Systematic approach to intradural tumors ventral to the brain stem
Intradural tumors that are situated anterior to the midbrain, pons, and medulla have historically been among the most inaccessible of all intracranial lesions. The classic approaches to the posterior fossa (e.g., suboccipital, retrosigmoid) provide only limited access to the anterior midline, primarily due to interposition of the cerebellum, brain stem, and numerous cranial nerves between the tumor and the viewpoint of the surgeon. A variety of techniques have been developed in recent years that create a craniotomy by removal of a portion of the lateral skull 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 petrosectomy, the extended transcochlear approach, involves removal of the entire petrous pyramid along with the lateral aspect of the clivus. This provides 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 midsegment of the brain stem, it carries substantial morbidity, including hearing loss and transient facial palsy, which typically recovers incompletely and with synkinesis. Over the past few years transcochlear procedures have been gradually supplanted, at the University of California Medical Center, by techniques that involve creating a simultaneous craniotomy of both the middle and posterior fossae fashioned around a more limited petrosectomy. These versatile procedures, in particular the middle fossa/retrolabyrinthine approach, provide excellent exposure of the region ventral to the midbrain and pons with less morbidity than the transcochlear approach. When tumors extend inferiorly, ventral to the 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) approach to the foramen magnum and the transjugular approach, both of which involve removal of the inferior portion of the petrous bone. To efficiently utilize these innovative surgical options the surgeon must decide which of the potential approaches optimizes resection while minimizing morbidity. An analysis of the anatomy of the tumor, the functional integrity of cranial nerves, and the extent of resection planned provides the surgeon with the information needed to arrive at a rational choice.