Zeiss IOLMaster Customer Service: 1-877-486-7473 ext.
IOLMaster — Evaluation of ALM Results.
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.
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
The displayed axial length values are thus
directly comparable to those obtained by immersion ultrasound!
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.
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.
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. The IOL Master measuring system is capable of resolving fine structures
on the fundus of the eye.
The IOLMaster measuring system is capable of resolving fine structures
on the fundus of the eye.
Depending on the anatomical conditions of the measured eye, the
measuring beam may also produce interferences when reflected at the
inner limiting membrane and/or the choroid.
Indications of this are:
Broader (smeared) signal peaks of the measuring curve
Variations of approximately 150 ... 350 μm in axial length data in one
measurement series and
Display of "Evaluation" in place of the mean value.
Such measuring curves or measurement series require immediate
verification, either between individual measurements (in ALM mode) or
in post-measurement editing (without the patient in front of the IOLMaster).
Interpretation or post-measurement editing should always be performed
with the help of the zoom function.
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.
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
||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
recognize the signal from the ILM.
In measurement series, such individual measurements stand out by
deviations in the range of approximately 150 ... 350 µm towards shorter
axial lengths. The measured value may be corrected by moving the
measuring cursor to the smaller peak (produced by the pigmented
epithelium). This manipulation is permissible only with the other signal
curves of this measurement series!
Signals From Choroid
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 ... 250 µm from the peak
of the pigmented epithelium.
In the above example, the signal from the RPE (middle peak) has the
highest amplitude. The automatic peak detection system has correctly
recognized 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
In 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.
The automatic peak detection system will find an axial length value that
is too short by approximately 150 ... 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.
In very rare cases signals may be produced by both the pigmented
epithelium and the choroid.
Here again, the automatic peak detection system has placed the
measuring cursor at the correct position, as the (correct axial length)
signal from the pigment epithelium has the greater amplitude. The
measuring cursor may not be moved.
Such a situation may only be evaluated correctly by viewing all
measuring curves of this eye and 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.