Optic nerve sheath meningiomas (ONSM) are rare benign tumors of the central nervous system (CNS). Albeit their growth is slowly progressive, their location is critical as it directly affects the anterior visual pathway and can lead to a severe visual loss. Treatment of this condition is still considered controversial, as the proximity of the tumor to the optic nerve makes it difficult to completely eradicate it without secondary complications and sequelae.
ONSM usually originate from the intraorbital or intercanalicular portions of the optic nerve sheath. The intraorbital portion is the most common site of origin (92%). About 95% of ONSM are unilateral. The rare bilateral ONSM tend to occur in patients with neurofibromatosis type 2.
ONSMs constitute about 2% of all orbital tumors and 1% to 2% of all meningiomas. These tumors, like their intracranial counterpart, are predominant in middle-aged females. They also tend to increase in size throughout pregnancy and during the menstrual cycle.
These tumors arise from the meningo-epithelial cells lining the optic nerve sheath. They typically grow circumferentially around the optic nerve and can significantly compress its pial vascular supply. These tumors tend to follow the path of least resistance, and as a result of that, they can invade the entire path of the optic nerve, from the optic canal to the globe. They can also diffuse intracranially.
Historically, the classification of meningiomas has been based upon cell shapes, cell patterns, and cell products. The macroscopic appearance of meningiomas may be hemispheric, bun-shaped, or globular, and they may vary in gross appearance as epithelial or mesenchymal. They usually are attached to the dura and invaginate into adjacent neural structures. Enveloped in a thin capsule derived from the adjacent meninges, they remain extraaxial and are separated easily from the brain or the spinal cord.
According to the World Health Organization (WHO) in 1993, three different types of meningiomas exist based on malignant behavior, as follows:
Benign (grade I) with a recurrence rate of 6.9%: Despite the invasion of the adjacent bony structures, grade I meningiomas do not invade the brain parenchyma.
Atypical (grade II) with a recurrence rate of 34.6%: This type of meningioma shows frequent mitosis and an increased nuclear-cytoplasmic ratio.
Malignant (grades III and IV) with a recurrence rate of 72.7%: This type shows even greater mitosis, necrosis, and invasion of brain parenchyma.
Malignant transformation is rare. Originally, malignancy was seen in anaplastic tumors, but they may arise from any of the meningioma variants or atypical meningiomas. Papillary histopathology is associated with local aggressiveness and increased incidence of late distant metastasis. The papillary type is considered malignant by definition and is encountered more frequently in children.
Earlier classification schemes used the term angioblastic meningioma for what is now considered to be a hemangiopericytoma. This neoplasm is distinctly separate from a meningioma, and it shows an extremely high propensity for recurrence and metastasis. Hemangiopericytoma is a sarcoma in the new WHO classification.
The natural history of ONSM is characterized by slowly painless and progressive visual loss in the affected eye. Left untreated, this tumor can lead to complete blindness. The mortality risk from ONSM is practically null.
Typically, there is a relative afferent pupillary deficit in the involved eye. In the early stages, patients can present with chronic optic nerve edema. This is seen in the context of the tumor compressing the intraorbital portion of the optic nerve. Eventually, the edema subsides, and optic pallor develops. Optociliary shunt vessels also can develop in about 30% of patients as the optic disc edema resolves. These represent venous collaterals which are connected to the choroidal circulation. They appear after chronic central retinal vein obstruction, such as in ONSM, but can also be found in other conditions such as central retinal vein occlusion, optic nerve glioma, and sphenoid wing meningioma.
Visual field defects can be present in the affected eye and are not specific; these can be seen as altitudinal defects, generalized constriction or enlarged blind spots.
Proptosis and resistance to retropulsion are also other signs seen on examination in ONSM, albeit not always present. Patients can also complain of transient visual loss associated with eye movement, or what is referred to as gaze-evoked amaurosis.
The diagnosis of ONSM is confirmed with MRI, especially with gadolinium-enhanced fat-suppression sequences. The MRI has become the gold standard for the diagnosis and has obviated the need for tissue biopsy.
ONSM is a sensitive lesion to gadolinium contrast. On MRI axial images, it will present with the characteristic "tram-track" sign, which corresponds to the enhancing outer ONSM encircling the inner non-enhancing optic nerve. On the coronal images, this will be seen a "doughnut" sign.
Other imaging features consist of tubular, fusiform, or globular enlargement of the lesion. Calcifications can also be seen in the ONSM.
Observation remains an acceptable conservative measure in cases where the visual function remains stable, especially in patients who maintain a central visual acuity of 20/50 or better. Neuro-ophthalmologists will closely follow these patients with a thorough neuro-ophthalmological examination, including serial testing with formal visual fields and peripapillary retinal nerve fiber layer optical coherence tomography. Repeat MRIs also are warranted.
Tumor resection is almost impossible without incurring a severe visual loss, due to the intimal relationship of the ONSM to the optic nerve. However, surgical resection can be justified in cases of disfiguring proptosis where the visual function has significantly decreased or in cases of intracranial extension.
Differential diagnoses include:
In cases where visual function decreases, stereotactic fractionated radiotherapy (SFRT) has become the modality of choice since it delivers the appropriate amount of radiation to the tumor in a localized fashion. SFRT can preserve visual function in a majority of treated patients, although risks of radiation-induced retinopathy or optic neuropathy are prevalent.
Current data indicate that radiation following surgery lowers the risk of recurrences and improves the success of obtaining visual improvement.
Chemotherapy is usually used in patients with unresectable disease or those who are not candidates for surgery. Many combination drugs have been used including 5-fluorouracil, levamisole, cisplatin, dacarbazine, and doxorubicin. Overall, the response after chemotherapy is marginal and the quality of life is poor. Many patients suffer from numerous adverse side effects of the drugs.
The overall survival rate for all patients at five years and ten years is approximately 90% and 60%, respectively.
Permanent vision loss is the most serious complication
Follow up with regular visual acuity and field testing are recommended.
An MRI with dye should be done every 12 months to assess for recurrence of disease.
Optic nerve sheath lesions are best managed by a multidisciplinary team of neurosurgeons, ophthalmologists, radiation oncologists, medical oncologists, and neurologists.