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
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.
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.
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
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.