By Prem Subramanian, M.D., Ph.D. (University of Colorado)
Introduction and Epidemiology
Optic nerve sheath meningiomas (ONSMs) are rare tumors of the anterior visual pathway that usually arise from the meninges that surround the optic nerve, or-less commonly- from the surrounding tissue (like cavernous sinus, falciform ligament, sphenoid wing, or pituitary fossae), that extended into the orbit. They represent around 2% of orbital tumors, 2% of meningiomas and 33% of optic nerve neoplasms (optic nerve gliomas being the most common).
Unlike optic nerve gliomas, ONSMs are rare in children with mean age of presentation around 40 years of age, and slight female predilection of 2:1. If they happen to affect children, though, they tend to be more aggressive with faster growth, higher chances of bilaterality, and higher incidence of malignant transformation and spread into the CNS.
ONSMs surround the optic nerve preventing axonal transport and interfering with pial blood supply causing optic nerve swelling. The most common symptom is painless decrease in visual acuity seen in 95% of the cases. Other symptoms include visual field defects, dyschromatopsias, and spontaneous or gaze-evoked amaurosis. Proptosis, lid edema, pain and headaches can also occur.
The classic triad of swelling or atrophy of optic nerve, vision loss and optociliary shunt vessels on exam is highly suggestive of ONSM but is rarely present. Visual field defects are usually present and can vary to include generalized constriction, altitudinal or arcuate defect, central scotoma and enlarged blind spot. Other signs include afferent pupillary defect, proptosis, limitation of extraocular muscle movement and conjunctival chemosis.
Pregnancy can cause the tumor to rapidly expand and patients may present with accelerating symptoms.
Diagnosis in most cases is achieved through neuroimaging in the setting of a suggestive clinical picture. Post gadolinium fat suppression MRI of the brain and orbit is the gold standard which shows a tumor that is separated from the optic nerve (unlike glioma), and delineates its course through the orbit, canal and into the intracranial region. On CT, calcifications can be seen in one third of the patients as linear bright lines extending over the course of the optic nerve (tram track sign). CT is not used routinely in diagnosis of ONSM. In the setting of the new technological advances in neuroimaging, biopsy is rarely needed for diagnosis.
Natural history suggest that ONSM are slow growing tumors and in many cases they remain stable in size over the course of several years. In the absence of vision loss i.e. incidental discovery during imaging for other reasons, patients may be observed. When vision loss is present, fractionated stereotactic radiotherapy results in vision improvement in 80% of patients. Surgery almost always cause loss of vision and is reserved to treat blind or uncomfortable eye. In all cases patients should be referred to orbital specialist or neuro-ophthalmologist to follow progression and decide on treatment modality.