Optic nerve glioma pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Simrat Sarai, M.D. [2]
Overview
Pathophysiology
Genetics
Optic nerve gliomas are classified as low-grade astrocytomas. Histologically, optic nerve gliomas are identical to pilocytic astrocytomas. Sporadic pilocytic astrocytomas usually have a tandem duplication of chromosome 7q34 and associated BRAF-KIAA fusion gene. Pilocytic astrocytomas associated with neurofibromatosis type 1 lack this fusion gene. There is characteristic loss of neurofibromin (which is a negative growth regulator for astrocytes), and increased RAS activation in patients with optic nerve gliomas associated with NF1. In some pilocytic astrocytomas there is loss of allelic chromosome, suggesting that they are clonal lesions that arise from inactivation of a tumor suppressor gene. There is some link between 17q gene and tumor development in optic nerve glioma, as loss of chromosome 17q has been demonstrated in some cases, even in patients without NF1 or NF2.
Gross pathology
On tumor resection, gross pathology reveals a smooth, fusiform intradural lesion. Macroscopically, these tumors may be cystic, solid or gelatinous.
Microscopic pathology
Generally, optic pathway gliomas are classified under the WHO’s classification of a grade I pilocytic astrocytoma, usually Juvenile Pilocytic Astrocytomas (JPA). Occasionally pathology is positive for grade II fibrillary astrocytoma. The tumors are slow growing and are usually not associated with metastatic deposits. High grade lesions are rare in children. In adult patients tumors tend to be more aggressive and tumor de-differentiation is rarely seen in younger children with optic nerve gliomas but may be observed in older children and adults. In rare cases, tumor may become an anaplastic astrocytoma or glioblastoma. Optic nerve gliomas an be further classified as either pilocytic and or fibrillary in nature. The majority of cases are pilocytic. About 60% of optic pathway astrocytomas are pilocytic and 40% are fibrillary. Hypothalamic tumors which invade the optic chiasm, show evidence of local invasion and histologically are not pilocytic in nature but they are very similar to cerebral hemisphere gliomas. Typical pilocytic histology consists of:
- Densely cellular areas alternating with loose cystic regions
- Immature spindle shaped pilocytic astrocytes and glial filaments
- Rosenthal fibers are common
- Eosinophilic granular bodies are seen
- Microcystic degeneration is seen
- Mitotic figures usually cannot be identified
- Microcalcifications can be seen in 50% of these tumors.
Several other histological patterns of these tumors have been described: Oligodendroglial and astrocytic proliferation Leptomeningeal invasion Reticular pattern that involves microcystic foci with mucous-like fluid Arachnoid hyperplasia & mucus-substance
A new subgroup of optic pathway gliomas has been defined: pilomyxoid astrocytomas Pilomyxiod astrocytomas have different histological features and have been shown to behave more aggressively than pilocytic astrocytomas.
- Pilomyxoid astrocytomas classically show a markedly myxoid matrix, with small, compact, piloid and highly monomorphous cells.
- Tumor cells are often arranged radially around vessels in a pattern that simulates the perivascular pseudorosettes seen in ependyomas
- Tumor samples appear solid without the presence of Rosenthal fibers and eosinophilic granular bodies.
- Satellitosis of the tumor cells in the surrounding neuropil can be seen,
- Mitotic figures can be seen occasionally
- 14% of patients with pilomyxoid astrocytomas had cerebrospinal fluid dissemination of their disease which was not recognized in patients with the pilocytic variant.
The growth pattern of tumor can be either perineural or intraneural in nature. Patients with NF1 tend to have a perineural growth pattern, whereas sporadic optic pathway glioma patients tend to have an intraneural growth pattern.