Gamma Knife Trigeminal Neuralgia Treatment
Trigeminal neuralgia (TN), also known as tic douloureux, is a pain syndrome recognizable by patient history alone. The condition is characterized by intermittent one-sided facial pain. The pain of trigeminal neuralgia typically involves one side (>95%) of face (sensory distribution of trigeminal nerve (V), typically radiating to the maxillary (V2) or mandibular (V3) area). Physical examination findings are typically normal; although mild light touch or pin perception loss has been described in central area of the face. Significant sensory loss suggests that the pain syndrome is secondary to another process, and requires high-resolution neuroimaging to exclude other causes of facial pain.
The mechanism of pain production remains controversial. One theory suggests that peripheral injury or disease of the trigeminal nerve increases afferent firing in the nerve perhaps by ephaptic transmission between afferent unmyelinated axons and partially damaged myelinated axons; failure of central inhibitory mechanisms may also be involved. Blood vessel-nerve cross compression, aneurysms, chronic meningeal inflammation, tumors, or other lesions may irritate trigeminal nerve roots along the pons. Uncommonly, an area of demyelination, such as may occur with multiple sclerosis, may be the precipitant. In some cases, no vascular or other lesion is identified rendering the etiology unknown. Development of trigeminal neuralgia in a young person (<45 years) raises possibility of multiple sclerosis, which should be investigated. Thus, although trigeminal neuralgia typically is caused by a dysfunction in the peripheral nervous system (the roots or trigeminal nerve itself), a lesion within the central nervous system may rarely cause similar problems.
The goal of pharmacologic therapy is to reduce pain. Carbamazepine (Tegretol) is regarded as the most effective medical treatment. Additional agents that may benefit selected patients include phenytoin (Dilantin), baclofen, gabapentin (Neurontin), Trileptol and Klonazepin.
Prior to considering surgery, all trigeminal neuralgia patients should have a MRI, with close attention being paid to the posterior fossa. Imaging is performed to rule out other causes of compression of the trigeminal nerve such as mass lesions, large ectatic vessels, or other vascular malformations.
The surgical options for trigeminal neuralgia include peripheral nerve blocks or ablation, gasserian ganglion and retrogasserian ablative (needle) procedures, craniotomy followed by microvascular decompression (MVD), and stereotactic radiosurgery (Gamma Knife®).
Percutaneous transovale needle techniques include radiofrequency trigeminal electrocoagulation, glycerol rhizotomy, and balloon microcompression. Microvascular decompression (MVD) is often preferred for younger patients with typical trigeminal neuralgia. High initial success rates (>90%) have led to the widespread use of this procedure. This procedure provides treatment of the cause of trigeminal neuralgia in many patients. Percutaneous techniques are advocated for elderly patients, patients with multiple sclerosis, patients with recurrent pain after MVD, and patients with impaired hearing on the other side, however some authors recommend needle techniques as first surgical treatment for many patients. It is generally agreed that MVD provides the longest duration of pain relief while preserving facial sensation. In experienced hands, MVD can be performed with low morbidity and mortality. Most authors offer MVD to young patients with trigeminal neuralgia.
Trigeminal Neuralgia Radiosurgery
Radiosurgery is performed by delivering a high dose of ionizing radiation in a single treatment session using multiple beams precisely focused at the target inside the brain. Several reports have documented the efficacy of Gamma Knife®‚ stereotactic radiosurgery for trigeminal neuralgia . Because radiosurgery is the least invasive procedure for trigeminal neuralgia, it is a good treatment option for patients with co-morbidities, high-risk medical illness, or pain refractory to prior surgical procedures.
Between 1992 and 2007, a more than 750 radiosurgical procedures for TN were performed at the University of Pittsburgh Medical Center. Our report summarizes the long-term outcome in 220 patients who had undergone Gamma Knife® radiosurgery for idiopathic, longstanding pain refractory to medical therapy. One hundred and thirty-five patients (61.4%) had prior surgeries including microvascular decompression, glycerol rhizotomy, radiofrequency rhizotomy, balloon compression, peripheral neurectomy, or ethanol injections. Eighty-six patients (39.1%) had one, 39 (17.7%) had two, and ten (4.5%) had three or more prior operations. For the other 85 patients, radiosurgery was the first surgical procedure. A maximum dose of 70 to 80 Gy was used.
The outcome of pain relief was categorized into four results (excellent, good, fair, and poor). Complete pain relief without the use of any analgesic medication was defined as an excellent outcome. Complete pain relief with still requiring some medication was defined as a good outcome. Partial pain relief (>50% relief) was defined as a fair outcome. No or less than 50% pain relief was defined as a poor outcome. Most patients responded to radiosurgery within six months (median, two months). At the initial follow-up within six months after radiosurgery, complete pain relief without medication (excellent) was obtained in 105 patients (47.7%), and excellent and good outcomes were obtained in 139 patients (63.2%). Greater than 50% pain relief (excellent, good, and fair) was obtained in 181 patients (82.3%).
Complications after Radiosurgery
The main complication after radiosurgery for trigeminal neuralgia was new facial sensory symptoms caused by partial trigeminal nerve injury. Seventeen patients (7.7%) in our series developed increased facial paresthesia and/or facial numbness that lasted longer than 6 months.
Trigeminal neuralgia patients who experience recurrent pain during the long-term follow-up despite initial pain relief after radiosurgery can be treated with second radiosurgery procedure. The target is placed anterior to the first target so that the radiosurgical volumes at second procedure overlaps with the first one by 50%. We advocate less radiation dose (50 to 60 Gy) for second procedure, because we believe that a higher combined dose would lead to a higher risk of new facial sensory symptoms.
Indications for Radiosurgery
The lack of mortality and the low risk of facial sensory disturbance, even after a repeat procedure, argue for the use of primary or secondary radiosurgery in this setting. Repeat radiosurgery remains an acceptable treatment option for trigeminal neuralgia patients who have failed other therapeutic alternatives.