Pain

Pain has been described as ‘an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage’. The physician initially confronts pain as the signal of some underlying damage and, as such, an important symptom of numerous diseases. Treatment is aimed at eradicating the disorder responsible for the pain. Occasionally the underlying problem- cannot be identified or eradicated. It may even represent damage to the nervous system itself. The above definition also takes account of the emotional aspects of pain which may be all consuming for the patient and suggests the importance of psychological factors in evaluation and treatment.

Patients with pain are usually divided into two groups:

  -Those with diseases that limit life expectancy (e.g. malignancies) and those with chronic benign pain. The management of each group is very different. The treatment of pain follows a continuum (Table 35.8) which is determined by the severity of the pain and its response to treatment. It is also essential that chronic pain patients are managed holistically by a pain team including pain-relief specialists and nurses, psycholo­gists, physiotherapists and occupational therapists in con­junction with a neurosurgeon. Drug therapy is aimed at suppressing pain at points along the pain pathways. Steroids and nonsteroidal anti-inflamma­tories act directly on chemoreceptors. Afferent pain fibres can be anaesthetised by infiltration with local anaesthetic. Descending pain modulation circuits are influenced by narco­tics and antidepressants. Psychotropic and antidepressant drugs (chlorpromazine and haloperidol) are used to treat the affective component of pain.

The optimal surgical treatment for any intractable pain would be to have its effect confined to the painful area, be simple and inexpensive to perform, and be associated with a low mortality and morbidity. In particular it should be associated with a low incidence of neurological deficit.

There are three principal methods of                neurosurgical management for pain:

1. operations that interrupt nociceptive pathways by creat­ing lesions in peripheral nerves, roots or ganglia, the spinal cord, various parts of the brain and brainstem, and the sympathetic nervous system;

2. electrical stimulation of pain suppressive systems or blocking pain pathways (peripheral nerve, spinal cord or brain);

3.  administration of various drugs to the intraspinal or intraventricular compartments of the CSF pathways.

Electrical stimulation of the central nervous system

Epidural stimulation

This is achieved by either totally implanted devices which are powered by a battery with a finite lifespan, or radiofrequency powered devices which have an implanted receiver which is activated by an external stimulator. These are connected to multichannel electrodes. By activating various electrodes and adjusting the pulse width, amplitude and frequency, beneficial stimulation is sought. The electrodes are positioned epidutally over the dorsal columns or more caudally over the cauda equina roots (Fig. 35.46). If paraesthasiae can be induced in the area of pain — significant benefit is virtually assured. This location is most effective for failed back surgery syndrome, arachnoiditis, peripheral vas­cular disease and angina. Spinal cord stimulation is based on the gate control theory of pain with inhibition of C-fibre conduction. The exact neurochemical and neurophysiolgical mechanisms are poorly understood. Electrodes can also be positioned epidurally over the motor cortex for more refractory pain syndromes such as thalamic and phantom limb pain. This can be of benefit in 50 pet cent of patients.

Deep brain stimulation

Stimulation of the lateral margin of the periaqueductal and periventricular grey matter is thought to affect a pathway running from the midbrain to the dorsal horn, inhibiting nociceptive neurons. The other target area is the ventro­posteromedial/ventroposterolateral nuclei of the thalamus which ultimately inhibit spinothalamic tract neurons. These electrodes are implanted stereotactically at specialist centres.

lntrathecal drug delivery

The opiate doses required to maintain patients with chronic pain frequently result in unacceptable side effects such as respiratory depression, drowsiness, urinary retention, nausea and vomiting, and eventually progress towards tolerance. Delivery of opiates into the CSF space via the lumbar or transventricular routes has a potentiated effect directly upon

the opiate receptors in the brain and spinal cord at a fraction -of the previous dose. This is achieved by implanted drug

delivery systems (Fig. 35.47). These can be either manually activated — pressurised systems controlled by capillary resis­tance utilising different concentrations — or self-activated programmable pumps delivering small aliquots.

Pain associated with spasticity following spinal cord injury can be treated with intrathecal baclofen.

Neuroablative procedures

Ablative procedures have proved effective in relieving certain types of pain but run the risk of producing a neurological deficit without relieving and potentially even exacerbating pain and are hence more suited for patients suffering from pain associated with malignant disease. Sympathectorny is used to treat causalgic and visceral pain. Coeliac ganglion blocks have been especially helpful in treating pain associated with pancreatic disease.

Surgically produced dorsal root entry zone lesions in the spinal cord are especially helpful in treating phantom limb pain, brachial plexus avulsion and discrete spinal cord lesions.

Cordotorny, the surgical interruption of the spinothalarnic tract, results in hemianalgesia below the level of the lesion. It is invariably effective in managing unilateral pain associated with metastatic disease but its efficacy diminishes with time. Within 1 year 20 pet cent of patients will develop painful dysaesthesias in the anaesthetic area. It is usually performed at C1—2 with the patient awake. Risks include sleep apnoea (Ondine’s curse), bladder, bowel and sexual dysfunction. Commissural rnyelotomy is a longitudinal sectioning of the spinal cord in the sagittal plane to disrupt the crossing fibres of the spinothalarnic tract.

Mesencephalotomy reduces the number of functioning ascending fibres in the newer specific and older nonspecific pain pathways. It is effective for pains of the neck, head and upper chest caused by cancer.

Stereotactive cingulotorny is the creation of bilateral medial frontal lesions modifying the patient’s response to pain. These central lesions do not remove the painful sen­sation but alleviate the concomitant suffering.

Alcohol injection, radiofrequency lesioning, balloon com­pression and avulsion of various components of the trigernin­al nerve are all usually effective in the treatment of ttigeminal neuralgia but invariably only temporising measures. Intra­cranial microvascular decompression and partial trigerninal rhizotomy offer long-term relief with lower recurrence rates.