Pain is not primarily a pathological phenomenon, but serves a protective function. Conditions with loss of pain perception exemplify this, resulting in frequent injuries, burns and subsequent mutilations, e.g. syringomyelia, hereditary sensory neuropathy, congenital insensitivity to pain.
Pathological conditions do, however, cause pain - as a symptom of cancer, injury or other disease.
The following conditions produce characteristic pain syndromes. CAUSALGIA
Causalgia is an intense, continuous, burning pain produced by an incomplete peripheral nerve injury. Touching the limb aggravates the pain, and the patient resents any interference or attempt at limb mobilisation. The skin becomes red, warm and swollen. Theoretical mechanism
Efferent sympathetic motor
Mixed peripheral nerve ^^ sensory
Afferent----^- ' At the site of damage, efferent sympathetic fibres may somatic link up to afferent somatic fibres producing a 'short circuit'
Causalgia only occurs with damage to peripheral nerves containing a large number of sympathetic fibres and responds in part to sympathetic blockade (pharmacological or surgical).
Following activation of a latent infection with varicella zoster virus lying dormant in the dorsal root or gasserian ganglion, the patient develops a burning, constant pain with severe, sharp paroxysmal twinges over the area supplied by the affected sensory neurons. Touch exacerbates the pain. Thick myelinated fibres are preferentially damaged, possibly opening the 'gate'.
Treatment of postherpetic neuralgia is particularly difficult. Carbamazepine and/or antidepressants may help. Ethylchloride spray over the affected area provides temporary relief. Topical capsicin is a promising new treatment.
Thalamic stimulation may produce or abolish pain depending upon the electrode site. A vascular accident which involves the inhibitory portion of the thalamus may result in pain - the thalamic syndrome.
Clinical features: Hemianaesthesia at onset contralateral to the lesion precedes the development of pain. This is burning and diffuse, and exacerbated by the touch of clothing.
^Caudate nucleus Thalamus
Treatment: Drug treatment gives poor results. A stereotactic procedure although increasing the sensory deficit may help.
Paradoxically the thalamic syndrome may occur following a thalamic stereotactic procedure for movement disorders.
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