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Findings: The T1 weighted sagittal images demonstrate a small posterior fossa with herniation of the cerebellar vermis and brainstem through the foramen magnum. Cerebellar tissue is also shown to herniate through a spina bifida defect posteriorly into a large CSF sac. T1 weighted axial images show the craniospinal defect and meningoenecephalocele.

Diagnosis:Occipital encephalocele

Discussion: An encelphalocele is a developmental abnormality in which part of the CNS herniates through a cranial defect. There is considerable variation in the incidence which ranges from 1 in 3000 (Southeast Asia) to 1 in 10,000 live births (North America). Encephaloceles can be classified as containing only CSF and meninges (meningocele), neural tissue and meninges (meningoencephalocele), or neural tissue, meninges, and ventricle (hydroencephalomeningocele). Encephaloceles represent approximately 10-15% of all neural tube defects (NTD). Similar to NTD's, their etiology and pathogenesis is poorly understood, but has been correlated with maternal use of sodium arsenate, clofibrate, and vitamin A. Of all encephaloceles, those located in the occipital region (~80%) are the most common in North America, and are easily diagnosed at birth. While routine use of ultrasound has been helpful in the prenatal diagnosis, maternal alpha-fetoprotein levels are often normal since the lesions are usually completely epithelialized and hence do not leak AFP. Prenatal ultrasound is valuable not only for those electing pregnancy termination, but also for C-section planning to prevent potential damage to vital neural tissue at time of delivery. Upon delivery diagnosis, further imaging is critical for treatment planning regardless of the size of the defect. MRI and MRA have become the standard in pre-operative evaluation. MRI/A is critical in determining the extent of neural tissue herniation into the defect - a factor which is generally considered one of the most important prognostic indicators. In addition, operative planning with regards to venous sinus drainage can be determined. Complete CNS imaging is important since 15-20% of children have additional severe CNS defects including NTDs, Dandy-Walker, Chiari II/III, hydrocephalus and optic nerve malformations, all of which can decrease long term functional status and overall operative success. Other severe congenital defects, including cardiovascular and multi-organ defects have also been reported (i.e. Meckel's syndrome) and may affect prognosis and treatment decisions. Untreated occipital encephaloceles are generally considered lethal (secondary to meningitis), but long-term uncorrected survival has been reported - but usually with severe neurological dysfunction, or secondary to uncertain initial diagnosis or minimal defect. Surgical correction is usually undertaken in patients with anatomically favorable lesions and few (if any) other congenital abnormalities. Despite advances in pre-operative imaging, complete diagnosis, (already stated) can often only be made at time of surgical correction. If sac contents are found to contain primitive glial tissue, then a normal neurological outcome can be expected, but if functional and vital CNS tissue are contained in the protruding sac or resection is limited by vascularity then the overall prognosis is less favorable. Overall, a poor prognosis and limited neurological functional capacity can be expected with significant neural herniation, and often corrective or palliative surgery is of little benefit.

References:
Simpson DA, David DJ, White J: Cephaloceles: Treatment, Outcome, and Antenatal
Diagnosis. Neurosurgery 15:14-21, 1984.

Mealey J, Dzenitis AJ, Hockey AA: The Prognosis of Encephaloceles. J. Neurosurgery
32:209-218, 1970.

Chapman PH, Swearingen B, Caviness VS: Subtorcular occipital encephaloceles. J.
Neurosurgery 71:375-381, 1989.

Muraszko KM: Encephaloceles. In Wilkins RH, Rengachary SS (eds): Neurosurgery. New
York: McGraw-Hill, 1996, pp 3573-3579.

James HE: Encephalocele, Dermoid Sinus, and Arachnoid Cyst. In McLaurin RL, Schut L,
et al. (Eds): Pediatric Neurosurgery. Philadelphia: W.B. Saunders Co., 1989, pp 97-106.

McComb JG: Encephaloceles. In Youmans JR (ed): Neurological surgery: A
comprehensive reference guide to the diagnosis and management of Neurosurgical
problems. Philadelphia: W.B. Saunders Co., 1996, pp 829-842.

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