Stereotactic Radiosurgery for Trigeminal Neuralgia

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CHAPTER 162 Stereotactic Radiosurgery for Trigeminal Neuralgia

Jason P. Sheehan, Jay Jagannathan, Stephen J. Monteith

The French surgeon Nicholas Andre described “tic doulourex,” or trigeminal neuralgia, in 1756. Trigeminal neuralgia is characterized by a temporary paroxysmal lancinating or electric shock–like pain in the trigeminal nerve distribution and is typically confined to one side of the face but may be bilateral in rare cases. Pain-free intervals are common and vary in length from weeks to years. The prevalence of trigeminal neuralgia is 4 to 5 per 100,000.¹ First-line treatment for trigeminal neuralgia is with medications. Medications typically employed include Tegretol (carbamazepine), Neurontin (gabapentin), Lyrica (pregabalin), and baclofen. Other kinds of facial pain broadly called atypical trigeminal neuralgia may occur with deafferentation, trauma, multiple sclerosis, somatoform pain disorders, and herpes zoster; more appropriate classification schemes exist.² Nonetheless, treatment options for these types of facial pain are fairly similar to those used for trigeminal neuralgia, but the treatment options are usually less effective.

Because most patients with trigeminal neuralgia present in their 50s or 60s, they typically have a greater than 10-year life expectancy. Over the years, it is not uncommon for a patient’s trigeminal neuralgia to become refractory to their medications. Those with medically refractory trigeminal neuralgia go on to neurosurgical treatment. One of the major causes of trigeminal neuralgia is compression of the nerve by a vascular structure. The first microvascular decompression was described by Taarnhoj in 1952 and refined by Janetta ³^–⁵; this procedure treats the causal agent of trigeminal neuralgia. Ablative neurosurgical procedures performed through percutaneous techniques (e.g., glycerol injection, thermocoagulation, radiofrequency rhizotomy) produce a partial lesion of the trigeminal nerve and may afford pain relief. The concept of stereotactic radiosurgery for trigeminal neuralgia was introduced as a minimally invasive option, but this approach necessitates a trigeminal nerve lesion to some degree.

Historical Perspective of Gamma Knife in Trigeminal Neuralgia

The initial interest in Gamma Knife radiosurgery for trigeminal neuralgia arose from Lars Leksell’s interest in using radiosurgery for the treatment of pain syndromes.⁶ Twenty years before building the Gamma Knife, Leksell used an orthovoltage stereotactic technique to treat patients with trigeminal neuralgia and achieved some degree of relief of symptoms.⁷

Leksell also described gamma thalamotomies, which were used not only for tremor but also for intractable pain.⁸ The gasserian ganglion was irradiated for trigeminal neuralgia, and gamma capsulotomies were performed to interrupt frontolimbic connections in the treatment of intractable anxiety and obsessive-compulsive disorders.⁹^,¹⁰ Leksell’s first patient was treated through an x-ray tube attached to a stereotactically centered device, and, in some patients, Leksell achieved pain relief for up to 17 years after treatment. The major challenge in targeting trigeminal neuralgia at that time was the limited visualization of the gasserian ganglion on plain x-rays. This hurdle, combined with the introduction of new drugs and the emergence of nonablative methods, caused a decline in the use of Gamma Knife for trigeminal neuralgia through the 1970s and early 1980s. However, with the introduction of high-quality magnetic resonance imaging (MRI), which permitted high-resolution imaging of the trigeminal nerve and root entry zone, interest began to increase again.

Rationale for Radiosurgery in Trigeminal Neuralgia and Factors Related to Successful Treatment

As early as 1941, Olivecrona understood (and described) that mechanical pressure along the root or at the level of the gasserian ganglion could be the cause of trigeminal neuralgia.¹¹ In their pioneering work, Granit, Leksell, and Skoglund (1944) demonstrated that local pressure on nerve fibers could result in painful afferent discharges from the injured neural segment,¹² evidence that has been supported by more contemporary work from Jannetta and colleagues, who have suggested that vascular compression of the trigeminal nerve may be a causal agent in trigeminal neuralgia.^5,^13,¹⁴ Despite these hypotheses, however, the fact that balloon compression of the trigeminal nerve can lead to symptomatic improvement in some patients with trigeminal neuralgia underscores the void in our understanding of the pathophysiology of this disease entity.¹⁵^,¹⁶

Radiosurgery appears to be more effective when used as the initial modality than as a salvage treatment for patients with trigeminal neuralgia. Maesawa and coworkers ¹⁷ demonstrated that patients who underwent radiosurgery following failed microsurgical decompression had high rates of recurrence, with only 60% of these patients maintaining complete pain relief at 1 year, a number that dropped to 53% at 2 years and 33% at 5 years. Régis and colleagues¹⁸ have reported that the probability of pain relief at 1 year in patients without previous surgery was 88%, compared with 82%, 80%, and 75% in patients with one, two, and three prior surgical interventions respectively.

All secondary treatments for trigeminal neuralgia are associated with poorer results than after the first procedure, a factor potentially related to the fact that patients who need retreatments (or alternative treatments) have more severe disease.

In patients who have not undergone previous surgery for trigeminal neuralgia, patients who have contact between a blood vessel and the trigeminal nerve revealed by high-resolution MRI have a particularly favorable response to radiosurgery.¹⁹ In addition to these factors, Pollock and associates²⁰ demonstrated that younger patient age and the length of the trigeminal nerve irradiated were related to improved radiosurgical outcomes (with patients who had longer segments radiated having improved outcomes). In contrast, Régis and associates²¹ noted that the probability of being pain free was lower in patients younger than 60 years of age (56% compared with 91%).

The presence of atypical facial pain (such as tingling or burning) has also been reported to have a poorer prognosis when radiosurgical treatment is used, with only 44% improvement of symptoms in this group, compared with 84% improvement in patients with typical pain according to Maesawa and colleagues.¹⁷ Atypical facial pain patients frequently represent a collection of patients with different types of underlying disease (e.g., postherpetic neuralgia, pain following sinus surgery, pain from facial trauma). The efficacy of Gamma Knife radiosurgery for relief of pain in atypical facial pain patients is not well defined by the literature; this is in part due to the heterogeneity of this patient population. Nevertheless, it is safe to conclude that the outcome of radiosurgical treatment for atypical facial pain patients is less favorable than for those with traditional trigeminal neuralgia.

Treatment with Gamma Knife radiosurgery in the setting of multiple sclerosis has been performed with lower rates of success according to some authors.¹⁸^,²² Régis and associates¹⁸ found a trend toward treatment failure in cases of multiple sclerosis (42.9% compared with 12.8%; P = .07) when the minimal nerve dose was lower. In contrast, Rogers and coworkers²³ achieved a high level of success in a study of 15 patients with multiple sclerosis undergoing Gamma Knife radiosurgery for trigeminal neuralgia. At a mean follow-up of 17 months, 80% of patients had experienced relief using an initial dose of 70 to 90 Gy. Five patients underwent repeat Gamma Knife radiosurgery to the same treatment area with a mean maximal dose of 48 Gy. All five patients attained some pain relief, and 60% were able to discontinue their trigeminal neuralgia pain medication.²³ More investigation is required to assess the long-term success and toxicity (in particular the development of troublesome facial numbness) of Gamma Knife radiosurgery in multiple sclerosis patients.

Radiosurgical Targeting

Accurate MRI is critical to the success of radiosurgical treatment. The trigeminal nerve is typically imaged using at least a 1.5-Tesla (T) MRI unit (Fig. 162-1); 3-T imaging during radiosurgery may offer improved resolution. Localization is performed using T1-weighted and fast spin echo T2-weighted axial images along with coronal images of the nerve (Fig. 162-2). The axial volume acquisition matrices are then divided into sections of 1 mm without gaps. In many centers, T1-weighted images are also repeated after administration of gadolinium. Three-dimensional reconstructions are also routinely performed (Fig. 162-3). When MRI is not feasible (i.e., in patients with pacemakers), computed tomography (CT) may be used for targeting, although this is not ideal. CT cisternography can be used to improve delineation of the radiosurgical target.

FIGURE 162-1 T1-weighted axial unenhanced magnetic resonance imaging demonstrating a Gamma Knife treatment plan for trigeminal neuralgia. The nerve is targeted with a single nonblocked isocenter (yellow circle; the 50% isodose is receiving 40 Gy), and the brainstem is outlined in red for dose-volume histogram computational purposes.

FIGURE 162-2 This is a constructive interference in steady state (CISS) sequence axial image of the same patient depicted in Figure 162-1. The brainstem is again contoured in red. CISS sequencing can be particularly useful in patients with prior open neurosurgical interventions around the area of the trigeminal nerve.

FIGURE 162-3 Three-dimensional reconstructions are possible using radiosurgical software. These permit differentiation and accurate mapping of the trigeminal nerve (green), the brainstem (red), and compressive lesions (e.g., vessels). They also allow for three-dimensional depiction of isodoses (50% isodose depicted in yellow).

With radiosurgery, the usual target for treating trigeminal neuralgia is the trigeminal nerve root entry zone at the level of the pons. Varying lengths of the cisternal segment of the trigeminal nerve can affect radiosurgical targeting. Longer segments of the nerve allow for placement of a single isocenter with minimal to no blocking yet with still a tolerable dose delivered to the brainstem. A patient with a shorter trigeminal nerve cisternal segment may necessitate blocking or beam shaping to reduce dose to the brainstem (Fig. 162-4). Using a gamma angle of 110 degrees frequently facilitates alignment of the isocenter with the axis of the trigeminal nerve.

FIGURE 162-4 T1-weighted axial magnetic resonance imaging is shown with the 50% isodose depicted in yellow and the brainstem contoured in red. The isocenter has been blocked to alter the normal spherical shape to a more elliptical one. This change helps decrease the brainstem dose in patients undergoing retreatment or in those with short cisternal segment trigeminal nerves.

Much has been written about whether the trigeminal nerve should be targeted more proximally or distally.²⁴^–²⁶ Régis and associates²¹ irradiated the cisternal (distal) portion of the trigeminal root and reported a high rate of effective pain control, with an 87% rate of excellent outcome and a 10% rate of painful relapse. This more anterior target choice showed fewer complications even with a maximal dose of 90 Gy. Nicol and associates²⁷ and Pollock and coworkers²⁰ targeted the trigeminal root entry zone with a dose of 90 Gy and found a higher rate of pain control, along with a higher rate of complications compared with when lower doses were given. Massager and colleagues²⁶ obtained higher pain control rates using the plexus triangularis as the target. However, the length of the nerve targeted in this study was significantly increased when compared to other reports, and, as a result, the rate of trigeminal nerve injury (38%) was higher than that reported by Nicol²⁷ and Pollock.²⁰

Dose selection for trigeminal neuralgia has been studied extensively. Most centers use maximal doses of 70 to 85 Gy.²⁸^–³³ Several authors have demonstrated a significant improvement in the rate of pain cessation and a reduction of recurrence rate when high maximal doses are used. Brisman and Mooij³⁴ demonstrated that treatments in which the brainstem received 20% or more of the maximal dose to a volume of 20 mm³ provided better pain control than those targeting smaller regions with lower doses. When higher doses are required, the isocenter is usually placed away from the brainstem to avoid injury. Because doses greater than 90 Gy have been associated with an increased risk for postradiosurgical complications, most radiosurgical teams tend to limit the maximal dose to 90 Gy or less.³³

Also important during planning is to ensure that the steepest gradient index of the dose distribution (between the 50% and 70% isodose lines) coincides with the periphery of tissue being treated. This may require several overlapping fields of radiation, each using a different collimator size and a separate stereotactic focal point. Changing the relative time of radiation at each target may also change the isodose distribution. Finally, the radiation field may be altered by blocking some of the radiation sources, also known as “plugging” or shielding.