Abstract
Purpose: :
Electroretinograms (ERGs), specifically 30 Hz flicker amplitudes (amp), are used to monitor retinal function in patients with Infantile Spasms (IS) taking vigabatrin (Vgb), a drug associated with visual field loss. Flicker amp is variable in this infant population decreasing its specificity in detecting retinal changes due to Vgb. Our purpose was to identify a more specific Vgb attributed marker of retinal changes.
Methods: :
Prospective cohort study of 18 patients with IS. ERG 30 Hz flicker amp, Photopic Negative Response (PhNR), cone sensitivity (SCONE) and cone maximum response (Rmax) were collected over the course of drug treatment. PhNRs were generated using a red flash (0.8 log P cd) on a blue background (100 scot cd) and measured as amp. The cone a wave model (Lamb and Pugh; modified by Hood and Birch) described SCONE (phot td-1 s-3) and Rmax (µV). To isolate the effects of Vgb treatment from other developmental changes on the tested retinal markers, linear functions describing the expected development of seizure affected children were generated using data recorded pre-Vgb treatment. All values are expressed as difference from this seizure expected value at test age and then converted to z-scores. For all z-scores, a more negatives value indicates worsening visual function.
Results: :
Contingency tables showed associations between flicker and PhNR (p<0.05) as well as between flicker and SCONE (p<0.01) but not between flicker and Rmax. Data from patients with worsening flicker and PhNR or worsening flicker and SCONE were compared over the course of drug treatment. The rate of change of flicker (-0.07 + 0.02, p<0.01) and Scone (-0.05 + 0.01, p=0.004), but not PhNR, were related to time on drug. Flicker amp z-score has a larger standard deviation at baseline than SCONE (1.0 vs. 0.6).
Conclusions: :
SCONE and flicker both decrease significantly with duration of Vgb treatment in IS patients, with similar rates of reduction over time. These results suggest that cone sensitivity decreases may be an important contributor to flicker abnormalities in this population. Decreased variability at baseline in cone sensitivity may increase specificity of identifying Vgb effects in IS patients.
Keywords: electroretinography: clinical • drug toxicity/drug effects • visual impairment: neuro-ophthalmological disease