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Neuroradiology Case of the Week

Case 87

Terry Chun, MD, Lena Ketonen, MD, PhD,
and P-L Westesson, MD, PhD, DDS

Clinical Presentation: The patient is a 6-year-old boy with new onset nystagmus, ptosis, and short term memory loss. He has a history of myelomeningocele and shunted hydrocephalus.

Radiological Findings:  The cerebellar tonsils extend far below the foramen magnum to the level of C3-4. The posterior fossa is small. The torcular herophili is low-lying with near vertical orientation of the straight sinus. There is towering of the cerebellum and beaking of the midbrain tectum. The massa intermedia is enlarged, and there is partial agenesis of the corpus callosum. Wrapping of the cerebellum around the brainstem is seen. There is widening of the tentorial incisura. There is interdigitation of the cortical gyri, particularly around the posterior aspect of the interhemispheric fissure due to a fenestrated or partially absent falx cerebri. Bilateral frontal clefts are present. A syrinx is seen in the cervical spine

Diagnosis: Arnold-Chiari II malformation

Discussion: Arnold-Chiari II malformation is a complex syndrome affecting the skull, dura, brain, and spinal cord. The etiology is believed to be abnormal neurulation that allows CSF to escape through a neural tube defect [1]. This changes the pressure and volume inductive effects on the surrounding developing tissue, resulting in a small posterior fossa [1]. The consequence of this leads to displacement and distortion of the posterior fossa contents, including herniation of the cerebellum inferiorly and causing it to “tower,” which leads to beaking of the tectum. The abnormal neurulation presents as a myelomeningocele, which is seen in nearly 100% of such patients at birth [2]. Other associated findings include a large massa intermedia, corpus callosum dysgenesis, a lacunar skull, fenestrations in the falx cerebri, and syringohydromyelia in 50-90% of patients [1].
     Clinical suspicion for this syndrome generally starts when the pregnant mother’s alpha-fetoprotein levels are found to be elevated. Newborn patients virtually always have a myelomeningocele. Infants usually have symptoms due to brain stem and cranial nerve dysfunction. These include neurogenic dysphagia with aspiration, apneic spells due to impairment of the ventilatory drive, arm weakness, nystagmus, and opisthotonos (abnormal posturing with the neck muscles stiffened and the head arched backwards) [3]. Hydrocephalus is a common finding for which CSF shunting is often required. If neurogenic dysphagia, stridor, or apneic spells occur, the shunt needs to be evaluated for patency. If the shunt is intact, then emergent posterior fossa decompression may be needed [3]. Respiratory arrest, meningitis/ventriculitis, and aspiration are the most common causes of death [3].

References:

1. Osborn AG. Diagnostic Neuroradiology. Mosby, St. Louis, 1994.
2. Osborn AG, Blaser SI, Salzman KL. Pocket Radiologist, Brain, Top 100 Diagnoses. Amirsys, Salt Lake City, 2002.
3. Greenberg MS. Handbook of Neurosurgery, Fifth Edition. Thieme, New York, 2001.