Stargardt Disease Presenting as Retinitis Pigmentosa: A Case of Monoallelic ABCA4-Associated Retinopathy.

Stargardt disease is an inherited retinal disorder caused by mutations in the ABCA4 gene and encompasses a wide range of phenotypes. Variability in age of onset, visual function, and retinal findings can make diagnosis challenging. We report a 48-year-old Hispanic female patient with a heterozygous ABCA4 variant, c.5603A>T (p.Asn1868Ile), previously classified as benign. The patient had well-preserved best-corrected visual acuity of 20/40 in both eyes and normal color vision testing. Fundus examination revealed optic disc pallor, arteriolar attenuation, and peripheral bone spicule-like pigmentation in both eyes. Fundus autofluorescence demonstrated central hypoautofluorescence with surrounding hyperautofluorescence, suggestive of retinal pigment epithelium dysfunction. Spectral-domain optical coherence tomography showed preserved central macular contour with central subfield thickness (CST) of 298 µm in the right eye (OD) and 312 µm in the left eye (OS). Additionally, cubic volume was decreased in the OD at 9.1 mm³ and within normal limits in the OS at 9.6 mm³. Visual field testing demonstrated an annular pattern of visual field loss with significant mean deviation reduction (-17.17 dB in the OD and -16.12 dB in the OS). This adult-onset, RP-like presentation highlights the phenotypic variability of ABCA4-associated retinopathies and underscores the importance of cautious interpretation of monoallelic variants. The novelty of this case lies not in the mutation itself, but in the distinct clinical phenotype and diagnostic challenge it presents, reinforcing the need for careful genotype-phenotype correlation in inherited retinal disorders.