3.2 Microvascular Decompression for Trigeminal Neuralgia

When a person suffers from trigeminal neuralgia, many options exist that can help them manage or eliminate their pain. Both medical and surgical options exist to meet differing needs, but patients should understand that each option has risks, benefits, and relative rates of and durations of success. Choosing to have intracranial surgery is always serious, and a thorough explanation by healthcare professionals should precede any such decision. As one of the most common surgeries for treating trigeminal neuralgia, microvascular decompression (MVD) is the surgical option discussed here.

Introduction. An MVD is a surgical procedure used to treat trigeminal neuralgia (TN), both TN type 1 (attacks of pain) and TN type 2 (some constant pain along with attacks of pain). It is also effective in treating other related disorders such as glossopharyngeal neuralgia, geniculate neuralgia, and hemifacial spasm, but for this discussion, we will limit the topic to TN. Of the surgical and procedural options, MVD has the distinction of being the most effective treatment for TN. This holds for general success rates as well as longevity of success, and it allows for preserved nerve function (no numbness). Most neuro surgeons believe that this is a logical outcome, as MVD is the only treatment for TN that –

• Corrects the anatomic problem causing the pain, namely, vascular compression of the trigeminal (fifth) nerve, and
• Does not include deliberate nerve damage as part of the procedure (all other procedures do, including radiation).

However, although the most effective, MVD is also the most invasive. Let us now take a closer look at the details behind TN and MVD surgery.

The trigeminal nerve is the fifth of 12 cranial nerves. Cranial nerves all emerge from or directly enter the brain or brain stem. The major function of the fifth nerve is to provide sensation from the face. A web of nerve endings comes together to flow through three openings in the skull: one above and one just below the eye and a large oval opening, the foramen ovale, at the base of the skull. These nerve endings converge on the ganglion, which is the energy center for each of the nerve fibers, in a small basin, Meckel’s cave, and then narrow in a thicker nerve root (like the trunk of a tree) that is located in the spinal fluid space until it enters the part of the brain stem called the pons. The trigeminal nerve provides sensation from the eyes, teeth, tongue, and inside of the nose and mouth. The nearest named blood vessel above the trigeminal nerve root is the superior cerebellar artery (SCA). This artery (and sometimes other vessels) can elongate (common with aging) and form a loop that makes contact with the trigeminal nerve root. In some people, when a vascular loop pulsates against the trigeminal nerve at or near where it leaves the brain stem, it can cause pain. Not everyone with this anatomic variant gets TN, and it is unclear why it occurs in those who do.

The trigeminal nerve undergoes an interesting structural change as it enters the brain (as do all nerves as they enter the brain or spinal cord). Picture nerves as wires in a circuit. The long segments of nerves (axons) connect the body to the brain. And just like wires, all nerves have insulation for protection. This protective insulation is composed of a fatty compound called myelin, which helps conduct impulses rapidly. This is where we get to the fascinating (and potentially terrifying) part of the trigeminal nerve: the mysterious root entry zone (REZ).

The REZ is the site where the nerve is just outside the brain. Here, the myelin that surrounds the axons transitions from a type made by one cell to a type made by another kind of cell. The myelin covering the trigeminal nerve in the central nervous system (inside the brain) is produced by oligodendroglia cells. These exist only in the brain itself. When the trigeminal nerve leaves the brain to become part of the peripheral nervous system, the myelin manufactured changes over to that produced by Schwann cells. These exist only outside the brain. You can understand that the brain is protected by the skull, so nerves inside it are well protected. However, outside the skull, the nerves are subject to injury. So, they have a thicker form of myelin, or insulation, around them. There is a transition zone as the nerve enters the brain in which there is still some thin insulation. Something about that transition makes the nerve vulnerable to compression. This is why physicians who understand TN make a big deal about finding “a vascular loop at the root entry zone.”

Although it is not known why the REZ is vulnerable, contact or compression of the nerve at this location can certainly create “abnormal conduction behavior”—that is, terrible face pain—even though, to appearances, there is nothing wrong with the face. Although the cause of the problem is pulsation near the trigeminal nerve REZ, few patients describe their TN pain in the character one would consider “pressure” or a “pressure sensation.” The pain of TN is typically a sudden, sharp, electric, or stabbing pain. It is a short circuit of the nerve caused by the presence of unprotected nerve fibers stripped of their insulation from the constant arterial pounding.

Now that we have a basic understanding of the trigeminal nerve when it is not functioning correctly, we can explore the journey of having an MVD.

Discussing an MVD with a Surgeon. In most cases, the physician orders preoperative imaging with thin-cut MRI scans and three-dimensional studies performed using one of the techniques known by the acronyms FIESTA and CISS. These are acronyms for computer-programmed sequences that will show the nerves, arteries, brain, spinal fluid and their relationships. Three-dimensional means that the images are demonstrated in three different planes at once: straight ahead, from the side, and from the top or bottom. This gives the reviewer the best ability to determine whether there is a vein or artery in contact with the trigeminal nerve. Understand that, if the neuroradiologist reviewing your MRI images, is not familiar with the anatomy of TN, they may call the nerve “normal,” which, to all appearances, it is. In addition to having an expert surgeon who knows what to do on “the inside,” the experienced surgeon will be able to provide their personal interpretation of the images, which will help determine whether to offer you an MVD versus other choices based on the MRI results, as well as other considerations.

When selecting a surgeon, the best place to start is with a provider who has the time to speak to you and answer all your questions. Unfortunately, we do see MVDs that have not been done well, so do your homework and research your provider. Nearly all neurosurgeons complete their training with at least a little experience performing MVDs. However, finding someone who is an expert and focuses their practice on this condition and this specific operation will serve you well. (See the previous article, “What Makes a Neurosurgeon an Expert?”) After surgical and nonsurgical options have been reviewed to your satisfaction and you feel that an MVD by an experienced surgeon is for you, it is time to prepare for surgery.

The Surgery. All MVDs are performed under general anesthesia. During MVD surgery, an opening is made in the bone behind the ear. Some spinal fluid is drained, and the space over the exposed brain (the cerebellum) is followed to the location where the nerve enters the pons and then on to the location of vascular compression. At this point, the magic occurs under the surgical microscope as the vascular compression is relieved (Figure 3-1). In the classical operation popularized by Dr. Jannetta, a tiny piece of Teflon felt was used to separate the blood vessel and nerve. In my particular practice, I try to avoid using Teflon, and instead, I employ a variation I call a microvascular transposition. Article 3.3 will provide more information about this Teflon-free MVD. The risks of MVD surgery include the need for general anesthesia, the possibility of bleeding, and infection, as in all surgeries, as well as risks related to the patient’s overall medical condition. Because the surgery takes place next to other cranial nerves, there is a small chance that hearing can be affected on the same side as surgery. To prevent this, monitoring of the nearby nerves is routinely performed; this is known as intraoperative neuromonitoring or intraoperative electromyography (EMG) and brain-stem auditory evoked response (BAER).

Figure 3-1. Intraoperative image of the trigeminal nerve with and without vascular compression.

After the decompression is complete, the covering (dura) of the brain is meticulously closed. The bone removed can be replaced and held in place with miniplates, screws, or cement, or an implant can be used instead. An inch of muscle at the base of the skull that was split is closed, and then the scalp above it is closed in layers.

The Recovery. Recovery from MVD surgery should focus on mobilization. I get my patients walking the night after their surgery. This helps reduce muscle spasms and prevents other problems such as pneumonia and blood clots in the legs and lungs. These latter two issues are risks after any surgery with a general anesthetic. Some patients have some nausea and dizziness after MVD surgery. These symptoms usually resolve in a few days and can be treated with medication, or a medication patch, worn behind the nonsurgical ear. Once the facial pain has stopped, patients can begin tapering off their medications. The facial pain medications are not stopped “cold turkey,” as this can cause significant side effects. Around 75% of MVD patients can leave the hospital the next day, possibly after two days. Postoperative pain medication is used for the discomfort at the surgical site behind the ear. Early mobilization and pain medication are the mainstay to support this recovery.

In conclusion, MVD is a successful surgery for TN. The first steps are obtaining a correct diagnosis; seeking good information prior to deciding; and of course, choosing an experienced surgeon.