IOLMaster — Axial Length Measurement.

Best Technique for ALM

Measuring axial length with the IOL Master allows a high degree of flexibility. Rather than simply positioning a small, in-focus image in the middle of a set of video screen cross hairs, you can instead maneuver the focusing spot anywhere within the measurement reticule, and even focus in, or focus out.

In this way, it is possible to sample different areas around the visual axis until the best axial length display is obtained. Then, once that best spot is discovered, all subsequent measurements are taken from that location. This technique is especially useful for eyes with small corneal scars, anterior cortical spokes, posterior subcapsular plaques, or other localized media opacities.

If the signal-to-noise ratio of the eye to be measured is borderline (2.0 to 1.6), focusing in, or focusing out, in such a way that that focusing spot enlarges to about the same size as the measurement reticule, will sometimes significantly improve the quality of the axial length display. This is possible with the IOL Master because axial length measurements by partial coherence interferometry are distance independent.

It is useful to take all 20 measurements. At least four of these measurements should be within 0.02 mm of one another, and should exhibit the characteristics of an Ideal Display. An ideal axial length display is more important than a high signal-to-noise ratio (SNR).

Remember to instruct your patient to look directly at the small red fixation light. In this way, axial length measurements will be made to the center of the macula, giving the refractive axial length, rather than the anatomic axial length.

If there is a high refractive error (more than ±4.00 D), you can even take several measurement with the patient's glasses in place to ensure adequate fixation. Measurements with and without glasses should be exactly the same. For eyes with high to extreme myopia, with a type 1 peripapillary posterior staphyloma, being able to measure to the fovea is an enormous advantage over conventional A-scan ultrasonography.