A novel KCNV2 mutation in a patient taking hydroxychloroquine associated with cone dystrophy with supernormal rod response

Background: Cone dystrophy with supernormal rod response (CDSRR) is a rare inherited retinal degeneration. A patient superimposed with medical conditions requiring use of hydroxychloroquine (HCQ) may obscure accurate diagnosis of CDSRR. Herein, we report a referral case for HCQ retinopathy screening. Comprehensive ophthalmic examinations, however, guided the diagnosis of CDSRR from a novel mutation in potassium voltage-gated channel modifier subfamily V member 2 (KCNV2) gene. Materials and methods: Comprehensive ophthalmic examinations were evaluated for two patients whose parents are first cousins. Direct sanger sequencing of KCNV2 was applied to confirm the mutation. Results: A 38-year-old male proband was referred for HCQ retinopathy screening after taking HCQ for systemic lupus erythematosus (SLE). Fundus examination showed bull's eye pattern, and photoreceptor loss in the foveal region of both eyes was noted on spectral domain-optical coherence tomography (SD-OCT). The full-field electroretinography (ffERG) revealed a disproportionate increase in scotopic maximal response with implicit time delay, as well as universal cone dysfunction. Proband's 24-year-old sister had similar ffERG pattern in both eyes. Direct sanger sequencing of KCNV2 gene revealed a novel homozygous mutation c.280_281 insG (p.Ala94GlyfsTer278), confirming a diagnosis of CDSRR. Conclusions: We report a novel KCNV2 mutation in a consanguineous family. The unique ffERG features of CDSRR are pathognomonic and thus crucial in guiding clinicians toward genetic testing of the KCNV2 gene. Altogether, multimodal imaging, ffERG, and detailed history taking are important diagnostic tools for differentiating between acquired and inherited retinal disorders.