Third ventriculostomy and fourth ventricle outlets obstruction.

A dilated fourth ventricle due to outlet obstruction is a clinical-radiologic entity with symptoms similar to those of a posterior fossa space-occupying lesion. Indeed, blockage of the foramina of Luschka and Magendie and of the aqueduct results in a "trapped" fourth ventricle. Continued cerebrospinal fluid (CSF) production within the fourth ventricle leads to cystic dilatation of the fourth ventricle. We could not, until now, understand the phenomenon, mainly on exploring endoscopically the permeability of the whole aqueduct. In adults, we call this condition the functional trapped fourth ventricle because in none of our cases have we found physical obstruction of CSF flow. Third ventricle-fourth ventriculostomy is by far the most frequently used technique for cannulation of the aqueduct in a trapped fourth ventricle. In our reported cases, we have introduced a silicone tube stent from below after accessing the fourth ventricle through a small suboccipital craniectomy, ascending it on the aqueduct in order to reach the third ventricle. Management of this infrequently isolated fourth ventricle, but communicated with the rest of ventricular system, remains a challenge for neurosurgeons. Lack of knowledge of the pathophysiology makes it difficult to treat a problem that we do not understand.