Cranial Nerve II: Optic Nerve

By:  Varsha Kumari

INTRODUCTION:

The optic nerve, also known as cranial nerve II, is different from other paired cranial nerves in that it is not part of the peripheral nervous system (PNS). Instead, it is a white-matter tract of the central nervous system (CNS) and contains around 1.2 million axons from retinal ganglion cells. Unlike peripheral nerves, the optic nerve is encased in all three layers of the meninges and is surrounded by cerebrospinal fluid (CSF), further highlighting its CNS origin (Atkins et al., 2011).

BASIC ANATOMY AND FUNCTION:

The optic nerve is divided into four parts: intraocular, intraorbital, intracanalicular, and intracranial. It is made up of nasal and temporal fibers, with the optic chiasm forming where the nasal fibers from both nerves cross, while the temporal fibers remain on the ipsilateral side. Following this, the optic tract extends posteriorly and laterally, winding around the ventrolateral surface of the cerebral peduncles and synapsing in the lateral geniculate nuclei. Finally, the optic radiation transmits visual information to the primary visual cortex in the occipital lobe (Freddi & Ottaiano, 2022).

OPTIC NERVE LESIONS:

The optic nerves are distinct from other CNS structures in terms of anatomy, size, location, and blood supply; therefore, they are uniquely susceptible to every pathological process that may disrupt the central and peripheral nervous systems, including inflammation, ischemia, compression, infiltration, toxic or hereditary metabolic conditions, trauma, and mechanical damage (Atkins et al., 2011).  

1. Optic neuritis:

Inflammation of the optic nerve is most associated with autoimmune diseases, such as multiple sclerosis (MS), but can also occur in other conditions like infections, inflammatory disorders, or due to the action of certain medications or toxins.

It usually presents with acute onset of monocular eye pain and vision loss in a young adult (Guier & Stokkermans, 2023).

• Anterior Ischemic Optic Neuropathy (AION):

It is caused by reduced blood flow to the anterior portion of the optic nerve, which is primarily supplied by the posterior ciliary artery seen frequently among people with vascular risk factors like hypertension or diabetes.
Clinical presentation is sudden, painless vision loss, usually affecting just one eye, with a distinct visual field defect. AION is classified into “non-arteritic” (more prevalent) and “arteritic” (linked to giant cell arteritis) variants (Atkins et al., 2011).

• Optic Nerve Compressive and Infiltrative Lesions:

Compressive lesions of the optic nerve can be unilateral or bilateral, anterior or posterior, neoplastic or inflammatory, infiltrating or noninfiltrating typically caused by tumors (e.g., pituitary adenomas, meningiomas), swelling, or other space-occupying lesions.

It presents with progressive loss of vision, especially peripheral vision, with potential visual field defects and the initial abnormalities on examination include an ipsilateral relative afferent pupillary defect (Decoding Visual Pathway Lesions – American Academy of Ophthalmology, n.d.).

• Leber’s Hereditary Optic Neuropathy (LHON):

LHON is an inherited condition that leads to mitochondrial dysfunction and typically presents in young males with progressive visual loss due to optic neuropathy.

LHON typically initiates painlessly in one eye, progressing to the second eye within a year, leading to profound visual impairment, color vision deficits, and central scotomas (Shemesh et al., 2024). 

• Papilledema (swelling of the optic disc):

Increased intracranial pressure, can result from conditions like brain tumors, hydrocephalus, or intracranial hemorrhages.

Presents with headache, nausea, and transient visual disturbances. It indicates swelling of the optic nerve head due to elevated intracranial pressure (Xie et al., 2022).

DIAGNOSIS:

Diagnosis is established based on the clinical history and clinical examination, of which certain features are particularly essential, including the onset of visual loss, the presence of discomfort and pain with eye movements, the visual acuity, and the retention of color vision. Modern developments in optic nerve imaging, notably retinal digital photography, fundoscopy, optical coherence tomography, and MRI strategies have revolutionized the diagnosis and monitoring of patients with optic neuropathy (Biousse & Newman, 2016).

MANAGEMENT: Management typically depends upon etiology and involves a multidisciplinary approach. The currently available treatment offered for acute optic neuritis (ON) is centered on speeding up visual recovery by the use of high-dose intravenous corticosteroids (Horton & Bennett, 2018). Treatment for papilledema patients with IIH (idiopathic intracranial hypertension) involves weight loss and oral acetazolamide and surgical options should be considered for refractory cases, with ventriculoperitoneal shunting being the preferred procedure of choice (Xie et al., 2022).

REFERENCES:

1. Atkins, E. J., Newman, N. J., & Biousse, V. (2011). Lesions of the optic nerve. Handbook of Clinical Neurology, 102, 159–184. https://doi.org/10.1016/B978-0-444-52903-9.00012-1

• Biousse, V., & Newman, N. J. (2016). Diagnosis and clinical features of common optic neuropathies. The Lancet Neurology, 15(13), 1355–1367. https://doi.org/10.1016/S1474-4422(16)30237-X/ATTACHMENT/941AD753-1E77-4EB2-A21C-DFF33716B6EB/MMC1.PDF

• Decoding Visual Pathway Lesions – American Academy of Ophthalmology. (n.d.). Retrieved December 3, 2024, from https://www.aao.org/eyenet/article/decoding-visual-pathway-lesions

• Freddi, T. de A. L., & Ottaiano, A. C. (2022). The Optic Nerve: Anatomy and Pathology. Seminars in Ultrasound, CT, and MR, 43(5), 378–388. https://doi.org/10.1053/J.SULT.2022.04.006

• Guier, C. P., & Stokkermans, T. J. (2023). Optic Neuritis. StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK557853/

• Horton, L., & Bennett, J. L. (2018). Acute Management of Optic Neuritis: An Evolving Paradigm. Journal of Neuro-Ophthalmology : The Official Journal of the North American Neuro-Ophthalmology Society, 38(3), 358–367. https://doi.org/10.1097/WNO.0000000000000700

• Shemesh, A., Sood, G., Blair, K., & Margolin, E. (2024). Leber Hereditary Optic Neuropathy (LHON). StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK482499/

• Xie, J. S., Donaldson, L., & Margolin, E. (2022). Papilledema: A review of etiology, pathophysiology, diagnosis, and management. Survey of Ophthalmology, 67(4), 1135–1159. https://doi.org/10.1016/J.SURVOPHTHAL.2021.11.007