Answer: Encephalocele

Histology: The specimen consists predominantly of mature neuroglial tissue. No dura or leptomeninges are noted. Contrasted with Case 2 above, there are two notable histologic differences. First, this tissue lacks the intersecting bands of fibroconnective tissue. Second, neurons are present. Indeed, the neurons are quite numerous and seem to be arranged into a distinct zonal pattern (i.e. gray and white matter).

Discussion: With the true encephalocele, the displaced neuroglial tissue is in direct continuity with the intracranial space. The only reliable way to distinguish the encephalocele from glial heterotopia is the finding of dura and/or leptomeninges, but even when these are absent there are other useful clues. In glial heterotopias, neurons are either sparse or absent. In encephaloceles, they tend to be abundant. In the present case, the neurons are arranged in a zonal pattern that resembles the gray/white matter interface. This degree of histologic organization is not retained in glial heterotopias. Although glial tissue may also be encountered in nasopharyngeal teratomas, teratomas (unlike encephaloceles) also harbor tissues from the other 2 germ cell layers. If an encephalocele is suspected, the surgeon should be contacted in an effort to correlate the histologic findings with the imaging findings, surgical findings, and the post-operative course.