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Scott E. Brodie, Jasmine H. Francis, Brian Marr, David H. Abramson; Scleral Depression Depresses the Photopic ERG. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5706.
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During ERG monitoring of patients undergoing treatment for retinoblastoma, acute depressions of ERG amplitudes were observed following laser treatment and cryotherapy. In an attempt to understand these findings, we studied the effect of scleral depression (without retinal ablation) on the ERG.
Patients being treated for retinoblastoma underwent ERG testing during regularly-scheduled examinations under anesthesia. ERGs were obtained according to the ISCEV standard ERG protocol, modified to minimize the duration of anesthesia. We report photopic response amplitude data, which is representative of the full protocol. ERG recordings were obtained as the initial procedure after anesthesia induction. Patients were then examined with scleral depression of one or both eyes, and the ERG was repeated. In a few cases, the ERG was repeated after 5-10 minutes without intevening scleral depression.
ERGs were obtained from 25 patients, some on more than one occasion. A total of 64 recordings were analyzed. Scleral depression reduced the ampltude of the response to 30-Hz photopic flicker by an average of 28.3 microvolts, or 19% (p=0.008). In 12 instances where scleral depression was performed unilaterally, the 30-Hz flicker response was also reduced by an average of 32 microvolts (20%) in the fellow eye (p=0.02). In addition to amplitude reductions, photopic waveforms were altered by scleral depression. In many cases, the photopic "off response" was apparently delayed. In six eyes for which the ERG was repeated after 5-10 minutes without scleral depression, the photopic single flash and 30-Hz flicker responses were unchanged.
Scleral depression appears to reduce the amplitude of the photopic ERG, at least when recorded during examination under anesthesia. After unilateral scleral depression, a comparable reduction in the ERG amplitude is also seen in the untouched fellow eye. This effect is not seen after an additional 5-10 minutes of anesthesia without ocular manipulation. This observation may explain some of the variability in the clinical ERG, if testing follows fundus examination with scleral depression.
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