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The optic canal is a funnel-like structure as part of the sphenoid bone that extends from the optic foramen to the orbital apex, the posterior-most end of the orbit. The orbital apex consists of the optic canal and the superior orbital fissure. The superior orbital fissure is bordered superomedially by the lesser wing of the sphenoid bone and inferolaterally by the greater wings of the sphenoid bone. The superior orbital fissure is the largest opening that connects the orbit with the middle cranial fossa.[rx] The optic canal connects the orbit to the middle cranial fossa and transmits the optic nerve, ophthalmic artery, meningeal sheaths, and sympathetic nerve fibers.
The optic nerve, also known as the cranial nerve II, transmits visual signal from the retina to the visual cortex. The ophthalmic artery, the first branch of the internal carotid artery, arises distal to the cavernous sinus and supplies mainly the orbit but also other structures in the face and meninges.
Blood Supply and Lymphatics
Understanding blood supply to the optic nerve is vital as the optic nerve is a vulnerable structure to compression within the limited space of the optic canal. The main vascularization of the optic nerve comes from the superior hypophyseal arteries and ophthalmic artery.[rx]
The hypophyseal arteries mainly supply the intracranial and intracanalicular part of the optic nerve while the ophthalmic artery supplies the intraorbital portion of the optic nerve through the long ciliary arteries and the central retinal artery.
A critical structure passing through the optical canal is the ophthalmic artery. The ophthalmic artery is the first main branch of the internal carotid artery. It originates from the distal dural ring intracranial, intracanalicular, and intraorbital sections. During its course, it runs inferolateral relative to the optic nerve within the optic canal.[rx]
- Central retinal artery: supplies the inner layer of the retina
- Long posterior ciliary artery: two branches arise that supply the iris through the circulus arteriosus major
- Short posterior ciliary artery: branches pierce the sclera to supply the choroid and the ciliary process.
- Lacrimal artery: supplies the lacrimal gland and the anterior portion of the eyeball and a portion of the eyelid
- Anterior ethmoidal artery: gives rise to the anterior meningeal artery. It supplies ethmoidal air cells and the periosteum
- Posterior ethmoidal artery: supplies the ethmoidal air sinuses and part of the nasal mucosa and septum
- Supraorbital artery: supplies part of the orbit and face – terminal branches: include the supratrochlear (or frontal) artery and the dorsal nasal artery, which supply the forehead and scalp
Understanding the lymphatic drainage of the various tissues in the eye are crucial in studying conditions that involve dysfunctional lymphatic systems, including inflammatory diseases, metastatic cancers, transplant rejection, lymphatic malformation, and surgical complications.
While most body tissues have an embedded lymphatic drainage system, the ocular lymphatic structure has a heterogeneous appearance. While the cornea, lens, retina, ciliary body, choroid, and sclera are mostly lymphatic-free, other tissues are not. Optic nerve sheath is considered a lymphatic-rich ocular structure.[rx] This area is rich with LYVE-1 (lymphatic vessel endothelial hyaluronan receptor-1).[rx]
Another crucial structure passing through the optical canal is the optic nerve. The optic nerve is the second cranial nerve surrounded by the cranial meninges and responsible for the transmission of sensory information for vision. The retinal ganglion cells receive impulses from the rods and cones and subsequently converge to form the optic nerve. After its formation, it leaves the bony orbit, passes through the optic canal and the sphenoid bone to enter the cranial cavity and run along the middle cranial fossa.
Within the middle cranial fossa, both optic nerves converge to form the optic chiasm. The following optic tracts emerge from the optic chiasm:
- Left optic tract: transmits signal from the left temporal retina and the right nasal retina
- Right optic tract: transmits signal from the right temporal retina and the left nasal retina
All optic tracts synapse at the lateral geniculate nucleus in the thalamus. Axons project from this region into two major optic radiation tracts[rx]:
- Upper optic radiation: these fibers travel through the parietal lobe and carry information from the superior retinal quadrants, which correspond to the inferior visual field quadrants.
- Lower optic radiation: these fibers travel through the temporal lobe and carry information from the inferior retinal quadrants, which correspond to the superior visual field quadrants
Each optic tract travels to its corresponding cerebral hemisphere to reach the lateral geniculate nucleus (LGN), a relay system located in the thalamus; the fibers synapse here.
Axons from the LGN then carry visual information via a pathway known as the optic radiation. The pathway itself divides into the following[rx]:
- Upper optic radiation – carries fibers from the superior retinal quadrants (corresponding to the inferior visual field quadrants). It travels through the parietal lobe to reach the visual cortex.
- Lower optic radiation– also known as the Meyers loop, carries fibers from the inferior retinal quadrants (corresponding to the superior visual field quadrants). It travels through the temporal lobe, via a pathway known as Meyers loop, to reach the visual cortex.
These optic radiations project to the visual cortex, where sensory data is processed and interpreted.
There are seven extraocular muscles within the ocular orbit[rx]:
- Levator palpebrae superioris and superior tarsal muscle: levator palpebrae superioris innervated by the oculomotor nerve; superior tarsal muscle innervated by the sympathetic nervous system; both elevate the upper eyelid
- Superior rectus: innervated by the oculomotor nerve; the primary function is the elevation of the eyeball.
- Inferior rectus: innervated by the oculomotor nerve; the primary function is a depression of the eyeball.
- Medial rectus: innervated by the oculomotor nerve; the primary function is the adduction of the eyeball.
- Lateral rectus: innervated by the abducens nerve; the primary function is the abduction of the eyeball.
- Superior oblique: innervated by the trochlear nerve; the primary function is depression, abduction, and medial rotation of the eyeball.
- Inferior oblique: innervated by the oculomotor nerve; the primary function is elevation, abduction, and lateral rotation of the eyeball.