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ESSENTIAL INFORMATION

Key Differential Diagnosis Issues
- Cisterna magna (CM) between medulla (anterior), occiput (posterior) (a.k.a. cerebellomedullary cistern) - Below/behind inferior vermis - Large medullary cistern masses may extend laterally, posteriorly into CM
- Most common adult lesions are tonsillar associated - Indirect (secondary effect on tonsil) > direct (lesion in tonsil)
- MR and clinical information helps DDx
Helpful Clues for Common Diagnoses
- Herniation Syndromes, Intracranial - Tonsillar herniation most often 2° to posterior fossa (PF) mass effect - Tonsils pushed down into CM - Peg-like configuration of tonsils - Tonsil folia usually oriented horizontally → become vertically oriented when herniated - 4th ventricle may obstruct → obstructive hydrocephalus
- Chiari 1 - Pointed cerebellar tonsils ≥ 5 mm below foramen magnum (FM) - Crowded FM + effaced CSF - ± brainstem compression - ± retroflexed odontoid, horizontal clivus, scoliosis - PF usually normal size - Syrinx in 30-70% - Treatment aim = restore normal CSF flow at FM
- Chiari 2 - Small PF → contents herniates inferiorly through FM and superiorly through widened incisura - Cascade of tissue (vermis, not tonsil) herniates downward through FM - Caudal brainstem herniation with cervicomedullary kink, tectal beaking, falx insufficiency - ± ventriculomegaly, subependymal gray matter heterotopia, &/or polymicrogyria - ~ 100% secondary to open spinal dysraphism, most often myelomeningocele
- Dandy-Walker Continuum - Dandy-Walker continuum is broad spectrum of cystic PF malformations - 2/3 have associated CNS &/or extracranial anomalies - All may have inner table occipital bone scalloping - Dandy-Walker malformation classic triad - Vermian agenesis/hypogenesis - Cystic dilatation 4th ventricle - Large PF with lambdoid-torcular inversion - ± hydrocephalus - Vermian hypoplasia - Varying degrees of vermian hypoplasia - ± rotation of vermis - No enlargement of PF - Blake pouch cyst - Open 4th ventricle - Elevated tegmentovermian angle - Normal size and morphology of vermis - Mega CM - Enlarged retrocerebellar cistern communicates with normal 4th ventricle - Normal vermis
Helpful Clues for Less Common Diagnoses
- Arachnoid Cyst - Sharply demarcated extraaxial cyst that follows CSF attenuation/signal - FLAIR suppresses; no diffusion restriction - Size varies from few millimeters to giant - Often asymptomatic, found incidentally - Cerebellopontine angle (CPA) location > CM
- Ependymoma - Anywhere in neuraxis: 2/3 in PF, 1/3 in supratentorial - 3rd most common malignant pediatric brain tumor arising in PF - Heterogeneous T1/T2 signal with mild to moderate enhancement - Soft or "plastic" tumor squeezes out of 4th ventricle foramina into cisterns - Calcification common (50%) - ± cysts, hemorrhage - Sagittal imaging can distinguish origin as floor vs. roof of 4th ventricle - CSF seeding may occur → image entire neuraxis
- Meningioma - CM rare PF location - CPA, medullary cisterns more common than CM - CM meningiomas usually arise from occipital squamosa - Well-demarcated, lobulated/rounded enhancing mass with dural attachment - Hyperostosis, tumoral calcifications, ↑ vascular markings
- Metastasis - Linear or nodular meningeal enhancement - MR CSF flow may be helpful establishing location and degree of CSF obstruction - Primary tumors include breast, lung, melanoma, prostate - Lymphoproliferative malignancy = lymphoma and leukemia - Primary CNS tumor seed basal cisterns (drop metastases) - Image entire neuraxis
- Intracranial Hypotension - Sagittal shows brain descent in 40-50% - Caudal displacement of cerebellar tonsils in 25-75% - Diffusely and intensely enhancing dura in 85% - Bilateral subdural fluid collections in 15% - Dural venous sinus may be distended (convex margins) - Enlarged pituitary - Frequently misdiagnosed syndrome of headache caused by ↓ intracranial CSF pressure from spontaneous spinal CSF leak
Helpful Clues for Rare Diagnoses
- Subependymoma - T2-hyperintense lobular, nonenhancing intraventricular mass - When arises from 4th ventricle floor, may extend posteroinferiorly into CM - More common in middle-aged, older adults - 0.7% of intracranial neoplasms
- Epidermoid Cyst - Congenital inclusion cysts - Lobulated, irregular, CSF-like mass with "fronds" insinuate cistern - FLAIR usually does not completely null; diffusion yields high-signal restriction - 0.2-1.8% of all primary intracranial tumors - CPA cistern (40-50%) > 4th ventricle (17%) > sellar/parasellar (10-15%) - Rare malignant degeneration into squamous cell carcinoma
- Dermoid Cyst - Fat appearance: Use fat suppression sequence to confirm - With rupture, fat droplets in cisterns, sulci, and ventricles with extensive MR enhancement possible from chemical meningitis - Rare: < 0.5% of primary intracranial tumors - Rupture can cause significant morbidity/mortality - Rare malignant degeneration into squamous cell carcinoma
- Hemangioblastoma - Intraaxial PF mass with cyst, enhancing mural nodule abutting pia - Classified as 1 of mesenchymal, nonmeningothelial tumors - Familial = von Hippel-Lindau - ≈ 30% of patients with hemangioblastoma have von Hippel-Lindau - 7-10% of PF tumors
- Neurenteric Cyst - Round/lobulated, nonenhancing, slightly hyperintense to CSF mass - Most intracranial NECs found in PF - Benign malformative endodermal CNS cyst - Part of split spinal cord malformation spectrum; persistent neurenteric canal - Location - Thoracic (42%), cervical (32%) - Others: Lumbar spine, basilar cisterns, brain parenchyma - Anterior medullary, CPA cisterns > CM

References

Selected References

Images

Selected Images

Herniation Syndromes, Intracranial Sagittal T1 C+ MR shows a cystic mass with an enhancing nodule , consistent with hemangioblastoma, pushing the tonsils inferiorly through the foramen magnum . Note effaced 4th ventricle , enlarged foramen of Monro , and enlarged 3rd and lateral ventricles, consistent with obstructive hydrocephalus.

Chiari 1 Sagittal T2 MR in a patient with occipital headaches shows a significantly pointed configuration of a low-lying cerebellar tonsil to the level of the midposterior ring of C2. Notice also the dorsally tilted dens , effacement of CSF at the foramen magnum, and associated cervicothoracic syrinx .

Chiari 2 Sagittal T2 MR shows a small posterior fossa with extension of the cerebellar peg through the foramen magnum to the level of C6. The cervicomedullary junction and 4th ventricle are low-lying and there is mild tectal beaking and moderate prominence of the massa intermedia .

Dandy-Walker Continuum Sagittal T2 MR demonstrates a markedly enlarged posterior fossa with cystic dilatation of the 4th ventricle , upward rotation of severely hypoplastic vermis , markedly enlarged posterior fossa, and elevated torcula.

Arachnoid Cyst Sagittal T1WI MR shows a CSF isointense arachnoid cyst filling the lower posterior fossa, effacing the cisterna magna, flattening the cervicomedullary junction , and extending caudally into the upper cervical canal.

Ependymoma Sagittal T1WI C+ MR shows enhancing tissue extruding through the foramen of Magendie, filling the cisterna magna . Resultant obstruction has caused enlargement of the cerebral aqueduct and dilated 3rd ventricle .

Meningioma Sagittal T1WI MR demonstrates dural-based tumor with significant mass effect compressing and displacing the cerebellum. Note the trapped CSF clefts . The tumor encroaches on the cisterna magna.

Metastasis Axial T1WI C+ MR shows a typical case of primary CNS lymphoma with subependymal tumor spread. Note a posterior fossa mass near the foramen of Luschka as well as a 2nd dural-based mass .

Intracranial Hypotension Sagittal T1WI C+ MR shows obliteration of the suprasellar cistern , sagging/fat midbrain with closed angle between the peduncles and the pons , dural enhancement , and tonsillar descent with effacement of the cisterna magna.

Subependymoma Sagittal T1WI C+ MR in 40-year-old man shows an enhancing mass at the bottom of the 4th ventricle filling the cisterna magna.

Epidermoid Cyst Sagittal T1WI MR shows a typical case of a large 4th ventricular epidermoid cyst , which follows CSF intensity but is often slightly brighter and mildly heterogeneous in signal.

Dermoid Cyst Axial T2WI MR demonstrates a T2- hyperintense mass encroaching laterally upon the ventral CSF at the foramen magnum. There was evidence of rupture (not shown) causing recurrent meningitis.

Hemangioblastoma Sagittal T1WI C+ MR shows an enhancing vermian mass protruding through the foramen magnum . An associated cyst is deforming the 4th ventricle .

Neurenteric Cyst Axial T1WI C+ MR shows a neurenteric cyst encroaching upon the prepontine cistern. Although most often these are located anteriorly, they may arise posteriorly at the cisterna magnum.

Additional Images

Chiari 2 Sagittal T1WI MR shows a small posterior fossa with extension of the cerebellar peg through the foramen magnum to the level of C3. The cervicomedullary junction and 4th ventricle are low-lying, and there is tectal beaking .

Dandy-Walker Continuum Sagittal T1WI MR demonstrates a markedly enlarged posterior fossa with cystic dilatation of the 4th ventricle , upwardly rotated hypoplastic vermis , and low-lying torcular Herophili .

Chiari 1 Sagittal T1WI MR shows inferior extension of the peg-like cerebellar tonsils well below the level of the foramen magnum , consistent with Chiari 1.

Herniation Syndromes, Intracranial Axial T2WI MR at the level of the foramen magnum demonstrates downward tonsillar herniation secondary to mass affect from brain death.

Herniation Syndromes, Intracranial Sagittal T1WI MR shows cerebellar tonsillar herniation from a large left posterior fossa mass. Note compression of 4th ventricle . Supratentorial ventricles are enlarged .

Chiari 1 Sagittal CINE phase-contrast CSF flow - diastolic shows diminished posterior CSF flow compared to anterior CSF flow at the site of tonsillar impaction.

Chiari 1 Sagittal T2WI MR shows a classic case of Chiari 1 with pointed cerebellar tonsils protruding through the foramen magnum and effacing the cisterna magna.

Chiari 2 Sagittal T2WI MR shows a small posterior fossa with caudal descent of cerebellar tonsillar tissue , elongation of the 4th ventricle , and cervical cord syrinx .

Chiari 2 Axial FSPGR image demonstrates caudally displaced cerebellar tissue wrapping around the brainstem. This is sometimes referred to as a "creeping cerebellum" due to a small posterior fossa.

Chiari 2 Sagittal T1WI MR shows caudal descent of cerebellar vermian tissue and elongated 4th ventricle as well as callosal dysgenesis and a small posterior fossa.

Chiari 2 Sagittal T1 C+ MR shows a small posterior fossa with extension of the vermis through the foramen magnum to the level of C1/2 . The 4th ventricle is small , and there is tectal beaking .

Dandy-Walker Continuum Axial T2WI MR shows a normal 4th ventricle and vermis; however, there is a prominent retrocerebellar CSF space , which does not cause compression but does remodel endosteum, leading to mega cisterna magna.

Arachnoid Cyst Sagittal T1WI MR demonstrates a CSF-intensity arachnoid cyst remodeling the endosteum and filling the cisterna magna .

Ependymoma Sagittal T2WI MR shows heterogeneous features of an ependymoma. Focal hyperintense regions correspond to focal necrosis and hypointense regions correlate to calcification .

Ependymoma Axial T1 C+ MR demonstrates a heterogeneously enhancing ependymoma containing cystic elements .

Ependymoma Sagittal T1 C+ MR shows an enhancing mass expanding the 4th ventricle . The mass is also distending the obex and protruding through the foramen of Magendie into the cisterna magna .

Meningioma Axial T2WI MR demonstrates a posterior fossa meningioma with significant mass effect upon the cerebellum with interposed trapped CSF clefts and vessels .

Meningioma Axial T1 C+ MR of a meningioma shows an avidly enhancing, dural-based mass splaying the cerebellar hemispheres, flattening the 4th ventricle , with dural tail .

Metastasis Coronal T1 C+ MR demonstrates the diffuse enhancement of meningeal carcinomatosis from metastatic xanthoastrocytoma spread throughout the posterior fossa, folia, and also more cephalad.

Intracranial Hypotension Sagittal T1WI MR shows tonsillar descent and dural thickening secondary to intracranial hypotension.

Epidermoid Cyst Axial DWI MR shows a hyperintense mass in the midline posterior fossa.

Hemangioblastoma Sagittal T1 C+ MR demonstrates a mostly solid hemangioblastoma involving the cerebellar tonsils and effacing the cisterna magna.

42 KiB Raw Permalink Blame History

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Helpful Clues for Common Diagnoses

Helpful Clues for Less Common Diagnoses

Helpful Clues for Rare Diagnoses