Transcerebellomedullary fissure approach with special reference to methods of dissecting the fissure.

OBJECT The purpose of the present study was to refine the transcerebellomedullary fissure approach to the fourth ventricle and to clarify the optimal method of dissecting the fissure to obtain an appropriate operative view without splitting the inferior vermis. METHODS The authors studied the microsurgical anatomy by using formalin-fixed specimens to determine the most appropriate method of dissecting the cerebellomedullary fissure. While dissecting the spaces around the tonsils and making incisions in the ventricle roof, the procedures used to expose each ventricle wall were studied. Based on their findings, the authors adopted the best approach for use in 19 cases of fourth ventricle tumor. The fissure was further separated into two slit spaces on each side: namely the uvulotonsillar and medullotonsillar spaces. The floor of the fissure was composed of the tela choroidea, inferior medullary velum, and lateral recess, which form the ventricle roof. In this approach, the authors first dissected the spaces around the tonsils and then incised the taenia with or without the posterior margin of the lateral recess. These precise dissections allowed for easy retraction of the tonsil(s) and uvula and provided a sufficient view of the ventricle wall such that the deep aqueductal region and the lateral region around the lateral recess could be seen without splitting the vermis. The dissecting method could be divided into three different types, including extensive (aqueduct), lateral wall, and lateral recess, depending on the location of the ventricle wall and the extent of surgical exposure required. CONCLUSIONS When the fissure is appropriately and completely opened, the approach provides a sufficient operative view without splitting the vermis. Two key principles of this opening method are sufficient dissection of the spaces around the tonsil(s) and an incision of the appropriate portions of the ventricle roof. The taenia(e) with or without the posterior margin of the lateral recess(es) should be incised.