Microvascular decompression of the root emerging zone for hemifacial spasm: evaluation by fusion magnetic resonance imaging and technical considerations

The root exit zone (RExZ) of the facial nerve has been considered to be the target in microvascular decompression (MVD) for hemifacial spasm. However, more proximal segments with oligodendrocyte-derived myelin, where the facial nerve root emerges at the pontomedullary sulcus and adheres to the brainstem surface (root emerging zone [REmZ]), may also be susceptible to neurovascular compression. This study evaluated the predictive value of magnetic resonance (MR) imaging in detecting and assessing the features of vascular compression, especially in the pontomedullary sulcus, and describes the technical considerations of MVD procedures for the more proximal segments of the facial nerve. Twenty patients treated with MVD underwent three-dimensional constructive interference in steady-state MR imaging and three-dimensional time-of-flight MR angiography. Their fusion images were used to evaluate the anatomical neurovascular relationships and intraoperative findings were analyzed. Most offending arteries at the REmZ and the RExZ of the facial nerve were correctly identified by fusion MR imaging. During surgery, neurovascular contacts were identified at one or more segments of the facial nerve in all patients. The REmZ of the facial nerve was affected in 55 % of the patients. The most common offending vessel at the REmZ was the posterior inferior cerebellar artery rather than the anterior inferior cerebellar artery. The key procedure to explore the deep-seated REmZ in the pontomedullary sulcus was full dissection of the lower cranial nerves to the brainstem origin. Our definition more correctly describes the specific anatomical relationship of the facial nerve origin from the brainstem and the clinically relevant target for MVD surgery. Fusion MR imaging is very useful to identify neurovascular contacts at both the RExZ and the REmZ of the facial nerve.