27 Chiasmal misrouting in infantile nystagmus syndrome (INS): phenotypes in patients with molecular diagnoses.

Chiasmal misrouting, once believed to be pathognomonic for albinism, has been reported in cases of INS, independent of melanin pathway disruption. The purpose of this study is to determine if there are clinical-electrophysiological parameters that correlate with particular genotypes in INS.A retrospective chart review at Moorfields Eye Hospital identified 71 patients with a molecular diagnosis relating to INS. Visual acuity; presence of nystagmus, signs of albinism and OCT foveal hypoplasia grade were recorded alongside flash and pattern VEP (Visual Evoked Potential) amplitude and peak time. VEP asymmetry was assessed using the Pearson Correlation Coefficient (r).Pathological variants in 8 genes (TYR, OCA2, HPS6, HPS3, HPS1, GPR143, FRMD7, SLC38A8, OCA1) were identified. Mean BCVA per group ranged from 0.38-0.74LogMAR F(0.72,3.5)=2.8; p=0.04 one-way ANOVA. All genotypes demonstrated foveal hypoplasia (mode grade 4) except FRMD7 (all grade 1). In this cohort, positive flash and pattern VEP amplitude/peak time asymmetry correlated with clinical signs of albinism (flash VEP, r=0.22(0-6yrs); pattern VEP, r=0.17(6-65yrs)). There was marked asymmetry in SLC38A8 patients (r = -0.85 to-0.93), a feature known to be associated with foveal hypoplasia 2.This study provides a detailed genotype-phenotype correlation of VEP findings in a molecularly characterised INS cohort - useful in selecting clinically guided genetic testing and counselling patients.