SYSTEMATIC-APPROACH TO INTRADURAL TUMORS VENTRAL TO THE BRAIN-STEM

Intradural tumors that are situated anterior to the midbrain, pons, an d medulla have historically been among the most inaccessible of all in tracranial lesions. The classic approaches to the posterior fossa (e.g ., suboccipita, retrosigmoid) provide only limited access to the anter ior midline, primarily due to interposition of the cerebellum, brain s tem, and numerous cranial nerves between the tumor and the viewpoint o f the surgeon. A variety of techniques have been developed in recent y ears that create a craniotomy by removal of a portion of the lateral s kull base. These procedures enhance exposure of the ventral surface of the brain stem while markedly reducing the need for brain retraction. An underlying theme of transbasal craniotomy is judicious removal of a portion of the petrous pyramid. The most radical form of petrosectom y, the extended transcochlear approach, involves removal of the entire petrous pyramid along with the lateral aspect of the clivus. This pro vides an unimpeded view of the ventral surface of the pons, including the basilar artery, vertebrobasilar junction, and both abducens nerves . Whereas this technique provides splendid exposure along the midsegme nt of the brain stem, it carries substantial morbidity, including hear ing loss and transient facial palsy, which typically recovers incomple tely and with synkinesis. Over the past few years transcochlear proced ures have been gradually supplanted, at the University of California M edical Center, by techniques that involve creating a simultaneous cran iotomy of both the middle and posterior fossae fashioned around a more limited petrosectomy. These versatile procedures, in particular the m iddle fossa/retrolabyrinthine approach, provide excellent exposure of the region ventral to the midbrain and pons with less morbidity than t he transcochlear approach. When tumors extend inferiorly, ventral to t he lower medulla and/or upper cervical spinal cord, augmented inferior exposure is required. Approaches to ventrally situated lesions at the craniovertebral junction include the far lateral (transcondylar) appr oach to the foramen magnum and the transjugular approach, both of whic h involve removal of the inferior portion of the petrous bone. To effi ciently utilize these innovative surgical options the surgeon must dec ide which of the potential approaches optimizes resection while minimi zing morbidity. An analysis of the anatomy of the tumor, the functiona l integrity of cranial nerves, and the extent of resection planned pro vides the surgeon with the information needed to arrive at a rational choice.