Autonomic responses and pain perception in Alzheimer's disease

We analysed the effects of electrical noxious stimulation on the autonomic nervous system of Alzheimer's disease (AD) patients who were assessed by means of the Mini Mental State Examination test (MMSE). To do this, we used electrical stimuli at two different intensities: just above pain threshold and twice pain threshold. We recorded heart rate and systolic blood pressure by using conventional electrocardiography and finger photo-plethysmography. When a pain stimulus just above threshold was delivered, AD patients were found to have blunted autonomic responses compared to controls of the same age. Similarly, prestimulus expectation produced a less pronounced increase of the responses in AD patients compared to the controls. However, when the painful stimulus was increased to twice the pain threshold, the systolic blood pressure increase of AD patients did not differ from the controls, whereas heart rate increase was still slightly diminished. By contrast, pain perception was similar in the two groups when the stimulus was at pain threshold, whereas it was blunted in AD patients when the stimulus was twice the pain threshold. These findings show that in AD mild noxious stimulation produces blunted autonomic responses and normal pain perception, whereas strong noxious stimulation produces quasi-normal autonomic responses and blunted pain perception. These results indicate that AD patients have an increased threshold for both autonomic activation and pain tolerance.