Optic Ischemia

Excerpt

Optic Ischemia Overview

Optic ischemia, otherwise known as ischemic optic neuropathy, encompasses vascular diseases affecting the optic nerve. The condition is categorized into anterior and posterior types. Anterior ischemic optic neuropathies often present with optic disc edema, while posterior ones do not.

Nonarteritic acute anterior ischemic optic neuropathy (NAION) is the most common form of optic ischemia, typically occurring in individuals with a small, crowded optic nerve. However, the exact cause of this disorder remains unknown, and curative or preventive treatment is not available. Posterior ischemic optic neuropathy is rare and typically associated with cardiovascular risk factors or perioperative conditions. This subset also lacks specific treatment.

Urgent evaluation for arteritic causes, notably giant-cell arteritis (GCA), is essential, with immediate intravenous methylprednisolone administration indicated to limit vision loss in the affected eye and prevent involvement of the other eye.

Optic Nerve Anatomy

The optic nerve spans from the eyeball to the chiasm and has 4 segments: intraocular (1 mm), intra-orbital (30 mm), intracanalicular (6-10 mm), and intracranial (10-16 mm). Nerve fibers arise from retinal ganglion cells and converge at the optic disc, commonly known as "the blind spot," before traveling approximately 1 mm within the globe and penetrating the sclera through a sieve-like structure called "lamina cribrosa." Encased by the optic nerve sheath, the optic nerve also sits amid extraocular muscles and fat tissue.

The optic nerve has a diameter of about 3 mm, while the optic nerve sheath measures approximately 1 mm thick. The sheath consists of the pia mater, subarachnoid space, arachnoid mater, and dura mater. This covering varies in thickness from 0.09 to 0.15 mm for the pia and arachnoid mater, to 0.1 to 0.29 mm for the subarachnoid space, and 0.3 to 0.5 mm for the dura. The subarachnoid space features a complex structure of trabeculae, septa, and pillars immersed in cerebral spinal fluid, with variations along the nerve's course.

The optic nerve is divided into anterior and posterior portions and may be damaged by ischemia anywhere along its path. The optic nerve area between the retina and sclera is supplied by a network of 6 to 12 short posterior ciliary arteries at the back of the globe. These vessels form a circumferential network called the "circle of Zinn-Haller." The optic nerve head is perfused in a centripetal and segmental fashion. Branches from the pial plexus supply the optic nerve's retrolaminar portion.

The anterior part contains trabeculae of 5 to 7 µm diameter, while the midsection exhibits septa and pillars dividing the space into communicating chambers, with pillars ranging from 10 to 30 µm in diameter. Both pillars and trabeculae are present in the posterior part, where the sheath traverses the optic canal. The right and left optic nerves cross each other within the cranium, forming the optic chiasm and connecting with the visual cortex within the occipital lobe.