I read the paper by Healy et al., which appears in the March 2013 issue,
1 with great interest. The authors describe a relationship between refractive error and the grade of foveal hypoplasia in patients with nystagmus. This finding is novel and interesting. However, the authors also conclude that their results suggest that impaired emmetropization is likely to be attributed to the whole eye effect in albinism and suggest that the fovea does not have a role in the emmetropization process.
The authors included 23 patients with albinism, all of whom had severe foveal hypoplasia (grade 3 or 4),
2 while only 3 out of 17 patients with other forms of nystagmus had severe foveal hypoplasia (grade 3). The remaining patients with nystagmus without albinism had no foveal hypoplasia (
n = 8) or mild foveal hypoplasia (
n = 6; grade 1).
2 The authors found a significant correlation only when statistics included all patients (with and without albinism). As the group without albinism, consisting of only three patients with severe foveal hypoplasia, showed no correlation, the authors concluded that impaired emmetropization may be related to albinism. Statistics based on only three subjects with severe foveal hypoplasia without albinism are unlikely to be sufficiently powered to support this conclusion. No power calculation was included in the paper.
1 In this paper, of the three subjects with grade 3 foveal hypoplasia without albinism, one had a refractive error as high as +5.88 diopters spherical equivalent, suggesting an emmetropization defect in foveal hypoplasia in the absence of albinism.
The conclusion that the fovea does not have a central role in emmetropization is surprising given that numerous retinal diseases, including foveal hypoplasia associated with
PAX6 mutations
3,4 or diseases specifically affecting cone function such as blue cone monochromatism
5 or cone dysfunction syndrome,
6 are described in the literature as having high refractive errors.