Intraocular Portion of the Optic Nerve

The intraocular portion of the optic nerve is visible on ophthalmoscopy as the optic disk. All the retinal nerve fibers merge into the optic nerve here, and the central retinal vessels enter and leave the eye here. The complete absence of photoreceptors at this site creates a gap in the visual field known as the blind spot.

Shape and size: The optic disk (Fig. 13.2) is normally slightly vertically oval with an average area of approximately 2.7 mm2 and a horizontal diameter of approximately 1.8 mm. There is a wide range of physiologic variability in the size of the optic disk; its area may vary by a factor of seven, and its horizontal diameter by a factor of two and one-half.

Color: The normal physiologic color is yellowish orange. The temporal half of the optic disk is usually slightly paler.

Margin: The margin of the optic disk is sharply defined and readily distinguished from the surrounding retinal tissue. On the nasal side, the greater density of the nerve fibers makes the margin slightly less distinct than on the temporal side. A common clinical observation is a crescent of pigment or irregular pigmentation close to the optic disk on the temporal side; sometimes the sclera will be visible through this crescent.

Prominence of the optic disk: The normal optic disk is not prominent. The nerve fibers are practically flush with the retina.

Neuroretinal Rim

Neuroretinal Optic cup rim

Fig. 13.2 Typical signs of a normal pupil include a yellowish orange neuroretinal rim sharply set off from the retina.

Neuroretinal Optic cup rim

Fig. 13.2 Typical signs of a normal pupil include a yellowish orange neuroretinal rim sharply set off from the retina.

13.1 Basic Knowledge 361

Neuroretinal rim (Fig. 13.2): This consists of the bundles of all the optic nerve fibers as they exit through the scleral canal. The rim has a characteristic configuration: The narrowest portion is in the temporal horizontal region followed by the nasal horizontal area; the widest areas are the vertical inferior and superior areas.

Optic cup: This is the slightly eccentric cavitation of the optic nerve that has a slightly flattened oval shape corresponding to that of the neuroretinal rim. It is the brightest part of the optic disk. No nerve fibers exit from it (Fig. 13.2). The size of the optic cup correlates with the size of the optic disk; the larger the optic disk, the larger the optic cup. Because enlargement of the optic cup means a loss of nerve fibers in the rim, it is particularly important to document the size of the optic cup. This is specified as the horizontal and vertical ratios of cup to disk diameter (cup/disk ratio). Due to the wide range of variability in optic disk size, it is not possible to specify absolute cup/disk ratios that indicate the presence of abnormal processes.

Central retinal artery and vein: These structures usually enter the eye slightly nasal to the center of the optic disk. Visible pulsation in the vein is normal. However, arterial pulsation is always abnormal and occurs with disorders such as increased intraocular pressure and aortic stenosis.

Cilioretinal vessels are aberrant vessels originating directly from the choroid (short posterior ciliary arteries). Resembling a cane, they usually course along the temporal margin of the optic disk and supply the inner layers of the retina (Fig. 13.2).

Blood supply to the optic disk (Fig. 13.3): The optic disk receives its blood supply from the ring of Zinn, an anastomotic ring of small branches of the short posterior ciliary arteries and the central retinal artery. Both groups of vessels originate from the ophthalmic artery, which branches off of the internal carotid artery and enters the eye through the optic canal. The central retinal artery and vein branch into the optic nerve approximately 8 mm before the point at which the optic nerve exits the globe. Approximately 10 short posterior ciliary arteries penetrate the sclera around the optic nerve.

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