Surgeon: Peter Janetta, MD
Microvascular decompression surgery is a procedure used to reduce pressure from offending vesselson cranial nerves. The trigeminal, facial and glossopharyngeal nerves are the most common cranial nerves affected by vascular compressions, where a vessel can be pressing directly on the nerve causing it to malfunction. This procedure invloves a retromastoid craniectomy which exposes the posterior fossa. Within this area, the brainstem and cerebellum are accessible as well as several of the lower cranial nerves, including CN V, VII, VIII, IX, X, XI and XII. Once the compressed nerve is located a piece of felt is placed between the nerve and the vessel, allowing the nerve to return to normal or near normal functioning. Neurolgical disorders such as trigeminal neuralgia and hemifacial spasms can be treated with the use of the microvascular decompression surgical procedure pioneered by Dr. Peter Janetta. It may be considered as a treatment for these debilitating conditions.
Intraoperative monitoring (IOM) has become an essential tool during this procedure because of the invasive nature of a craniectomy and the specific location of the surgical exposure, putting structures at risk. The Auditory Brainstem Respnse (ABR) has shown to be very valuable in the assessment of the VIIIth cranial nerve. During the exposure stage of the surgery, retractors are placed in the surgical opening which puts pressure on the cerebellum and brainstem. When the retractors are placed, CN VIII can suffer either temporary or permanent damage that is observed immediately in the ABR. If this does occur the IOM staff is able to notify the surgeon to make an adjustment to the retractors to prevent permanent damage and preserve hearing. The cases invloving hemifacial spasms can be monitored with the use of spontaneous and evoked EMG's. The evoked EMG's are seen in the form of a lateral spread. When there is a normally functioning VIIth nerve, no lateral spread will be seen. Therefore, during the decompression of CN VII the lateral spread should disappear. The use of IOM in this case is to verify that the nerve has been completely decompressed.