Preoperative identification of the facial nerve achieved using fast spin-echo MR imaging: can it help the surgeon?

In this issue of the AJNR, Dr. Sartoretti-Schefer et al (page 810) present a novel study investigating imaging of the facial nerve and its relationship with acoustic tumors. They point out that it has been reported in the literature that the facial nerve lies anteriorly to the tumor 98% of the time. With the T2-weighted fast spin-echo MR technique, they could identify consistently the entire course of the facial nerve in small tumors and found its position where expected (anterosuperior). Their article calls further attention to the kind of detailed preoperative imaging anatomy that is now available to the neuro-otologist with this technique. Other useful imaging data for the surgeon includes distance from the lateral tumor margin to the fundus of the internal auditory canal (IAC), and in smaller tumors, the nerve origin. One of the major advances in the treatment of acoustic tumors has been the development of transtemporal approaches (translabyrinthine and middle fossa) for their removal. These new surgical approaches allow positive facial nerve identification in the proximal fallopian canal (labyrinthine segment) where it is normal, and not involved with the tumor. Routine facial nerve preservation in most published series exceeds 90% as a result of the surgeon’s confident facial nerve identification (1). For small tumor removal through the retrosigmoid approach, facial nerve identification in the lateral IAC is also routinely possible. The other important advance in acoustic tumor treatment has been intraoperative facial nerve monitoring. This consists of continuous electromyographic monitoring of facial muscle activity, and allows identification and mapping of the course of the facial nerve by using electrical stimulation during surgery. This technique has led to improved postoperative facial nerve function (2). With the combination of anatomic facial nerve location and intraoperative facial nerve monitoring, the facial nerve can be identified routinely during acoustic tumor surgery. Preoperative imaging of the facial nerve probably would not change the surgical approach or the intraoperative surgical technique. For acoustic tumors, the choice of surgical approach is usually based on tumor size, tumor location, and hearing level. At our institution, we use the translabyrinthine approach for patients with poor hearing or who have large tumors. For patients with good hearing, we use the middle fossa approach for small tumors involving the lateral IAC, and the retrosigmoid approach for small tumors in the cerebellopontine angle and the medial IAC (3). There is debate regarding the tumor type associated with the unusual posterior course of the facial nerve (approximately 2% of cases, as stated by the authors). In my experience, this unusual facial nerve position is found only with facial nerve neuromas and meningiomas, and is not seen in association with acoustic tumors. We have found preoperative fast spin-echo MR imaging to be helpful in our center in predicting the course of the facial nerve (and the eighth nerve) in patients with posterior fossa meningiomas. In these cases, the relationship of the facial nerve to the tumor is quite variable, and preoperative knowledge of its course can influence surgical technique and selection of surgical approach (4). Depending on the course of the facial nerve in relationship to the meningioma, an approach may be selected that leads to decreased manipulation of the nerve, improved visualization of the nerve, or transposition and rerouting of the nerve to augment access.