Detailed anatomy of the optic canal is important to decipher the various pathologies of the region as well to guide surgical procedures and therapeutic options. The optic canal transmits the optic nerve, ophthalmic artery, and sympathetic nerve fibers.
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. 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.
During the third month of gestation, the cartilaginous optic foramen forms. The cartilaginous optic foramen gets ossified between week 12 and 17 of gestation. During the fifth month of fetal development, the bony optic foramen transforms into the bony optic canal. This transformation is dependent on the normal development of the optic strut, the lower part of the sphenoidal wing, into an anterior-inferior and a posterior-superior segment.
Orbital development exhibits a linear growth. The growth rate is different at various stages of fetal growth. The optic canal reaches its peak growth potential when a fetus reaches 400 mm in length. Male fetuses tend to present with a larger diameter of orbit than female counterparts. In a study, researchers determined that the length of the optic canal was 1.61 cm (1.1 to 2.3 cm) in male participants and 1.39 cm (0.7 to 2.0 cm) in female participants.
While the diameter of the cranial opening portion of the optic canal grows the most during the gestational period, this diameter continues to grow during childhood and into adulthood.
Assessing the proper formation and growth of the optic canal is essential when studying the various ocular malformations such as ocular hypertelorism, hypotelorism, anophthalmos, and microphthalmos. These congenital abnormalities affect the shape of the optic canal. Orbital defects in fetuses can be assessed using ultrasound or MRI.
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.
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.
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 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. This area is rich with LYVE-1 (lymphatic vessel endothelial hyaluronan receptor-1).
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:
All optic tracts synapse at the lateral geniculate nucleus in the thalamus. Axons project from this region into two major optic radiation tracts:
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:
These optic radiations project to the visual cortex, where sensory data is processed and interpreted.
There are seven extraocular muscles within the ocular orbit:
Optic canal variants include duplicated optical canal and the key-hole anomaly. Duplicated optic canal is a rare finding present in 0.64% of the population. In the key-hole anomaly, the optic canal has a grooved floor with a keyhole appearance in plain film radiographs. This variant was present in 2.65% of studied orbits.
In some cases of the duplicated optic canal, the origin of the ophthalmic artery is the intracavernous part of the internal carotid artery. In this variant, the ophthalmic artery could pass through the superior orbital fissure.
The optical canal has crucial surgical implications given the important function of the structures within the canal. A significant surgical procedure involving the optic canal is optic nerve decompression. Some of the indications for optic nerve decompression include traumatic optic neuropathy, non-traumatic optic neuropathy (e.g. Graves ophthalmopathy, space-occupying lesions/tumors of the orbit, periorbital sinuses, and the surrounding structures), idiopathic intracranial hypertension (also known as pseudotumor cerebri), neoplasms (sinonasal tumor, meningioma, orbital apex tumor), and osseous lesions.
Decompression of the optic nerve is achievable through several approaches, including transcranial and transsphenoidal routes. Endoscopic optic nerve decompression is an option for both traumatic and non-traumatic optic neuropathies. However, its utilization has been far less common for non-traumatic optic neuropathy. While many approaches exist, the endoscopic endonasal approach has decreased morbidity and recovery time compared to other methods.
Another important structure within the optical canal is the ophthalmic artery. Ophthalmic artery aneurysms pose a serious surgical challenge. By the time patients become symptomatic enough for a diagnosis, they are usually emergency cases. Embolization using the coiling technique is a standard of care. Embolization is contraindicated in cases of large ophthalmic artery aneurysms, atherosclerotic, or hypoplastic internal carotid artery.
The optic canal is a very important structure due to the structures that pass through this canal, mainly the optic nerve and the ophthalmic artery. Damage to the optic nerve can lead to permanent loss of vision. Disorders of the optic nerve can be classified into two main categories:
Hereditary optic nerve disorders:
Non-hereditary optic nerve disorders:
Another essential structure within the optic canal is the ophthalmic artery. Some disorders of the ophthalmic artery include the following:
Meningeal sheets cover the optic nerve: pia mater, arachnoid mater and dura mater. Subarachnoid space surrounds the optic nerve. The optic canal is the narrowest point of the optic nerve subarachnoid space. The subarachnoid space surrounding the optic nerve is continuous with the rest of the CSF flow. The CSF flow surrounding the optic nerve can be analyzed using magnetic resonance imaging (MRI). A rise in intracranial pressure is transmitted by the cerebrospinal fluid (CSF) to the back of the eyeball. Therefore, the retinal ganglion cells and their axons have exposure to two types of pressures: intraocular pressure and intracranial pressure. The rise in either pressure can lead to detrimental effects for the retinal ganglion cells and the optic nerve. Increased pressure on optic nerve leads to papilledema. It is important to distinguish papilledema from pseudopapilledema. Pseudopapilledema is the congenital elevation of the optic disc(s). Features of pseudopapilledema include dome-shape disc elevation, lack of edema around the peripapillary nerve fiber layer, and abnormal peripapillary vessels. Ancillary testing may be needed to distinguish one from the other. A-scan ultrasound and optical coherence tomography could be used to differentiate papilledema from pseudopapilledema. Optical coherence tomography can e be useful to measure peripapillary retinal nerve fiber layer thickness. A-scan ultrasound can be used to measure the retrobulbar optic nerve sheath diameter.
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