Myopic Optic Neuropathy
The optic nerve is subject to a variety of mechanical forces in highly myopic eyes. Alterations in the circulatory supply of the nerve can be demonstrated in these eyes as well, making them susceptible to optic nerve damage. However, recognition of optic
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Kyoko Ohno-Matsui and Richard F. Spaide
Nearly every feature of the eye is influenced by the development of high myopia. For some structures, such as the sclera, there are well-established abnormalities that have been evaluated in humans, and our knowledge has been amplified by experiments using multiple animal models. For others the changes associated with high myopia are less clear. The anatomic changes in the optic nerve head and surrounding structures are readily evident by imaging, but the functional changes induced and the possible pathophysiologic mechanisms are not clearly understood or defined. Ocular imaging is improving rapidly and has provided clues suggesting there may be classes of abnormalities in optic nerve structure and function in high myopia. This chapter explores possible abnormalities of the optic nerve associated with high myopia. Considerations about the possible pathophysiology involved hinges on detailed knowledge of the anatomic and physiology of the optic nerve and associated structures and incorporates analysis of changes induced by high myopia.
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the optic stalk, called the choroidal fissure, begins to close to form a tube. Failure of closure would lead to an optic nerve coloboma. The retinal nerve fibers converge on the optic disc through a complicated interaction between attractive and repulsive forces acting on their growth [1]. By the seventh week axons line the inner wall of the lumen of the optic stalk, and by the eighth week the stalk is filled with axons that extend back to a primitive chiasm. The cells on the inner side of the stalk are destined to form glial cells between the nerve fibers while the external cells form the glial mantle around the nerve. The scleral development is covered in more detail in Chaps. 5 and 8, but the collagen fibers in the sclera develop in a sequence from the front of the eye to the back to eventually reach the already formed nerve. There is penetration of the nerve by collagen fibers starting in the fourth month. The lamina cribrosa then develops and is not fully formed until after birth [2]. The meningeal coverings of the optic nerve start to become evident as layers in the twelfth week. Myelination of the nerve begins somewhat before the sixth month of gestation.
Embryology of the Optic Nerve
The optic vesicle evaginates from the prosencephalon but remains connected by a short optic stalk. Invagination of the optic vesicle forms the optic cup, and the fetal fissure closes not only the optic cup but also the optic stalk. The hyaloid artery and vein enter the stalk medially and continues into the eye to come into contact with the primary vitreous. The hyaloid artery exits the hyaloid canal on the inner aspect of the disc. Between the sixth and seventh weeks, the fissure in K. Ohno-Matsui, MD, PhD (*) Department of Ophthalmology and Visual Science, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510, Japan e-mail: [email protected] R.F. Spaide, MD Vitreous, Retina, Macula Consultants of New York, 460 Park
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