anterior capsulotomy for obsessive-compulsive disorder, thalamotomies for pain and movement disorders, and irradiation of the trigemi-nal ganglion for neuralgia. Subsequently, the technique became increasingly used for vascular and neoplastic lesions, as discussed above.
However, over the last few years the fastest growing indication for radiosurgery, particularly in the USA, has been trigeminal neuralgia. This revival was made possible by the development of imaging enabling the identification of, and thus targeting the entry zone of, the trigem-inal nerve, or indeed the nerve itself. The reader will be familiar with the wide range of treatments and procedures in use for this condition. The reason to add another was the perceived surgical risk of microvascular decompression on one hand, and the recurrence rate with the alternative operations on the other. In practical terms the procedure is particularly straightforward as there is very little shape matching necessary when planning this treatment. Most gamma knife centers use a single 4 mm colli-mator field, prescribing 70 or 80 Gy to the isocenter. It was observed that irradiation of a longer section of the nerve using two adjacent fields may increase the risk of post-operative facial sensory impairment. In terms of efficacy, gamma knife surgery does live up to the expectations: in primary cases 85-90% pain-free state is achieved. The only drawback is the 3-12 weeks lag period before the cessation of pain. Salvage procedures after previous destructive operations are less effective but may result in good and excellent outcome (pain free with or without continued medication) in 65-70%. The permanency of the results appears to mirror microvascular decompressions rather than the percutaneous rhizotomies. Whether in the long run the recurrence rate remains as low as it appears at present is not yet clear. The risk of facial dysesthesia or numbness is low, and the other risks of surgery (e.g. cerebrospinal fluid rhinorrhea, meningitis, hearing loss, etc.) are avoided. The attraction of a day-case procedure under local anesthetic for an elderly or infirm patient is obvious.
After treating AVMs associated with focal-onset epilepsy, many observed an improvement or cessation of epileptic activity. This prompted treatment of small indolent lesions and good seizure-control results were observed. Since 1995, Regis presented several patients with mesial temporal sclerosis who had been treated using focused radiation with the gamma knife. His results were very convincing, achieving seizure control in a high proportion of cases. Other centers, including the authors of this review, followed suit, treating small series of patients. Our experience showed that seizure control at 3 years following irradiation is very similar to that achieved with temporal lobe resections, and thus this method could be advocated for those who are unable or unwilling to undergo resective surgery. However, our observation was that there was a long, 12-36-month time lag to achieve the seizure-free state. Furthermore, some patients found it difficult to cope with the slow alteration of their seizure pattern and the appearance of frequent auras for some months. In our small series we have not observed permanent neurological deficit, but about 50% of patients required several weeks of dexamethasone treatment owing to focal cerebral swelling after about a year. The method certainly deserves to be exposed to wider scale trials, but its role is not yet established.
Sporadic reports are available concerning the revival of radiosurgery for traditional functional indications. Thalamotomies, anterior corpus callosotomies and even cingulotomies have all been described using radiosurgery. Admittedly, it would be attractive to make the lesions currently made using radiofrequency techniques without the need for a burr hole and physical penetration of the brain. The limitation of radiosurgery for these indications is the lack of feedback obtained by stimulation and recording. Although the need to adjust the target coordinates during open procedures is infrequent, the reassurance before making the lesion is required by most neurosurgeons. An additional problem is the relative variability of the size of the brain lesion made even with a standard radiation dose. These indications should be considered as experimental.
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