Annulus of Zinn

The intraorbital part of the optic nerve lies within the muscle cone. Before passing into the optic canal, the nerve is surrounded by the annulus of Zinn, which is formed by the origins of the rectus muscles. The superior and medial rectus muscles partially share a connective tissue sheath with the optic nerve. This connection may partly explain why patients with retrobulbar neuritis report symptoms of pain on eye movement.

Meningeal sheaths

The internal sheath, the innermost meningeal sheath of the optic nerve, is continuous with the pia mater and arachnoid mater, which cover the brain and spinal cord (Fig 3-7). It is a vascular connective tissue coat, covered with meningothelial cells, that sends numerous septa into the optic nerve, dividing its axons into bundles. (The meningothelial cells can give rise to optic nerve sheath meningioma.) The septa continue throughout the intraorbital and intracanalicular regions of the nerve and end just before the chiasm. They contain collagen, elastic tissue, fibroblasts, nerves, and small arterioles and venules. The septa provide mechanical support for the nerve bundles and nutrition to the axons and glial cells. A mantle of astrocytic glial cells prevents the pia and septa from having direct contact with nerve axons.

The arachnoid sheath, which is composed of collagenous tissue and small amounts of elastic tissue, lines the dural sheath and is connected to the internal sheath across the subarachnoid space by vascular trabeculae. The subarachnoid space ends anteriorly at the level of the lamina cribrosa. Posteriorly, it is usually continuous with the subarachnoid space of the brain.

Because the central retinal vessels cross this space, a rise in intracranial pressure (ICP) can compress the retinal vein and raise the venous pressure within the retina above the intraocular pressure. This situation causes intraocular venous dilatation and the loss of spontaneous venous pulsation (SVP) at the nerve head. The presence of SVP indicates normal ICP. However, some individuals have normal ICP and absent SVP. Thus, the loss of previously documented SVP is more indicative of elevated ICP.

The external, or dural, sheath of the optic nerve is the thick outermost meningeal sheath and is continuous with the dura mater in the brain. It is 0.3–0.5 mm thick and consists of dense bundles of collagen and elastic tissue that fuse anteriorly with the outer layers of the sclera.

Figure 3-7 Meningeal sheaths. The dural sheath, which is the outer layer, is composed of collagenous connective tissue. The arachnoid sheath, the middle layer, is made up of fine collagenous fibers arranged in a loose meshwork. The internal sheath, the innermost layer, is made up of fine collagenous and elastic fibers and is highly vascularized. Elements from both the arachnoid and the internal sheaths are continuous with the optic nerve septa (Masson trichrome stain, ×64).

(Courtesy of Thomas A. Weingeist, PhD, MD.)

The meninges of the optic nerve are supplied by sensory nerve fibers, which account in part for the pain experienced by patients with retrobulbar neuritis or other inflammatory optic nerve diseases.

Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.