• Synonyms: Chiari III, rhombencephalocele
Diagnostic checklist
• Low occipital or high cervical meningoencephalocele containing cerebellum ± brainstem, meninges, vessels, CSF
• Midline bone defect within supraoccipital bone, opisthion
• Skin-covered upper cervical meningoencephalocele containing cerebellum
TOP DIFFERENTIAL DIAGNOSES
• Isolated occipital encephalocele
• Other occipital encephaloceles
○ Iniencephaly
○ Syndromic occipital encephalocele
PATHOLOGY
• Severity classified by sac contents
• Cephalocele contents: Meninges, cerebellum, brainstem ± cervical cord, occipital poles, vasculature
○ Disorganized (neuronal migration anomalies, cortical dysplasias) and gliotic brain tissue
○ Lining of sac may show gray matter heterotopias
• Associated abnormalities: Corpus callosum anomalies, gray matter heterotopia, syringohydromyelia, tethered cord
CLINICAL ISSUES
• Microcephaly, severe developmental delay, spasticity, hypotonia, seizures
• Mechanical brainstem traction, respiratory deterioration lower cranial nerve dysfunction
DIAGNOSTIC CHECKLIST
• Occipitocervical cephalocele containing cerebellum ± brainstem in conjunction with C1-C2 spina bifida = Chiari 3 malformation
Imaging gallery
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Sagittal T2WI MR reveals a large defect in the ventral chondral portion of the supraoccipital bone and opisthion of foramen magnum. Gliotic cerebellar tissue protrudes into a large sac. Note the displacement of the brainstem and basilar artery .
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Sagittal T1WI MR shows a large meningoencephalocele composed of meninges, CSF, cerebellum , brainstem , and upper cervical spinal cord herniated through a bone defect in the lower occiput and upper cervical spine.
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Bone CT 3D shaded surface rendering of the posterior calvaria/upper cervical spine in a Chiari 3 malformation (CM3) patient shows a large defect of the ventral chondral and squamous supraoccipital bones in conjunction with upper cervical spina bifida .
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| Sagittal MRV demonstrates typical venous abnormalities of Chiari 3. The straight sinus and vein of Galen is severely hypoplastic. Large occipital sinuses rather than transverse sinuses are present. |