Anatomy, Head and Neck, Optic Canal

There are seven extraocular muscles within the ocular orbit[12]:

• 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 depression of the eyeball.  
• Medial rectus: innervated by the oculomotor nerve; the primary function is 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.

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.[1]

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.[1][13][14]

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.[15] While many approaches exist, the endoscopic endonasal approach has decreased morbidity and recovery time compared to other methods.[16]

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.[17]

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: 

• Autosomal dominant optic atrophy: This is the most common hereditary optic neuropathy. It leads to bilateral optic nerve degeneration. The genetic defect has been attributed to the coding of inner mitochondrial membrane proteins. Some of the genes responsible are OPA1, OPA3, OPA4, OPA5, and OPA8. The disease course is highly variable but it can lead to insidious vision loss, especially during the first decade of life in those with associated extra-ocular features. It can also present with central, centrocecal and paracentral scotomas, color perception deficits. A visual evoked potential test could demonstrate reduced amplitudes and prolonged latencies. Currently, there is no established treatment for this condition.[18]
• Leber hereditary optic neuropathy: an inherited mitochondrial disorder characterized by bilateral, painless vision loss during young adulthood. Vision loss occurs simultaneously or sequentially. There is a severe reduction of visual acuity by central or centrocecal scotoma. Other symptoms include postural tremor, peripheral neuropathy, and myopathy. This condition results from the degeneration of retinal ganglion cells and their projecting axons forming the optic nerve. The diagnosis is through genetic testing. Most commonly identified genes are MT-ND1, MT-ND4, or MT-ND6. There is no cure, and the management is supportive.[19]

Non-hereditary optic nerve disorders: 

• Glaucoma: the worldwide leading cause of progressive vision loss. It classifies into open-angle glaucoma and angle-closure glaucoma, with the latter being more common. They can also classify into primary and secondary diseases. Secondary glaucoma could result from trauma, corticosteroids, inflammation, or tumor.[20] Risk factors for primary open-angle glaucoma include increased cup-to-disc ratio and asymmetry, disc hemorrhage, elevated intraocular pressure (IOP), family history of glaucoma, black race, and old age.[21] Most important preventive modality is the reduction of intraocular pressure. In open-angle glaucoma, the resistance to aqueous outflow through the trabecular meshwork becomes increased while in angle-closure glaucoma, the drainage sites get obstructed. The resulted increased in intraocular pressure transmits strain to the posterior structures of the eye, damaging the retinal ganglion cells and the optic nerve in the process. Medications used to lower the IOP include the following:
  • Prostaglandin analogs (e.g., latanoprost, travoprost, tafluprost, unoprostone, bimatoprost): Increase outflow of aqueous humor. 
  • Beta-adrenergic blockers (e.g. timolol, levobunolol, carteolol, metipranolol, betaxolol): Decrease aqueous humor production.
  • Alpha-adrenergic agonists (e.g., brimonidine, apraclonidine): Decrease aqueous humor production and increase outflow. 
  • Carbonic anhydrase inhibitors (e.g., dorzolamide, brinzolamide, acetazolamide): Decrease aqueous humor production.
  • Cholinergic agonists (e.g., pilocarpine, carbachol): Increase aqueous humor outflow.
• Optic neuritis: inflammation of the optic nerve. It could be caused by a variety of conditions such as multiple sclerosis, neuromyelitis optica, ischemic optic neuropathy, or infection. Common symptoms include visual loss, periocular pain, and dyschromatopsia. The first line of treatment is intravenous steroids. Spontaneous visual improvement is possible in greater than 90% of the cases.[22]
• Ischemic optic neuropathies: includes anterior ischemic optic neuropathy (AION) affecting the optic disc and posterior ischemic optic neuropathy (PION) affecting the retrobulbar part of the optic nerve. Both types are further subdivided into arteritic and non-arteritic. Arteritic ischemic optic neuropathy is caused by giant cell arteritis mediated by arterial inflammation whereas non-arteritic ischemic optic neuropathy is caused by small arterial occlusive disease. Early treatment with corticosteroid therapy is necessary to avoid permanent vision loss.[23]
• Papilledema: the swelling of the optic disc (most often bilateral) as a result of increased intracranial pressure. Possible underlying causes include space-occupying brain lesions, cerebral hemorrhage, head trauma, hydrocephalus, meningitis, cerebral venous sinus thrombosis, and idiopathic intracranial hypertension. Treatment should be tailored based on the underlying etiology.[24]
• Optic nerve sheath meningiomas (ONSM): a rare finding. ONSM is a benign neoplasm of the meningothelial cells of the meningeal sheath around the optic nerve, either intraorbital or intracanalicularly. While the growth itself is benign, mass compression can lead to compression of the optic nerve or the surrounding blood vessels. This complication can lead to progressive vision loss.[25] Treatment options include observation, radiotherapy, surgery, local chemotherapy, and systemic chemotherapy.[26]

Another essential structure within the optic canal is the ophthalmic artery. Some disorders of the ophthalmic artery include the following: 

• Ophthalmic artery occlusion: severe cases can lead to ocular ischemic syndrome. 
• Ophthalmic artery aneurysm: 5% of cerebrovascular aneurysms are ophthalmic artery aneurysms. It tends to be asymptomatic until it becomes large enough to become symptomatic. Most common symptoms are diplopia and headache. When symptomatic, they are surgical emergencies in most cases. Embolization techniques are the standard of treatment.[17]

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).[27] A rise in intracranial pressure is transmitted by the cerebrospinal fluid (CSF) to the back of the eyeball.[28] 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.[29]