Interruption of ascending pathways carrying various sensations results in loss of sensory perception (anaesthesia) over parts of the body concerned. In case of peripheral nerves the area of anaesthesia following injury is often much less than the area of distribution of the nerve. This is so because of considerable overlap in the areas supplied by different nerves. The area of skin supplied from one spinal segment is called a dermatome. Dermatomes for adjoining segments overlap, a given area of skin being innervated by two or more segments.
In the case of spinal cord lesions, the level of disease can be inferred from the level of sensory loss. In this connection it must be remembered that the finer modalities of touch are carried by the posterior column tracts which are uncrossed. Crude touch, pain and temperature are carried by the spinothalamic tracts which are crossed. Thus, a unilateral lesion in the spinal cord can result in loss of the power of tactile localisation, tactile discrimination and of stereognosis on the side of lesion; with loss of crude touch, pain and temperature on the opposite side. Because of a double pathway for touch, loss of sensations of pain and temperature is often more obvious than interference with touch. We have seen that, while crossing the midline, fibres of the spinothalamic tracts do not run horizontally. They ascend as they cross so that their path is oblique. The degree of obliquity is greater in the case of fibres carrying touch as compared to those carrying pain and temperature. Because of this there can be a difference of a few segments in the level at which (or below which) these sensations are lost when the crossing fibres are interrupted by a lesion. In a disease calledsyringomyelia, the region of the spinal cord near the central canal undergoes degeneration with formation of cavities. Fibres of spinothalamic tracts crossing in this region are interrupted. Sensations of pain and temperature are lost over the part of the skin from which fibres are interrupted, but touch is retained as there is an additional pathway for it through the posterior column tracts. This phenomenon is called dissociatedanaesthesia.
Sensory disturbances can also result from lesions in the brainstem because of damage to the medial lemniscus or to the spinal and trigeminal lemnisci. Lesions of the thalamus can produce bizarre sensory disturbances. Lesions in the internal capsule can cause sensory loss in the entire opposite half of the body as thalamocortical fibres pass through this region. Pressure on sensory areas of the cerebral cortex can result in various abnormal sensations,or in anaesthesia over certain regions. Damage to pathways carrying special sensations of smell, vision and hearing can result in various defects.
Some other terms are used to describe sensory disturbances. Reduced perception for touch is hypoaesthesia, for pain it ishypoalgesia. Increased perception for touch is hyperaesthesia.Abnormal sensations are referred to as paraesthesias.
It sometimes happens that when one of the viscera is diseased pain is not felt in the region of the organ itself, but is felt in some part of the skin and body wall. This phenomenon is called referredpain. This pain is usually (but not always) referred to areas of skin supplied by the same spinalsegments which innervate the viscus. Some classical examples of referred pain are as follows.
1. Pain arising in the diaphragm, or diaphragmatic pleura, is referred to the shoulder (C4).
2. Pain arising in the heart is referred to the lower cervical and upper thoracic segments. It is felt in the chest wall and along the medial side of the left arm. It may also be referred to the neck or jaw.
3. Referred pain from the stomach is felt in the epigastrium (T6 to T9); and that from the intestines is felt in the epigastrium and around the umbilicus (T7 to T10). Pain from the ileocaecal region is felt in the right iliac region. Pain from the appendix is felt in the umbilical region.
4. Pain from the gall bladder is referred to the epigastrium. It may also be referred to the back just below the inferior angle of the right scapula.
5. Pain arising in the area of distribution of one division of the trigeminal nerve may be referred along other branches of the same division, or even along branches of other divisions.
1. Threshold for appreciation of touch pain or temperature is lowered.
2. Sensation that are normal may appear to be exaggerated or unpleasant.
3. There may be spontaneous pain.
4. Emotions may be abnormal.
Sometimes a patient may be in severe pain that cannot be controlled by drugs. As an extreme measure pain may be relieved by cutting the spinothalamic tracts. The operation is called cordotomy.
The ligamentum denticulatum serves as a guide to the surgeon. For relief of pain the incision is placed anterior to this ligament (anterolateral cordotomy).
Pain can also be relieved by cutting the posterior nerve roots in the region. This operation is called posterior rhizotomy.
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