What is Pachychoroid?
Pachychoroid is a structural and functional choroidal phenotype characterized by dilated choroidal vessels (pachyvessels) in Haller's layer with overlying choriocapillaris attenuation, representing a pathologic state that predisposes to various exudative maculopathies rather than simply an abnormally thick choroid. 1, 2
Core Pathologic Features
The defining characteristic of pachychoroid is not merely increased choroidal thickness, but rather the presence of hypertrophic or congested vessels in the choroid under areas of reduced or absent choriocapillaris in the posterior pole. 2 This represents a shift from simply measuring thickness to identifying specific morphological abnormalities with functional implications. 1
Key Structural Components:
- Pachyvessels: Dilated choroidal vessels in Haller's layer (the outer choroidal vascular layer) 3, 1
- Inner choroidal attenuation: Thinning or loss of the choriocapillaris and inner choroidal layers overlying the dilated vessels 1, 2
- Choroidal vascular hyperpermeability: Functional abnormality visible on indocyanine green angiography 4
- Increased choroidal thickness: Often present but not required for diagnosis; subfoveal choroidal thickness ≥400 μm is used as one diagnostic criterion in some contexts 3
Clinical Spectrum of Pachychoroid Disease
Pachychoroid represents a spectrum of related disorders that share common choroidal alterations but differ in their clinical manifestations and severity. 1, 5
Disorders Within the Pachychoroid Spectrum:
- Central serous chorioretinopathy (CSC): Serous neuroretinal detachment with focal RPE leakage 3, 1
- Pachychoroid pigment epitheliopathy: RPE alterations without serous fluid 3, 1
- Pachychoroid neovasculopathy: Type 1 neovascularization developing over areas of pachychoroid 3, 1, 6
- Polypoidal choroidal vasculopathy: Polypoidal dilations with underlying pachychoroid features 3, 1
- Peripapillary pachychoroid syndrome: Maximal choroidal thickness near the optic nerve with nasal macular fluid 3, 1
- Focal choroidal excavation: Localized depression in choroidal contour 1, 5
Pathophysiology
The exact pathogenesis remains incompletely understood, but choroidal congestion and vascular dysfunction appear central to disease development. 4 The dilated choroidal vessels create increased hydrostatic pressure, leading to choriocapillaris compromise and subsequent RPE dysfunction. 1 This cascade can progress from simple RPE alterations to fluid accumulation, neovascularization, and ultimately photoreceptor atrophy. 2
Clinical Significance and Implications
Recognizing pachychoroid is critical because it represents a distinct pathogenic pathway for macular neovascularization, separate from drusen-related age-related macular degeneration. 4 Eyes previously categorized as typical neovascular AMD or polypoidal choroidal vasculopathy can be sub-categorized based on presence or absence of pachychoroid versus drusen, which may have treatment implications. 4
Diagnostic Imaging Features:
- OCT: Dilated Haller's layer vessels, increased choroidal thickness, choriocapillaris thinning 1, 2
- ICGA: Choroidal vascular hyperpermeability, hyperfluorescent placoid areas 3, 4
- OCT-A: Visualization of neovascular networks over pachyvessels when present 3, 6
- Fundus autofluorescence: RPE alterations corresponding to areas of choriocapillaris loss 6
Important Clinical Pitfalls
Do not diagnose pachychoroid based solely on increased choroidal thickness measurements. 2 The critical finding is the presence of dilated choroidal vessels with overlying choriocapillaris attenuation, which can occur even with normal choroidal thickness in some cases. 2
Distinguish pachychoroid-related neovascularization from drusen-related AMD, as these may respond differently to anti-VEGF therapy, with pachychoroid neovasculopathy potentially benefiting from photodynamic therapy in addition to or instead of anti-VEGF injections. 3