Blood Vessels and Nerves of the Eye
The human eye is a highly evolved structure of our anatomy and has many coexisting and interdependent elements. It is capable of moving and follow the objects along with accommodating to near and far; the eyes also can see in varying light, and in colour. Our two eyes working together give us stereoscopic vision, and depth perception. This article covers the anatomy, function and clinical relevance of the vessels and nerves of the eye.
Anatomy of the Eye
The eyeball is filled with vitreous humor, with the aqueous humor lying in the small anterior chamber of the eye. The eye itself is coated with three layers:
- the sclera and cornea (opaque and transparent layer respectively)
- the choroid (filled with blood vessels)
- the retina (with the rod cells for black and white, and the cone cells for colour)
The optic nerve feeds in the posterior surface of the eye and carries all visual information from the retina, ultimately to the primary orbital cortex, which lies within the depths of the calcarine sulcus on the occipital lobe of the brain.
The fovea centralis is the region of the retina with a high concentration of cone cells (colour detecting cells). The fovea and the surrounding area are not supplied by direct branches from the ophthalmic artery, and are in fact supplied directly from the choroid. As well as staying wet enough to maintain a transparent layer (tear film), the eye must also be supplied with nutrients, and be able to move for our vision to be effective. The pupil must be able to dilate and constrict to accommodate to light and autonomic stimuli, and the lens must be able to relax and contract for near and far vision.
Blood Supply and Drainage of the Eye
The eye is supplied by the ophthalmic artery, which is the first branch of the internal carotid artery, when it has travelled passed the cavernous sinus. The ophthalmic artery has numerous branches that supply the muscles that move the eye and surround the eye, the eyelid and the eyeball itself. The branches of the ophthalmic artery are divided into the orbital (supply the orbit and related structures) and the optical group (supply the eye and its muscles).
This artery arises from the ophthalmic artery near the optic canal. It runs with the lacrimal nerve to supply the lacrimal gland. The terminal branches of the artery pass through the lacrimal gland and supply the eyelids and conjunctiva as the lateral palpebral arteries, and pass medially to supply the upper and lower eyelids respectively. These arteries then anastomose with the medial palpebral artery and form a complete arterial circle.
This artery arises from the ophthalmic artery just as it passes over the optic nerve, and runs forwards, along the medial border of levator palpebrae superioris and superior rectus muscles. It then passes through the supraorbital foramen in order to supply the:
- upper eyelid
- frontal sinus
- levator palpebrae superioris
- part of the scalp
Posterior Ethmoidal Artery
Once the ophthalmic artery reaches the medial wall of the orbit, it runs forwards and gives off the posterior ethmoidal artery. This artery enters the nasal cavity by passing through the posterior ethmoidal canal, and supplies the posterior ethmoidal sinuses as well as continuing to enter the skull and supply the meninges.
Anterior Ethmoidal Artery
This artery branches from the ophthalmic artery within the orbit, and accompanies the nasociliary nerve through the anterior ethmoidal foramen into the middle and anterior air cells as well as the frontal sinus. Before the artery passes into the cranium, it provides a meningeal branch to supply the dura mater. There are also nasal branches that pass into the nasal cavity via a small opening adjacent to the crista galli, and goes on to supply a section of the dorsum of the nose.
Medial Palpebral Artery
This artery has two branches i.e. the superior and inferior palpebral arteries. They arise opposite the superior oblique muscle. They supply the upper and lower eyelids respectively.
Long Posterior Ciliary Arteries
These arteries branch from the ophthalmic artery near the optic nerve, and run anteriorly on both sides of the eyeball. They run between the sclera and the choroid layers, and they run to supply the ciliary muscle where they divide further. These two arteries merge and form the circulus arteriosus major around the iris, which run inwards to form a smaller circle of arteries (the circulus arteriosus minor). In all they supply the:
- ciliary body
Short Posterior Ciliary Arteries
There are around 6-12 of these arteries for each eye. They branch from the ophthalmic artery as it curves medially to cross over the optic nerve. They pierce the back of the eye and run between the sclera (which they supply) and choroid, and supply up to the ciliary processes. They also give off smaller branches that supply the optic disc. They do this by forming an arterial ring known as the circle of Zinn-Haller.
Anterior Ciliary Arteries
There are 7 of these arteries per eye, and they supply the sclera, and rectus muscles. They branch from the ophthalmic artery and pass forwards to the anterior aspect of the eyeball, where they pierce the sclera, near the cornea, and terminate in the circulus arteriosus major, that surrounds the iris. Medial, inferior and superior rectus are supplied by two branches each, with the lateral rectus receiving the remaining single branch.
Central Retinal Artery
This artery runs underneath the optic nerve and lies within the dural sheath of the nerve to reach the eyeball. It pierces the optic nerve itself near the back of the eye, and sends numerous branches over the internal aspect of the retina. In diabetic retinopathy, there can be haemorrhages and aneurysms that can form in this artery and its branches.
The central retinal vein runs through the optic nerve. From here it drains into the cavernous sinus or superior ophthalmic vein. The superior ophthalmic vein is usually the largest and is the principal vein. It is formed when the supraorbital and angular veins unite just behind the trochlea (pulley like structure). The veins that drain into it are named as the arteries of the region (medial palpebral, lacrimal, anterior ethmoidal, inferior ophthalmic, central retinal and muscular).
The inferior ophthalmic vein runs over the surface of the inferior rectus muscle, and drain to the cavernous sinus or the superior ophthalmic vein. It drains the inferior rectus muscle, inferior oblique muscle, lacrimal sac and lower lid.
There is sometimes a middle ophthalmic vein which some consider a second inferior ophthalmic vein. The veins of the region run with the arteries and follow a similar course.
Nerve Supply to the Eye
The special sense of sight is transmitted by cranial nerve II, the optic nerve. This nerve leaves the skull via the optic canal, and provides us with our sense of sight. The pair of optic nerves are in fact a direct structure of the brain, and not technically a distinct cranial nerve like the other 11 pairs. They unite together at the optic chiasm that lies superior to the pituitary gland (which lies in the sella turcica).
The peripheral fields/nasal retinal fields cross over at the chiasm. The optic tracts then travel to synapse in the lateral geniculate nucleus of the thalamus, and from there they travel through Meyer’s loop (from inferior retina/superior visual field) and Baum’s loop (from the superior retina/inferior visual field). From there, they go to the primary visual cortex.
This occurs when the eye is exposed to light. This originates from the Edinger Westphal nucleus which carries parasympathetic fibers that run as the outer part of the oculomotor nerve, and eventually synapse with the ciliary ganglion (which is a parasympathetic ganglion that lies in the posterior orbit. This then gives off the short ciliary nerves, which innervate the constrictor pupillae.
Lens Relaxation & Tensing/Accommodation
The short ciliary nerves described above also innervate the ciliaris muscle, which contract, and release the tension on the zonular fibers, causing the lens to become more convex (rounder) and hence focus on near objects. When we need to focus on a distant object, the impulse to the ciliary muscles is withdrawn, the suspensory ligaments become taut, and the lens tenses and become broader.
This occurs when the eye is in the dark, when we are experiencing sympathetic output i.e. adrenaline (fight, flight and fright response), and in response to some drugs. Anatomically, the long ciliary nerves (sympathetic nerves) mediate the reflex.
Cranial nerve III is the oculomotor nerve, and as the name suggests it moves the eye. It leaves the skull through the superior orbital fissure. It innervates the inferior rectus, medial rectus and inferior oblique. When the eye is deviated away from the midline the inferior rectus is responsible for depressing the eyeball. The medial rectus acts to adduct the eyeball. When the eyeball is adducted, the inferior oblique acts to elevate the eyeball.
Cranial nerve IV, the trochlear nerve originates from the posterior aspect of the brainstem (the only cranial nerve to do so), and is a long slender nerve that winds around the brainstem to run forwards. It then leaves the skull via the superior orbital fissure and innervates the superior oblique muscle. This muscle runs through a pulley, and attaches the eyeball in an oblique fashion. This muscle is responsible for depressing the adducted eye. If the muscle acts alone when the eye is facing directly forwards, it moves the eye down and out. Hence it is known as the ‘tramp’s’ muscle.
Cranial nerve VI is the abducens, which innervates the lateral rectus muscle, and leaves the skull via the superior orbital fissure. The muscle it supplies abducts the eye, and its function links with the name of the nerve that supplies it.
Sensation of the eye is possible via the V1 (ophthalmic) division of the trigeminal nerve. This nerve exits the skull via the superior orbital fissure. It provides sensation to the:
- upper eyelid
- ridge of the nose as far down as the nasal tip
Oculomotor Nerve Palsy- The eye appears downward and abducted (due to paralysis of the inferior oblique, and medial rectus). There is ptosis (upper eyelid droop, due to denervation to the levator palpebrae superioris), and the pupil is dilated (as the constrictor pupillae is denervated). If there is a rise in intracranial pressure, the pupil is dilated and unresponsive. This is because the parasympathetic short ciliary nerves lie on the outside of the oculomotor nerve and are affected before the motor component of the nerve.
Uveitis- This is inflammation of the uveal tract (middle layer) of the eye. Symptoms include pain on eye movement.
Glaucoma- This is a rise in the intraocular pressure of the eye. The patient will feel a pressure within the eye. Treatment includes medication to reduce intraocular pressure.
Horner Syndrome- When the sympathetic nucleus of Budge located in the intermediolateral horn of the thoracic spinal cord becomes damaged, the sympathetic supply to the eye is compromised. Horner’s syndrome is when something damages or disrupts the sympathetic chain. Symptoms include miosis, anhydrosis and ptosis.
Lateral Rectus Palsy- This is caused by Abducens nerve damage. The eye is unable to abduct beyond the midline.
Pituitary tumours- These tumours expand upwards, and compress the optic chiasm from below. The temporal visual fields (nasal retinal fields) cross over at the chiasm and therefore result in a bitemporal hemianopia.
Retinal hemorrhages- Diabetes and hypertension cause damage to the retinal arteries. This can cause blindness if it goes untreated.
Ocular Ischaemic syndrome- This is caused by severe occlusive disease of the carotid artery. Symptoms include visual loss and ocular pain. Ocular treatments include photocoagulation to reduce neovascularisation of the retina as well as intravitreal steroids. Systemic treatment includes antiplatelet therapy, systemic steroids and thrombolytic therapy.