Arteries and Veins of the Orbit
However, there are a number of additional ancillary arteries and veins that help support proper structure and functioning of the eyes.
Arteries of the Orbit
Ophthalmic Artery & Branches
Our discussion of the arteries and veins will begin with the ophthalmic artery, which is the first branch off of the internal carotid artery distal to the cavernous sinus. Branches of the ophthalmic artery supply all of the structures within the orbit, in addition to other structures found in the nose, face and meninges. In some cases, the ophthalmic artery will branch off just before the internal carotid exits the cavernous sinus.
While this artery usually arises from the internal carotid along the medial length of the anterior clinoid process (part of the sphenoid bone, which gives attachment to the tentorium cerebelli) and runs anteriorly through the optic canal along with and inferolaterally to the optic nerve, it can also pass superiorly to the optic nerve in rare cases. At the posterior third of the orbit cone, the ophthalmic artery will make a sharp turn to run medially along the orbit wall.
The arteries in the paragraphs that follow are branches of the ophthalmic artery.
Central Retinal Artery
First, the central retinal artery is the first (and one of the smaller) branches of the ophthalmic artery, which runs in the dura mater inferior to the optic nerve. When this central retinal artery approaches the globe posteriorly and is roughly 12.5 millimeters (0.5 inches) away, it will take a turn superiorly and penetrate the optic nerve. It then continues along, posterior to the optic nerve, within its dural sheath to enter the eye and supply the inner retinal layers.
The lacrimal artery is the next branch of the ophthalmic artery that arises as the ophthalmic artery enters the orbit, and runs along the superior edge of the lateral rectus muscle. This is one of the largest branches and the artery functions to supply the lacrimal gland, eyelids and conjunctiva.
Posterior Ciliary Arteries
When the ophthalmic artery turns medially, it branches off into 1 to 5 posterior ciliary arteries (PCA) that will subsequently branch into the long (LPCA) and short (SPCA) posterior ciliary arteries. The LPCA and SPCA perforate the sclera posteriorly near to the optic nerve and macula to supply the posterior uveal tract.
Muscular branches of the ophthalmic artery can arise in one of two ways:
- when the ophthalmic artery is continuing medially, the superior and inferior muscular branches can arise from the artery, or
- a single trunk from the ophthalmic artery can divide into the superior and inferior branches, which function to supply the extraocular muscles.
As the ophthalmic artery passes superiorly over the optic nerve, the supraorbital artery can branch off from it and will pass anteriorly along the medial border of the superior rectus and levator palpebrae, and then further through to the supraorbital foramen to supply muscles and skin of the forehead.
Posterior Ethmoidal Artery
Upon reaching the medial wall of the orbit where the ophthalmic artery will turn anteriorly again, the posterior ethmoidal artery will enter the nose via the posterior ethmoidal canal and supply the posterior ethmoidal sinuses, and then finally enter the skull to supply the meninges, which helps protect the central nervous system.
Anterior Ethmoidal Artery
And as the ophthalmic artery process travels anteriorly, it will also branch off into the anterior ethmoidal artery, which enters the nose after traversing the anterior ethmoidal canal and supplies the anterior and middle ethmoidal sinuses, as well as the frontal sinus. The anterior ethmoidal artery will also enter the cranium to supply the meninges.
Moving along, the ophthalmic artery will continue anteriorly into the trochlea, where the medial superior and inferior palpebral arteries will arise and supply the eyelids.
Frontal & Dorsal Nasal Arteries
Finally, the ophthalmic artery will terminate in two branches: the supratrochlear (or frontal) artery and the dorsal nasal artery. Both of these arteries exit the orbit medially to supply the forehead and scalp.
In close proximity to the orbit, the infraorbital artery branches off the maxillary artery and emerges through the infraorbital foramen, just under the orbit of the eye.
To summarize, the following arteries are associated with the orbit of the eye:
- the ophthalmic artery
- internal carotid artery
- central retinal artery
- lacrimal artery
- posterior ciliary arteries
- muscular branches of the ophthalmic artery
- supraorbital artery
- anterior ethmoidal artery
- posterior ethmoidal artery
- medial palpebral arteries (superior and inferior)
- supratrochlear (or frontal) artery
- dorsal nasal artery
- infraortibal artery
- maxillary artery
Veins of the Orbit
Central Retinal Vein
The central retinal vein (retinal vein) is a relatively short vein that runs through the optic nerve, but leaves the optic nerve before arriving at the eyeball. It drains blood from the capillaries of the retina into either the superior ophthalmic vein or into the cavernous sinus directly. Markedly, anatomy of the veins of the orbit of the eye will vary between individuals, and in some cases, the central retinal vein drains into the superior ophthalmic vein, and in other cases it drains directly into the cavernous sinus.
The ophthalmic veins are the veins that drain the eyes, and can be broken down into the superior ophthalmic vein and the inferior ophthalmic vein.
Finally, the vorticose veins, which are clinically known as the vortex veins, drain the ocular choroid. The number of vortex veins varies from 4 to 8 with over half of the normal population having 4 or 5, and in most cases, there is at least one vortex vein in each quadrant of the eye.
Typically, the entrances to the vortex veins in the outer layer of the choroid (lamina vasculosa) can be observed funduscopically. The vortex veins run posteriorly in the sclera, and exit the eye posterior to the equator. The path of drainage can also vary between the vortex veins, especially since there is usually collateral circulation between the superior and inferior orbital veins.
Occlusion found in the ophthalmic artery or its branches can produce pathologies and conditions that threaten eyesight. A visit to a optometrist or ophthalmologist is recommended for those who experience changes in their vision. Severe and immediate changes are often signs of a more serious health condition.
Painless loss of monocular vision is the usual presenting symptom of retinal artery occlusion, which is an ophthalmologic emergency. Ocular stroke is commonly caused by embolism of the retinal artery. Emboli can travel to distal branches of the retinal artery and cause loss of vision in only a small area of the visual field.
Delay in treatment may result in permanent loss of vision, while immediate attention can improve the chances of vision recovery (however, prognosis remains poor). Since the central retinal vein is the venous equivalent of the central retinal artery, this vein can also suffer from occlusion—known as central retinal vein occlusion, which will result in presentations similar to ocular ischemic syndrome.
When performing an ophthalmic/optical fundus exam, the arteries appear brighter red and narrower than do the veins. Although, since the retinal arteries lack a muscular coat, they are more accurately referred to as arterioles.
Since the posterior cilliary arteries (PCAs) arise directly from the ophthalmic artery, and are end arteries that will anastomose with any other artery, sudden occlusion of any PCA will produce an infarct in the region of the choroid supplied by that particular PCA. Occlusion of a short or long PCA will produce a smaller choroidal infarct(s) within the larger area supplied by the specific parent PCA.