  Purpose: To quantify choriocapillaris flow alterations in early Sorsby Fundus
Dystrophy (SFD) and to investigate the relationship of choriocapillaris flow
with the choroidal and outer retinal microstructure. Methods: In this
prospective case-control study, 18 eyes of 11 patients with early SFD and 32
eyes of 32 controls without ocular pathology underwent multimodal imaging
including spectral-domain optical coherence tomography (OCT)followed by
deep-learning-based layer segmentation. OCT-angiography (OCT-A) was performed
to quantify choriocapillaris flow signal deficits (FDs). Differences in
choriocapillaris flow area percentage between SFD patients and controls were
determined and a structure-function correlation with outer retinal layer
thicknesses were analyzed based on mixed model analysis. Results: SFD patients
exhibited a significantly greater choriocapillaris FDs area percentage than
controls (estimate [95% CI] 32.05% [24.31-39.80] vs. 23.36% [20.64-26.09],
P<0.001), even when adjusting for age. Choroidal thickness was a structural OCT
surrogate of the choriocapillaris FD area percentage (-0.82% per 100
micrometer, P=0.017), whereas retinal-pigment-epithelium-drusen-complex
thickness was not informative regarding choriocapillaris FDs (P=0.932). The
choriocapillaris FD area percentage was associated with an altered
microstructure of the overlying photoreceptors (outer-segments, inner-segments
and outer-nuclear-layer thinning of -0.31, -0.12 and -0.47 $\mu$m per %FD,
respectively, P<0.001). Conclusions: Patients with early SFD exhibit pronounced
abnormalities of choriocapillaris flow signal on OCT-A, which are not limited
to areas of sub-RPE deposits seen in OCT imaging. Thus, analysis of the
choriocapillaris flow may enable clinical trials at earlier disease stages in
SFD and possibly in mimicking diseases with an impaired Bruchs membrane
including age-related macular degeneration.
