Cephalocele

According to the type of the herniating structure cephaloceles may be classified into: meningoencephaloceles (herniation of meninges, brain tissue and CSF); meningocele (herniation of the meninges and CSF only); atretic cephalocele (formes frustes consisting of dura, fibrous tissue and degenerated brain tissue); and glioceles (a glial-lined cyst containing CSF).

A second type of classification takes into consideration location and site of the cephalocele.

Occipital cephaloceles are the most common; both supra- and infratentorial structures (occipital poles and horns, cerebellum and fourth ventricle), together with the tentorium and major venous sinuses may herniate through the abnormal opening.

Frontoethmoidal cephaloceles are anterior congenital midline anomalies, frequently associated with nasal dermoids and nasal gliomas (Fig.1). They are thought to have an embryologic derivation resulting from lack of normal regression of a projection of dura that extends through the embryologic foramen caecum, between the developing cartilage and nasal bone.

Parietal cephaloceles are uncommon, usually associated with significant brain anomalies like Dandy Walker malformation, callosal agenesis, Chiari II malformation and holoprosencephaly and have a poor prognosis. A high percentage of parietal cephaloceles are atretic and present as small midline masses near the vertex. Atretic occipital cephaloceles are small nodular masses above the external occipital protuberance that communicate with the intracranial cavity through a small calvarial defect; the tract usually terminates in the falx cerebri or tentorium.

Nasopharyngeal cephaloceles are uncommon, usually occult, and diagnosed later than encephaloceles in other locations, when the child manifests difficulty in nose breathing secondary to obstruction of the nasopharynx.

Other locations for cephaloceles are frontal, temporal (along the superior surface of the petrous ridge), sphenomaxillary (through orbital fissures into the pterygopalatine fossa) and spheno-orbital (through a defect in the sphenoid bone or optic canal/orbital fissure into orbit). The defects in embryogenesis that explain the formation of cephaloceles are different for the different locations and not yet completely clarified.

The aim of the neuroradiological diagnosis is partly to confirm the presence of the lesion, which is frequently quite obvious, but mainly to precisely define the structures involved, particularly vascular, associated anomalies of the nervous system, and occult lesions. MR is the modality of choice; the midline sagittal T1-weighted image is particularly useful for midline anomalies; CT better shows the extent and characteristics of the bony defect while angiography may be necessary to demonstrate the position of dural venous sinuses.

GS