CHAPTER 15

Optic Nerve

Topography

The optic nerve, embryologically derived from the optic stalk, is a continuation of the optic tract; thus, the pathology of the optic nerve reflects that of the central nervous system (CNS). The optic nerve extends from the eye to the optic chiasm and is 35–55 mm in length (intraocular portion, 0.7– 1.0 mm; intraorbital portion, 25–30 mm; intracanalicular portion, 4–10 mm; intracranial portion averages 10 mm). Optic nerve axons originate from the retinal ganglion cell layer and have a myelin coat posterior to the lamina cribrosa.

Oligodendrocytes, astrocytes, and microglial cells are glial cells (glia = glue). Oligodendrocytes produce and maintain the myelin sheath of the optic nerve. Astrocytes are involved with support and nutrition. Microglial cells (CNS histiocytes) have a phagocytic function.

The meningeal coat that covers the optic nerve includes the dura mater (which merges with the sclera), the cellular arachnoid layer, and the vascular pia mater. The pial vessels extend into the optic nerve and subdivide the nerve fibers into fascicles. The subarachnoid space, which contains cerebrospinal fluid (CSF), ends blindly at the termination of the meninges (Figs 15-1, 15-2). See BCSC Section 2, Fundamentals and Principles of Ophthalmology, and Section 5, NeuroOphthalmology, for additional discussion.

Congenital Anomalies

Numerous congenital defects can involve the optic nerve, including optic nerve hypoplasia, optic nerve head pit, morning glory disc anomaly, Bergmeister papilla, and optic nerve coloboma. These are discussed in BCSC Section 5, Neuro-Ophthalmology, and Section 6, Pediatric Ophthalmology and Strabismus.

Colobomas

Typical colobomas of the optic nerve head result from defective closure of the embryonic fissure. They are observed inferonasally in the optic nerve head and are associated with colobomatous defects of the retina/choroid, ciliary body, and iris, which may occur at any point along the course of the embryonic fissure (Fig 15-3A).

Figure 15-1 Longitudinal section of normal optic nerve. Axons of the retinal ganglion cells (R) become axonal fibers of the optic nerve. Optic nerve axons pass through the fenestrations in the lamina cribrosa (arrowheads), which is continuous with the anterior sclera (S). The posterior sclera is continuous with the dura (D). C = choroid, A = arachnoid, P = pia, CRA = central retinal artery, CRV = central retinal vein.

Figure 15-2 Transverse or cross section of the normal optic nerve. The axons of the optic nerve are segregated into fascicles by the delicate fibrovascular pial septa. The nuclei of oligodendrocytes, astrocytes, and microglia are present between the eosinophilic axons. The subdural space (asterisk) is relatively narrow in a normal optic nerve. (Courtesy of Tatyana

Milman, MD.)

Histologically, an optic nerve coloboma consists of a large defect in the optic nerve, involving the retina, retinal pigment epithelium, and choroid. An atrophic and gliotic retina lines the defect. The sclera is ectatic and bowed posteriorly. The wall of the defect may contain adipose tissue and even smooth muscle (Fig 15-3B).