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  CHIASM AND PARASELLAR REGION

  
  
  
  
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    Signs and symptoms: neuroanatomic correlation

    
    
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    Disorders of the chiasm

    
    
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    Evaluation and management

  
  
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  OPTIC TRACT

  
  
  
  
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    Anatomy

    
    
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    Signs and symptoms

    
    
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    Disorders of the optic tract

  
  
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  LATERAL GENICULATE NUCLEUS

  
  
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  OPTIC RADIATIONS

  
  
  
  
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    Parietal lobe signs and symptoms

    
    
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    Temporal lobe signs and symptoms

  
  
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  OCCIPITAL LOBE

  
  
  
  
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    Anatomy and pathophysiology

    
    
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    Signs and symptoms

  
  
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  EVALUATION AND MANAGEMENT OF PATIENTS WITH HOMONYMOUS VISUAL FIELD LOSS

  
  
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  KEY POINTS

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.

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.

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 the sella turcica of the sphenoid. The hypothalamus is directly above, and the pituitary stalk (infundibulum), connecting the hypothalamus and pituitary, is posterior. The cavernous sinuses form the lateral walls of the sella turcica. The third ventricle extends to the posterior notch of the chiasm.

The chiasm is positioned within the circle of Willis and receives its blood supply from multiple sources so infarction is uncommon. Mass lesions arising from sellar and parasellar regions (pituitary adenomas, craniopharyngiomas, meningiomas, and aneurysms) are the most common disease processes affecting the chiasm. Mass lesions cause injury to chiasmal axons or their myelin sheath from compression, vascular compromise, or both.

SIGNS AND SYMPTOMS: NEUROANATOMIC CORRELATION

The Chiasm

The patterns of visual field loss that arise from disorders of the chiasm are a logical consequence of the axonal decussation, and are discussed in Chapter 3 (see Figures 3–11 and 3–12). The classic pattern consists of bitemporal visual field defects that respect the vertical meridian. Since disorders that affect the chiasm are usually mass lesions, they rarely attack with pinpoint accuracy. Therefore, complex visual field defects resulting from insults to various combinations of optic nerve, the body of the chiasm, and optic tract are not uncommon. Curiously, trauma to the chiasm from head injury can occasionally produce “perfect” bitemporal visual field defects.

Patients who have dense bitemporal visual field defects may have no overlapping visual field to allow their eyes to obtain fusion and lock together. This situation can cause the hemifield slide phenomenon, in which each half of the binocular visual field slips and slides with respect to the other. Horizontal objects (eg, lines of print) may overlap or separate, or appear broken, dynamically slipping up and down, or even moving horizontally with respect to each other (Figure 5–2A). Related visual complaints include double vision and difficultly with tasks that require alignment, such as adding up a column of numbers. Reading can be particularly difficult because of misalignment of text and words that appear to fall off the line (see Figure 5–2B).

Another feature of bitemporal visual field loss is a postfixation scotoma. Objects located just beyond a near fixation point fall into the blind temporal hemifields of both eyes, and are therefore not seen (Figure 5–3). This condition creates difficulty with tasks such as cutting fingernails or toenails or threading a needle, and can cause individual words or letters to disappear with reading.

Lesions affecting the chiasm can cause retrograde axonal atrophy, which results in visible optic disc pallor and nerve fiber layer loss over time. Disorders affecting the body of the chiasm injure the crossing axons from each eye; axons that originate from each nasal hemiretina (representing the temporal hemifields in each eye). Retrograde axonal degeneration causes optic nerve pallor primarily at the midnasal and midtemporal portions of the optic disc; the nasal part of the disc receives axons from all ganglion cells located nasal to the optic disc, and the temporal portion of the disc receives maculopapillary axons from ganglion cells between the foveola and the disc. The axons at the superior and inferior poles, which sweep around from the unaffected temporal hemiretina, are preserved. This selective atrophy results in a horizontal band of optic disc pallor (bow-tie atrophy) in each eye, a pattern that can be seen in the eye(s) with temporal hemifield loss from chiasmal or optic tract lesions (Figure 5–4).

A relative afferent pupillary defect (RAPD) is often present with chiasmal disease, depending on the density and asymmetry of the visual field loss between the two eyes. The RAPD is present in the eye with the greatest visual field loss, not necessarily the eye with the worst visual acuity.

The Sellar and Parasellar Region

The pituitary gland is inferior to the chiasm within the sella turcica. In approximately 80% of individuals the body of the chiasm is directly above the pituitary gland. In approximately 10% the body of the chiasm is located more anteriorly, over the tuberculum sella (prefixed chiasm), and in the remaining 10% the chiasm is more posterior than the sella (the postfixed chiasm), lying directly above the posterior wall or the dorsum sellae. The position of the chiasm determines whether the optic nerves, body of the chiasm, or optic tracts will be primarily affected by masses that grow upward from the sellar region. On average, there is at least 1 cm between the dorsum sellae and the body of the chiasm. This space is the inferior chiasmatic cistern. Thus, tumors arising from the region of the sella may extend unimpeded superiorly toward the chiasm, but must be at least 1 cm in vertical height to begin to compress the chiasm and have any effect on vision (see Figure 5–1A). In addition to visual loss, chronic headache is common but not invariable among all types of sellar tumors.

The paired cavernous sinuses form the lateral walls of the sella turcica. This dural venous sinus contains the cavernous segment of the internal carotid artery; cranial nerves (CNs) III, IV, VI, V_1, and V₂; and sympathetic nerves (see Figure 5–1B). Sellar masses (eg, pituitary tumors) can extend laterally into the cavernous sinuses, causing diplopia (affecting CN III, IV, VI), ptosis or anisocoria (affecting CN III or the sympathetic nerves), and pain or facial numbness (affecting CN V).

Other Neighboring Structures

The posterior notch of the chiasm is immediately adjacent to the anterior extent of the third ventricle. Noncommunicating hydrocephalus due to aqueductal stenosis can cause expansion of the third ventricle, stretching the posterior aspect of the chiasm and impinging on the posterior decussating axons and causing central bitemporal visual field loss. Signs and symptoms associated with hydrocephalus may also be present including papilledema, CN VI palsies, headache, gait disturbances, and somnolence. Additionally, lesions that cause cerebral aqueductal stenosis may also produce symptoms of dorsal midbrain syndrome (discussed in Chapter 10).

The hypothalamus is directly contiguous with the chiasm superiorly, and diseases may seamlessly involve both structures (eg, chiasmal glioma or neurosarcoidosis). Disorders that affect the hypothalamus may be life threatening. Early symptoms of hypothalamic involvement include diabetes insipidus, marked behavior changes, and lethargy.

The ascending intracranial internal carotid arteries are lateral to the body of the chiasm. Arterial ectasia and aneurysm of the carotid arteries are rarely a source of chiasmal compression.

Chiasmal lesions are occasionally associated with acquired seesaw nystagmus; the reason is unclear, but it may be due to simultaneous involvement of adjacent brainstem structures.

DISORDERS OF THE CHIASM

As discussed above, extrinsic mass lesions are the most common cause of chiasmal visual field loss, and include pituitary tumors, meningiomas, craniopharyngiomas, other tumors (Figure 5–5), and occasionally giant aneurysms. In addition, processes that commonly affect the intracranial optic nerve, such as demyelination, gliomas, and inflammatory disorders, can also affect the chiasm. Table 5–1 lists disorders that can affect the chiasm.

Pituitary Adenomas

Pituitary macroadenomas are the most common extrinsic masses that cause chiasmal compression (and the most common lesion overall affecting the chiasm). Pituitary macroadenomas extend beyond the sella; microadenomas are contained within the sella, and therefore do not cause visual dysfunction. Pituitary tumors may be associated with hypersecretion (secreting pituitary adenomas), hyposecretion (compression of otherwise normal portions of the gland), or normal pituitary function. Most patients with visual loss from pituitary masses have nonsecreting adenomas, because patients with secreting adenomas generally seek medical help because of symptoms of endocrine dysfunction before the tumor is large enough to compress the chiasm and cause visual loss (Figures 5–6 and 5–7).

Many pituitary macroadenomas are surgically accessible by transsphenoidal microsurgery. This approach minimizes surgical morbidity and recovery time by approaching the pituitary fossa extracranially through the nose and sphenoid sinus. Successful decompression often results in improved vision, but patients who have developed optic atrophy prior to surgery have a less favorable prognosis, and typically have some degree of permanent visual field loss. Thus, early diagnosis and treatment are important to maximize visual recovery. Bromocriptine (eg, Parlodel) and cabergoline (eg, Dostinex) are dopamine agonists that reduce the size of prolactin-secreting pituitary tumors, but regrowth is the rule when the medication is discontinued. Radiation therapy may be a treatment alternative or adjunct in some cases.

Pituitary Apoplexy

Acute enlargement of a pituitary adenoma can occur from spontaneous infarction and hemorrhage. The rapid expansion of the tumor within the sella can extend outward into the cavernous sinuses causing cranial neuropathies and motility disturbances, or upward into the optic nerves/chiasm/optic tracts, resulting in visual loss. Anterior extension may cause epistaxis or cerebrospinal fluid (CSF) rhinorrhea; posterior rupture can cause inflammatory meningitis from blood and debris. The acute compromise of pituitary function (especially corticotropins) may be life threatening, and thus requires prompt diagnosis and medical treatment.

During pregnancy the pituitary gland enlarges, and it defervesces postpartum. The vascular changes associated with the pituitary gland’s response to pregnancy can predispose patients to pituitary apoplexy, particularly after delivery (Sheehan syndrome).

Craniopharyngioma

Craniopharyngiomas are mixed solid and cystic midline tumors that arise from embryonic remnants of Rathke pouch, and are located in the region of the pituitary gland and stalk. These tumors can occur at any age, but the incidence is bimodal: more common in young patients (<20 years), with a second peak in the fifth and sixth decades of life (Figure 5–8). Tumor cysts contain desquamation products, necrotic tissue, and blood, often with punctate dystrophic calcification (a helpful sign on computed tomography (CT)). These large suprasellar tumors frequently cause pituitary and hypothalamic dysfunction, as well as hydrocephalus. Surgical treatments include shunting procedures for hydrocephalus, cyst aspiration, cyst shunting to a subcutaneous reservoir, and tumor debulking. Radiation (stereotactic radiotherapy) may be an alternative or adjunctive treatment.

Intracranial Meningiomas

Meningiomas that arise intracranially near the optic foramen (medial sphenoid ridge, tuberculum sellae) can cause optic nerve compression with monocular symptoms. Tumors that arise or extend posteriorly cause compression of the chiasm and result in bilateral visual field defects that are usually asymmetric between eyes. Visual loss is typically progressive, but may wax and wane. Growth may be accelerated during pregnancy because of progesterone and estrogen receptors that may be present in these tumors.