Chronic pain causes changes in the human brain

‘Chronic pain causes permanent alterations in the human primary somatosensory (SI) and motor (M1) cortices,’ says docent Nina Forss. ‘These alterations can be used as objective indicators of pain that shapes the human brain,’ she continues. Nina Forss works at the Helsinki University of Technology Low Temperature Laboratory: the laboratory’s Brain Research Unit was appointed a Centre of Excellence in Research in 1995.

Each body part has its representation area in the somatosensory cortex

The LTL Brain Research Unit launched in 2000 a research project aimed at exploring the mechanisms of pain processing in the human brain. One of the key instruments is a thulium laser that is used for selective and controlled stimulation of the pain fibres in the skin. The stimulus is repeated at intervals of five seconds, causing to the healthy volunteer a short sharp sensation of pain, resembling the prick of a needle. The cortical response is measured by means of multichannel magnetoencephalography (MEG). The objective is to find out which areas of the cerebral cortex are activated to pain and in what order.

Normally each body part has its own area of representation in the cortex: that area will first receive the sensory information arriving from the respective body part. The size of the area depends upon the density of nerve fibres in that body part. For instance the thumb has a much larger representation area in the somatosensory cortex than the back. Earlier studies have shown that the size of these areas may continue to change in adulthood depending on the amount and frequency of sensory stimuli received in each area. ‘For instance the representation area of an amputated arm will disappear altogether; its place will be taken over by the adjacent area, usually that of the fac,e, Nina Forss explains.

It has been shown earlier that chronic pain is associated with alterations in the representation areas. This was seen for instance in amputees who showed phantom pain in the amputated limb. The intensity of the pain increased in proportion to the extent of the changes in the cortical representation areas. So far researchers have been unable to establish to what extent pain alone, without the loss of a limb or a nerve connection, can cause permanent changes in the somatosensory cortex.

Results promise new methods of rehabilitation for chronic pain patients

For their studies of how the somatosensory cortex is affected by chronic pain, the BRU investigated six patients contacted through the Orton Hospital in Helsinki. These patients had intense pain in one upper extremity without a nerve injury or other obvious reason. The patients also showed changes in their sense of touch, and any movement of the limb added to the sense of pain. In addition, they had difficulty using their hand on account of clumsiness and reduced muscle strength.

To locate the representation areas of the patients’ thumb and little finger, the researchers applied a light sensory stimulus to their fingers. On the side of the healthy upper extremity the areas at the cortex were located at a distance of 1.5 cm from each other, just as in the healthy subjects in the control group. By contrast on the side of their painful arm the representation areas of the thumb and littlefinger were exceptionally close to each other.

The same result was seen in all subjects, i.e. chronic pain had changed the way that sensory stimuli were processed in the somatosensory cortex. This kind of alteration may contribute to impaired hand movements and clumsiness; such symptoms were also observed in the subjects. In other words the results provided indication not only of changes in the somatosensory cortex, but also of changes in the function of the motor cortex in connection with chronic pain.

The results also point at new possibilities of rehabilitation. For instance, the stimulation of individual fingers could restore the normal representational areas of the cortex and thus improve the function of the hand. With this approach it might even be possible to reduce the perceived pain.