Increased spinal nociception and reduced pain communication in dementia

Patients suffering from Alzheimer's disease (AD) complain less often about acute and chronic pain. The reduce pain report might either be due to a diminished ability to communicate pain or might be due to decreased nociception caused by neuropathological changes in the central pain pathways. Previous studies revealed that the pain threshold is unchanged in patients with AD, whereas the tolerance threshold is markedly increased. In addition to verbal pain ratings we aimed to assessed the nociceptive flexion reflex (NFR). The NFR is a spinal physiological reflex of protection which underlies spinal and supra spinal control. It is known to correlate well with the subjective pain threshold in healthy men. Twenty patients with dementia (Alzheimers's disease, vascular dementia and mixed dementia) and 51 age matched control participants took part in this study. For recording of the NFR the sural nerve was stimulated electrically. The motor response was recorded from the biceps femoris muscle by surface electrodes. Following the calculation of the reflex threshold (by employing the staircase method), we measured the parameters of ten averaged reflexes 5 mA above threshold. In parallel we assessed the verbal pain ratings on a 6-step categorical scale. Complete verbal pain ratings could only be obtained in 45 per cent of the demented patients. The NFR threshold was found to be significantly lower in the group of demented patients compared to the group of healthy subjects (13.9±7.5 mA vs. 21.1±9.6 mA, p<0.05). No significant group difference in the NFR parameters 5mA above threshold and in pain ratings were obtained. Finding the NFR threshold to be markedly decreased in dementia indicates an increased nociception on the spinal level. We assume that the degenerative process in both forms of dementia contributes to reduced inhibitory influences. Since there is no significant spinal pathology in dementia, this might be a result of a reduced descending inhibition due to degeneration of cortical neurons. From the present data we conclude that pain perception is enhanced in dementia while the reduced reliability of verbal pain report might lead to an underestimation of the perceived pain.