Retina as a window to systemic hypertension: From hypertensive retinopathy to artificial intelligence-driven microvascular biomarkers

Table 2 Comparison of retinal imaging modalities for hypertension-related microvascular assessment

Modality	Principal biomarkers	Resolution/depth	Relative cost	Accessibility	Acquisition burden	Clinical readiness
Non-mydriatic fundus photography	HR grade; CRAE, CRVE, AVR; fractal dimension, tortuosity, branching angle; AI-derived BP and CVD risk	Surface, approximately 10 µm lateral	Low (portable and smartphone systems available)	High; already embedded in national DR screening programmes	Seconds; no dilation; non-specialist operator	Deployable now; the only modality realistically scalable to population screening
Optical coherence tomography	Peripapillary RNFL and macular GC-IPL thickness	Cross-sectional, approximately 5 µm axial	Moderate to high	Widespread in ophthalmology; rare in primary care	1-2 minute; trained operator	Established in ophthalmic practice; not a hypertension screening tool
OCTA	Superficial and deep capillary vessel density; FAZ area; choriocapillaris flow voids	Capillary plexus level, depth-resolved	High	Specialist ophthalmology centres	1-3 minute; motion artefact common; trained operator	Research and specialist use; device-dependent metrics preclude screening
Adaptive optics SLO	Arteriolar wall thickness; wall-to-lumen ratio	Cellular, minute 2 µm	Very high	Few research centres worldwide	Prolonged; expert operator; small field of view	Research tool only

HR: Hypertensive retinopathy; CRAE: Central retinal arteriolar equivalent; CRVE: Central retinal venular equivalent; AVR: Arteriolar-to-venular ratio; BP: Blood pressure; CVD: Cardiovascular disease; DR: Diabetic retinopathy; RNFL: Retinal nerve fibre layer; GC-IPL: Ganglion cell-inner plexiform layer; FAZ: Foveal avascular zone; SLO: Scanning laser ophthalmoscopy; AI: Artificial intelligence; OCTA: Optical coherence tomography angiography.