The pathology specimen shows disorganized tissue with microcyst formation (Fig. 5a), dense fibrillary tissue and elongated spindle cells (Figs. 5b and 5c), and degenerating axons (Rosenthal fibers, Fig. 5d). The appearance is that of a typical benign pilocytic astrocytoma involving the optic nerve.
Pilocytic astrocytoma (WHO grade I) of the optic nerve.
There are numerous causes of acute visual loss. A complete history and physical examination and, in many cases, the course of the visual loss (i.e., progressive, stable, or improvement) are fundamental in reaching the correct diagnosis. This patient was first diagnosed with presumed anterior ischemic optic neuropathy (AION) because of the acute presentation and the ophthalmic findings of optic disc swelling with peripapillary hemorrhages. The vision, however, proceeded to deteriorate to no light perception, which is not typical of non-arteritic AION but that can occur with arteritic AION. Thus, after the ESR and CRP were found to be slightly elevated and despite the lack of constitutional symptoms typically experienced by patients with giant-cell (temporal) arteritis, the patient was placed on a short course of systemic steroids and a temporal artery biopsy was performed. Red flags were also raised when the optic disc remained swollen months after the onset of loss of vision, thus raising the suspicion of another process causing the loss of vision. The finding on MRI of enlargement and enhancement of the right optic nerve suggested an inflammatory process such as sarcoidosis or an infiltrative tumor such as a lymphoma or glioma. Accordingly, the patient was treated with a short course of high-dose steroids. When the MRI appearance of the nerve did not improve, an optic nerve biopsy was performed, revealing a low-grade pilocytic astrocytoma.
Optic nerve gliomas are rare in adults. These tumors can range from low-grade tumors as in this case to infiltrating tumors such an anaplastic astrocytoma (WHO grade III) and glioblastoma (WHO grade IV). Pilocytic astrocytomas are usually seen in children and young adults and are present in 15% to 20% of patients with neurofibromatosis type 1 (1). Wulc et al. reported seven cases of optic nerve pilocytic astrocytoma in patients ranging from 18 to 61 (2). One of their cases had NF1. Other cases have been reported in adults (3-4), but none appear to have been as old as ours. Unlike benign optic nerve gliomas, malignant optic nerve gliomas (MONGs) are most often reported in older individuals. Wabbels et al. reviewed 45 cases of MONG in the literature (12). The mean age of diagnosis was 54 with a mean survival time of just over 8 months.
The typical presentation of adult optic nerve PAs is progressive loss of vision and variable proptosis (2-4). The vision can range from 20/30 to no light perception. Most cases are associated with slowly progressive visual loss. Sudden visual loss, as seen in our patient, may occur from acute hemorrhage within the infiltrated nerve or from ischemia in the region of maximum compression. Sudden visual loss can also occur in patients with other types of optic nerve tumors such as optic nerve sheath meningioma (5) and malignant optic nerve gliomas (6). Other ocular findings in both children and adults with optic nerve PAs include an ipsilateral relative afferent pupillary defect, optic disc swelling or pallor, peripapillary hemorrhages, and retinochoroidal (shunt) vessels (2).
The diagnosis of a PA is based on a combination of clinical findings, neuroimaging, and, ultimately, pathologic findings. The pathology is characteristic of a low-grade astrocytoma (WHO grade I), including spindle-shaped (hair-like) cells, a fibrillary background (see Figs. 5a-c), degenerating axons (Rosenthal fibers; see Fig. 5d), eosinophilic granular bodies, and, in some cases, areas of cystic degeneration (7). Mitotic figures and necrosis are absent, thus differentiating these tumors from higher grade gliomas, although the cellular proliferation rate as evidenced by the Ki67 index, is somewhat variable. Although some authors consider these tumors to be “hamartomas,” they are, in fact, true tumors and should be considered as such with respect to assessment and management.
Both computed tomographic (CT) scanning and MRI can detect these tumors. Both typically reveal enlargement and enhancement of the nerve; however, of the two, MRI is the study of choice due to its ability to delineate the extent of involvement. CT is useful to evaluate the size of the optic canal and to assess the presence of neural calcification that might be associated with an optic nerve sheath meningioma. In our case, the initial MRI 4 months after symptom onset demonstrated uniform high T2 signal and uniform enhancement confined to the optic nerve with mild associated mass effect. The imaging differential diagnosis of this appearance includes both inflammatory and neoplastic conditions. Sarcoidosis and, rarely, tuberculosis and syphilis can present with this picture. Wegener granulomatosis also can involve the optic nerve, but rarely in isolation. Within the neoplastic category, lymphoma can present as an isolated optic neuropathy with this imaging appearance (11). In our patient, a meningioma could be excluded based on the imaging appearance, as meningiomas typically demonstrate low T2 signal and a “tram-track” or “doughnut” enhancement of the optic nerve sheath with no enhancement of the nerve.
The management of optic nerve pilocytic astrocytomas is somewhat controversial. For children, most authors recommend observation alone with radiation therapy, chemotherapy, or surgical resection used in cases of progressive visual loss not due to amblyopia, an increase in the size of the tumor by neuroimaging, or disfiguring proptosis. The number of cases of PAs reported in adults is too small to warrant a definitive recommendation; however, the cases reported suggest that the visual and systemic prognoses generally are good, and thus are similar to the prognoses in children. Thus, treatment is used when necessary to protect the fellow eye from being affected by extension of the tumor to the optic chiasm and, hopefully, improve vision in the affected eye. Radiation, chemotherapy or surgery may be used in selected cases (8). In adults more than children, careful monitoring by clinical examination and neuroimaging is essential to identify the rare cases of malignant transformation that can be occur spontaneously or following radiation (9,10,13). It has been recommended that our patient receive radiation to protect the vision in his fellow eye from chiasmal involvement.
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