Transcerebellomedullary fissure approach with special reference to methodsof dissecting the fissure

Object. The purpose of the present study was to refine the transcerebellome dullary fissure approach to the fourth ventricle and to clarify the optimal method of dissecting the fissure to obtain an appropriate operative view w ithout splitting the inferior vermis. Methods. The authors studied the microsurgical anatomy by using formalin-fi xed specimens to determine the most appropriate method of dissecting the ce rebellomedullary fissure. While dissecting the spaces around the ton sils a nd making incisions in the ventricle roof, the procedures used to expose ea ch ventricle wall were studied. Based on their findings, the authors adopte d 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 w as composed of the tela choroidea, inferior medullary velum, and lateral re cess, which form the ventricle roof. In this approach, the authors first di ssected the spaces around the tonsils and then incised the taenia with or w ithout the posterior margin of the lateral recess. These precise dissection s allowed for easy retraction of the tonsil(s) and uvula and provided a suf ficient view of the ventricle wall such that the deep aqueductal region and the lateral region around the lateral recess could be seen without splitti ng the vermis. The dissecting method could be divided into three different types, including extensive (aqueduct), lateral wall, and lateral recess, de pending on the location of the ventricle wall and the extent of surgical ex posure required. Conclusions. When the fissure is appropriately and completely opened, the a pproach 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 t he lateral recess(es) should be incised.