Optic nerve
Special sensory function
Afferent system for vision.
Anatomy
1. Optic nerve
The optic nerve is an extension of the brain, not a true nerve. It is formed by retinal ganglion cell axons. It is myelinated by oligodendrocytes (and not Schwann cells like other cranial nerves). It is divided into 4 segments: intraoccular, intraorbital, intracanalicular and intracranial.
The intraoccular segment is 1 mm long and occupies lamina cribrosa of the sclera, where the ganglion cell axons exit the globe.
The intraorbital segment is 20-30 mm long and is covered by all 3 meningeal layers of the brain. Increases in intracranial pressure are transmitted via the subarachnoid space. The central retinal artery and vein enter the optic nerve about 1 cm posterior to the globe.
The intracanalicular segment is 4-9 mm long and runs within the optic canal. The ophthalmic artery runs inferior to the nerve in this segment. The dura fuses with the periosteum of the orbit at this level, tethering the nerve thus making this segment susceptible to shear injury in head trauma.
The intracranial segment is ~10 mm long. It is surrounded by CSF and covered by pia. The ophthalmic artery runs inferolateral to this segment.
2. Optic chiasm
Fibers from the nasal half of each retina cross to the opposite site. The most posterior/medial fibers in the chiasm do not originate from the optic nerves. They are fibers connecting the medial geniculate bodies on each side forming the ventral supraoptic decussation or commissure of Gudden.
3. Optic tract
The optic tract is composed of a medial and a lateral band of fibers which curve around the cerebral peduncles. The lateral band carries most fibers and terminates in the lateral geniculate body of the thalamus. The medial band reaches the medial geniculate body from where it projects to pretectal nuclei and the superior colliculus.
4. Optic radiation
The optic radiation or geniculocalcarine tract is composed of fibers from the lateral geniculate body to the primary visual cortex around the calcarine fissure in the occipital lobe. The optic radiation is composed of two distinct parts:
Meyer’s loop carries fibers from the inferior part of the retina. These fibers pass through the temporal lobe by looping around the inferior horn of the lateral ventricle. They carry information from the superior part of the visual field.
Baum’s loop carries fibers from the superior part of the retina. These fibers pass through the parietal lobe in the retrolenticular limb of the internal capsule. They carry information from theinferior part of the visual field.
5. Visual cortex
The primary visual cortex is located medially in the occipital lobe, along the calcarine fissure. The gyrus superior to the calcarine sulcus collects afferents from the superior half of the retina (lower visual field). The gyrus inferior to the calcarine sulcus collects afferents from the inferior half of the retina (upper visual field).
Clinical implications
- Lesions anterior to the optic chiasm cause unilateral visual deficits. Lesions of or posterior to the optic chiasm cause bilateral visual deficits.
- Damage to the optic chiasm causes bitemporal hemianopia.
- Damage to the optic tract causes contralateral homonymous hemianopia.
- Damage to Meyer’s loop causes contralateral homonymous superior quadrant quadrantanopia (“pie in the sky”).
- Damage to Baum’s loop causes contralateral homonymous inferior quadrant quadrantanopia (“pie on the floor”).
- Damage to the visual cortex causes bilateral hemianopia. Sometimes macular sparing is observed (5-10 degrees of central visual field spared) because 1) the macular region may receive bilateral arterial supply and 2) the macula may have larger cortical representation.
- Anterior ischemic optic neuropathy (AION) affects the anterior part of the optic nerve and disc. It usually presents with unilateral, acute onset, painless, rapid vision loss. The majority of AION are non-arteritic (NAION) and are associated with diabetes mellitus, elevated intraocular pressure, sleep apnea, dyslipidemia, acute hypotension (eg in cardiac arrest) and certain drugs such as amiodarone and interferon-alpha. Most patients present with defect of the lower half of the visual field (inferior altitudinal loss). Arteritic AION (AAION) occurs in patients with systemic arteritis such as polyarteritis nodosa, Churg Strauss vasulitis, rheumatoid arthritis or granulomatosis with polyangiitis (Wegener’s granulomatosis).
- Posterior ischemic optic neuropathy (PION) affects the retrobulbar part of the nerve, without initial disc involvement. It is less common than AION. PION is a watershed infarction of the optic nerve caused by precipitous and prolongued fall in blood pressure. It is usually bilateral.
- Other causes of optic neuropathy include inflammatory, infiltrative, compressive, traumatic, toxic (eg methanol or ethyl alcohol ingestion), nutritional (eg in vitamin B12 deficiency) and hereditary (eg mitochondrial optic neuropathies).