Facial Nerve Elongation in Vestibular Schwannomas: A Useful Aspect with Regard to Functional Preservation

Background The facial nerve is mostly displaced and stretched in an anterior direction in vestibular schwannomas. Preoperative visualization may be partially achieved by Gadolinium-enhanced CISS MRI (Gd-Ciss). The individual tumor shape and extension might be of relevance for tumor-induced changes of the nerve structure and was the main target of this study.

Methods In 50 patients (22 men, 28 women, mean age: 51 years), standard preoperative MRI (Gd-T1) was supplemented by enhanced CISS MRI (Gd-Ciss). Visibility of the facial nerve on the healthy and on the tumor side was investigated. Tumor extensions (along the nerve axis and perpendicular to it), intrameatal extension, and anterior tumor extensions at the level of the facial nerve course were correlated with early postoperative facial nerve function by House Brackmann scale 1 week after surgery.

Results Gd-Ciss MRI showed the tumor extension in the coronal plane, along the nerve axis, of 23.4 mm (significantly larger than that in standard Gd-T1, p < 0.03), 18.9 mm in sagittal and 17.6 mm in cranio-caudal extensions. The tumor growth along the nerve axis was of predictive importance with 22.1 and 23.4 mm resulting in HB 1 or HB 2 to 3 respectively, while a mean of 27.6 mm resulted in HB 4 to 5. The anterior tumor length at the nerve level was even more clearly correlated with outcome, with 28.4 mm resulting in HB 1, 30.7 mm in HB 2 to 3, and 38.2 mm in HB 4 to 5 (p < 0.05). Intra-meatal tumor extension showed a mean of 10.3 mm in Gd-Ciss versus 5.3 in Gd-T1, but no correlation with pre- or postoperative nerve function. Cranial nerve VII was visible in Gd-Ciss MRI in 28 cases (56%) as dark lines running over the enhanced tumor surfaces, while in 22 cases these could not be visualized. Out of the 28 visible cases, in 12 the nerve origins could be identified, in 4 the origin and the intra-meatal portion, in 8 some nerve segments, and in 4 cases the nerves were discernible at their whole length.

Conclusion Gd-Ciss MRI enables direct visualization of the facial nerve in a considerable number of cases, while it remains nonvisible in Gd-T1. Hereby, Gd-Ciss allows a better perspective of the likely nerve course to be encountered at surgery. We suggest the anterior tumor length as a new parameter to signify the lengthening of the facial nerve as it shows a close correlation with early postoperative facial function and has an even higher predictive power than the tumor extension along the nerve axis.