Radiological Category Neuroradiology Principal Modality 1 MRI Principal

Radiological Category: Neuroradiology Principal Modality (1): MRI Principal Modality (2): Case Report 653 Submitted by: Bradley Birt, M. D. Faculty reviewer: Jeanie Choi, M. D, University of Texas Medical School at Houston Date accepted: 14 Feburary 2010

Case History 6 week-old male with cranial deformity, MRI is requested for further evaluation.

Radiological Presentations

Radiological Presentations

Radiological Presentations

Radiological Presentations

Radiological Presentations

Test Your Diagnosis Which one of the following is your choice for the appropriate diagnosis? After your selection, go to next page. • Isolated occipital encephalocele • Chiari I malformation • Chiari III malformation

Findings and Differentials Findings: There is a large defect in the occipital bone through which dysplastic portions of bilateral occipital lobes protrude. The cerebellar tonsils protrude inferiorly through the foramen magnum by more than 2 cm. Segmentation abnormalities are present in the cervical spine with fusion of C 2 -C 4. Two small cystic structures are seen anterior to the brainstem. Differentials: • Isolated occipital encephalocele • Chiari I malformation • Chiari III malformation

Discussion A Chiari III malformation consists of a high cervical or low occipital meningoencephalocele as well as the intracranial findings of Chiari II malformation. These intracranial abnormalities include: a small posterior fossa forcing the contents down into the cervical spinal canal, elongation of the pons, scalloping of the clivus, hydrocephalus, interdigitated cerebral gyri, and towering of the cerebellum. This case meets the criteria of Chiari III malformation with an occipital encephalocele, and cerebellar tonsil herniation. Vertebral body segmentation anomalies are present which are more commonly associated with Chiari I malformations. Chiari III malformations are very rare, and unfortunately they are associated with high early mortality rates and severe deficits. When possible, surgical treatment is aimed at early operative closure and CSF shunting. Preoperative evaluation should include assessment of position of the brainstem and medulla as well as mention of any anomalous venous drainage pathways associated with the encephalocele. When large encephaloceles are present, CSF diversion may be constructed immediately with delayed closure of the encephalocele defect. This allows the ventricular system to decompress before closure is obtained.

Discussion There is a large bony defect through the occipital bone, allowing herniation of the occipital lobe (red arrow). The cerebellar tonsils (yellow arrow) descend caudally well below the foramen magnum.

Discussion Chiari I Malformation: The findings in Chiari I consist of caudal protrusion of the cerebellar tonsils below the foramen magnum by more than 5 mm, a finding common to all Chiari malformations. A syringomyelia is found in 20 -25% of these patients. The additional finding of a high encephalocele excludes this diagnosis. Chiari II Malformation: This malformation includes caudal protrusion of the cerebellar tonsils, but also has a near absolute association with myelomeningocele, most often lumbar in location. The additional finding of a high encephalocele excludes this diagnosis. Isolated Occipital Encephalocele: While isolated occipital encephaloceles may occur as congenital or post-traumatic defects, the addition intracranial findings of a Chiari II are diagnostic of a Chiari III malformation.

Diagnosis Chiari III malformation.

References Grossman RI, Yousem DM. Neuroradiology The Requisites. 2 nd edition. Philadelphia, PA: Mosby, 2003; 436 -440. Lee R, Tai K, Cheng P, et al. Chiari III Malformation: Antenatal MRI Diagnosis. Clinical Radiology 2002; 57: 759 -767. Snyder W, Luerssen T, Boaz J, et. al. Chiari III Malformation Treated with CSF Diversion and Delayed Closure. Pediatric Neurosurgery 1998; 29: 117 -120.