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The optic nerve extends from the posterior pole of the eye to the optic chiasm (Fig. 13.1). After this characteristic crossing, the fibers of the optic nerve travel as the optic tract to the lateral geniculate body. Depending on the shape of the skull, the optic nerve has a total length of 35 – 55 mm. The nerve consists of:
An intraocular portion.
An intraorbital portion.
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 thecentral 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 blindspot.
The optic disk (Fig. 13.2) is normallyslightly vertically ovalwith 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 thesize of the optic disk; its area may vary by a factor of seven, and its horizontaldiameter by a factor of two and one-half.
The normal physiologic color isyellowish orange. The temporal half ofthe optic disk is usually slightly paler.
The margin of the optic disk issharply definedand readily distin-guished 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; some-times the sclera will be visible through this crescent.
The normal optic disk is not prominent. Thenerve fibers are practically flush with the retina.
Neuroretinal rim (Fig. 13.2):
This consists of the bundles of all the optic nervefibers as they exit through the scleral canal. The rim has a characteristic con-figuration: The narrowest portion is in the temporal horizontal region fol-lowed by the nasal horizontal area; the widest areas are the vertical inferior and superior areas.
This is theslightly eccentric cavitationof the optic nerve that has aslightly 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 documentthe 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 inoptic disk size, it is not possible to specify absolute cup/disk ratios that indi-cate the presence of abnormal processes.
These structures usually enter the eyeslightly nasal to the center of the optic disk. Visible pulsation in the vein is normal. However, arterial pulsation is always abnormal and occurs with dis-orders 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).
The optic disk receives its bloodsupply 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 inter-nal carotid artery and enters the eye through the optic canal. The central reti-nal 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.
The intraorbital portion begins after the nerve passes through a sieve-like plateofscleralconnectivetissue,thelaminacribrosa. Insidetheorbit,theoptic nerve describes an S-shaped course that allows extreme eye movements.
After the optic nerve passes through the optic canal, the short intracranialportion begins and extends as far as the optic chiasm. Like the brain, theintraorbital and intracranial portions of the optic nerve are surrounded by sheaths of dura mater, pia, and arachnoid (see Fig. 13.3). The nerve receives its blood supply through the vascular pia sheath.
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