Long posterior ciliary arteries

The long posterior ciliary arteries are two major arteries in the eye that arise from the ophthalmic artery and play a crucial role in supplying blood to the anterior segment of the eye, particularly the ciliary body and the iris. These arteries are distinct from the short posterior ciliary arteries due to their longer course and different area of supply.

Location

The long posterior ciliary arteries originate from the ophthalmic artery, just as it passes through the orbit. There are typically two long posterior ciliary arteries, one located medially and one laterally. These arteries travel along the outer layer of the eyeball, piercing the sclera near the optic nerve. They then course forward, running between the sclera and choroid, to reach the ciliary body and iris. They eventually anastomose with the anterior ciliary arteries to form part of the major arterial circle of the iris.

Anatomy

The long posterior ciliary arteries (LPCAs) are critical arteries that supply blood to the anterior segment of the eye, particularly the ciliary body, iris, and parts of the choroid. There are typically two LPCAs, and they have a longer course than the short posterior ciliary arteries. Below is a detailed description of their anatomy.

Origin

The long posterior ciliary arteries originate from the ophthalmic artery, which itself is a branch of the internal carotid artery. The ophthalmic artery enters the orbit through the optic canal and branches into various arteries, including the long posterior ciliary arteries. The LPCAs usually arise in pairs, with one positioned medially (nasal side) and one laterally (temporal side) relative to the optic nerve.

Course

The course of the long posterior ciliary arteries can be divided into several segments as they travel toward the anterior part of the eye:

• Posterior Segment (Intraorbital Segment): After branching from the ophthalmic artery, the LPCAs travel anteriorly alongside the optic nerve. Unlike the short posterior ciliary arteries, which pierce the sclera around the optic disc, the LPCAs penetrate the sclera at a more distal point, slightly farther from the optic nerve head.
• Scleral Penetration: The LPCAs penetrate the sclera near the posterior pole of the eye, but they do so at two points: one medially and one laterally. After entering the sclera, they travel between the sclera and choroid along the meridian of the eyeball, extending toward the front of the eye.
• Anterior Segment: The LPCAs continue to course forward through the suprachoroidal space (the space between the sclera and the choroid). They run longitudinally along the eye’s outer surface, parallel to the eyeball’s axis, until they reach the ciliary body and iris in the anterior segment of the eye. Their journey is longer compared to the short posterior ciliary arteries, hence their name.
• Termination and Anastomosis: Upon reaching the anterior segment, the LPCAs divide into smaller branches that contribute to the vascular supply of the ciliary body and the iris. These branches anastomose with the anterior ciliary arteries to form the major arterial circle of the iris, a circular vascular structure located at the root of the iris. This anastomosis ensures that the anterior segment of the eye receives a rich and consistent blood supply.

Branches of the Long Posterior Ciliary Arteries

The LPCAs give off small branches along their course, and these branches serve various parts of the eye, particularly the anterior segment:

• Branches to the Choroid: As the LPCAs travel through the suprachoroidal space, they give off small branches to the choroid, especially in the mid-peripheral and anterior regions. These branches help supply blood to the choroid, though the short posterior ciliary arteries are the main suppliers of the posterior choroid.
• Branches to the Ciliary Body: Upon reaching the anterior segment of the eye, the LPCAs provide branches to the ciliary body, which is involved in controlling the lens shape for accommodation (focusing) and the production of aqueous humor.
• Anastomosis with Anterior Ciliary Arteries: The LPCAs ultimately anastomose with the anterior ciliary arteries to form the major arterial circle of the iris. This vascular ring provides an essential blood supply to the iris and the ciliary body, ensuring proper circulation in the anterior segment.

Anatomical Relations

The long posterior ciliary arteries have important relationships with nearby structures within the orbit and eye:

• Optic Nerve: The LPCAs travel alongside the optic nerve as they course toward the eye but do not penetrate the sclera near the optic disc. Instead, they pierce the sclera farther away from the optic nerve, at a more peripheral point. This differentiates them from the short posterior ciliary arteries, which have a closer anatomical relationship with the optic nerve.
• Sclera and Choroid: After entering the sclera, the LPCAs travel between the sclera and the choroid in the suprachoroidal space, which is a key anatomical region separating these two layers of the eye. As they progress toward the front of the eye, the arteries give off small branches to the choroid but primarily continue to the anterior segment.
• Ciliary Body and Iris: The LPCAs terminate by supplying the ciliary body and iris, two critical structures located in the anterior segment of the eye. The ciliary body is responsible for controlling the lens’s shape and producing aqueous humor, while the iris regulates the amount of light entering the eye by adjusting the size of the pupil.

Anastomoses

The long posterior ciliary arteries form important anastomoses with other vessels, particularly in the anterior segment of the eye:

• Major Arterial Circle of the Iris: The LPCAs anastomose with the anterior ciliary arteries (which arise from the muscular branches of the ophthalmic artery) to form the major arterial circle of the iris. This circle is located at the root of the iris and provides a critical blood supply to both the ciliary body and the iris.
• Choroidal Anastomoses: Although the LPCAs do not primarily supply the posterior choroid, they provide small branches that contribute to the blood supply of the mid-peripheral and anterior choroid, helping maintain its vascular health.

Variability

In most individuals, there are two long posterior ciliary arteries—one medial and one lateral. However, there may be anatomical variations in the exact number and location of these arteries. The general course and function of the arteries remain consistent, but small differences in branching patterns may exist between individuals.

Termination

The long posterior ciliary arteries terminate by contributing to the major arterial circle of the iris and supplying blood to the ciliary body and iris. These arteries provide critical blood flow to the anterior segment of the eye, ensuring the proper functioning of the structures in this region.

Function

The long posterior ciliary arteries (LPCAs) have a critical role in supplying oxygenated blood to the anterior segment of the eye, particularly to the ciliary body, iris, and parts of the choroid. Below is a detailed explanation of the various functions of these arteries, with subheadings highlighting their specific contributions.

Blood Supply to the Ciliary Body

Nourishing the Ciliary Muscle

• The long posterior ciliary arteries are the primary source of blood for the ciliary body, specifically the ciliary muscle, which controls the shape of the lens for accommodation (focusing on objects at varying distances). This muscle adjusts the lens’s curvature to enable the eye to focus on near or distant objects.
• The LPCAs ensure that the ciliary muscle receives a constant supply of oxygen and nutrients, enabling it to function effectively in accommodation. A disruption in this blood supply can impair the ability of the ciliary muscle to contract, leading to difficulties with focusing.

Aqueous Humor Production

• The LPCAs also supply blood to the ciliary processes, part of the ciliary body responsible for producing aqueous humor, the clear fluid that fills the anterior and posterior chambers of the eye. Aqueous humor plays a critical role in maintaining intraocular pressure and providing nutrients to avascular structures like the lens and cornea.
• By nourishing the ciliary processes, the LPCAs support the production and regulation of aqueous humor, ensuring that the eye maintains proper pressure and that nutrients are delivered to essential structures. Without adequate blood flow from the LPCAs, the production of aqueous humor would be compromised, potentially leading to issues such as increased intraocular pressure or poor nutrient delivery to the cornea and lens.

Blood Supply to the Iris

Regulation of Pupil Size

• The LPCAs contribute to the blood supply of the iris, the colored part of the eye responsible for regulating the size of the pupil. The iris controls the amount of light entering the eye by adjusting the pupil’s size in response to changes in ambient lighting.
• The long posterior ciliary arteries, along with the anterior ciliary arteries, form the major arterial circle of the iris, providing the iris with a robust blood supply. This ensures that the sphincter pupillae (for constriction) and dilator pupillae (for dilation) muscles within the iris have the necessary nutrients and oxygen to regulate the pupil’s size effectively.

Support for Light Regulation

By maintaining the blood flow to the iris, the LPCAs play an indirect role in regulating the amount of light that enters the eye. Proper blood flow ensures that the muscles within the iris can contract or dilate appropriately, responding to changes in light conditions. This is crucial for visual adaptation in different lighting environments, such as moving from a dark room to a bright area.

Blood Supply to the Anterior Choroid

Nourishing the Mid and Anterior Choroid

• Although the short posterior ciliary arteries primarily supply the posterior choroid, the LPCAs contribute to the blood supply of the anterior portion of the choroid. The choroid is the vascular layer between the sclera and the retina that nourishes the outer retinal layers, especially the photoreceptors.
• The branches of the LPCAs help supply blood to the anterior regions of the choroid, ensuring that these areas receive adequate oxygen and nutrients to support the health of the outer retina.

Maintaining Photoreceptor Health

The LPCAs indirectly support the retinal pigment epithelium (RPE) and the outer layers of the retina by supplying the anterior choroid. The RPE is responsible for maintaining photoreceptor cells, recycling visual pigments, and absorbing excess light. Proper nourishment of the choroid by the LPCAs is vital for the RPE to function effectively, which in turn supports the health of the photoreceptor cells responsible for vision.

Contribution to the Major Arterial Circle of the Iris

Formation of the Major Arterial Circle

• One of the most important functions of the LPCAs is their contribution to the formation of the major arterial circle of the iris, an anastomotic network that supplies blood to the iris and the ciliary body. This vascular circle is formed by the anastomosis of the LPCAs with the anterior ciliary arteries, creating a rich blood supply for the anterior segment of the eye.
• The major arterial circle ensures that the iris and ciliary body receive a steady supply of oxygenated blood, even in cases where blood flow through one artery may be compromised. This anastomotic network helps maintain the physiological processes necessary for normal eye function.

Ensuring Collateral Blood Supply

The anastomoses between the LPCAs and the anterior ciliary arteries provide a collateral circulation in the anterior segment of the eye. This redundancy ensures that if one artery is blocked or damaged, the other can compensate by maintaining the blood supply. The major arterial circle serves as a backup system that protects the eye’s vital anterior structures from ischemia.

Thermoregulation and Metabolic Support

Maintaining Temperature and Metabolic Balance

The LPCAs contribute to the thermoregulation of the anterior segment of the eye. The eye is a metabolically active organ, and blood flow is essential for dissipating heat generated by metabolic processes. The LPCAs help regulate temperature by providing a continuous flow of blood to the ciliary body and iris, preventing overheating and ensuring that these tissues maintain optimal metabolic conditions.

Removal of Metabolic Waste

The LPCAs also play a role in removing metabolic byproducts from the tissues they supply, including the ciliary body and iris. By providing a constant flow of oxygenated blood and carrying away carbon dioxide and other waste products, the LPCAs help maintain the metabolic balance necessary for the normal functioning of these structures.

Clinical Significance

The long posterior ciliary arteries (LPCAs) are clinically significant due to their crucial role in supplying blood to the anterior segment of the eye, particularly the ciliary body and iris. Any disruption in the LPCAs’ blood flow can lead to serious conditions, such as anterior ischemic optic neuropathy (AION) or uveitis, both of which may cause significant visual impairment.

In cases of occlusion or ischemia of the LPCAs, the ciliary body may be affected, leading to impaired accommodation (focusing ability) and reduced aqueous humor production, which can affect intraocular pressure and potentially result in glaucoma. Additionally, reduced blood flow to the iris can result in pupil abnormalities and difficulties in regulating the amount of light entering the eye.

Diseases affecting the LPCAs, such as inflammatory conditions or systemic vascular diseases like giant cell arteritis, can compromise the vascular supply to the anterior segment, leading to vision-threatening complications. Therefore, the LPCAs are vital in maintaining normal ocular function, and damage to these arteries can result in profound ocular pathology and vision loss.