nerve extending to the chiasm and even infiltrating the hypothalamus. Treatment was by enucleation and extirpation of the optic nerve within the orbit. Since that time, the disease has been characterized and defined more precisely as a distinct clinical entity.

CLINICAL CHARACTERISTICS

Demographics

The mean age at presentation for all optic gliomas is 8–9 years with a median age of 7 years (1). It has been reported in patients from birth to 79 years of age, but 70% are in the first decade of life. Although several studies suggest that age predilections are associated with specific tumor locations, this has not been confirmed with larger series. There is no sex predilection, with 50% of patients being male and 50% female.

Location

Gliomas may occur at any location along the visual pathway from the optic nerve to the occipital cortex. Reliable data are available only from more recent series where computer tomographic (CT) or magnetic resonance imaging (MRI) studies were obtained. Many older series relied upon clinical data to deduce tumor location. Among 600 cases where reliable data are available, 7% are confined to the chiasm alone, and 69% occur in the chiasm and adjacent optic nerve or brain

(1). Only 24% of gliomas are confined to the optic nerve. Rarely, the tumor may extend to the optic disk. It has been stated that in patients with neurofibromatosis, there is a greater incidence of pure optic nerve gliomas, and this has been reiterated in several more recent studies.

Association with Neurofibromatosis

An association between neurofibromatosis and optic pathway gliomas was first noted as early as 1873, and has since been confirmed in all larger series. Among patients with neurofibromatosis, the incidence of optic gliomas determined on CT scan is

about 15%, although only about 20% will show evidence of visual impairment. It is therefore important that all patients with neurofibromatosis type 1 have periodic neuroimaging studies.

The exact incidence of neurofibromatosis among patients with optic pathway gliomas has been a matter of some debate, with estimates varying from 10% to 70%. When all data are taken together, however, about 29% of children with optic gliomas show stigmata of neurofibromatosis type 1 (1). The true incidence may be higher since many studies fail to mention the presence or absence of such findings.

PRESENTING SIGNS AND SYMPTOMS

The signs and symptoms of optic pathway gliomas depend largely on tumor location. When confined to the orbital optic nerve, symptoms are those associated with an orbital tumor, including unilateral proptosis, diplopia, ptosis or eyelid retraction, and optic disk edema (Fig. 1A). With tumors of the chiasm and little or no orbital component, there may be minimal or no orbital signs.

Regardless of tumor location, most patients experience some degree of visual dysfunction. Overall, 88% show some visual loss with 25% having good vision of 20=40 or better, and 55% having significant visual loss of 20=300 or worse. More than 75% of patients show optic nerve disturbance with a relative afferent papillary defect, visual field defect, and decreased color vision. Optic atrophy is common, seen in 60% of patients, most with involvement of the orbital optic nerve.

With extension of tumor into the midbrain, hypothalamic signs may predominate. These include precocious puberty, obesity, diabetes insipidus, panhypopituitarism, and dwarfism. Hydrocephalus may follow third ventricle involvement. Seizures and diencephalic syndrome have also been reported.

RADIOGRAPHIC IMAGING

With the advent of CT imaging, diagnostic accuracy for this lesion has increased considerably. On CT, the orbital glioma

appears as a well-outlined enlargement of the optic nerve that is usually fusiform, but may be more rounded or even multilobulated (Fig. 1B). Increased tortuosity or kinking of the nerve is a common finding (3). The optic canal may be enlarged compared to the contralateral side when the canalicular portion of the nerve is involved. The tumor is isodense to brain but typically shows a heterogeneous structure. Less dense cystic spaces correspond to areas of mucinous accumulation. Small high-attenuation foci of calcification are rare. Following contrast administration, enhancement is heterogeneous and variable from imperceptible to moderate. For patients with neurofibromatosis, it should be kept in mind that a normal CT scan in younger children does not preclude the future development of an optic nerve glioma, and these studies should be repeated at intervals.

On the T1-weighted MRI study, gliomas are isointense or slightly hypointense with respect to cortical gray mater (Fig. 2) (4). A dilated subarachnoid space filled with cerebrospinal fluid may image as a hypointense zone surrounding the tumor. Low signal hypointense areas within the lesion represent cysts of mucinous degeneration and necrosis. On T2-weighted images, the signal may be more variable. Small fusiform tumors can be homogeneously hyperintense due to the proton-rich water component and prolonged relaxation time. Larger lesions are usually heterogeneous with a peripheral zone of hyperintense arachnoidal hyperplasia and cerebrospinal fluid, and a hypointense inner zone of optic nerve and glial cells. There is moderate to marked enhancement with gadolinium, but less than with meningioma.

ULTRASOUND

Standardized echography allows the evaluation of the orbital optic nerve that is complimentary to radiographic studies. B-scan typically shows widening of the optic nerve void (Fig. 3A). Large gliomas appear as a smooth, fusiform, or oval mass of low density replacing the normal wedge-shaped optic nerve shadow (5). The optic nerve may be kinked behind