Neuroradiology Case of the Week

Case 12

William T. Kuo, MD

Clinical Presentation: A 19-month-old male with history of myelomeningocele, shunted hydrocephalus and seizures.

MR Findings: There is crowding in the posterior fossa with inferior displacement of the vermis through a gaping foramen magnum and superior herniation of the cerebellum through a hypoplastic tentorium. The 4th ventricle is elongated and caudally displaced, and there is a low-lying torcular herophili. The cervical-medullary junction is kinked. An enlarged massa intermedia, concave clivus and beaked tectum are present. There is a hypoplastic, fenestrated falx with an indistinct interhemispheric fissure; and there is dysgenesis of the corpus callosum with absence of the rostrum and splenium. Stenogyria is present in the occipital lobe.

Figure 1: Sagittal T1 image shows a crowded posterior fossa, towering cerebellum, low-lying torcular herophili (1), elongated 4th ventricle and inferiorly displaced vermis (2) through a gaping foramen magnum. An enlarged massa intermedia (3), concave clivus (4) and beaked tectum (5) are present. There is dysgenesis of the corpus callosum (6) with absence of the rostrum and splenium. Stenogyria (7) is present at the medial aspect of the occipital lobe. The cervical-medullary junction is kinked (8).
Figure 2: Axial T1 image through the posterior fossa shows herniation of the cerebellum through a gaping, heart-shaped tentorial incisura (small arrows).
Figure 3: Coronal T1 image demonstrates a towering cerebellum herniating through a hypoplastic tentorium cerebelli. The lateral ventricles have a characteristic serrated appearance.
Figure 4A	Figure 4B
Figure 4A&B: Axial T2 images show parallel configuration of the lateral ventricles coinciding with dysgenesis of the corpus callosum. The interhemispheric fissure is indistinct secondary to a hypoplastic, fenestrated falx.

Diagnosis: Chiari II Malformation

Discussion: The Chiari II malformation includes a spectrum of anomalies affecting the brain, ventricles, dura, skull, spine and cord. The hallmark is development of the cerebellum and brainstem within an abnormally small posterior fossa. This is thought to result from a neural tube defect which leads to decompression of the primitive ventricular system and subsequent loss of induction on the surrounding mesenchyme, resulting in a small posterior fossa.
    Common associations of the Chiari II malformation include presence of a myelomeningocele (95-100%), obstructive hydrocephalus (50-98%), lacunar skull (85%), long 4th ventricle caudally displaced (>90%), corpus callosum dysgenesis (80-90%), hypoplastic fenestrated falx (~100%), and dysplastic tentorium (95%). Less commonly, patients may have compression of the brainstem at the foramen magnum or the C-1 level. These patients may experience dysphagia, stridor, apnea, and arm weakness. Patients with brainstem dysfunction have a higher mortality which is not affected by surgical decompression. Epilepsy is another clinical association of Chiari II, but this occurs in only 17% of patents with myelomeningoceles.

References:

1. Osborn AG: Diagnostic Neuroradiology. St. Louis, MO: Mosby; 1994: 15-24.
2. Barkovich AJ: Pediatric Neuroimaging. Philadelphia, PA: Lippincott, Williams & Wilkins; 2000: 330-337.