Cone dystrophy with supernormal rod response

Cone dystrophy with supernormal rod response (CDSRR), also known as cone dystrophy with supernormal rod electroretinogram or supernormal scotopic electroretinogram with cone dystrophy, is a rare inherited retinal dystrophy that primarily affects the cone photoreceptor cells of the retina while producing a characteristic and paradoxical enhancement of rod photoreceptor responses on electroretinography (ERG). The disease affects the visual system, specifically the retina, leading to progressive loss of central vision, decreased visual acuity, color vision deficiency (dyschromatopsia), and photophobia (light sensitivity). Patients typically experience difficulty with tasks requiring detailed central vision, such as reading and recognizing faces. Nyctalopia (night blindness) may also be present in some individuals despite the supernormal rod ERG findings. The hallmark diagnostic feature of CDSRR is the electroretinographic pattern: cone-mediated (photopic) ERG responses are markedly reduced or absent, reflecting cone dysfunction, while rod-mediated (scotopic) ERG responses — particularly the b-wave amplitude — are abnormally large or 'supernormal,' which is a unique and distinguishing characteristic among retinal dystrophies. This paradoxical supernormal rod response helps differentiate CDSRR from other forms of cone dystrophy. The condition is caused by biallelic pathogenic variants in the KCNV2 gene, which encodes a voltage-gated potassium channel subunit (Kv8.2) expressed in photoreceptor cells. This channel plays a critical role in modulating the electrical activity of both rod and cone photoreceptors. Onset of symptoms typically occurs in childhood, with progressive deterioration of cone function over time. There is currently no cure or specific treatment for CDSRR. Management is supportive and includes low-vision aids, tinted lenses to reduce photophobia, and regular ophthalmologic monitoring. Gene therapy approaches are being explored in preclinical research as a potential future treatment strategy given the well-defined genetic basis of the condition.

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