Anatomy of Malformations of the Anterior Skull Base

Abstract

Various types of cephalocele are recognized. One type, the meningocele, is a cerebrospinal fluid-filled hernial sac that is covered only by skin or surrounding tissue. If the sac also contains brain tissue, it is called a meningoencephalocele. An encephalocele contains brain tissue only, the dura mater usually merging with the outer periosteum at the margin of the bony defect. In an encephalocystocele the sac contains portions of the ventricles in addition to brain. Combined forms may occur. The various malformations cannot always be distinguished clinically, because the tissue in the hernial sac is greatly altered and may even be difficult to identify histologically. The anterior skull base region may be affected by frontoethmoid cephaloceles, which emerge anterior or lateral to the cribriform plate of the ethmoid bone in the area of the foramen cecum. From there the sac may project forward, presenting over the nasal root. This may be associated with a splitting or diastasis of the nasal bones or the frontal process of the maxilla. Nasoethmoid cephaloceles project downward between the ethmoid bone and the frontal or nasal bone and are commonly associated with asymmetries of the anterior fossa. Naso-orbital cephaloceles project forward into the anterior part of the orbit between the frontal, ethmoid, and lacrimal bones (anterior orbital cephalocele). Generally the sac displaces the globe laterally downward while displacing the lacrimal sac medially. Spheno-orbital cephaloceles (rare) may protrude between the orbital surface of the frontal bone and the lesser wing of the sphenoid or may traverse the superior orbital fissure or optic canal to enter the posterior part of the orbit (posterior orbital cephalocele), producing the symptoms of a retrobulbar mass. Some of these lesions remain clinically silent until adulthood. A nasomaxillary cephalocele protrudes through the same opening but then passes through the inferior orbital fissure into the pterygopalatine fossa. Nasopharyngeal cephaloceles may penetrate the cribriform plate or pass between the cribriform plate and the developing anterior border of the sphenoid downwards into the nasopharynx. A sphenopharyngeal cephalocele protrudes into the pharynx through the former synchondrotic zones of the sphenoid.