venerdì 12 ottobre 2007
Figure 1: Sagittal MR image demonstrates frontal lobe tissue protruding through a defect in the anterior calvarium. A portion of the umbilical cord is seen wrapping around the fetus’s neck.
Figure 2: Axial MR image demonstrates a soft tissue mass protruding between the orbits, with hypertelorism and bilateral hydrocephalus.
Diagnosis: Frontoethmoid encephalocele
A frontoethmoid encephalocele is herniation of intracranial contents through a defect of the skull in the nasofrontal region (1). These are usually detected first on prenatal screening or diagnostic US performed for further evaluation of prenatal screening lab abnormalities, usually elevated maternal serum alpha-fetoprotein levels (2). Sonographic clues include hypertelorism and a facial mass. The best diagnostic clue is visualizing brain parenchyma herniating through an anterior skull defect. Prenatal MR can be used to better delineate the anatomy in a suspected encephalocele, and can better determine size, location, and communication with intracranial structures (1).
Small encephaloceles can be missed on prenatal ultrasound. Other imaging clues include hypertelorism and other associated anomalies which include dysgenesis or absence of the corpus callosum, heterotopia, microcephaly, hydrocephalus, microophthalmos, beaked tectal plate, flattened basiocciput and an interhemispheric lipoma (2). In the postnatal period, they can present as a skin-covered facial or nasal mass (3).
The pathogenesis of an encephalocele is a late neurulation defect during the fourth gestational week. There is a disturbance of separation of the surface ectoderm and neuroectoderm in the midline just following neural fold closure (3). As a result there is failure of the fonticulus frontalis to close with herniation of intracranial parenchyma through persistent embryologic relationships. For unclear reasons, occipital lesions are most common (80%) in North America and Europe (1:35,000 live births) and frontoethmoidal lesions are more common in Southeast Asia (1:5000 lives births) (3).
Prognosis depends on the presence of other associated congenital or developmental brain anomalies. Prognosis is often better with frontoethmoidal lesions than occipital or parietal lesions. An overall mortality of 7-20% has been seen, with a favorable developmental outcome (3). Treatment to prevent infection and further herniation of intracranial contents involves excision of the dysplastic brain tissue with watertight closure of the dura and reconstruction of the skull defect (3). Most of the time, the glial tissue can be resected without causing additional neurological deficits. The exception is in the case of transphenoidal encephaloceles which can contain pituitary gland, hypothalamus or part of the Circle of Willis (3). Hydrocephalus should be treated first (3).