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Findings: The patient is status post decompression; however, herniation of the cerebellar tonsils is still appreciated. In addition, the fourth ventricle is caudally displaced and elongated. Tectal beaking is present.
Diagnosis: Chiari II malformation.
Discussion: Multiple findings can be seen in a Chiari II malformation. The posterior fossa is decreased in volume, and the foramen magnum is widened. The cerebellar tonsils can herniate through the foramen magnum a variable distance. The vermis may also herniate. A hypoplastic tentorium with a widened tentorial incisura can result in upper herniation of the superior portion of the cerebellum or downward herniation of the supratentorial parenchyma. Varying degrees of cerebellar dysplasia can be present. The fourth ventricle is caudally displaced and elongated. The cervical medullary junction may be kinked. Interdigitation of gyri can be seen due to a hypoplastic falx or fenestrated falx. Tectal beaking and elongation of the midbrain may be present. The lateral ventricles are dysmorphic and the septum pellucidum can be absent in 40% of patients. An enlarged massa intermedia is seen in greater than two thirds of cases. Hydrocephalus is present in up to 90% of cases, and aqueductal stenosis is seen in 50%. A myelomeningocele is present in nearly all patients with a Chiari II malformation. They are most prominent in the lumbar region. Partial or complete agenesis of the corpus callosum is present in one third of cases. Hydromyelia is only identified on MRI in 5% of cases; however, it is reported in 20 to 80% of cases on pathology reports. Regular honeycombed ridges along the inner table of the skull may also be present.
References:
Taveras JM. Brain Congenital Anomalies. In: Neuroradiology.
Williams and Wilkins, Baltimore; 1996:166-175.
Wolpert SM, Anderson M, Scott RM, et al: Chiari II Malformation:
MR Imaging Evaluation. AJNR 2987; 8:783-792.
Woodruff WW. Pediatric Neuroimaging. Fundamentals of Neuroimaging.
Woodruff WW. W.B. Saunders Co., Philadelphia; 1993:513-517.
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