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Editor,—Acute zonal occult outer retinopathy (AZOOR) is a clinical syndrome described as acute zonal loss of outer retinal function associated with photopsia, minimal or no fundal findings initially, full field ERG abnormalities, and visual field loss.1
The diagnosis of the patient reported here is consistent with AZOOR. However, she showed a normal full field flash ERG and, interestingly, an abnormal multifocal ERG.
A 33 year old white myopic female presented in Italy in September 1996 with the sudden appearance of a dark spot in the visual field of the right eye associated with photopsia. She was first seen at Princess Alexandra Eye Pavilion, Edinburgh, in November 1996 with the same complaint. She had no systemic problems and was taking no medications. Vision was 6/5 N5 in each eye with correction. She has no afferent pupillary defect. There was no inflammation in the anterior chamber or the vitreous. The right fundus showed retinal pigment epithelial atrophy superiorly. Neurological examination was normal. Visual field test showed inferonasal scotoma in the right eye. Fundus fluorescein angiography (FFA) and indocyanine green angiography (ICG) done in Italy showed peripheral “hyperfluorescent spots” which were interpreted as choriocapillaris in the same eye. Brain magnetic resonance imaging was normal.
The system used for the multifocal ERG recording was similar to the commercially available VERIS system.3 However, it was custom built to run on Microsoft/PC platform. Care was taken to assure maximum filter bandwidth as differentiation of the response is known to lead to signal distortion.4 Ground electrodes were placed on the forehead, reference electrodes were placed on the outer canthi, and active electrodes were placed in the lower lid.5
Conventional Ganzfield ERGs were recorded using our custom build system triggering a Ganzfield stimulator.6 Multifocal and conventional ERGs were repeated after 5 months.
Electrophysiological findings from full field ERG recordings along with confidence ratings are summarised in Table 1. These results indicate that although some disparity is evident between the two eyes, both responses fall within normal limits. The multifocal ERG results are reported as topographical maps of retinal function. The evoked response maps in Figure 1 are a good record of the initial and subsequent recording performed after a 5 month interval. These responses indicate localised dysfunction in the inferonasal field of the right eye. Repeat measurements indicated no significant improvement or deterioration over this short period.
Given the suggested outer retinal dysfunction associated with AZOOR, the electroretinogram, it would appear, is an ideal test for assessing and diagnosing such pathologies as its record is a direct measure of photoreceptor and bipolar cell function.7 However, if suggestions are correct that AZOOR represents a localised dysfunction in the outer retinal system and since the full field flash ERG represents a global response to a diffuse stimulus, information derived from the full field flash ERG in the investigation of localised retinal pathologies is of limited value.
The recent advances in the application of pseudo random binary sequences (PRBS) to signal averaging has addressed the limitation of local ERG measurements and made them a routine clinical practicality.3 8-10
It should be mentioned however that the multifocal technique, in its present form, is not a replacement for either full field electrophysiology or its subjective partner perimetry. It merely complements the arsenal of investigative techniques available to clinicians.