|Many surgical procedures have been undertaken in an attempt to alleviate chronic pain, and most involve ablation or excision of neural tissue. Present neurosurgical techniques, based on current understanding of the pathophysiology of pain, attempt to modify or modulate pain pathways. It is sometimes possible to treat the cause of chronic pain, and investigations must always be directed at this possibility. Only when treatable causes have been excluded should other treatment modalities for chronic pain be considered.
Neuroablation, or destruction, of neural tissue has been the mainstay of surgical procedures for chronic pain. However, the recognition that nerve injury is often the cause of chronic pain syndromes has resulted in reluctance to induce further nerve damage. Despite this, some neuroablative procedures remain useful in the treatment of chronic pain.
Interruption of the spinothalamic tract in the anterolateral quadrant of the spinal cord abolishes nociceptive pain from the contralateral side of the body. However, this effect is not always long-lasting and the procedure is mainly used to treat pain caused by malignant disease which proves resistant to drug treatment. There are several methods of performing a cordotomy, but the most commonly used techniques are a percutaneous cervical cordotomy or an open thoracic cordotomy.
Percutaneous cervical cordotomy — under local anaesthesia, using radiological control (either fluoroscopy or, more recently, computed tomography), a lateral cervical puncture is carried out at the C1/2 level, following which an electrode is placed in the anterolateral quadrant of the spinal cord. The optimal position of the electrode is verified by electrical stimulation, which produces sensory effects on the contralateral side. A thermal lesion is then created by applying a radiofrequency current to the tip of the electrode, which should result in loss of pain sensation in the opposite side of the body. The percutaneous method has the advantage of being a relatively minor procedure but requires good patient cooperation and tolerance.
Open cordotomy is usually undertaken under general anaesthesia, although techniques of waking the patient during the procedure to confirm satisfactory lesioning of the cord have been described. A small upper thoracic laminectomy is carried out and the anterolateral quadrant of the spinal cord is sectioned with specially designed blades.
Indications for cordotomy
Improvements in the drug treatment of pain mean that cordotomy is less commonly undertaken, but it remains a useful method of pain control for malignant disease. As the analgesic effects reduce with time and there is a risk of delayed dysaesthetic pain syndromes, cordotomy is seldom carried out in those with pain resulting from benign disease. The distribution of nerve fibres in the spinothalamic tract makes it impossible to obtain pain control above C4/5. In cervical cordotomy, the reticulospinal fibres responsible for spontaneous respiration are also destroyed, and if carried out bilaterally there is a risk of respiratory failure. This is also the situation if there is significant lung disease on the side of the cordotomy, and caution must be exercised in patients with pulmonary malignancy.
Dorsal root entry zone lesions The superficial part of the grey matter in the dorsal horn of the spinal cord has a complex of neural networks that are thought to modulate pain pathways. Lesions have been made in the area where the dorsal root enters the spinal cord (dorsal root entry zone) in an attempt to control a variety of pain syndromes. The operation requires a laminectomy to expose the dorsal root entry zone, followed by either a series of radiofrequency coagulations or an incision and diathermy of the area. Dorsal root entry zone lesions have been used for the treatment of a variety of neuropathic pain syndromes but brachial plexus avulsion remains the best indication. It is useful but less effective in peripheral brachial plexus injuries. Other indications include boundary zone pain in spinal cord injury and some peripheral nerve injury pain syndromes. It has been used for post-herpetic pain but the results are variable. The nucleus caudalis, in the lower brainstem and upper spinal cord, forms part of the descending pathway of the trigeminal nerve. Dorsal root entry zone lesions of the nucleus caudalis have been used for the treatment of neuropathic facial pain.
Other neurosurgical ablative procedures
Other ablative procedures in the spinal cord that are sometimes performed include commissural myelotomy for lower limb cancer pain and cordectomy for spinal cord injury pain.
Few intracranial ablative procedures are carried out for chronic pain. Mesencephalic tractotomy may be an option in patients with head and neck cancer. Patients with malignant disease in whom cordotomy has failed may occasionally benefit from lesions placed in the thalamic and subthalamic sensory pathways.
Pituitary ablation, with either a percutaneous alcohol injection or a trans-sphenoidal hypophysectomy, has been undertaken for pain caused by disseminated breast and prostatic malignancy. The mechanism of action of pituitary ablation is unclear but is thought to be due either to modification of the hormonal status or to a direct action on the hypothalamus.
The current trend in neurosurgery for chronic pain has been to replace ablative techniques with those that attempt to modulate the generation or transmission of pain impulses. The two methods that are widely used are neurostimulation and drug delivery systems.
Deep brain stimulation — chronic electrical stimulation of subcortical structures — was hampered in the early days by unreliable hardware and limited imaging tools. The resurgence of deep brain stimulation for movement disorders has led to the development of improved hardware for stimulation and improved stereotactic surgery planning systems. There is now a renewed interest in the use of deep brain stimulation for the management of neuropathic pain syndromes that do not respond to other neuromodulation techniques. The stimulation targets are the sensory pathways (sensory nuclei in the thalamus, the medial lemniscus or the internal capsule) or the periventricular/periaqueductal grey areas.
Motor cortex stimulation
Hyperactivity in the neurones of the pain afferent pathway above the level of a central lesion led to the use of cortical stimulation for pain control. It was later found that greater pain inhibition could be obtained by stimulation of the motor rather than the sensory cortex. In motor cortex stimulation, a variety of imaging and neurophysiological methods are used to direct the placement of an epidural electrode over the precentral gyrus for chronic stimulation. Success has been reported in a number of conditions, but the best indication appears to be central pain following a stroke.
Trigeminal neuralgia Trigeminal neuralgia is probably the most common chronic pain syndrome requiring neurosurgical treatment. Surgical treatment must be considered if the pain proves refractory to drug treatment or if there is intolerance to drug treatment. The cognitive side-effects of drug treatment are often underestimated, and this is particularly important in younger patients, in whom a lifetime of anticonvulsant medication can be avoided by surgery.
Percutaneous neuroablative procedures: the trigeminal ganglion and rootlets can be accessed percutaneously by an entry lateral to the angle of the mouth and directed under radiological control to the foramen ovale at the base of the skull. Percutaneous procedures are relatively minor and well tolerated, especially in medically compromised patients. The disadvantages are the short-lived effects, with the need for frequently repeated procedures. The incidence of denervation pain syndromes (e.g. anaesthesia dolorosa) is high and they are effectively untreatable. However, these techniques are extremely useful in secondary trigeminal neuralgia (e.g. multiple sclerosis) and in physiologically compromised patients.
During intermittent intravenous anaesthesia, a radiofrequency electrode is inserted through the foramen ovale and the position verified by electrical stimulation. When the appropriate divisions of the nerve have been identified, a radiofrequency thermal lesion is produced to damage the more sensitive pain fibres while preserving the touch fibres.
Alcohol and phenol have been largely replaced by glycerol for chemical neurolysis. The procedure can be undertaken under local anaesthesia and under ideal circumstances selective ablation of pain fibres can be achieved.
A small arterial embolectomy catheter is placed in Meckel’s cave and is repeatedly inflated to crush the trigeminal ganglion gently and lesion the pain fibres.
Other ablative procedures
Stereotactic radiosurgery — using stereotactic surgical techniques; a controlled dose of radiation is administered precisely to the trigeminal root entry zone in the brainstem. This is a relatively new procedure, and is non-invasive, but the long-term effects are uncertain.
Open selective rhizotomy
Open section of the trigeminal nerve was previously undertaken in the middle cranial fossa but is now largely carried out in the posterior cranial fossa. Good long-term denervation can be achieved, although the risks of denervation pain syndromes are high.
Vascular compression of the trigeminal nerve at the root entry zone is now largely accepted as the main aetiological factor in trigeminal neuralgia, and can be radiologically confirmed by magnetic resonance imaging (Figure 2). Microvascular decompression treats the cause of the disease without inducing nerve damage. Following a retromastoid craniectomy, a microsurgical exposure of the trigeminal nerve in the cerebellopontine angle is undertaken. The vascular compression is usually identified by the brainstem at the root entry zone and the offending vessel is mobilised away from the nerve. A synthetic sponge is then inserted between the vessel, the nerve and the root entry zone.
Microvascular decompression is a major neurosurgical operation with the attendant risks but, despite this, it is well tolerated, even by elderly patients. Risks such as cerebrospinal fluid leaks, infection, haemorrhage and unexpected neurological deficits remain, although the specific risks to the auditory and facial nerves can be reduced by meticulous microsurgical techniques and intraoperative neurophysiological monitoring. These risks are counterbalanced by the fact that the technique attempts to treat the cause of the condition and is associated with good long-term results. The avoidance of iatrogenic nerve damage and denervation pain syndromes is a significant advantage.
Copyright © 2005 The Medicine Publishing Company Ltd