What are the various pressures in the vascular system of the retina and choroid?

Vascular Pressures in the Retinal and Choroidal Circulations

Key Pressure Relationships

The choroidal circulation operates at a lower mean arterial pressure compared to the retinal circulation, a critical distinction that explains differential vulnerability to vascular occlusions. 1, 2

Retinal Circulation Pressures

Arterial Pressure Characteristics

• The retinal circulation maintains higher mean arterial perfusion pressure than the choroidal system, originating from the central retinal artery with robust autoregulatory capacity. 2
• The retinal vasculature compensates effectively for changes in intravascular pressure through intrinsic autoregulation mechanisms. 1, 2
• Retinal blood flow is characterized as relatively low flow with high resistance compared to choroidal circulation. 3

Venous Pressure Dynamics

• Increased intravascular retinal venous pressure can impede cilioretinal artery flow when combined with the lower choroidal arterial pressure, particularly in central retinal vein occlusion. 1
• Normal retinal veins maintain standard caliber in arterial occlusions, contrasting with the dilated, tortuous appearance in venous occlusions. 1

Choroidal Circulation Pressures

Arterial Pressure Features

• The choroidal circulation has lower mean arterial pressure relative to retinal arterial pressure due to multiple collateral flow channels within the choroidal vasculature. 1, 2
• The cilioretinal artery, when present (15-25% of eyes), demonstrates lower intravascular perfusion pressure than the central retinal artery because it originates from the short posterior ciliary arteries with extensive choroidal collateralization. 1, 2
• Choroidal blood flow is extremely high at 696±110 mg/min throughout the entire choroid, reflecting a low-resistance, high-flow system. 2

Regulatory Limitations

• The choroidal circulation lacks autoregulatory mechanisms that compensate for pressure changes, instead relying predominantly on sympathetic innervation for regulation. 1, 2, 3
• The absence of autoregulation means choroidal perfusion is more directly dependent on absolute systemic blood pressure and intraocular pressure. 3

Clinical Pressure Relationships

Perfusion Pressure Dynamics

• Ocular perfusion pressure (OPP) depends on both mean arterial pressure (MAP) and intraocular pressure (IOP), with complex interactions affecting blood flow. 4
• In the optic nerve head, blood flow correlates with MAP at IOP ≤25 mmHg (P<0.001), but this correlation disappears at IOP >25 mmHg (P=0.79). 4

Pathological Pressure States

• Cilioretinal artery occlusion occurs when elevated retinal venous pressure combines with lower choroidal arterial pressure, creating a pressure gradient that impedes flow through the higher outflow resistance. 1
• Starting at IOP of 30 mmHg, retinal blood flow diminishes linearly with increasing pressure and is nearly extinguished at 100 mmHg. 5
• Choroidal filling remains unaffected until IOP reaches 60 mmHg, then decreases to 20% of baseline at 100 mmHg, demonstrating greater pressure tolerance than retinal circulation. 5

Anatomical Basis for Pressure Differences

Vascular Architecture

• The retinal circulation features non-fenestrated capillaries with tight junctions forming the inner blood-retinal barrier, creating higher resistance. 2
• The choroidal circulation has fenestrated capillaries allowing free passage of molecules up to 500 kDa, contributing to lower resistance and higher flow. 2

Regulatory Infrastructure

• The choroid lacks the glial cells, has markedly reduced pericyte ensheathment, and lacks intermediate filaments in pericytes compared to retinal vasculature, explaining its limited autoregulatory capacity. 6
• The retinal vasculature possesses robust neurovascular coupling with astrocytes and pericytes that enable pressure autoregulation. 6, 3