We read with interest on the article by Gliem and associates, which reported statistically significant decreased subfoveal choroidal thickness and loss of small choroidal vessels in patients with pseudoxanthoma elasticum (PXE) as compared to age-controlled normal subjects. However, there are relevant factors that we would like to point out.
Gliem and associates classified PXE patients into certain disease characteristics “assuming more severe Bruch membrane alterations with more progressed disease stages.” However, the authors did not provide any data for qualitative or quantitative changes in the Bruch membrane. A previous study identified breaks in the Bruch membrane with alterations in the overlying retinal pigment epithelium (RPE) in spectral-domain optical coherence tomography (SD OCT) among PXE patients. Hence, the number, extent, location, and length of the breaks within the Bruch membrane could have been further studied. Moreover, in advanced PXE with large areas of fibrosis and atrophy, breaks in a thickened Bruch membrane might not be detected on fluorescein angiography or fundus photograph. However, noninvasive SD OCT may still detect breaks in the Bruch membrane in advanced PXE.
In their study, Gliem and associates classified PXE patients into 3 distinct groups based on PXE-associated fundus changes, choroidal neovascularization, or chorioretinal atrophy. It is known that PXE patients might present with a mix of the above features resulting in visual loss. Hence, such a classification may not account for subjects that have overlapping features.
Additionally, Gliem and associates qualitatively assessed the vascular changes in the inner and outer choroid using enhanced depth imaging OCT volume scans to reconstruct en face views 60 μm below the RPE. They did not explain why 60 μm was taken as the cutoff. There could be a potential misinterpretation of the vascular changes of the choroid, especially so when choroid thickness ranges from 331.4 ± 23.9 μm in controls to 103.8 ± 27.9 μm in Group 3 PXE patients. Instead, a better solution might have been to delineate the Sattler and Haller layer of the choroid. Sim and associates have described the method of measurement. Subsequently, the delineation of the Sattler and Haller layer of the choroid may enable a better assessment while providing measurements within the different layers.
Gliem and associates studied choroidal thickness taken on a single horizontal and vertical line scan centered on the fovea. Previous studies have reported on macular choroidal thickness in normal subjects based on an ETDRS grid. It will be interesting to note if there are also differences in mean sectorial choroidal thickness in other areas of the grid among PXE patients.
We were glad that Gliem and associates took into consideration factors that could influence choroidal thickness like age, axial length, refractive error, previous retinal pathologies or surgeries, and diurnal variation.
In conclusion, this paper has provided important information on choroid changes in PXE patients. However, the evaluation of the Bruch membrane could be better assessed on SD OCT to investigate if changes in the Bruch membrane directly impact choroidal changes.