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CHAPTER OUTLINE

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  • CHIASM AND PARASELLAR REGION

    • Signs and symptoms: neuroanatomic correlation

    • Disorders of the chiasm

    • Evaluation and management

  • OPTIC TRACT

    • Anatomy

    • Signs and symptoms

    • Disorders of the optic tract

  • LATERAL GENICULATE NUCLEUS

  • OPTIC RADIATIONS

    • Parietal lobe signs and symptoms

    • Temporal lobe signs and symptoms

  • OCCIPITAL LOBE

    • Anatomy and pathophysiology

    • Signs and symptoms

  • EVALUATION AND MANAGEMENT OF PATIENTS WITH HOMONYMOUS VISUAL FIELD LOSS

  • KEY POINTS

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INTRODUCTION

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This chapter explores the chiasm and parasellar region. As in the previous chapter addressing the optic nerve, the clinical expression of disease is discussed in the context of neuroanatomy. Visual field defects resulting from disorders of the chiasm and parasellar region were addressed in the discussion of the organization of the visual system in Chapter 3.

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CHIASM AND PARASELLAR REGION

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The chiasm is formed by the confluence of the right and left optic nerves. Axons from the optic nerves are re-routed in the chiasm to form the right and left optic tracts (Box 5–1). The intracranial optic nerves and chiasm ascend at an angle of 45° from the skull base (Figure 5–1A). From a superior perspective, the chiasm is shaped like the Greek letter chi (χ), the origin of its name. The chiasm is approximately 4-mm thick, 12-mm wide, and 8-mm long.

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BOX 5–1. DESTINATION OF AXONS PASSING THROUGH THE CHIASM

  1. The overwhelming majority of axons passing through the chiasm enter the right or left optic tract (depending on which half of visual space they represent) and synapse in the lateral geniculate nucleus (retino-geniculate pathway).

  2. Several small bundles of axons exit from the dorsal and posterior surface of the body of the chiasm and ascend bilaterally to synapse in the suprachiasmatic, supraoptic, and paraventricular nuclei of the hypothalamus. These fibers are likely involved in controlling diurnal rhythms and circadian neuroendocrine systems, but have no clinically testable visual function.

  3. Some axons passing through the chiasm and into the optic tract exit in the brachium of the superior colliculus just before reaching the lateral geniculate nucleus. The major destination of axons in this pathway is the pretectal brainstem nuclei that participate in the afferent limb of the pupillary light reflex; other axons in this pathway (of uncertain function) synapse in the superior colliculus and the accessory optic nucleus of the midbrain, and the pulvinar.

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Figure 5–1.
Anatomy of the chiasm and parasellar region.

(A) Midsagittal view showing relationship of the chiasm to the pituitary gland and third ventricle. (B) Coronal perspective shows the location of the pituitary gland relative to the cavernous sinus.

Graphic Jump Location
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Lesions affecting the chiasm, and their accompanying signs and symptoms, are readily understood when one considers the structures around the chiasm. The chiasm is located about one centimeter above the pituitary gland, which rests in ...

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