Lesions of the chiasm can be either extrinsic or intrinsic.
Extrinsic lesions affecting the chiasm
The most common lesions producing the chiasmal syndrome include pituitary adenoma (Fig 4-27), parasellar meningioma, craniopharyngioma, and parasellar internal carotid artery aneurysm. Other CNS mass lesions can produce third-ventricle dilation and secondary posterior chiasmal compression.
Pituitary adenomas are the most common cause of chiasmal compression and may occur at any age, although they are rare in childhood. Patients with nonsecreting tumors typically present with vision loss, their tumors having reached a relatively large size without causing other symptoms. Tumors that actively secrete hormones (such as prolactin or growth hormone) are often detected before vision loss occurs because of their systemic endocrine symptoms. Pituitary tumors may enlarge during pregnancy and produce chiasmal compression.
Figure 4-26 Junctional scotoma. A, Visual field patterns from kinetic perimetry testing and Humphrey 30-2 program testing (insets). Note the central scotoma in the patient’s left eye along with the superotemporal depression in his right eye. B, C, Postcontrast, T1-weighted (TR = 650 ms; TE = 14 ms) MRI scans using a slice thickness of 3 mm. B, Coronal image of a section in front of the optic chiasm showing a tumor compressing the prechiasmic segment of the left optic nerve (long arrow) but not the right optic nerve (short arrow). C, Coronal image at the level of the optic chiasm showing minimal rostral displacement (arrow) but no notable direct mass effect.
(Reprinted with permission from Karanjia N, Jacobson DM. Compression of the prechiasmal optic nerve produces a junctional scotoma. Am J Ophthalmol. 1999;128(2):256–258.)
Figure 4-27 Pituitary adenoma. A, B, Visual field patterns from a patient with a pituitary tumor, showing bitemporal depression worse superiorly, with margination along the vertical midline. C, A T1-weighted coronal MRI scan shows an intrasellar enhancing mass, with extension into the suprasellar cistern and upward displacement and compression of the chiasm (arrow).
(Parts A, B courtesy of Steven A. Newman, MD; part C courtesy of Sophia M. Chung, MD.)
Acute hemorrhage or infarction of the pituitary tumor, known as pituitary apoplexy, is a potentially life-threatening event heralded by severe headache, nausea, and altered consciousness. Neuro-ophthalmic findings include diplopia and loss of vision or visual field (Fig 4-28). Sudden expansion of the tumor into the adjacent cavernous sinuses can cause dysfunction of CNs III, IV, V, and VI (CN III is most commonly affected). Superior extension causes severe visual loss ranging from normal visual acuity with bitemporal hemianopia to no light perception. Extravasation of blood into the subarachnoid space causes numerous symptoms, including a decreased level of consciousness and vasospasm with secondary stroke. The acute endocrine abnormalities may lead to numerous complications, including adrenal crisis. Therefore, recognition of pituitary apoplexy is crucial so that neurosurgical treatment can be initiated promptly.
Figure 4-28 Acute compressive optic neuropathy in pituitary apoplexy. Coronal (A) and sagittal (B) MRI scans show a large pituitary tumor with suprasellar extension. Inhomogeneity within the tumor represents hemorrhage and infarction.
(Courtesy of Steven A. Newman, MD.)
Parasellar meningiomas occur most often in middle-aged women. They frequently arise from the tuberculum sella, planum sphenoidale, or anterior clinoid. Very commonly, they cause asymmetric bitemporal vision loss. Parasellar meningiomas may also enlarge and produce chiasmal compression during pregnancy.
Craniopharyngiomas are common in children but may present at any age, with a second incidence peak in adulthood. Often arising superiorly (ie, in the suprasellar or suprachiasmatic space), these tumors more frequently cause inferior bitemporal visual field loss.
Internal carotid artery aneurysms, particularly in the supraclinoid region, may produce a markedly asymmetric chiasmal syndrome, with optic nerve compression on the side of the aneurysm.
Therapy of parasellar tumors is complex and depends on the age of the patient; the nature, location, and extent of the tumor; its hormonal activity; and the severity of symptoms, particularly the presence of vision loss. Treatment modalities include
observation only, if visual field is normal
surgery (usually transsphenoidal, or craniectomy)
medical therapy (primarily bromocriptine or cabergoline for prolactin-secreting pituitary tumors)
radiation therapy (as either a primary or adjunctive therapy for incompletely resectable tumors)
After surgical resection of the tumor and relief of anterior visual pathway compression, vision recovery is usually rapid and may be dramatic, even in patients who experienced severe vision loss. Medical therapy for pituitary adenomas has a slower effect, taking days to weeks, but also shrinks the tumor and improves visual function in responsive cases. Prognosis is poor if mean RNFL thickness is less than 75 μm, as revealed on OCT scans (see Chapter 3, Fig 3-10). The ophthalmologist’s role in the management of parasellar tumors is crucial, because vision loss may be the first sign of recurrence. Visual field, visual acuity, and color vision testing should be performed 2–3 months after treatment and at intervals of 6–12 months thereafter, depending on the course. Visual acuity and visual fields should be rechecked more often if the patient reports any ongoing change. Periodic neuroimaging is essential.
Delayed vision loss after therapy for parasellar lesions should prompt the following considerations:
delayed radionecrosis of the chiasm or optic nerves
chiasmal distortion due to adhesions or secondary empty sella syndrome with descent and traction on the chiasm
chiasmal compression from expansion of intraoperative overpacking of the sella with fat
Neuroimaging effectively helps differentiate among these entities and guides further management decisions.
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