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(A) Illustration of the unique features of a normal fovea detectable on optical coherence tomography. (B) Illustration of typical and atypical grades of foveal hypoplasia. All grades of foveal hypoplasia have incursion of inner retinal layers. Atypical foveal hypoplasia also has incursion of the inner retinal layers. In OCA all four grades of foveal hypoplasia are observed; however, in OA and Hermansky-Pudlak syndrome, only grades 3 and 4 have been observed [Kuht et al 2022a].

Grade 1: shallow foveal pit (near-normal pit in grade 1a vs shallow indent in grade 1b), outer nuclear layer (ONL) widening, and outer segment (OS) lengthening relative to the parafoveal ONL and OS length, respectively

Grade 2: all the features of grade 1 present except the presence of a foveal pit

Grade 3: all the features of grade 2 present except the widening of the cone OS

Grade 4: all the features of grade 3 are present except no widening of the ONL at the fovea

Atypical foveal hypoplasia: shallower foveal pit with disruption of the inner segment ellipsoid (Note: Not observed in albinism, but seen in other retinal disorders such as achromatopsia [Thomas et al 2011a, Thomas et al 2011b].)

Adapted with permission from Thomas et al [2011b]