Meningiomas are the most common extra-axial tumours of the central nervous system and account for 14-20% of all intracranial neoplasms 4. They are a non-glial neoplasm that originates from the arachnoid cap cells of the meninges.
This article is a general discussion of meningiomas, and focuses on the imaging findings of intracranial disease. For spinal disease refer to spinal meningioma.
Meningiomas are more common in women, with a ratio of 2:1 intracranially and 4:1 in the spine. They are uncommon in patients before the age of 40 and should raise suspicion of neurofibromatosis type 2 (NF2) when found in young patients.
Many small meningiomas are found incidentally and are entirely asymptomatic. Often they cause concern as they are mistakenly deemed to be the cause of vague symptoms, most frequently headaches. Larger tumours, or those with adjacent oedema or abutting particularly sensitive structures can present with a variety of symptoms. Most common presentations include 8:
- headache: 36%
- paresis: 22%
- change in mental status: 21%
- focal neurological deficits
Meningiomas may also become clinically apparent due to complications dependent on location including:
- convexity/parasagittal: seizures and hemiparesis
- basisphenoid: visual field defect
- cavernous sinus: cranial nerve deficit(s)
- frontal: anosmia (although often become very large before becoming symptomatic)
- dural venous sinus invasion/dural venous sinus thrombosis (usually this occurs gradually and even occlusion is asymptomatic, with collateral veins having time to enlarge)
- intraosseous extension: may be hyperostotic or osteolytic and may result in local mass effect (e.g. proptosis)
Although the majority of tumours are sporadic, they are also seen in the setting of previous cranial irradiation and of course in patients with neurofibromatosis type II (NF2) (Merlin gene on Chromosome 22). Additionally meningiomas demonstrate oestrogen sensitivity and may grow during pregnancy.
They are also divided histologically into 3,8:
- fibroblastic: abundant reticulum and 'stout' collagen
- transitional: whorl formation
- syncytial: poorly formed polygonal cells arranged in lobules
- angioblastic: now classified separately as a haemangiopericytoma
- clear cell: high rate of local recurrence 6
- microcystic 12
- papillary: has a high rate of local recurrence 8
- mixed type
In general there are two main macroscopic forms: globose and en plaque.
Globose are rounded, well defined dural masses, likened to the appearance of a fried egg seen in profile.
En plaque meningiomas on the other hand are extensive regions of dural thickening.
- arise from meningothelial arachnoid cells
- histological sub types include
- papillary and rhabdoid: have a propensity to recur
- haemangiopericytoma (CNS): previously angioblastic sub type
- intraosseous meningioma: sclerotic or lucent
- degeneration into: (rare)
- intraventricular meningioma
- atypical meningioma (WHO II): have an increased mitotic rate, only make up approximately 7% of all meningiomas 4, but have a greater tendency to recur. They generally have more restricted diffusion on DWI.
malignant meningioma (WHO III)
- uncommon accounting for only 2.4% of all meningiomas 4, and demonstrate intraparenchymal invasion, rapid growth, and a high mitotic rate or sarcomatous degeneration.
- like atypical meningiomas they too demonstrate restricted diffusion on DWI.
- they are thought to originally be standard meningiomas which undergo malignant degeneration.
- the papillary subtype appears to do so more frequently than others.
radiation induced meningioma
- more frequently multiple, and typically occur ~35 years after radiotherapy.
- meningiomas are a much more frequent complication of radiotherapy compared to sarcomas or gliomas.
- clear cell meningioma: have up to a 60% recurrence rate and occur in younger patients 6
- microcystic meningioma: rare, and are typically very high on T2 weighted imaging and are more commonly associated with atypical features and adjacent brain oedema 12
Generally follows the WHO classification for CNS tumours 7,11:
- WHO I: meningioma ~88-95 %
- WHO II: atypical meningioma (atypical, clear cell, chordoid) ~ 5-6%
- WHO III: malignant meningioma (rhabdoid, anaplastic, papillary) ~1%
- WHO IV: meningioma with sarcomatous degeneration, extremely rare 11
There is also a Simpson grade for meningiomas.
Meningiomas are located anywhere that meninges are found, and in some places where only rest cells are presumed to be located. Locations include:
- 85-90% supratentorial 8
- 5-10% infratentorial
- <5% miscellaneous intracranial
- <1% "extra dural"
- sinonasal cavity: most common
- intraosseous and may involve scalp
- parotid gland
Plain films no longer have a role in the diagnosis or management of meningiomas. Historically a number of features were observed, including:
- enlarged menigeal artery grooves
- hyperostosis or lytic regions
CT is often the first modality employed to investigate neurological signs or symptoms, and often is the modality which detects an incidental lesion:
- 60% slightly hyperdense to normal brain
- 20-30% have some calcification 8
- 72% brightly and homogenously contrast enhance 8, less frequent in malignant or cystic variants
- typical for meningiomas that abut the base of skull
- need to distinguish reactive hyperostosis from skull vault invasion (eventually involves the outer table too)
- lytic regions
As is the case with most other intracranial pathology, MRI is the investigation of choice for the diagnosis and characterisation of meningiomas. When appearance and location is typical, the diagnosis can be made with a very high degree of certainty. In many instances however the appearances are atypical.
Meningiomas typically appear as extra-axial masses with a broad dural base. They are usually homogeneous and well circumscribed, although many variants are encountered.
Signal characteristics include:
- isointense: ~60-90% 3,8, 13
- somewhat hypointense: ~10-40% compared to grey matter
- T1 C+ (Gd): usually intense and homogenous enhancement
- isointense: ~50% 3,8,13
- hyperintense: ~35-40%
- usually correlates with soft textures and hypervascular tumours 13
- very hyperintense lesions may represent the microcystic variant 12
- hypointense: ~10-15% compared to grey matter
- DWI: atypical and malignant subtypes may show greater than expected restricted diffusion although recent work suggests that this is not useful in prospectively predicting histological grade 15-16
Helpful signs include:
- CSF cleft sign, which is not specific for meningioma, but helps establish the mass to be extra-axial
- dural tail seen in 60-72% 2 (note that a dural tail is also seen in other processes)
Meningiomas typically narrow arteries which they encase. This is a useful sign to distinguish a meningioma from a pituitary macroadenoma which will not.
Oedema can be seen and correlates with:
- rapid growth
- location (convexity and parasagittal > elsewhere)
- invasion in the case of malignant meningiomas
The underlying mechansims for the oedema may relate to:
- venous stasis/occlusion/thrombosis
- compressive ischaemia
- aggressive growth/invasion
- parasitisation or pial vessles
MR spectroscopy (MRS)
Usually MRS does not play a significant role in diagnosis but can help distinguish meningiomas from mimics. Features include:
- increase in alanine (1.3-1.5 ppm)
- increased glutamine/glutamate
- increased choline (Cho): cellular tumour
- absent or significantly reduced N-acetylaspartate (NAA): non-neuronal origin
- absent or significantly reduced creatine (Cr)
- good correlation between volume transfer constant (k-trans) and histological grade
- mother-in-law sign: "comes early, stays late, very dense", tumour blush
- dual blood supply from both
- pial (ICA) supplies periphery
- meningeal vessels (ECA) supplies core
- spoke wheel appearance
- dense venous filling
- preoperative embolisation : especially skull base, particles are favoured; 7-9 days prior to surgery
Treatment and prognosis
Treatment is usually with surgical excision. If only incomplete resection is possible (especially at the base of skull) then external-beam radiation therapy can be used 8.
Recurrence rate varies with grade and length of follow-up 8
- 5 year follow up: 5%
- 10 year follow up: 5%
- 32 year follow up: 5%
Metastatic disease is rare, but has been reported 8.
History and etymology
The term "meningioma" was first introduced by Harvey Cushing (Neurosurgeon) in 1922 9.
The differential diagnosis largely depends on location:
- cerebellopontine angle
- parasellar region
In the setting of hyperostosis consider:
In the setting of lucent intraossous meningioma the differential is essentially that of a solitary lucent lesion of the skull.
- imaging signs
- by location
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