Oligoastrocytomas are much more common in young adults, with a peak incidence occurring in the third to fifth decade. The incidence among males is higher than females, and there are currently no known risk factors. Prognosis is worse with increased age of onset. Oligodendrogliomas have a 5 year survival of greater than 50% and a 10 year survival of 25-34%. The prognosis for Oligoastrocytomas is similar to that for Anaplastic Astrocytomas, which typically have a 3-5 year life expectancy.
Oligoastrocytomas are infiltrative by nature but often infiltrate without affecting function, thus typically achieve large volumes prior to becoming symptomatic. As with other intracranial mass lesions, common symptoms include headaches, nausea, visual changes, and progressive focal neurological deficits. Systemic symptoms of fever, malaise, and weight loss are more suggestive of metastatic lesions versus a primary brain tumor. Headaches associated with increased intracranial pressure have a characteristic pattern of worsening with recumbency as well as occurring in a holocephalic and episodic pattern. With continued increased intracranial pressure, the headaches can become continuous but will often maintain episodic variations in intensity.
Oligoastrocytomas typically occur in white matter, and because there are more oligodendrocytes in the frontotemporal areas of the cerebrum, this is the most common location, consistent with this case. They also are commonly not contrast enhancing on MRI, thus may be mistaken for a low-grade glioma or stroke. MRI is an essential neuroimaging technique for the diagnosis as it is the most sensitive imaging modality for detecting gliomas. Gliomas may alter the blood-brain barrier and thus are frequently accompanied with surrounding edema, which is easily seen on T2 weighted images. Post-contrast images should be utilized as another sensitive modality for identifying a disrupted blood-brain barrier. High grade Gliomas typically enhance with contrast while low-grade Gliomas and Oligoastrocytomas usually do not.
A tissue biopsy must be performed prior to arriving at a final diagnosis. The term Mixed Glioma, or Oligoastrocytoma is assigned if a mixed histology including both astrocytic and oligodendroglial features exists. In general, oligodendrogliomas are more benign than astrocytomas, thus a predominance of oligodendrocytes over astrocytes confers a better prognosis for these rare brain tumors. The histologic diagnosis of an Oligoastrocytoma is difficult to make because many Oligodendrogliomas contain an area that resembles an Astrocytoma. Thus the diagnosis relies upon the percentage of oligodendroglial cells that are be required to classify them as pure Oligodendrogliomas versus mixed Oligoastrocytomas. The fraction of oligodendrocytes needed varies from ~51% to >90% to be a 'pure' oligodendroglioma.
Treatment of these tumors continues to be controversial. Clinical trials are underway to better define the effects of current treatment modalities, which include surgical reduction, radiotherapy and chemotherapy. Corticosteroids and seizure prophylaxis are commonly employed as well. New specific chemotherapy protocols are showing good results in treating anaplastic oligodendroglioma.
Reference(s): Olson JD, Riedel E, DeAngelis LM. Long-term outcome of low-grade oligodendroglioma and mixed glioma. Neurology 2000;54:1442-1448
Runge, Val M. Clinical Magnetic Resonance Imaging. J.B. Lippincott Company; Philadelphia; 1990.
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