Three-dimensional reconstructions were found to provide much clearer characterization of this area than traditional preoperative imaging. Therefore, Teton et al., suggest that use of these reconstructions in the preoperative setting has the potential to help identify appropriate surgical candidates, guide preoperative planning, and thus improve outcome in patients with HFS 1).
Many ablative procedures are effective for hemifacial spasm (HFS) (including sectioning of divisions of the facial nerve), however, this leaves the patient with some degree of facial paresis. The current procedure of choice for HFS is microvascular decompression (MVD) wherein the offending vessel is physically moved off of the nerve, and a sponge (e.g. Ivalon®, polyvinyl formyl alcohol foam) is interposed as a cushion. Other cushions may not prove to be as satisfactory (muscle may disappear, and Teflon felt may thin 2)).
Most often, the offending vessel approaches the nerve at a right angle, and causes grooving in the nerve. Compression must occur at the root exit zone; decompression of vessels impinging distal to this area is usually ineffective.
Intra-operative brainstem auditory evoked potentials (BAER), 3) or more applicable, direct VIII nerve monitoring 4) may help prevent hearing loss during MVD for 7th or 8th nerve dysfunction. Furthermore, monitoring for the disappearance of the (delayed) synkinetic response may aid in determining when adequate decompression has been achieved (generally reserved for teaching institutions) 5).
The facial nerve should not be manipulated, and one should avoid dissection around the VII and VIII nerves near the IAC 6). Vessels must be preserved, especially the cochlear artery and small perforators. Place gentle medial traction on the cerebellum (<1 cm is recommended 7) ), and incise the arachnoid membrane between the flocculus and the eighth nerve (to avoid tension on nerves that could cause post-op deficit). The IX nerve may be followed medially from the jugular foramen to locate the origin of the VII nerve (the origin of VII is 4 mm cephalad and 2 mm anterior to that of the IX nerve 8)).
Redo MVD remains a feasible treatment option for HFS patients who have failed to benefit from prior MVD, but is associated with higher risks of cranial nerve and vascular injuries 9).
A video demonstrates the surgical steps of a MVD at left facial REZ in a 41-year-old man who presented with typical hemifacial spasm on the left side due to VIIth nerve REZ compression by PICA. A classical retromastoid and infrafloccular approach was performed to avoid stretching of the VIIIth nerve and access the VIIth nerve ventro-caudally. The next step is insertion-along the brainstem, VII-VIIIth nerves REZ, and flocculus-of a plaque made of Teflon felt (Edward-type) which is semi-rigid, and by principle does not exert direct compression on the facial REZ, thus avoiding compression and/or transmission of pulsations on the VIIth nerve. The patient's postoperative period was uneventful and clinical outcome good 10).