Congenital Anomalies of The Skull Base
Susan I. Blaser, MD, FRCPC
DIFFERENTIAL DIAGNOSIS
Common
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Internal Jugular Vein Asymmetry
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Jugular Bulb Diverticulum
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Chiari 1
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Chiari 2
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Neurofibromatosis Type 1
Less Common
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Aberrant Internal Carotid Artery
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Persistent Stapedial Artery
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Carotid Artery, Sphenoid Migration
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Agenesis Internal Carotid Artery
Rare but Important
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Craniostenoses
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4th Occipital Sclerotome Anomalies
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Persistent Craniopharyngeal Canal
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Medial Basal Canal (Basilaris Medianus)
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Chiari 3
ESSENTIAL INFORMATION
Helpful Clues for Common Diagnoses
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Internal Jugular Vein Asymmetry
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Key facts: Right internal jugular vein (IJV) dominant in 68-75%
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Left IJV commonly smaller than right
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IJV asymmetry < in larger cranial vault
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Imaging
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Most commonly seen normal asymmetry: Right sigmoid sinus, jugular bulb, and IJV larger than left
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Increased signal of left IJV due to compression of left brachiocephalic vein during respiratory cycle
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Jugular Bulb Diverticulum
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Key facts: Asymptomatic normal variant
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Imaging
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Coronal best: Bone CT, MRV, or T1 C+
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“Pouch” projects from jugular bulb
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High signal on MR may simulate mass
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Chiari 1
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Key facts: Mismatch between posterior fossa size and cerebellar tissue
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Imaging
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Low-lying pointed (not rounded) peg-like cerebellar tonsils
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Tonsils project below (≥ 5 mm) OR are impacted in foramen magnum
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Chiari 2
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Key facts
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100% with neural tube closure defect
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Imaging
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Small bony posterior fossa
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“Scalloped” petrous pyramid
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“Notched” clivus
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Low-lying tentorium/torcular
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Large funnel-shaped foramen magnum
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Neurofibromatosis Type 1
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Key facts: Neurocutaneous disorder
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Characterized by diffuse neurofibromas, intracranial hamartomas, benign and malignant neoplasms
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Imaging
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Progressive sphenoid wing dysplasia
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Enlarged optic foramina and fissures
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Foramen magnum and skull base defects
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Helpful Clues for Less Common Diagnoses
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Aberrant Internal Carotid Artery
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Key facts: Displaced ICA courses through middle ear
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Imaging
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Enters posterior middle ear through enlarged inferior tympanic canaliculus
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Courses anteriorly across cochlear promontory
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Joins horizontal carotid canal through dehiscent carotid plate
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Absent carotid foramen & vertical segment of petrous ICA
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Look for associated persistent stapedial artery (PSA) in 30%
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Persistent Stapedial Artery
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Key facts
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Embryologic stapedial artery persists
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PSA becomes middle meningeal artery
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Imaging
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Enlarged CN7 canal
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Small canaliculus leaving carotid canal at genu of vertical and horizontal petrous ICA
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Absent ipsilateral foramen spinosum
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Carotid Artery, Sphenoid Migration
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Agenesis Internal Carotid Artery
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Key facts
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Isolated or syndromic (PHACES, morning glory, Goldenhar, clefting syndromes)
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Imaging
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Absent or hypoplastic vertical and horizontal petrous portions of ICA
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Helpful Clues for Rare Diagnoses
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Craniostenoses
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Key facts
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Syndromic (fibroblastic growth factor, TWIST, and MSX2 mutations) + nonsyndromic premature osseous obliteration of cranial sutures
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Imaging
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Early fusion of occipital sutures surrounding foramen magnum
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Enchondral skull base: Achondroplasia, syndromic bicoronal synostoses, kleeblattschädel
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4th Occipital Sclerotome Anomalies
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Synonym: Proatlas anomalies
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Imaging: 4th OSA finding
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Short clivus and atlas assimilation
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Craniovertebral bony anomalies
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Persistent Craniopharyngeal Canal
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Key facts
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Look for associated canal atresia or stenosis, moyamoya, coloboma
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Imaging
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Small: Nonpituitary tissue containing remnant channel
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Large: Contains pituitary gland or frank encephalocele or artery
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Medial Basal Canal (Basilaris Medianus)
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Key facts
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Notochord remnant cephalic terminus
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Imaging: Midline clivus
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Posterior to sphenooccipital synchondrosis
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Currarino types A-F denote completeness and location of defect
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Tortuous canal may indent superior or inferior aspect of clivus
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Chiari 3
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Key facts: Intracranial Chiari 2 + meningoencephalocele
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High cervical meningoencephalocele
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Imaging
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Occipital squama defect may involve upper cervical vertebrae
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Bony features of Chiari 2 in addition
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Image Gallery
![]() (Left) Axial bone CT in a teenager with achondroplasia shows asymmetry of the jugular foramina
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