A quantitative somatosensory testing of pain threshold in individuals with mental retardation.

The commonly held view, mainly based on behavioral observations, is that individuals with mental retardation (MR) have a decreased sensitivity to pain. However, the sensitivity to noxious stimuli was not systematically measured in these individuals. For this purpose we developed an experimental protocol with which we trained individuals with mild MR (unspecified MR and Down's syndrome) in heat-pain threshold (HPT) measurement on the hand, and then performed the measurement using both the method of limits (MLI) which relies on reaction time (RT) and the method of levels (MLE) which is RT-free. This allowed for an indirect assessment of the RT and conduction velocity (CV) of these individuals. We found that HPT in individuals with unspecified MR (41.23+/-1.86 degrees C) and Down's syndrome (40.96+/-2.93 degrees C) was significantly lower than that of controls (42.86+/-2.42 degrees C) when measured with the MLE (P < 0.05). With the MLI no significant differences in HPT were found between the groups. However, the RT and CV values of individuals with unspecified MR and Down's syndrome were significantly lower compared to controls (e.g. mean RT of 1.86 and 2.55 compared to 1.2 s, respectively, P < 0.01). From this work it would appear that individuals with MR are not only pain-sensitive, but also more sensitive to heat-pain than normal. It is suggested that computerized quantitative testing of pain threshold is feasible in individuals with MR preferably by using RT-free methods (e.g. the MLE) due to the low RT and CV values exhibited by them.