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Interpretation of Axial Length Measurements

As a rule, an interference signal is produced if the measuring light is reflected by the tear film and the retinal pigmented epithelium of the eye. This signal is utilized for axial length measurements.

Note Ultrasonic biometrical instruments measure the axial length as the distance between the cornea and the inner limiting membrane, because the sound waves are reflected at this membrane. To ensure that the measured values obtained with the IOLMaster are compatible with those obtained through acoustic axial length measurement, the system automatically adjusts for the distance difference between the inner limiting membrane and the pigmented epithelium. The displayed axial length values are thus directly comparable to those obtained by immersion ultrasound. Deviations may nevertheless occur between the displayed axial lengths and ultrasonic readings (particularly in the applanation procedure). At this point the importance of re-personalizing the "lens constants" should be stressed, because the IOLMaster is based on a new, more precise measurement technology. Refer to the specialist literature and publications by the originators of the IOL formulas regarding the personalization of lens constants. Updated information is available in the Internet at: http://www.meditec.zeiss.com/iolmaster and/or http://ocusoft.de/ulib/

With an optimally aligned device, relatively clear eye media and weak ametropia (< 6 D), the secondary maxima will be detected symmetrically on each side of the actual measuring peak. These are caused by the measuring light source used and maintain a constant distance of approximately 0.8 mm to the measurement signal and to each other, irrespective of the specific circumstances of the measured object. For this reason, the secondary maxima are similarly always visible in measurements of the supplied test eye.

Such measuring curves or measurement series require immediate verification, either between individual measurements (in ALM mode) or in post-run editing (without the patient in front of the IOLMaster). Interpretation or post-run editing should always be performed with the help of the zoom function (see Zooming Display).

Note The resolution of fine retinal structures is clearly distinguishable from the previously mentioned secondary maxima, which are further away from the multiple peaks and symmetrical to them. The distance between the maximum peak and internal limiting membrane or choroid is 350 μm (whereas the secondary maxima are about 800 μm from the maximum peak!).

Signals From Inner Limiting Membrane (ILM)

The measuring beam is relatively often reflected at the inner limiting membrane, likewise producing an interference signal. The respective signal peak lies to the left of the actual measurement peak (to the shorter axial lengths). The distance of the peak generated by the reflection on the inner limiting membrane from the measurement peak is between 150 and 350 µm. Both peaks can be observed separately in a zoom view of the graph.

Double peak produced at inner limiting membrane (triple zoom)

Usually, the signal amplitude of the peak from the inner limiting membrane is smaller than that of the interference on the pigmented epithelium. In such a case the automatic algorithm finds the correct axial length.

WARNING: Never move the measuring cursor manually to the (left) peak produced by the inner limiting membrane (see above)!

In rare cases the amplitude of the signal from the inner limiting membrane may be higher than that of the reflected light from the pigmented epithelium. In this case, the automatic peak detection will recognise the signal from the ILM.

Signal curve with higher signal from inner limiting membrane (double zoom)

In measurement series, such individual measurements stand out by deviations in the range of approx. 150 to 350 μm towards shorter axial lengths. The reading can be corrected by dragging the measurement cursor in the composite signal to the lower peak (that of the pigmented epithelium). This manipulation is only permissible in the context of the single signals of this series of measurements!

Signals From Choroid

Triple Peaks

In rare cases, the measuring beam may also be reflected by the vessels of the choroid.

The measuring peak produced by the choroid appears shifted towards longer axial lengths by approximately 150 to 250 μm from the peak of the pigmented epithelium.

WARNING: In the above example, the signal from the RPE (middle peak) has the highest amplitude. The automatic peak detection system has correctly recognised this measured value as the axial length, so that the measuring cursor may not be moved.

This type of rare triple peak clearly differs from the secondary maxima produced through the light source by the distance from the RPE reflected peak.

In very rare cases, depending on the anatomical conditions of the measured eye, the signal produced by the pigmented epithelium may not be the one with the highest amplitude.

Double peak produced by pigmented epithelium and choroid (double zoom)

The automatic peak detection system will find an axial length value that is too short by approximately 150 to 350 μm.

WARNING: Following the comparison of all measured values and curves for this eye, the measuring cursor must be moved manually to the middle (smaller) peak produced by the RPE. This measured value is thus corrected and shown in the display field with an asterisk.

Double peaks

In very rare cases signals may be produced by both the pigmented epithelium and the choroid.

Double peak produced by pigmented epithelium and choroid (double zoom)

WARNING: Here again, the automatic peak detection system has placed the measuring cursor at the correct position, as the (correct axial length) signal from the pigmented epithelium has the greater amplitude. The measuring cursor may not be moved.

Note Such a curve may only be evaluated correctly by viewing all measuring curves of this eye. Such a curve must be clearly distinguished from double peaks produced by the inner limiting membrane and the RPE (see Signals from the inner limiting membrane (ILM), above). It may be advisable to perform further measurements. Up to 20 measurements may be taken on one day. Updated information is available in the Internet at: http://www.meditec.zeiss.com/iolmaster and/or http://ocusoft.de/ulib/