary measurement. All spikes should be tall, sharp, and well-defined to indicate proper probe alignment.

How does silicone oil in the vitreous affect A-scan measurements?

Silicone oil transmits sound much more slowly than normal vitreous humor. If the standard vitreous velocity setting is used in an eye filled with silicone oil, the machine will significantly underestimate the axial length (because it thinks the longer travel time means the eye is shorter). A silicone oil mode or adjusted velocity setting is required to obtain accurate measurements.

A-Scan Ultrasonography: Biometry for Cataract Surgery

What Is A-Scan Ultrasonography?

A-scan ultrasonography (A for amplitude) is an ocular ultrasound technique used primarily to measure the axial length of the eye, which is the distance from the front of the cornea to the retinal surface. This measurement is the single most important variable in calculating the power of an intraocular lens (IOL) before cataract surgery.

The device emits high-frequency sound waves (around 10 MHz) through the eye. When these waves encounter an interface between two tissues of different acoustic density (such as the front and back of the lens, or the retina), some waves reflect back as echoes. The machine precisely measures the time of flight of these echoes and converts the time intervals into distance measurements using known sound velocities for each ocular medium.

The A-Scan Waveform

The output appears as a series of spikes on a screen, each representing a reflective interface. The key spikes in a diagnostic A-scan are:

Corneal spike (initial interface)
Anterior lens spike
Posterior lens spike
Retinal spike (the most important for axial length)
Scleral spike
Orbital spike

The time between the corneal spike and the retinal spike gives the axial length. The clinician or technician must verify that the spikes are sharp, properly peaked, and perpendicular to the probe axis. Broad or low-amplitude spikes indicate poor alignment or probe position issues.

For accurate biometry, the retinal spike must be tall and sharp, not broad or sloping. If it looks irregular, reposition the probe and repeat. A poor spike leads to an inaccurate axial length and a post-surgical refractive surprise.

Contact vs. Immersion A-Scan

There are two techniques for performing A-scan biometry, and their difference has a significant clinical impact:

Feature	Contact A-Scan	Immersion A-Scan
Probe contact	Directly on cornea	In saline bath (no corneal touch)
Corneal compression	Possible	None
Axial length bias	Tends to be shorter (compressed)	More accurate
Setup	Simpler, faster	Slightly more involved
Clinical preference	Common in busy offices	Preferred for accuracy

With contact technique, the probe rests directly on the anesthetized cornea. Even gentle pressure indents the cornea slightly, compressing the axial length reading and producing a falsely short measurement. Over-powered IOLs (too much plus power) result, leaving the patient myopic after surgery.

With immersion technique, a scleral shell or fluid-filled eye cup holds saline between the probe tip and the cornea. The probe never touches the cornea. This eliminates the compression artifact and produces more accurate axial length measurements.

Forgetting that contact A-scan compresses the cornea and underestimates axial length is a classic exam error. Immersion is more accurate because it removes this source of bias.

Average Axial Length Values

A typical adult emmetropic (normally refractive) eye has an axial length of approximately 23.5 to 24.5 mm. However, this varies considerably:

A highly myopic eye may measure 26 mm or more.
A highly hyperopic eye may measure 21 mm or less.
Even a difference of 0.1 mm in axial length translates to approximately a 0.25 diopter change in the resulting IOL power calculation.

This level of precision explains why accurate axial length measurement is so critical. For a patient expecting to see clearly without glasses after cataract surgery, a small measurement error can mean the difference between achieving that goal and needing glasses.

Sound Velocity Settings

Each ocular medium transmits sound at a different velocity, and the A-scan machine must use the correct velocity for each segment to calculate distances accurately. The velocity through the aqueous and vitreous humors differs from the velocity through the crystalline lens. When a patient has a dense cataract or silicone oil fill, velocity settings must be adjusted accordingly. Silicone oil in particular has a much slower sound velocity and requires a special mode to avoid gross underestimation of axial length.

Key Takeaways

A-scan ultrasonography measures axial length by analyzing the time of flight of ultrasound echoes between ocular interfaces.
Axial length is the most critical variable in IOL power calculation.
Contact technique risks corneal compression; immersion technique is more accurate.
Normal adult axial length is approximately 23.5-24.5 mm.
A 0.1 mm error in axial length results in approximately 0.25 D error in IOL power.