Indications of GK thalamotomy for the treatment of intractable tremor are determined in accordance with more conventional stereotactic surgery. While DBS or conventional thalamotomy is available, the use of GK thalamotomy has been reserved for specific patients, such as those who are poor candidates for conventional surgery (e.g., due to advanced age or ongoing anticoagulant therapy).

Intractable tremors could be classified as components of ET, PD, or multiple sclerosis. The most common disease treated by GK thalamotomy is ET. ET is a common adult movement disorder, with prevalence estimates derived from population studies ranging from 0.4 to 5 % [17, 18]. The incidence and prevalence of ET both increase with advancing age [18], and conventional thalamotomy or DBS is not often recommended for aged patients with ET.

PD is another indication for GK thalamotomy; however, subthalamic DBS improves symptoms of rigidity, bradykinesia and motor fluctuation, as well as tremor [19]. In a systematic review of PD progression, the non-tremor dominant subtype at disease onset appeared to favor rapid progression to Hoehn and Yahr stage III [20]. Tremor seems to predict a slow progression of PD. If intractable tremor causes significant functional impairment, GK thalamotomy is an option for tremor reduction [7].

Although the efficacy of GK thalamotomy for intractable tremor has been evaluated, the prescription dose and target selection varied greatly among the studies reviewed (see Table 55.1). Previous studies have selected the ventralis intermedius nucleus (VIM) as the target for treating intractable tremor. In the majority of studies, target determination was based on the anterior commissure-posterior commissure (ACPC) line. Anterior-posterior coordination was 6–8 mm anterior to PC, or 25 % of the ACPC line from PC, while right-left coordination was 11–13 mm lateral to the ACPC line, and rostral-caudal coordination was 2–4 mm above the ACPC line. Anatomical investigation of formalin-fixed human brains indicated the anterior tip of ventroposterior nucleus [21]. Ohye et al. adjusted the tentative target based on the ACPC line, so that the center of the radiation beam would cover 45 % of the longitudinal axis of the thalamus for actual irradiation [22]. No neurophysiological information aids the refinement of the target and is in fact a disadvantage in GK thalamotomy. Sato et al. performed depth recordings around the region of the supposedly optimum target for tremor, as determined in usual thalamotomy, to provide indirect support for GK thalamotomy, and demonstrated a rhythmic discharge time locked to tremor, and the presence of kinesthetic neurons within the expected target area [23]. Extensive experience using stereotactic thalamotomy with microelectrode recordings might support GK thalamotomy, which has the disadvantage of involving no neurophysiological examination.

Accurate lateral coordinates are crucial to prevent radiation injury to the internal capsule. Duma et al. adjusted the 50 % isodose line medially to the internal capsule [24]. Kondziolka et al. retained 20 % of the isodose line of the 4-mm collimator medially to the internal capsule [14]. The tolerance of the internal capsule thus remains unclear. However, Maruyama et al. used diffusion tensor tractography of the pyramidal tract when dose planning of GK surgery for treatment of an arteriovenous malformation and calculated the tolerance of the pyramidal tract [25, 26]. The maximum dose to the pyramidal tract and 20- or 25-Gy volume in the pyramidal tract significantly correlated with the motor complication. Interestingly, the internal capsule was found to be more fragile than the coronal radiation. A plugging technique to reduce the dose of radiation to the internal capsule was applied in several recent studies (Fig. 55.1) [7, 14, 16]. It is now generally accepted that the maximum dose of radiation to the internal capsule should be less than 20–25 Gy.

GK treatment has been commonly performed using a 4-mm collimator. The prescribed dose varied among the different studies (see Table 55.1). Steiner et al. reported that a dose of 150 Gy was necessary for the reproducible creation of a human brain lesion, using an autopsy series of GK thalamotomy that used to treat intractable cancer pain [27]. In the reports published in the early 1990s, a relatively high dose was selected for lesioning; however, in more recent reports, a maximum dose of 130–140 Gy was recommended for thalamotomy using GK [7, 14, 16]. Okun et al. reported five cases with complications of GK thalamotomy, including two cases of bilateral lesioning [6]. Radiation effects were found to have expanded into the internal capsule and mesencephalon, which caused further motor complications and pseudobulbar symptoms. However, the prescribed dose should be noted; the target was irradiated at a maximum dose of 200 Gy. Such a high dose is not currently used for the patients with intractable tremor.

Previous reports demonstrated the effectiveness of GK thalamotomy in treatment of tremor; however, the evaluation of tremor reduction was not uniform (Table 55.1). The Unified Parkinson’s Disease Rating Scale (UPDRS) [28], the Fahn-Tolosa-Marin Tremor Rating Scale [29], and tremor-cessation rates were all used to grade tremor and other symptoms, before and after treatment. Overall, 70–90 % of the tremor patients obtained an anti-tremor effect from GK surgery. Differences in the outcomes of GK surgery in different primary diseases remain unclear due to insufficient numbers of patients.

In order to clarify the safety and ascertain optimally effective conditions for performing unilateral GK thalamotomy for treatment of intractable tremors, a prospective multi-institutional study was published by the Japan Leksell Gamma Knife Society (JLGK0301). The study included blinded videotaped assessments and demonstrated the efficacy of GK surgery for the treatment of both tremor and rigidity in PD; however, no effects on bradykinesia or gait disturbance were found [7].

A slow improvement over several months tended to characterize the clinical course following GK thalamotomy, in contrast to the immediate effect of radiofrequency coagulation. The period over which improvements in clinical symptoms were observed varied widely, from several days to approximately 1 year (Table 55.2) [7–9, 24, 30–32]. A single session of high-dose irradiation requires time to produce the desired lesion. Leksell described autopsy cases in his first report of GK thalamotomy [3]. Lesions irradiated at 200–250 Gy were confirmed pathologically 2 and 5 months after treatment, and the lesions contained necrosis with a marginal gliotic zone. Radiosurgical lesions are dependent on the dose received, volume of tissue irradiated, and time [33, 34]. The low dose of irradiation (130–140) used more recently would require some time to produce the lesion following GK thalamotomy. However, some of the patients treated by GK thalamotomy experienced treatment effects even before coagulation necrosis occurred. Kondziolka demonstrated that the lower doses (80 Gy) used to treat trigeminal neuralgia cause incomplete axonal degeneration in a baboon model [35]. Early changes, prior to coagulative necrosis in the thalamus, might occur after GK surgery and cause an early improvement in clinical symptoms.

The anti-tremor effects of GK thalamotomy should be compared with those of other surgical treatment options. However, a head-to-head comparison with other surgical procedures was limited, because various methods of tremor evaluation and variable follow-up terms were applied in each study. Based on tremor-cessation rates, VIM DBS resulted in a tremor-absent state in 67–96 % of both PD and ET patients [36, 37]. VIM thalamotomy also abolished tremor in 65–89 % of PD and ET patients. The tremor-cessation rate achieved with GK thalamotomy was, however, relatively low; 79 % of the patients had good tremor control, including slight or infrequent tremors, which is similar to the results of VIM thalamotomy [37, 38].

Some failed cases of GK thalamotomy have been reported. Some of these cases received subsequent conventional thalamotomy, or DBS. In these cases, intraoperative monitoring revealed the radiation effects through electrical activity in the thalamus. Normal neuronal and tremor rhythm were demonstrated in and adjacent to the lesion of the previously irradiated target [39]. Target deviation to the anterior and medial direction, or rhythmic tremor discharge located close to the internal capsule, was less effective. This seems to reflect technical limit of GK thalamotomy at present. We have relatively limited knowledge of the effects of radiation on neuronal activity in the thalamus. Terao et al. reported their microelectrode findings after GK thalamotomy and demonstrated a reorganization of kinesthetic cells in the irradiated VIM [40]. Changes in the somatotopography of kinesthetic cells might make it difficult to perform a secondary surgery on these failed cases.

Reported complications from GK thalamotomy varied between 1.4 and 46.7 % (Table 55.1). These complications mainly consisted of hemiparesis and speech disturbances. Most of the reported complications were transient; however, some of patients suffered from severe sequelae [6]. A high prescribed dose of radiation seemed to be associated with more complications.

Siderowf et al. reported involuntary movements resulting from GK thalamotomy [41]. Magnetic resonance (MR) images revealed extended edema involving the thalamus, basal ganglia, and midbrain. Dystonia and choreoathetosis emerged, despite a good response to treatment with a marked reduction in the tremor.

Okun et al. reported pseudobulbar laughter following GK thalamotomy [42]; in this case, the tremor persisted following GK thalamotomy. MR images demonstrated extended edema, involving the internal capsule and posterior thalamus. The prescribed dose was unknown in both of these cases. Extended perifocal edema to the neuronal structures surrounding the target site is the expected cause of these complications. Ohye et al. reported thalamic reaction on MR images following GK thalamotomy [7]. Extended T2 elongation surrounding the GK lesion occurred in 10.9 % of the patients, and the prediction of a thalamic reaction was difficult in spite of a uniform treatment protocol.

Posteroventral pallidotomy (PVP) has been used to treat patients with advanced PD, since Laitinen et al. [43] reintroduced PVP and confirmed the results that Leksell and Svennilson et al. [44] and others described earlier [45, 46]. Reviews of controlled studies demonstrated an improvement in quality of life following surgery [47, 48]. Procedures without the need for battery replacement, risk of infection, or hardware problems seemed to be beneficial for some patients with PD. To date, DBS of the subthalamic nucleus and pallidum has become the dominant surgical procedure used for the treatment of PD.