CHIASM AND PARASELLAR REGION
Signs and symptoms: neuroanatomic correlation
Disorders of the chiasm
Evaluation and management
Signs and symptoms
Disorders of the optic tract
LATERAL GENICULATE NUCLEUS
Parietal lobe signs and symptoms
Temporal lobe signs and symptoms
Anatomy and pathophysiology
Signs and symptoms
EVALUATION AND MANAGEMENT OF PATIENTS WITH HOMONYMOUS VISUAL FIELD LOSS
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.
CHIASM AND PARASELLAR REGION
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.
BOX 5–1. DESTINATION OF AXONS PASSING THROUGH THE CHIASM
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).
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.
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.
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.
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).
Hemifield slide with bitemporal hemianopia.
Dense bitemporal visual field defects have no overlapping visual field to allow the two eyes to obtain fusion and lock together; therefore, the two halves of the binocular visual field slip and slide with respect to one another. Horizontal objects (eg, lines of printed text) may appear broken, dynamically slipping up or down, or even moving horizontally with respect to each other (A). Patients have complaints of double vision and difficultly with tasks that require alignment, such as adding up a column of numbers. Text may appear misaligned with words that appear to fall off the line. A boat may appear to be in danger of falling off the edge of the water (B)!
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.
Postfixation scotoma with bitemporal hemianopia.
Objects located just beyond a near fixation point will fall in the blind temporal hemifields of both eyes and are therefore not seen. This creates difficulty with tasks such as cutting one’s nails or threading a needle, and patients may complain of disappearing words and letters while reading. Abbreviation: VF, visual field.
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).
Bilateral bow-tie atrophy from trauma to the chiasm.
An 11-year-old boy with head trauma from an automobile accident had bitemporal visual field defects. (A) Goldmann visual fields show a complete bitemporal hemianopia, suggesting midline trauma to the body of the chiasm. (B) Bilateral bow-tie atrophy of the optic discs is evident (see Figure 3–18).
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 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, V1, 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).
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.
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.
Junctional scotoma from pilocystic astrocytoma.
A 15-year-old boy presented with left monocular oscillopsia and vision loss. Visual acuity was 20/400 in the right eye and 20/20 in the left eye. Neuroimaging revealed a mass at the right anterior chiasm, and subsequent pathology from surgical resection revealed a pilocystic astrocytoma. Visual field testing on presentation was consistent with a right junctional scotoma. (A) Goldmann perimetry of the right eye showed a dense central scotoma. (B) Humphrey perimetry demonstrated a temporal visual field defect (total deviation and grayscale plots shown) in the asymptomatic left eye. (C) Magnetic resonance imaging of the brain (coronal view, T1 with contrast) shows the mass, which involves the right anterior chiasm.
DIFFERENTIAL DIAGNOSIS OF CHIASMAL DISORDERS
|Pituitary tumors/apoplexy||Discussed in detail in this chapter.|
|Chiasmal gliomas||Often involve the optic nerve(s) as well. See discussion in Chapter 4.|
|Other tumors/masses||Metastases, nasopharyngeal carcinomas, chordomas, dysgerminomas, hemangiomas, arachnoid cysts, sphenoid sinus mucoceles.|
|Sarcoidosis and other granulomatous diseases||See Box 4–4. Hypothalamic involvement can cause death.|
|Radiation neuropathy||Acute visual loss months to years after radiation.|
|Vascular causes||Suprasellar aneurysms, dolichoectatic vessels, arteriovenous malformations, cavernomas, and (rarely) infarction from arteritis.|
|Optochiasmatic arachnoiditis||Foreign body (postsurgical), infectious, idiopathic.|
|Trauma||May have associated diabetes insipidus and basal skull fractures.|
|Demyelination||MS can cause chiasmal demyelination.|
|Posthypophysectomy empty sella||Herniation of the chiasm into an empty sella following surgical evacuation of a sellar mass can compress the intracranial optic nerves. Adhesions and inflammation can cause downward traction.|
|Posterior compression by an enlarging third ventricle||Acute noncommunicating hydrocephalus can cause expansion of the third ventricle into the posterior notch of the chiasm resulting in a central bitemporal visual field defect. Papilledema is usually present.|
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).
Bitemporal visual field loss from a pituitary adenoma.
A 34-year-old man described blurred vision, predominantly in his left eye. Visual acuity was 20/20 in the right eye and 20/50 in the left eye. The optic discs were normal. (A) Automated perimetry shows bilateral temporal visual field defects, denser in the left eye. (B) Magnetic resonance imaging (coronal view, T1-weighted image with contrast) reveals a mass arising from the sella and extending superiorly (arrows) compressing the chiasm. The cystic areas seen within the mass are regions of tumor necrosis.
Acromegaly and visual loss.
A 34-year-old man described gradual loss of vision in his left eye over 9 months. Acromegalic features were noted. (A) Goldmann perimetry shows temporal hemifield loss in the right eye and profound visual field loss in the left eye with only a temporal island of vision remaining. Visual acuity was 20/20 in the right eye and 1/200 in the left eye. (B) The optic discs were relatively unremarkable, with only a hint of temporal pallor of the left optic disc. (C) The patient’s hands showed disproportionate thickening of his fingers. (D) Coarse facial features including enlarged brow, nose, and jaw are demonstrated in this clinical photo (from a different patient with acromegaly). (E) Magnetic resonance imaging (T1 weighted, sagittal view) revealed a large sellar mass extending superiorly and elevating the floor of the third ventricle (arrows). A prominent brow and thickened skull are also evident on this scan. Subsequent evaluation confirmed the diagnosis of a growth-hormone-secreting pituitary adenoma.
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.
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).
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.
A 14-year-old boy complained of headache and blurred vision in his right eye. (A) The left optic disc showed temporal pallor; the right disc was relatively normal. (B) Goldmann perimetry showed a right homonymous, incongruous visual field defect. Visual acuity was 20/30 in the right eye and 20/100 in the left eye. (C) Magnetic resonance imaging (T1-weighted image, sagittal view) revealed a large inhomogeneous sellar mass (large arrows). The fluid material within the cystic mass has settled out into two layers (remember the patient is supine) (small arrow). Additional studies showed panhypopituitarism, and subsequent pathology showed the mass to be a craniopharyngioma. The visual function improved dramatically after resection, but the patient required repeated resections of solid tumor and drainage of cysts.
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.