The Somatosensory Hand Representation is Shrunken in Thalidomide-Induced Dysmelia

Abstract

It is well established that functional representations in the cerebral cortex expand due to excessive use]. Little is known, however, about the reverse. In this study we investigated the relationship between congenitally missing fingers and the size of the somatosensory hand representation in thalidomide embryopathy. Twenty-one subjects participated in our study, ten with normal, and eleven with malformed upper extremities due to intrauterine exposure to thalidomide. The number of developed fingers varied between two and four in the dysmelic subjects. Fingers were missing systematically from radial (D1 first finger to be missing) to ulnar (D5 last finger to remain). The somatosensory hand representation was studied using electrical stimulation of the most distant fingers of each hand in combination with MEG and fMRI. The localization of the finger representations was based on equivalent current dipoles (ECD) with localization in SI (MEG) and significant (p<0.001) activation peaks on the lateral surface of the postcentral gyrus (fMRI). To estimate the size of the somatosensory hand area and the effect of the malformation disorder, Euclidian distances between the representations of the most distant fingers and correlations between the size of the hand representation and the number of fingers were calculated. Both methods showed an agreement with similar results: The mean size of the hand area was significantly smaller in the dysmelic subjects (5.67 and 8.6mm, respectively) as compared to controls (11.26 and 20.4mm). There was a significant rank correlation (p<0.05) between the number of developed fingers and the Euclidian distance between the cortical representations of the most distant fingers (rs=0.62 and 0.69). Both methods showed a somatotopic arrangement of cortical finger representations. In the dysmelic subjects, however, the representation of the ulnar finger was shifted towards the thumb representation of the controls. Our data show an effect of congenitally missing fingers on the size of the sensory hand area. In contrast, when fingers were experimentally amputated or webbed, the size of the sensory hand area appeared unchanged. We suggest that our findings reflect the negative counterpart of lifelong use-dependent plasticity related to dysmelia-induced impairment in hand use.