brain and the high risk of long-term secondary malignancy and vascular malformations. Secondary malignancies include meningioma formation within the orbit or base of the skull. Vascular malformations include progressive stenosis of the proximal middle cerebral artery at the junction of the internal carotid artery, also known as moyamoya disease [15]. Since the NF1 gene is a growth regulator and tumor suppressor gene, radiation therapy is usually avoided when possible at any age, and it is used only when patients have a poor response to chemotherapy.

34.5 Chiasmal and Hypothalamic Glioma

34.5.1 Description and Clinical Issues

As previously mentioned, the incidence of optic pathway tumors in NF1 patients is 15%, but what proportion of those tumors are optic chiasm tumors is unknown. Tumors of the optic chiasm present with imaging features similar to those of intraorbital optic nerve glioma, and intraorbital tumors often extend through the optic canal to the chiasm and, sometimes, the optic tract. Symptoms associated with active chiasmal glioma are quite different from those of pure intraorbital tumors because of the bilaterality of visual loss. Symptomatic optic chiasmal gliomas are most often true juvenile pilocytic gliomas.

Visual symptoms of optic chiasm glioma include unilateral or, more often, bilateral optic nerve defects and bitemporal and homonymous visual field defects. Optic atrophy is often present, which is sometimes associated with a “bow–tie” pattern. Afferent pupillary defects are often present in the eye with the most visual field loss. Formal visual field testing is the preferred approach to clinical testing, but again, the early age of presentation makes this problematic.

34.5.2 Evaluation and Management

MRI demonstrates enlargement of the chiasm that is either symmetric or asymmetric [16]. The optic chiasm may enlarge sufficiently to merge with the hypothalamic structures just caudal to the chiasm, so high-resolution MRI with thin cuts through the chiasm, pituitary, and hypothalamus is usually the preferred modality. Progressive or active tumors frequently show enhancement on T1 postcontrast images. T2 flair images frequently show increased signal intensity of the chiasm and adjacent hypothalamic structures as well as the more distal regions of the optic pathway extending into the brain parenchyma to the region of the posterior thalamus. Positive response to treatment is judged by reduction of size of the chiasmal mass, stability of the chiasmal mass, or reduction of T1 postcontrast enhancement. T2 flair images may remain unchanged after treatment, and therefore flair changes are a less reliable sign of treatment response. Not infrequently, chiasmal gliomas grow into one or both of the optic tracts.