Foramen Magnum Mass
Karen L. Salzman, MD
DIFFERENTIAL DIAGNOSIS
Common
Acquired Tonsillar Herniation
Dolichoectasia (Vertebrobasilar)
Chiari 1
Chiari 2
Diffuse Astrocytoma, Low Grade
Less Common
Meningioma
Schwannoma
Ependymoma
Metastases, Intracranial, Other
Subependymoma
Hemangioblastoma
Intracranial Hypotension
Skull Base Masses
Chordoma, Clivus
Chondrosarcoma, Skull Base
Giant Invasive Pituitary Macroadenoma
Rare but Important
Epidermoid Cyst
Dermoid Cyst
Syringomyelia
Neurenteric Cyst
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
Foramen magnum (FM) is posterior skull base aperture in occipital bone
Transmits medulla oblongata, vertebral arteries & accessory nerves (CN11)
FM mass can be divided into intra-axial, extra-axial, & skull base masses
Cisterna magna is skull base cistern between medulla anteriorly & occiput posteriorly
Helpful Clues for Common Diagnoses
Acquired Tonsillar Herniation
Secondary to posterior fossa mass effect or severe hydrocephalus
Tonsils pushed inferiorly, impacted into FM
Cisterna magna obliterated
4th ventricle may obstruct causing hydrocephalus
Dolichoectasia (Vertebrobasilar)
Dilated, ectatic vessels in older patient
Typically affects vertebrobasilar system
May mimic a FM mass
Chiari 1
Small posterior fossa, crowded FM
Low-lying, pointed cerebellar tonsils; > 5 mm below FM
Chiari 2
Complex malformation of hindbrain with lumbar myelomeningocele
Tissue herniates through FM behind upper cervical cord
Elongated, “straw-like” 4th ventricle
Associated with dural abnormalities, “beaked” tectum, “towering” cerebellum, dysgenic corpus callosum
Diffuse Astrocytoma, Low Grade
Primary astrocytic brain tumor with intrinsic tendency for malignant progression
50% of brainstem “gliomas” are low-grade astrocytoma; occur in pons & medulla of children
T2 hyperintense mass; ± enhancement
Helpful Clues for Less Common Diagnoses
Meningioma
Extra-axial, enhancing, dural-based mass with dural “tails”
Often occur along clivus with extension through FM
Schwannoma
Benign encapsulated nerve sheath tumor composed of differentiated neoplastic Schwann cells
Enhancing extra-axial mass; T2 hyperintense
Often occur along cranial nerves at skull base with extension into FM
Ependymoma
Soft or “plastic” tumor, squeezes out through 4th ventricle foramina
2/3 infratentorial, 4th ventricle
Heterogeneously enhancing 4th ventricle mass
Metastases, Intracranial, Other
Enhancing mass, usually multiple
Primary tumor typically known
Subependymoma
Rare, benign, well-differentiated, intraventricular, ependymal tumor
Intraventricular, inferior 4th ventricle typical (60%)
T2 hyperintense lobular mass
Usually middle-aged or elderly male
Hemangioblastoma
Posterior fossa mass with cyst, enhancing mural nodule (60%); 40% solid mass
80% cerebellar hemispheres; 15% vermis, 5% medulla, 4th ventricle
Often extend through FM
Intracranial Hypotension
Brain descent with tonsillar herniation &/or “sagging midbrain” in 40-50%
Diffuse, intense dural enhancement 85%
Headache caused by reduced intracranial CSF pressure
Chordoma, Clivus
Rare malignant tumor arising from remnants of primitive notochord
Destructive, T2 hyperintense midline mass in clivus
May extend into FM
Chondrosarcoma, Skull Base
Chondroid malignancy of the skull base, typically centered on petro-occipital fissure
Chondroid matrix on CT 50%; > 50% bone destruction
T2 hyperintense mass, heterogeneous enhancement
May extend into FM from adjacent bone
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