Overall Introduction, Problem Definition, Incidence

ONSM is a rare tumour. It is usually separated into the primary ONSM (pONSM) (intraorbital or intracanalicular) and secondary (sONSM) (intra­ cranial). The tumour can also originate within the optic nerve sheath in the orbit or optic canal and may grow intracranially to involve various struc­ tures there. ONSM can also present in a bilateral form. pONSM represent approximately 96% of all intraorbital and approximately 1%–2% of all intracranial meningiomas. It is the second most common orbital tumour after optic nerve gliomas, but represents only 10% of all ONSM, all the oth­ ers (90%) being secondary ONSM. Of the pONSM, approximately 96% are true pONSM and only 4% were considered ectopic; pONSM typically de­ velop in middle aged women with the proportion of females ranging from 70 to 80, although they do occur in children as well. overall introduction, problem definition, incidence