Mirror, Mirror, Move My Manu!

Since the original observation that viewing movements of the intact arm induced illusory kinesthetic percepts of the phantom limb of some arm amputees, leading to amelioration of phantom pain, mirror therapy (MT) has received increasing interest as a neurorehabilitative intervention. Other avatars of MT soon followed, and the therapy was shown to be helpful for patients with type 1 complex regional pain syndrome or poststroke hemiparesis. In this issue, Dohle and coworkers report the results of a randomized study investigating the effect of MT on arm paresis following stroke. A total of 36 patients completed the study, involving simultaneous performance of movements with both arms, according to a protocol intended to progressively improve arm function. During therapy, half the patients viewed the good arm through a sagittally placed mirror, while the others experienced a control condition in which they viewed the paretic arm without a mirror. Thus, both groups underwent essentially similar motor training, the only difference being the type of visual input (mirrored vs direct arm view). Although there was overall improvement in motor abilities as assessed by the upper limb subscores of the Fugl-Meyer test, no significant difference emerged between the MT and control groups. However, in the subgroup of 25 patients with complete paralysis of the distal upper limb, MT had a significantly greater effect on finger movement, compared to the control procedure. Moreover, the Fugl-Meyer subscore for light tactile sensibility improved significantly more in the MT group compared to the control group. Also, in the subgroup of 20 right-handed patients with right hemispheric lesions manifesting signs of neglect prior to therapy, MT led to significantly greater improvement on a composite neglect score than the control intervention. Each of the three main benefits found in this study is of interest. In the first benefit, the observed improvement of hand (manu) motor function means that MT is an alternative to the now well-established method of constraint-induced movement therapy (CIMT). Indeed, as the authors point out, in contrast to CIMT, which requires some preservation of distal movement capability to be effective, MT seems to be especially beneficial for those without any detectable motor function of the distal upper extremity. Thus, MT may be useful for the initial treatment of patients with complete paralysis of the hand, and these patients may later become candidates for CIMT. The relative roles of MT and CIMT in the rehabilitation of hemiparesis remain to be worked out, but the prospect of multiple therapeutic options is highly encouraging to clinicians caring for patients with hemiparesis. The second benefit, that of improved tactile sensation, is perhaps even more intriguing. Dohle et al admit that the FuglMeyer sensory subscore is not the best measure of tactile capability. Yet this finding complements an earlier report of the efficacy of MT in a patient with profound somatosensory loss after a thalamic stroke. In this patient, prolonged application of MT resulted in improved motor coordination. No improvement in sensory function was seen, although this was not tested quantitatively. Clearly, the sensory effects of MT merit further study. Given that the key ingredient of MT is a sensory manipulation, it would not be very surprising to find major changes in sensory processing stemming from MT. Again, MT needs to be evaluated for its precise role relative to other sensory interventions that have been proposed, such as tactile coactivation, comprising simultaneous vibrotactile stimulation of a region of skin, which can improve tactile acuity and mitigate its age-related decline. The third benefit, that of attenuation of left hemineglect, is also noteworthy. While the effect was measured using a composite neglect score that has not been validated as such, the potential for treatment of neglect is of obvious clinical relevance. Neglect, when present, can be a major obstacle to successful neurorehabilitation. Thus, the addition of another weapon in the armamentarium of therapists is welcome. Further work is necessary to carefully assess the benefit accruing from MT for the manifestations of neglect, and to compare the effectiveness of MT with that of other modalities of intervention, such as prism adaptation. Finally, it will be important to work out the mechanisms by which MT and, for that matter, various other modes of therapy act, to pave the way for eventual rational prescription of particular therapies for individual patients based on neurophysiological principles, akin to rational pharmacotherapy. Is a brave new world, rather different from that envisioned by Aldous Huxley, in sight?