MODULE 11
The OPTIC CHIASM
- - - This small region is of enormous clinical importance - in part because of the reorganization of the visual pathway that takes place here, and in part because of the many structures that lie almost in contact with the chiasm. Let's consider the general anatomy of the region first.
- - - - - Figure 11-2 shows this region rather well, and Figure 1-27 is a drawing of the same basal view. Note that the two internal carotid arteries lie just lateral to the chiasm, and that the anterior cerebral artery runs medially, just above the optic nerve.
- - - - - Figure 2-14 is a similar view, with the chiasm removed to reveal the tissue of the hypothalamus, just above it. The small, dark "hole" in the midline is the slight extension of the third ventricle down within the infundibular part of the pituitary.
A few sagittal views will help to remind you that the pituitary lies just "underneath" the chiasm, and the hypothalamus lies just "above" it.
- - - - - Figure 1-3 is a drawing in which the hypophyseal fossa, tuberculum sellae and clinoid processes are labeled.
- - - - - Figure 1-37 is the real thing, but in this case the brain has been displaced upward and backward somewhat, so that the relationship of the chiasm and infundibulum to the hypophyseal fossa is distorted.
- - - From the point of view of the visual system, the important thing that takes place within the chiasm is a "regrouping" of the fibers conveying information to the lateral geniculate nucleus. In essence, the fibers from the nasal half of each retina cross to the contralateral side of the brain in the chiasm, while the fibers from the temporal half of the retina do not, remaining on the ipsilateral side of the brain. We have tried to indicate the process on three slides through the region. You can call them up by using Figure 11-3 or by clicking on the slides below.
- - - - - Cu Slide 40 depicts the situation just "in front of" the chiasm. In the upper left we show the color code we will use to describe the visual field. In the center is a little diagram to remind you of what happens (illustrated for the vertical plane only) when the image is inverted. The diagram on the right shows what this pattern of light would actually look as it reaches the retina. It's easy, however, to confuse retinal maps with visual fields and since information about visual deficits is always expressed in terms visual fields we will omit retinal maps on most views.
- - - - - Cu Slide 38 shows the fibers from the nasal retina of the left eye crossing in the chiasm. Note the layering of the pathway. This means that pressure on the chiasm from below (a pituitary tumor, perhaps) might cause a somewhat different visual field loss from that caused by pressure from above (a hypothalamic mass, for example).
- - - - - Cu Slide 36 shows the regrouping that has taken place "behind" the chiasm, within the optic tract.
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