Last year I reviewed the Eidon system and was impressed with the clarity of the images easily obtained from both retinas of a patient with just one single click of a button. This year, Haag Streit UK have released the Compass which is essentially the Eidon unit with a visual fields analyser incorporated within. This allows you to take an accurate image of the fundus and to measure the sensitivity of points across the retinal view so offering essential information about the structure and function of the eye. I recently spent a day putting the instrument through its paces and found the results both easy to obtain and of excellent quality.
Compass
The Compass (figure 1) is an instrument that, according to the manufacturers, is ‘for measuring retinal sensitivity, for a quantitative assessment of fixation characteristics, as well as to capture infrared and colour images of the retina without the use of a mydriatic agent’. In essence it is a high quality scanning laser ophthalmoscope and visual fields analyser combined – the former allows tracking of the image to ensure the latter is both accurate and allows sequential assessment to be taken in a way that progression is clearly detectable. Table 1 summarises its key features.
The patient is positioned (figure 2) without an occluder (each eye is separately assessed in one sitting) and their refraction corrected by internal mean sphere lenses – avoiding the risk of supplementary lens artefacts.
Figure 2: The Compass positioned
New patients are registered on the integral tablet screen and the usual data is required in order for the normative database to calculate each patient’s results. Existing patients may be reassessed by selecting their previous results (figure 3a) and the appropriate fields test chosen (figure 3b). The 24-2 employs a ZEST algorithm (zippy estimation by sequential testing, analogous to ZATA or SITA) while the 10-2 option is used for macular assessment in a way similar to some microperimeters.
Figure 3a
Figure 3b
Once the test begins, a reference image is captured and appears on screen – this needs to be of good quality to ensure accuracy. If acceptable, you move an onscreen circle to coincide with the optic disc of the patient shown. The patient then has to look between four green dots until a white light is first detected on fixation. This is a measure of foveal sensitivity – once complete, they fixate on a central target and simply click the button as stimuli appear as with most current field analysers.
Figure 4a
As the fields are assessed, progress appears on screen within a green circle (figure 4a) and is ceased whenever fixation strays, as indicated by a red circle (figure 4b). Once completed, the instrument automatically repositions (figure 5) to then take the retinal image (figure 6) which is stored as both full colour and infra-red.
Figure 4b
Figure 5
Figure 6
Performance
Figures 7a and 7b show the retinal images from one of my healthy subjects – the resolution is excellent and allows easy magnification and storage of areas of interest (figure 8) or enhancement of images (figures 9a and 9b). The data printout for a normal 24-2 (figure 10) is easily recognisable and includes threshold values (top left), cluster mean defects, total and pattern deviation plots (right), and also a record of fixation (bottom left). A retinal image or a glaucoma predictive display can also be included (bottom right).
Figure 7a
Figure 7b
Figure 8
Figure 9a
Figure 9b
Figure 10
For the macular plot (figure 11) I had my guinea pig deliberately eccentrically fixate (hence the blind spot scotoma) and the fixations are clearly more significant. Tracking allows repeat assessments and the plotting of any significant change (figure 12). I hope to publish a case revealing disease progression in future.
Figure 11
Figure 12
Intuitive
The Compass is easy to use and should be attractive not only for glaucoma monitoring but as a useful replacement for out of date fields and imaging systems but with half the footfall. This is likely to be the first step towards greater integration of data to help clinical management.
Further information from haagstreituk.com
