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Microvascular decompression surgery (MVD) is a surgical intervention that aims to alleviate nerves that are being compressed usually by arteries and veins, but they can also be compressed by tumors. These blood vessels or tumors press or push the nerves, thus causing them to become irritated and/or inflamed. This irritation of the neural tissue causes the nerve to become damaged, resulting in the loss of partial or total capacity of its function. The most common cranial nerves affected by this vascular compression are facial, trigeminal, and glossopharyngeal nerves (causing - Hemifacial spasm, trigeminal neuralgia, and glossopharyngeal neuralgia, respectively). Therefore, some of the most common symptoms produced by this neurological pathology are - spasms, pain, twitching, and facial paralysis.
The basic mechanism through which this surgical procedure is performed is placing (after the patient has been induced into general anesthesia) a Teflon sponge between the two affected tissues (the cranial nerve and the blood vessel). And this sponge can be inserted through a small incision behind the ear (of the affected side) right after the surgeon has isolated the affected cranial nerve.
Microvascular decompression is a commonly used neurosurgical treatment for different neuropathologies. It is widely recommended to patients in which other less risky treatment routes are not improving their current symptoms. Some of the risks that accompany this delicate surgery are, for example - hemorrhages, infections, numbness and/or weakness of the facial musculature, hearing loss, leaking of the spinal fluid, etc. Furthermore, there is always a risk that the previously mentioned iatrogenic side effects become permanent (although very unusual).
Electromyography (EMG) and Brainstem auditory evoked potentials (BAEP) are some of the most commonly used intraoperative neuromonitoring tools during microvascular decompression to treat the neuralgias and spams previously mentioned. EMG is used to record the live electrical activity of muscles. EMG’s features primarily used during these surgeries are AMR or abnormal muscle responses. As its name implies, AMR picks up any abnormal signal coming from a muscle thus, whenever it is used intraoperatively and AMR signals vanish completely, it usually means that the targeted blood vessel with the affected cranial nerve has been successfully separated from each other by the surgeon. Therefore, it can be used as a clear indicator and guiding factor that provides reliable information for the surgical team in charge. This is typical for MVD for the facial nerve.
On the other hand, BAEPs are a neuromonitoring test generally used for the preservation of hearing, especially in microvascular decompression surgeries of facial and trigeminal nerves. This IONM modality monitors the pathway of auditory signals (measuring a sound signal from the inner ear to the brainstem) and is usually very sensitive. Any abnormalities detected during BAEPs usually cause changes with waveform V. Thus; it is a very useful test in the detection mechanism to prevent intraoperative hearing loss since it is one of the most common iatrogenic events caused during microvascular decompression surgery. Multimodality intraoperative neurophysiological monitoring can be very beneficial in guiding the surgeon during the surgical procedure and minimizing postoperative neurological deficits.
References:
- Fernández-Conejero I, Ulkatan S, Sen C, Deletis V. Intra-operative neurophysiology during microvascular decompression for hemifacial spasm. Clin Neurophysiol. 2012 Jan;123(1):78-83. doi: 10.1016/j.clinph.2011.10.007.
- Jahangiri FR. Surgical Neurophysiology: A Reference Guide to Intraoperative Neurophysiological Monitoring (IONM). Second Edition. Jahangiri FR (ed): CreateSpace Independent Publishing Platform, Charleston, SC, USA; 2012.
- Lee, Cheng-Chia, et al. “Brainstem Auditory Evoked Potential Monitoring and Neuro-Endoscopy: Two Tools to Ensure Hearing Preservation and Surgical Success during Microvascular Decompression.” Journal of the Chinese Medical Association, vol. 77, no. 6, 2014, pp. 308–316., https://doi.org/10.1016/j.jcma.2014.02.016.
- Microvascular Decompression. (2022, Sep 26). Retrieved from Columbia University Irving Medical Center: https://www.neurosurgery.columbia.edu/patient-care/treatments/microvascular-decompression.
- Microvascular Decompression Surgery. (2022, Sep 26). Retrieved from University of Rochester Medical Center: https://www.urmc.rochester.edu/neurosurgery/services/treatments/microvascular-decompression.aspx.
- Ying, Ting-Ting, et al. “The Value of Abnormal Muscle Response Monitoring during Microvascular Decompression Surgery for Hemifacial Spasm.” International Journal of Surgery, vol. 9, no. 4, 2011, pp. 347–351., https://doi.org/10.1016/j.ijsu.2011.02.010.
