Optic Nerve Sheath Meningioma
Optic Nerve Sheath Meningiomas (ONSM) are rare, benign neoplasms originating from the meningothelial cells of the meninges surrounding the optic nerve. The tumor may arise from either the intraorbital or intracanalicular portions of the optic nerve sheath. Although considered benign tumors, ONSMs cause slow, progressive vision loss secondary to compression of the adjacent optic nerve and its blood supply.
Epidemiology and Risk Factors
Although rare, ONSM is the second-most common primary optic nerve tumor and represents 1-2% of all meningioma.. They account for one-third of primary optic nerve tumors. The incidence of ONSM is highest in adult women in the fourth or fifth decades of life, with women being three times more likely than men to be affected. ONSM can rarely occur in children: a review of cases by Dutton showed that only 4% of tumors occurred in patients younger than 20 years.
The etiology of ONSM has not been clearly identified. Exposure to ionizing radiation has been associated with meningiomas.. ONSM have been associated with neurofibromatosis type 2. The most common cytogenetic abnormality found in meningiomas is the loss of the long arm of chromosome 22, including the region containing the NF2 gene.
ONSMs consist of proliferations of meningothelial cells which are thought to originate from the arachnoid villi of the arachnoid mater. Grossly, they appear as rounded masses compressing adjacent tissues with well-defined borders. Typically ONSMs grow circumferentially around the optic nerve without invasion of the nerve tissues. They may spread along the length of the nerve and have potential to enter the intracranial space, in which case involvement of the contralateral visual pathway becomes a concern. Histologically, ONSM may have one of several appearances, including syncytial, fibroblastic, transitional, psammomatous (exhibiting psammoma bodies), secretory, or microcystic. These patterns do not convey prognostic information, however. ONSMs may rarely show malignant, invasive characteristics along with a higher rate of recurrence.
Symptomatic patients with ONSM most often present with complaints of slowly worsening, painless monocular vision loss. Visual acuity at presentation can vary considerably. Dutton's review found that 24% of patients had visual acuity of counting fingers or worse, while 45% of patients had visual acuity of 20/40 or better. Evidence of optic nerve dysfunction such as reduced color vision or relative afferent pupillary defect will be present. Patients may demonstrate the classic, pathognomonic clinical triad for ONSM, consisting of painless, slowly progressive vision loss, optic atrophy, and optociliary shunt vessels. These findings are seen together in only a minority of cases, however. Optociliary shunt vessels are non-specific and are seen in only 30% of patients with ONSM because they can be observed in any condition creating chronic retinal venous outflow resistance. Patients may also have proptosis or extraocular motility deficits, depending on the location of the tumor. Funduscopic exam may also reveal optic disc swelling.
Diagnosis of ONSM can be confirmed with neuroimaging. CT scan findings include diffuse, tubular, contrast-enhancing enlargement of the affected optic nerve. Calcification may be present within the tumor as well. MRI studies with gadolinium contrast as well as fat suppression are ideal for evaluating ONSM. MRI will again demonstrate diffuse, tubular thickening of the optic nerve sheath encasing the optic nerve, often producing a characteristic "tram track" sign on axial cuts or a "doughnut" sign on coronal cuts. The tumor enhances with contrast infusion. MRI is particularly useful for delineating the extent of the tumor and for evaluating for intracranial extension. Biopsy of the tumor is not necessary for diagnosis and carries a high risk of damage to the optic nerve.
Optic Nerve Glioma, Metastatic disease, Leukemic infiltration, Neurosarcoidosis, Tuberculosis, Gummatous syphilis
The optimal timing of treatment for ONSM is not clear due to the difficulty in predicting the natural course of an individual tumor. One case series found a high degree of variability of the change in visual acuity from baseline for 16 patients diagnosed with ONSM whom were then followed expectantly for an average of 6.2 years. There were additionally no clear variables identified which could help predict the patients' visual outcomes other than an already poor initial visual acuity. The authors suggested that due to this uncertainty, perhaps observation with regular follow-up and re-assessment is a reasonable initial management step in order to help avoid side effects of treatment.
Historically, management options have included observation, surgical excision, or radiation therapy. A retrospective study by Turbin et al compared visual acuity outcomes in patients who underwent observation, surgical therapy, radiation therapy, or a combination of surgery and radiation. The groups of patients all had statistically similar initial visual acuities. At the end of the follow-up period, however, only the patients who received radiation treatment alone had visual acuity levels which were not significantly decreased. This and other studies have led to the establishment of fractionated radiation therapy as the treatment of choice for ONSM once the decision has been made to intervene.
Surgical intervention is generally discouraged due to the high incidence of post-operative blindness after tumor excision. This is due to the common pial blood supply shared by the optic nerve and the ONSM. However, surgical excision could be considered in cases where there is intracranial spread or threat to the contralateral visual pathway. Palliative surgery could additionally be considered when there is advanced vision loss along with disfiguring proptosis.
The natural history of ONSM is that of slow, relentless ipsilateral vision loss. As noted, above, however, the exact rate at which this occurs is difficult to predict and can be highly variable among patients. The mortality rate from ONSM is virtually zero, however.
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