Figure 3-9 Forms of anterior chamber angle injury associated with blunt trauma, showing cross-sectional and corresponding gonioscopic appearance. A, Angle recession (tear between longitudinal and circular muscles of ciliary body). B, Cyclodialysis (separation of ciliary body from scleral spur) with widening of suprachoroidal space. C, Iridodialysis (tear in root of iris). D, Trabecular damage (tear in anterior portion of meshwork, creating a flap that is hinged at the scleral spur).
(Reproduced with permission from Shields MB. Textbook of Glaucoma. 3rd ed. Baltimore: Williams & Wilkins; 1992.)
Other findings that may be visible by gonioscopy are
microhyphema or hypopyon
retained anterior chamber foreign body iridodialysis
sclerostomy site
angle precipitates suggestive of glaucomatocyclitic crisis pigmentation of the lens equator
other peripheral lens abnormalities intraocular lens haptics
ciliary body tumors/cyst
Alward WLM. Color Atlas of Gonioscopy. San Francisco: Foundation of the American Academy of Ophthalmology; 2001. Campbell DG. A comparison of diagnostic techniques in angle-closure glaucoma. Am J Ophthalmol. 1979;88(2):197–204. Fellman RL, Spaeth GL, Starita RJ. Gonioscopy: key to successful management of glaucoma. Focal Points: Clinical Modules for
Ophthalmologists. San Francisco: American Academy of Ophthalmology; 1984, module 7.
Savage JA. Gonioscopy in the management of glaucoma. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2006, module 3.
The Optic Nerve
The entire visual pathway is described and illustrated in BCSC Section 5, Neuro-Ophthalmology. For further discussion of retinal involvement in the visual process, see Section 12, Retina and Vitreous.
Anatomy and Pathology
The optic nerve is the neural connection between the neurosensory retina and the brain, primarily the lateral geniculate body. An understanding of the normal and pathologic appearance of the optic nerve allows the clinician to detect glaucoma, as well as to follow glaucoma cases. The optic nerve is composed of neural tissue, glial tissue, extracellular matrix, and blood vessels. The human optic nerve consists of approximately 1.2–1.5 million axons of retinal ganglion cells (RGCs), although there is significant individual variability. The cell bodies of the RGCs lie in the ganglion cell layer of the retina. The intraorbital optic nerve is divided into 2 components: the anterior optic nerve and the posterior optic nerve. The anterior optic nerve extends from the retinal surface to the retrolaminar region, just where the nerve exits the posterior aspect of the globe. The average diameter of the optic nerve head is approximately 1.5–1.7 mm as measured with planimetry, but it varies widely among individuals and ethnic groups; the optic nerve expands to approximately 3–4 mm immediately upon exiting the globe. The increase in size is accounted for by axonal myelination, glial tissue, and the beginning of the leptomeninges (optic nerve sheath). The axons are separated into fascicles within the optic nerve, with the intervening spaces occupied by astrocytes.
In primates, there are 3 major RGC types involved in conscious visual perception: magnocellular neurons (M cells), parvocellular neurons (P cells), and koniocellular neurons (bistratified cells). M cells have large-diameter axons, synapse in the magnocellular layer of the lateral geniculate body, are
sensitive to luminance changes in dim illumination (scotopic conditions), have the largest dendritic field, primarily process information related to motion perception, and are not responsive to color. In comparison to the M cells, the P cells account for approximately 80% of all ganglion cells; they are concentrated in the central retina; and they have smaller-diameter axons, smaller receptive fields, and slower conduction velocity. They synapse in the parvocellular layers of the lateral geniculate body. P cells subserve color vision, are most active under higher luminance conditions, and discriminate fine detail. The cells are motion-insensitive and process information of high spatial frequency (high resolution). The bistratified cells (koniocellular neurons) process information concerned with blueyellow color opponency. This system, which is likely preferentially activated by short-wavelength perimetry, is inhibited when red and green cones (yellow) are activated and stimulated when blue cones are activated. Bistratified and large M cells each account for approximately 10% of RGCs.
Figure 3-10 Anatomy of retinal nerve fiber distribution. Inset depicts cross-sectional view of axonal arrangement. Peripheral fibers run closer to the choroid and exit in the periphery of the optic nerve, while fibers originating closer to the nerve head are situated closer to the vitreous and occupy a more central portion of the nerve. (Reproduced with permission from Shields MB. Textbook
of Glaucoma. 3rd ed. Baltimore: Williams & Wilkins; 1992.)
The distribution of nerve fibers as they enter the optic nerve head is shown in Figure 3-10. The arcuate nerve fibers entering the superior and inferior poles of the disc seem to be more susceptible to glaucomatous damage. This susceptibility explains the frequent occurrence of arcuate nerve fiber bundle visual field defects in glaucoma. The arrangement of the axons in the optic nerve head and their differential susceptibility to damage determine the patterns of visual field loss seen in glaucoma, which are described and illustrated later in this chapter.
The anterior optic nerve can be divided into 4 layers:
nerve fiber prelaminar laminar retrolaminar
The most anterior zone is the superficial nerve fiber layer region, which is continuous with the nerve fiber layer of the retina. This region is primarily composed of the axons of the RGCs in transition from the superficial retina to the neuronal component of the optic nerve. The nerve fiber layer can be viewed with the ophthalmoscope when the red-free (green) filter is used. Immediately posterior to the nerve fiber layer is the prelaminar region, which lies adjacent to the peripapillary choroid. More posteriorly, the laminar region is continuous with the sclera and is composed of the lamina cribrosa, a structure consisting of fenestrated connective tissue lamellae that allow the transit of neural fibers through the scleral coat. Finally, the retrolaminar region lies posterior to the lamina cribrosa, is marked by the beginning of axonal myelination, and is surrounded by the leptomeninges of the central nervous system.
The lamina cribrosa is composed of a series of fenestrated sheets of connective tissue and elastic fibers. The lamina cribrosa provides the main support for the optic nerve as it exits the eye, penetrating the scleral coat. The beams of connective tissue are composed primarily of collagen; other extracellular matrix components include elastin, laminin, and fibronectin. Neural components of the optic nerve pass through these connective tissue beams. In addition, relatively large, central fenestrations allow transit of the central retinal artery and central retinal vein. The fenestrations within the lamina have been described histologically as larger superiorly and inferiorly as compared with the temporal and nasal aspects of the optic nerve. It has been suggested that these differences play a role in the development of glaucomatous optic neuropathy. The fenestrations of the lamina cribrosa (laminar dots) may often be seen by ophthalmoscopy at the base of the optic nerve head cup. Between the optic nerve and the adjacent choroidal and scleral tissue lies a rim of connective tissue, the ring of Elschnig. The connective tissue beams of the lamina cribrosa extend from this surrounding connective tissue border and are arranged in a series of parallel, stacked plates.
The vascular anatomy of the anterior optic nerve and peripapillary region has been extensively studied (Fig 3-11). The arterial supply of the anterior optic nerve is derived entirely from branches of the ophthalmic artery via 1 to 5 posterior ciliary arteries. Typically, between 2 and 4 posterior ciliary arteries course anteriorly before dividing into approximately 10–20 short posterior ciliary arteries prior to entering the posterior globe. Often, the posterior ciliary arteries separate into a medial and a lateral group before branching into the short posterior ciliary arteries. The short posterior ciliary arteries penetrate the perineural sclera of the posterior globe to supply the peripapillary choroid, as
well as most of the anterior optic nerve. Some short posterior ciliary arteries course, without branching, through the sclera directly into the choroid; others divide within the sclera to provide branches to both the choroid and the optic nerve. Often a noncontinuous arterial circle exists within the perineural sclera, the circle of Zinn-Haller. The central retinal artery, also a posterior orbital branch of the ophthalmic artery, penetrates the optic nerve approximately 10–15 mm behind the globe. The central retinal artery has few, if any, intraneural branches, the exception being an occasional small branch within the retrolaminar region, which may anastomose with the pial system. The central retinal artery courses adjacent to the central retinal vein within the central portion of the optic nerve.
