How do you distinguish retinal arteries from retinal veins?

Distinguishing Retinal Arteries from Retinal Veins

Retinal arteries are narrower, lighter red in color, have a more prominent central light reflex, and typically cross over veins at arteriovenous crossing points, while veins are wider, darker red, have a less prominent or absent light reflex, and pass beneath arteries at crossings.

Key Distinguishing Features

Caliber and Size

• Arteries are approximately 2/3 to 3/4 the diameter of veins at comparable locations in the retinal vasculature 1
• The inferior temporal vessels are typically larger than superior temporal vessels in both arteries and veins 1
• Vessel diameter measurements at the optic disc border and 2mm from disc center can help differentiate vessel types 1

Color and Appearance

• Arteries appear lighter red (more orange-red) due to oxygenated blood
• Veins appear darker red (more burgundy) due to deoxygenated blood
• This color difference is one of the most reliable clinical features for differentiation

Central Light Reflex

• Arteries have a more prominent, narrower central light reflex that appears as a bright stripe along the vessel center 2
• Veins have a wider, less distinct light reflex or may lack one entirely 2
• Small vessels (<50 microns) may not show visible light streaks regardless of type 2
• The ratio of light reflex width to blood column width is greater in small veins compared to small arteries 2

Anatomical Relationships at Arteriovenous Crossings

• At arteriovenous crossing points, arteries typically cross over (anterior to) veins 3
• Branch retinal vein occlusions commonly occur at these arteriovenous crossings where vessels share a common adventitial sheath 3
• These crossing points are most frequently found in the superior temporal quadrant 3

Clinical Examination Approach

Direct Ophthalmoscopy

• Examine vessels radiating from the optic disc, noting caliber differences
• Trace vessels from the disc peripherally to identify branching patterns
• Arteries typically have more acute angle branching compared to veins

Advanced Imaging Confirmation

• Fluorescein angiography can definitively distinguish arteries from veins based on filling patterns, with arteries filling first in the early arterial phase (10-15 seconds after injection) and veins filling later in the arteriovenous phase 3, 4
• Optical coherence tomography angiography (OCTA) can detect vascular abnormalities and help identify vessel types 3
• Automated classification systems using deep learning achieve 91-95% accuracy in artery-vein classification 5, 6, 7

Common Pitfalls to Avoid

• Do not rely solely on light reflex in small vessels (<100 microns), as the reflex characteristics overlap significantly between small arteries and veins 2
• Avoid assuming all narrower vessels are arteries in pathological conditions like glaucoma, where arterial caliber decreases significantly with disease progression 1
• In retinal vein occlusions, venous dilation and tortuosity can make veins appear more prominent than usual, potentially causing confusion 3
• Peripheral vessels are more challenging to classify than central vessels; when uncertain, trace the vessel back toward the optic disc where identification is clearer 6