Images below require Macromedia's Flash Player to view

Neuroradiology Case of the Week

   Case 64

Karin Westesson, Kha Nguyen, George Mangalasseril,
and PL Westesson, MD, PhD, DDS

Clinical Presentation: A 17-month-old girl with a known Dandy Walker variant presents for MR of the head as follow-up.

Radiological Findings: There is a posterior fossa cyst that communicates with the fourth ventricle. The cerebellar vermis is hypoplastic and is associated with a high tentorium. The temporal horns are slightly prominent, but the ventricular system is otherwise unremarkable. The cerebral aqueduct is well seen and patent. This patient also exhibits micrognathia.

Diagnosis: Dandy Walker variant

Discussion:  A Dandy Walker complex is a genetically sporadic disorder that occurs one in every 25,000 live births in mostly females. It accounts for one to four percent of hydrocephalus cases. Two-thirds of patients with a Dandy Walker complex also have a CNS and/or extra cranial abnormalities [1-3]. Since there is a common association with facial cardiovascular variances, the suggested onset of the complex is during the migration of neural crest cells, which is generally three to four weeks post-ovulation [4]. The characteristic posterior fossa malformation occurs when there is an outflow obstruction of the 4th ventricle [6] caused by an intrauterine insult to the developing alar plates [5]. When the 4th ventricle grows, it goes in between the developing cerebellar hemispheres, preventing their fusion. Without this fusion, the vermis cannot form. Depending on how enlarged the 4th ventricle is, the occipital bone may be stimulated to grow, enlarging the posterior fossa [6]. Symptoms are generally developmental delay, distended head circumference, and common hydrocephalus symptoms [1].
   There are three types of Dandy Walker complexes. The malformation is the most severe. The posterior fossa is enlarged and the tentorium is in high position. There is agenesis of the cerebellar vermis. There is also cystic dilation of the 4th ventricle, which fills the posterior fossa. The second type is a mega cisterna magna. The posterior fossa is enlarged but it is secondary to an enlarged cisterna. The cerebellar vermis and the 4th ventricle are normal.
   The third type is the variant, which was seen in this patient, is less severe than the malformation. The 4th ventricle is only mildly enlarged and there is no enlargement of the posterior fossa [1]. The cerebellar vermis is mildly hypoplastic and has a variably sized cyst space. This is caused by open communication of the posteroinferior 4th ventricle and the cisterna magna through the enlarged vallecula [3]. Patients exhibit hydrocephalus in 25% of cases and supratentorial CNS variances are uncommon, only present in 20% of cases. There is no torcular-lambdoid inversion, as usually seen in patients with the malformation [2]. The 3rd and lateral ventricles as well as the brain stem are normal [3].
   In general, patients are treated for the symptoms of the disorder. To reduce intracranial pressure, a CSF diversion is used. Prognosis is normally good, but it depends on the supratentorial abnormalities and how severe the hydrocephalus is [4].

References:

1. Barkovick AJ. Pediatric Neuroimaging. New York: Raven Press, 1995.
2. Grossman RI, Yousem DM. Neuroradiology: The Requisites. St. Louis: Mosby, 1994.
3. Osborn, AG. Diagnostic Neuroradiology. St. Louis: Mosby, 1994.
4. Osborn, AG, Blaser SI, Salzman KL. Pocket Radiologist: Brain Top 100 Diagnosis. WB Saunders, 2002.
5. Robertson R, Caruso PC, Truwit CL, Barkovich AJ. Disorders of brain Development. IN: Magnetic Resonance Imaging of the Brain and Spine, 3rd ed. Ed. Scott W. Atlas. Philadelphia: Lippincott, Williams and Wilkins, 2002; 320-323.
6. Truwit, CL, Barkovich AJ. Disorders of brain development. IN: Magnetic Resonance Imaging of the Brain and Spine, 2nd ed. Ed. Scott W. Atlas. Philadelphia: Lippincott-Raven Publishers, 1996; 217-218.