The optic chiasma, or optic chiasm, derived from the Greek word for crossing, is the point at which the two optic nerves, coursing backward from each eye, intersect at the base of the brain, forming an x-shaped structure. In each eye, the temporal retina, the most lateral half of the retina, provides information about light entering the eye from the nasal side, the opposite side. On the contrary, the nasal retina of each eye provides information about light entering the eye from the same side of the body as the eye. Fibers running from the nasal retinal nerve cells cross in the chiasm to join the fibers from the temporal retinal nerve cells of the opposite side. In this way, the brain merges all of the data concerning the same side of the body together in the optic chiasma to form an optic tract, which travels from the chiasm to the visual centers of the brain.
Compression by a tumor or disruption of blood flow to the area may damage the optic chiasma. Mass lesions produce 95 percent of chiasmal problems, largely due to the optic chiasm being located approximately 0.394 inches (10 millimeters) above the pituitary gland. Patients with tumors in the chiasmal area may have either no symptoms at all or a variety of symptoms, including headaches, double vision, and decreased vision. Pituitary tumors, meningiomas, aneurysms, gliomas, and multiple sclerosis can all produce optic chiasm compression. In addition to a physical examination, physicians should also perform head computed tomography scans, laboratory tests, and visual field analyses to fully assess the patient.
Lesions to the optic chiasm typically produce characteristic visual field defects, or spatial zones with decreased perception. Tumors producing widespread compression on the chiasm most commonly damage the nasal fibers crossing in the center of the chiasm, which carry the temporal visual field information. The patient with a chiasmal lesion will often have a bitemporal hemianopia, with blackened areas of the field on each lateral side. A junctional defect occurs if the patient has a problem in the most anterior and lateral portion of the chiasm. This characteristic defect occurs due to compression of one optic nerve and the opposing nasal fibers, which cross and course to the anterior chiasm before curving back toward the brain in an arc-like structure called Von Willebrand’s knee.
Rarely, compression by the internal carotid arteries or the anterior cerebral arteries will injure the lateral fibers on each side of the optic chiasma, generating a binasal hemianopia. Angiography, a radiographic examination of the blood vessels using dye, will reveal the source of the defects. Treatment for a chiasmal syndrome depends on its etiology. Appropriate management may include radiotherapy, standard surgery, hormone replacement or systemic steroids. In most cases, the prognosis is poor.