Abstract
The normal foveal anatomy consists of the extrusion of the plexiform layers, a formed foveal pit, cone photoreceptor outer segment (OS) lengthening, and outer nuclear layer (ONL) widening relative to the parafoveal OS and ONL.1 Foveal development begins with the centrifugal displacement of the inner retinal layers, followed by the centripetal migration of the cone photoreceptors toward the incipient fovea and finally cone photoreceptor specialization (PRS) (Fig 1A–E ).2 3 , 4 Foveal pit formation begins midgestation and continues postnatally characterized by pit deepening and widening. Simultaneously, outer retinal specialization occurs at a rapid rate in the first 2 years of life, but ONL widening continues up to the age of 13 years.4 Lengthening of the OS layer represents cone PRS and reflects peak cone photoreceptor density.5 The maturation of the human fovea is a complex process and is critical for visual function.4 Many genes are involved in the development of the retina. Pathogenic variants in these genes often cause disruption to the foveal developmental process, resulting in foveal hypoplasia (FH), which describes the underdevelopment of the fovea.Foveal hypoplasia is characterized by the continuation of inner retinal layers posterior to the foveola, and progressive loss of these foveal elements is represented by increasing grades of FH. Three key foveal development stages form the basis of the Leicester Grading System for FH, developed by Thomas et al1 (Fig 1F–H). The Leicester Grading System is divided into 4 grades of typical FH (grades 1–4) and 1 grade of atypical FH, which is associated with photoreceptor degeneration. The Leicester Grading System can be applied to a diverse range of genetic disorders, including albinism, achromatopsia, and those caused by pathogenic variants in PAX6, SLC38A8, FRMD7, and AHR. In addition to providing insight into the degree of foveal development, identifying the grade of FH has diagnostic and prognostic implications.1 ,7 Rufai et al7 reported that identifying the grade of FH can predict future visual acuity (VA) in preverbal children with nystagmus. The introduction of OCT has revolutionized ophthalmic diagnosis with the ability to promptly visualize retinal morphology in a high-resolution, noninvasive manner.8 ,9 Furthermore, the advent of handheld OCT has facilitated the foveal examination of pediatric patients.10 OCT is now available in most ophthalmology clinics and subsequently has rapidly become part of routine ophthalmic assessment.8 Thus, the necessary technology to identify foveal developmental abnormalities is now accessible in most centers. The phenotypic spectrum of albinism has been reported, including the variance in degree of arrested retinal development.11 ,12 Furthermore, high grades of FH (grades 3 and 4) have recently been consistently associated with SLC38A8 variants13 ; however, to date, the full spectrum of FH linked with variants of known genes involved in retinal development has not been investigated. Thus, it is unclear whether variants of certain genes known to be related to FH and nystagmus are associated with a more underdeveloped foveal morphology. We performed a comparative study to investigate and characterize the genotypic and phenotypic spectrum of FH in albinism, achromatopsia, PAX6, SLC38A8, FRMD7, and AHR variants.
