This page contains a Flash digital edition of a book.
Gamma Knife Radiosurgery for Trigeminal Neuralgia—A Review GKRS was first used by Leksell to treat TN in 1971.19 Since this initial


experience, GKRS has been used to treat TN in thousands of patients worldwide. Unlike other TN treatments, GKRS is effectively non-invasive. It is similar to the percutaneous techniques in that its mechanism treats the trigeminal nerve through lesioning and bypasses the cause (i.e. vascular compression). Interestingly, although GKRS is very effective at eliminating the paroxysmal pain of TN, the risk of facial anesthesia or dysesthesias is low. It was originally suggested that the mechanism of action was a selective effect of radiation on certain populations of axons. Following irradiation of the trigeminal nerve by GKRS, MRI shows evidence of contrast enhancement at the radiosurgical target site.20


Histological evaluation of the trigeminal nerve


After a high radiation dose (100 Gy), nerve necrosis was identified. Thus these data demonstrated the effect of GKRS is non-selective.


in primates following GKRS showed axonal degeneration and mild edema, with both large and small myelinated and unmyelinated axons seeing effects.20


Our experience at the University of Pittsburgh treating TN with GKRS has spanned nearly three decades. In our recent report by Kondziolka et al. we reviewed our 25-year experience in over 500 medically refractory patients with idiopathic TN.21


Table 2: The Barrow Neurological Institute Score Grade


Result I II


IIIa IIIb IV V


No trigeminal neuralgia pain, requires no medication Occasional pain, requires no medication No pain, requires continued medication


Some pain, controlled adequately with medication


Pain improved, but not adequately controlled with medication Continued severe pain without relief


Figure 1: Axial Magnetic Resonance Imaging Images (Left T1 and Right T2) Showing the Radiosurgical Target at the Right Trigeminal Nerve in a Patient with Trigeminal Neuralgia


The median age of this cohort was 72 and nearly half had recurrent pain after previous procedures. Although the gasserian ganglion had previously been the radiosurgical target, the most common target at present is the cisternal portion of the trigeminal nerve. Specifically, the nerve was targeted 3–8 mm distal to its exit site from the pons. In the majority of patients, a single 4 mm isocenter was targeted to the nerve and a dose of 80–85 Gy was administered (see Figure 1). In order to compare results across studies, many investigators have adopted the Barrow Neurological Institute (BNI) score as a standardized vehicle to indicate the success of a radiosurgical treatment (see Table 2).22


Following


GKRS, 89 % of patients initially responded to treatment (BNI scores I–IIIb) with a median latency of one month. Only 11 % failed to achieve a good result (BNI scores IV and V). Forty per cent of the responding patients achieved complete pain relief (BNI score I). Results were durable in most patients but tended to decline as follow-up lengthened. The probabilities of maintaining adequate pain relief (BNI scores I–IIIb) at one, two, five, and 10 years were 80, 71, 46, and 30 %, respectively. Pain recurred in 193 patients (38 %) after a median of about two years, but only 147 (29 %) required additional surgical treatment for better pain control. We observed no early complications related to GKRS. Around 10 % of patients eventually developed increased facial sensory dysfunction in the form of paresthesias or sensory loss. Interestingly, patients who developed worsened facial sensory dysfunction had a significantly lower rate of pain recurrence (19 % of these patients recurred). Only one patient developed deafferentation pain (anesthesia dolorosa, 0.2 %) and this patient had pre-existing sensory loss after an MVD done prior to GKRS. Overall, most patients experienced good pain relief and had very low associated morbidity. Similar results have been reported in other large series.23–25


Several avenues of investigation have been taken in an attempt to improve upon results with GKRS. Pollock et al. sought to improve patient pain relief through dose escalation.26


In this study, patients receiving 70 Gy were


compared with those receiving 90 Gy. With the higher dose there was a significant reduction in treatment failure, with a trend toward improved pain control. However, 90 Gy resulted in a significant increase in permanent sensory dysfunction (54 versus 15 %) with 32 % of patients developing bothersome dysesthesias. The conclusion of this study was that dose


US NEUROLOGY


The inner circle represents the 50 % isodose line (35 Gy) and the outer circle shows the 12 Gy line.


prescription should be kept below 90 Gy. Flickinger et al. performed a prospective double-blind, randomized trial comparing short- and long-segment GKRS for TN.27


Patients were treated with either one or two


4 mm isocenters to the cisternal segment of the trigeminal nerve. Although pain relief rates were similar between the two groups, there was increased facial numbness and bothersome paresthesias in the two-isocenter group. Thus single isocenter treatments are favored. Other groups have evaluated whether proximal or distal target sites improve outcome or change complication rates.28–30


Matsuda et al. performed the only comparison,


although retrospective in nature, between anterior and posterior targeting of the trigeminal nerve.30


They observed that with posterior targeting of


The target is based on the variable distance between the pons and the cavernous sinus with the goal of minimizing the dose to the lateral pons.


the nerve patients had improved early pain relief and a lower rate of complications. However, other groups advocate for anterior cisternal targeting because of its improved pain relief and low complication rate.28,29 Thus, this question remains unanswered. It is our routine to target the nerve 3–8 mm anterior to the junction of the nerve with the pons such that the brainstem surface is irradiated at the 20 % isodose line or less.21


Which Patients Should Receive Which Therapy? Choosing which patients should undergo GKRS for TN requires consideration of a number of factors including the suspected pain etiology, patient age, surgical fitness, previous treatments, severity of pain, patient preference, and understanding of each technique’s success and complication rates. Ten years after initial MVD for medically refractory TN, 64% of patients continue to have nearly complete relief of pain without the need for medication.4 for GKRS after 10 years.21


This is twice the rate of durable pain relief Cohort analyses comparing the efficacy of


151


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108