Brain stem mapping

11.1.1

What is brain stem mapping?

BSM is a neurophysiological technique that helps localize CMN on the surgically exposed floor of the fourth ventricle ( Fig. 11.1 ). During surgery for a brain stem tumor, the surgeon must know the location of CMN to avoid damaging this area.

Previously, during surgeries in the fourth ventricle, surgeons were guided by anatomical landmarks when trying to prevent injury to the CMN and other structures. Using these landmarks, “safe entry zones” to the brain stem through the floor of the fourth ventricle were determined to be the supra- and infrafacial triangles. Their anatomical importance has been discussed elsewhere . There are two main problems that the surgeon encounters using anatomical landmarks as guidelines for safe entry to the brain stem. First, normal anatomy is usually distorted by tumor. Second, anatomical landmarks on the floor of the fourth ventricle (facial colliculus and striae medullares) cannot be easily recognized in some patients with brain stem tumors.

The idea of “safe entry zone” to the brain stem was first advocated by Kyoshima et al. in 1993 . The brain stem is packed full of vital structures and functions, so the surgeon has limited space to access the tumor safely. They demonstrated supra- and infrafacial triangles as “safe entry zones” to the brain stem through the floor of the fourth ventricle by using anatomical landmarks as guides. Series of morphometric and cytoarchitectonic studies confirmed the safe entry zone and its exact location on the floor of the fourth ventricle ( Fig. 11.2 ) . The idea of a “safe entry zone” to the brain stem gained great appreciation from neurosurgeons and ignited challenges to operate on more brain stem lesions .

The role of BSM is to locate CMN on the floor of the fourth ventricle. BSM enables neurosurgeons to preserve CMN before getting into the brain stem. Direct damage to CMN would be avoided by BSM, because it guides neurosurgeons to where the “safe entry zone” is located and how it shifts because of the location of the lesion . It should be reminded that BSM is a mapping technique to localize CMN and its intramedullary root. It does not reflect the functional integrity of the whole motor pathway of CMN including the CBT. Functional preservation of the sensory part of the motor cranial nerves as well as brain stem reflexes is not assured by BSM. Nevertheless, BSM plays a critical role for brain stem surgeries since functional preservation of CMN is the major concern for neurosurgeons during removal of brain stem lesion.

In our previous study , we showed that the facial colliculi could be visualized in 3 of 12 patients. This study was done in seven patients with medullary tumors that did not influence the anatomy of the facial colliculi and in five patients with a pontine tumor. In 9 of 14 patients, the striae medullares could be visualized (9 patients with medullary tumors and 5 patients with pontine tumors). BSM was used to localize CMN and to provide the surgeon with anatomical guidance, even when anatomical landmarks were not visible on floor of the fourth ventricle . Thus, it provided guidance as to where to make incisions on the floor of fourth ventricle and/or when to stop tumor resection at the bottom of the tumor cavity.

11.1.2

Anatomical background for brain stem mapping

Under near normal anatomical conditions of the floor of the fourth ventricle, CMN VII is mapped around the facial colliculus. It is actually not the facial nucleus that is electrically stimulated, but the intramedullary root of the facial nerve at its closest point to the floor of fourth ventricle . CMN XII is almost always mapped near the obex of the fourth ventricle. If CMN IX/X is mapped, it is localized at the area rostrolateral to the obex ( Fig. 11.3 ).

The original “safe entry zone,” defined by Kyoshima et al. using anatomical landmarks, is described as follows ( Fig. 11.2 ) :

• Suprafacial triangle

  • Medial border: the medial longitudinal fascicle (MLF)

  • Caudal border: the facial nerve

  • Lateral border: the superior and inferior cerebellar peduncles

• Infrafacial triangle

  • Medial border: the MLF

  • Caudal border: the striae medullares

  • Lateral border: the facial nerve

Strauss et al. performed a detailed morphometric anatomical study to measure the structural distances on the floor of the fourth ventricle in order to localize the “safe entry zones” . Bogucki et al. investigated the cytoarchitectonic relationship between the morphometric landmarks and intramedullary structures. They concluded that the abducens nucleus is located within the facial colliculus and CMN XII is located in the hypoglossal triangle . Summarizing the analyzed data of both papers, it would be safe to set the caudal border of the suprafacial safe entry zone as about 20 mm from the obex and between 2 and 4 mm from the midline. For the infrafacial triangle, it is located between 7 and 16 mm from the obex and between 2 and 4 mm from the midline . Anatomically defined “safe entry zones” are more limited areas than original defined ( Fig. 11.2 ). Furthermore, the problem with using anatomical landmarks is that those structures can be lost or difficult to recognize during surgery because of a distorted brain stem caused by the lesion .

11.1.3

Corticobulbar tracts

Anatomical knowledge of the CBT is required to understand the role of BSM and its limitations. Based on Krieg’s description, the CBT consists of seven major branches . All of the branches were from the corticospinal tract and run in a ventrodorsal direction toward CMN . Fig. 11.4 demonstrates each branch of the CBT. CMN supplies several branches of the CBT. For example, CMN VII is innervated by the medial, the leminiscal, the trigemino-facial, and the facial branches of the CBT . This anatomical background tells us that direct damage to the CBT is unlikely when a brain stem lesion is approached through the floor of the fourth ventricle. Furthermore, multiple innervations of CMN suggest that even when a branch of the CBT is damaged during surgery, the function of the CBT will be taken over by another branch of the CBT. The possibility of permanent damage to the CBT by brain stem surgery seems minimal as long as the brain stem lesion is approached from the fourth ventricle .