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Stereotactic Radiosurgery for Trigeminal Neuralgia
Trigeminal neuralgia (TN) is characterized by unilateral paroxysmal stabbing pain in one or more branch distributions of the fifth cranial nerve. Classical TN typically occurs as a result of neurovascular compression from branches of the superior cerebellar artery or other nearby vasculature. Patients can often identify triggers for facial pain, including chewing, talking, washing, shaving, smoking, or toothbrushing. The pain can be debilitating and have serious consequences on quality of life. Between pain episodes, patients tend to be asymptomatic, although a minority continue to experience background pain. , Although there are several interventions for the treatment of medically refractory TN—including percutaneous rhizotomy and microvascular decompression (MVD)—stereotactic radiosurgery (SRS) can also serve as an effective minimally invasive option.
Historical Background
In the late 1800s, around the time when X-rays (XRs) were introduced, Hermann Moritz Gocht realized the potential benefit of ionizing radiation for therapeutic purposes and treated several TN patients with radiotherapy. The advent of pharmacotherapy for TN and the damaging effects of radiation noted in the mid-1900s led this technique to fall out of favor for some time. However, when SRS was introduced, the application of radiotherapy for TN was revisited. Leksell reported on two patients with classic TN whom he treated in 1953 with a single dose of ionizing radiation targeted toward the gasserian ganglion. Results were unsatisfactory until the target was moved from the gasserian ganglion to the proximal trigeminal root near the pons. The effects of stereotactically delivered radiotherapy at the root entry zone (REZ) produces focal axonal degeneration at the nerve sheath transition between central myelin (oligodendrocytes) and peripheral nerve myelin (Schwann cells), which is thought to be the region most sensitive to both neurovascular compression and therapeutic radiation.
Techniques falling under the umbrella of SRS include Gamma Knife, CyberKnife, and linear accelerator (LINAC) therapy, which are becoming ubiquitously available treatment options. SRS became widely accepted as a beneficial treatment option for TN in 1996, after a multiinstitutional study demonstrated favorable results utilizing Gamma Knife radiosurgery. In 2003, Romanelli and colleagues reported the first use of the CyberKnife to treat TN. In the same year, Smith and colleagues reported the first series of patients with TN treated with LINAC radiosurgery. Although several types of SRS have been shown to be beneficial in the treatment of TN, no single technique has demonstrated clear superiority.
Indications
Initial treatment for TN is conservative medical therapy, with carbamazepine being the gold standard and oxcarbazepine, a keto derivative of carbamazepine, a suitable alternative with more tolerable side effects. SRS is recommended for patients with medically refractory TN who are not surgical candidates, since MVD is associated with the highest rates of pain control in patients who can safely undergo surgical intervention. Studies comparing the efficacy of SRS and MVD for TN have determined higher initial and long-term success rates with MVD. , Hence the use and timing of SRS for TN varies among clinicians. Several factors should be considered prior to intervention, including patient age, medical comorbidities, prior procedures, and patient preference. For example, the benefits of SRS for TN in the elderly has been clearly demonstrated for both initial and recurrent therapy. SRS may be used as a first-line therapy, and it has been reported that early and initial SRS provides superior pain relief when compared with late SRS. Nonetheless, SRS is frequently used as a repeat or salvage treatment for those who continue to have pain following other procedures, including previous SRS. ,
It is important to note that pain relief following SRS is often not immediate and may take months to become evident. Ideally, SRS should be performed in patients who can undergo magnetic resonance imaging (MRI) for treatment planning. For patients unable to undergo MRI, computed tomography (CT) with contrast cisternography can be used. As with other therapies for TN, patients with typical TN symptoms fare best after SRS. Therefore an accurate diagnosis is critical, because SRS has been shown to have less efficacy in the treatment of atypical facial pain compared with classical TN. For example, multiple sclerosis (MS)–related TN can be treated with SRS but not with the same success as that noted with classic TN.
Gamma Knife Technique
Gamma Knife radiosurgery is a frame-based stereotactic technique that delivers radiation from a fixed cobalt source. Planning and treatment occur on the same day in a single session. On the day of treatment, the first step involves placement of the stereotactic Leksell frame under local anesthesia. Typically, four points (two anterolateral and two occipital) are chosen to ensure appropriate fixation of the frame ( Fig. 89.1 ). The insertion points are injected with a mixture of lidocaine with sodium bicarbonate for local anesthesia. Bupivacaine can be injected concurrently for longer-term analgesia. The screws are then inserted into the frame and skull. The appropriate screw should be selected so that it sits flush with the frame. If a screw is too long, it will lie outside of the frame, creating additional artifact during the MRI, which could interfere with the treatment beams. It is also critical that the anterior screws be applied while the patient ‘s eyes are closed. If the screws are applied while the patient’s eyes are open, complete eye closure may not be possible, leading to patient discomfort and a risk of corneal ulceration. After application of the Leksell frame is complete, a 1-mm-thick MRI slice, using gadolinium, is obtained. T1, T2, and fast imaging employing steady-state acquisition (FIESTA) sequences allows for the evaluation of trigeminal nerve anatomy, including the nerve’s relationship with adjacent blood vessels.
The SRS target is typically a single 4-mm isocenter positioned at the trigeminal nerve’s REZ 2 to 4 mm from the junction of the nerve and the pons ( Fig. 89.2 ). , , The use of multiple isocenters has not been demonstrated to provide additional benefit in the treatment of TN, as additional nerve dysfunction may occur. Maintenance of this buffer zone between the nerve and the pons keeps the radiation delivered to the brain stem between 20% and 30% of the isodose. Final dose selection and calculation of dosage delivery plans are performed in conjunction with a radiation oncologist and a physicist. Once planning is complete, treatment can proceed. The patient is placed on the treatment bed and the collimator helmet is affixed ( Fig. 89.3 ). The target coordinates for the beam are confirmed and the treatment commences. Once the treatment is complete, the Leksell frame is taken off by sequentially removing the screws opposite each other. The pin-site defects are covered with gauze and antibiotic ointment.
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