The cilioretinal artery (CRA) is a unique blood vessel that plays a specialized role in the circulation of the retina. The retina is the light-sensitive tissue lining the back of the eye, requiring a constant supply of oxygen and nutrients to function. While the central retinal artery supplies most of the retina, the CRA represents an important variation in this system. This small vessel often acts as a secondary path for blood flow to the most sensitive part of the visual system.
Unique Anatomy and Prevalence
The eye’s blood supply is divided into the retinal circulation and the ciliary circulation. The central retinal artery supplies the inner layers of the retina, but the cilioretinal artery is unusual because it typically originates from the ciliary circulation, specifically the short posterior ciliary arteries, or directly from the choroid. This distinct origin means the CRA is not a branch of the main central retinal artery. The artery usually enters the retina at the edge of the optic nerve head, often in a temporal position, and can be seen with an ophthalmoscope.
The presence of a cilioretinal artery is an anatomical variation, not a disease, and acts as a natural bypass mechanism for a portion of the retina. Prevalence estimates vary depending on the detection method. However, the artery is generally observed in approximately 15% to 30% of people, meaning it is absent in the majority of individuals.
Essential Role in Central Vision
When present, the cilioretinal artery contributes significantly to the blood supply of the macula, the central area of the retina responsible for sharp, detailed central vision. The macula contains the fovea, the point of highest visual acuity. The CRA often supplies the papillomacular bundle, a cluster of nerve fibers that relay information from the macula to the optic nerve.
The macula’s blood supply is extremely important for fine detail tasks. In eyes without a CRA, the macula’s inner layers rely on the central retinal artery branches. When a functional CRA is present, it provides an additional, direct source of blood flow to this visually sensitive area. This supplementary circulation can lead to a measurable improvement in visual acuity compared to eyes without the artery.
Cilioretinal Artery Occlusion
A blockage in the cilioretinal artery is known as cilioretinal artery occlusion. This event results from the same underlying causes as other retinal artery blockages, most commonly an embolus traveling from a larger vessel, such as the carotid artery, and lodging in the small vessel. Inflammatory conditions that affect the blood vessel walls are also potential causes.
The immediate symptom of a CRA occlusion is a sudden, painless loss of vision localized to the central field of view. Vision loss is variable, ranging from moderate blurring to a severe central blind spot, depending on the area supplied by the occluded vessel. Since the artery supplies a sector of the macula, the resulting visual defect is often sector-shaped. Diagnosis requires a full medical workup to identify the source of the embolus and address systemic health risks, such as high blood pressure, to prevent future events.
Protection During Central Retinal Artery Occlusion
The most significant clinical importance of the cilioretinal artery is its ability to protect central vision during a central retinal artery occlusion (CRAO). CRAO occurs when the main central retinal artery is blocked, leading to a sudden and often permanent loss of vision across the entire eye. This blockage causes severe damage as the retina is deprived of oxygen.
When a functional cilioretinal artery is present, its separate blood supply from the ciliary system remains unaffected by the CRAO. This mechanism, known as macular sparing, allows blood to continue flowing to the macula and fovea, preserving sharp central vision despite the rest of the retina being compromised. Studies show that approximately 26% of eyes experiencing CRAO exhibit macular sparing due to a functional CRA. Patients with this protective anatomy experience a significantly better visual prognosis and a higher likelihood of retaining useful visual acuity.