Lesions of the parietooccipital regions of one cerebral hemisphere sometimes have peculiar effects on the patient’s capacity to feel and react to pain. Under the title of pain hemiagnosia, Hecaen and Ajuriaguerra described several cases of left-sided paralysis from a right parietal lesion, which, at the same time, rendered the patient hypersensitive to noxious stimuli. When pinched on the affected side, the patient, after a delay, became agitated, moaned, and seemed distressed but made no effort to fend off the painful stimulus with the other hand or to withdraw from it. In contrast, if the good side was pinched, the patient reacted normally and moved the normal hand at once to the site of the stimulus to remove it. The motor responses seemed no longer to be guided by sensory information from one side of the body.
There are also two varieties of rare individuals who from birth are totally indifferent to pain coupled with anhidrosis (“congenital insensitivity to pain”) or are incapable of feeling pain (“universal analgesia”). The former have been found by Indo and colleagues to have a mutation in the neural tyrosine kinase receptor, a nerve growth factor receptor; those in the second group suffer from either a congenital lack of pain neurons in dorsal root ganglia or from a mutation in the sodium channel discussed earlier. A similar loss of pain sensibility is encountered in the Riley-Day syndrome (congenital dysautonomia, see Chap. 25). Neuropathic pain syndromes predicated on sodium channel abnormalities have already been mentioned.
The phenomenon of asymbolia for pain is another rare and unusual condition wherein the patient, although capable of distinguishing the different types of pain stimuli from one another and from touch, is said to make none of the usual emotional, motor, or verbal responses to pain. The patient seems totally unaware of the painful or hurtful nature of stimuli delivered to any part of the body, whether on one side or the other. The current interpretation of asymbolia for pain is that it represents a particular type of agnosia (analgognosia) or apractagnosia (see Chap. 21), in which the person loses his ability to adapt his emotional, motor, and verbal actions to the consciousness of a nociceptive impression. Prefrontal lobe lesions from stroke, trauma, tumor, or in former times, frontal lobotomy, can produce a version of this syndrome.
Once the nature of the patient’s pain and underlying disease has been determined, therapy must include some type of pain control. Initially, of course, attention is directed to the underlying disease with the idea of eliminating the source of the pain by appropriate medical, surgical, or radiotherapeutic measures. When the primary disease is not treatable, the physician should, if time and the circumstances permit, attempt to use the milder measures for pain relief first—for example, nonnarcotic analgesics and antidepressants or antiepileptic drugs before resorting to narcotics, local nerve blocks or contemplating surgical approaches for pain relief. Not all situations allow this graduated approach, and large doses of narcotics may be required early in the course of some illnesses—for example, to treat the pain of visceral and bone cancer. The same measured strategy is appropriate in the treatment of neuropathic pain and of pain of unclear origin except that one generally stops short of ablative procedures that irrevocably damage nerves or parts of the central nervous system.
The field of pain relief was altered by the introduction of analgesic procedures that block nerves, alter neural conduction, or administer conventional medications in new ways. In addition, a number of procedures such as intrathecal pumps and spinal cord stimulators and medications are effective for pain relief but are unique to specific situations. For example, trigeminal neuralgia may be relieved by microvascular decompression of a branch of the basilar artery or by controlled damage of the gasserian ganglion; painful dystonic disorders may be relieved by the intramuscular injection of botulinum toxin. The following discussion provides some guidance for the physician who is asked to undertake or participate in the treatment of chronic pain or of neuropathic pain.