In a recent paper published in the journal Neurology, Swiss scientists took advantage of arm immobilization after arm injury as a proxy for constrained-induced therapy in non-stroke patients. They investigated the effect of limb immobilization on brain structure, especially on gray and white matter plasticity.
The increased use of the non-dominant left arm due to right arm immobilization elicited a thickening of the right sensorimotor regions. This increase is similar to the increase in cortical thickness observed during the learning of juggling (Draganski et al. 2004) or learning in other task (see Zatore et al. 2012 for a recent review). This increase was correlated with the degree of left hand dexterity improvement.
More interestingly, the sensory and motor deprivation of the right arm due to its immobilization led to a decrease in cortical thickness in the left sensorimotor regions and also in a decrease in fractional anisotropy in the left corticospinal tract, i.e. the information was not conveyed as fast and efficiently as before immobilization. While beneficial for left hand skills, sensorimotor regions linked to the immobilized limb were severely affected by immobilization.
"Use it or Lose it" is a nice summary of the paper. The immobilization of the right arm had negative effects on the right sensorimotor regions although it is unknown how it affected right limb functions afterwards. Therefore, while it has a positive influence on the affected limb, constrained-induced therapy might deteriorate the motor functions of the unaffected hand, which is immobilized during this therapy. Unfortunately, the authors did not study how the brain recovered once the immobilization period was over. It is also unknown how much of these changes are related to the injury itself.
Draganski, B., Gaser, C., Busch, V., Schuierer, G., Bogdahn, U., & May, A. (2004). Neuroplasticity: changes in grey matter induced by training. Nature, 427(6972), 311–312. Nature Publishing Group. doi:10.1038/427311a
Zatorre, R. J., Fields, R. D., & Johansen-Berg, H. (2012). Plasticity in gray and white: neuroimaging changes in brain structure during learning. Nature Neuroscience, 15(4), 528-536. doi:10.1038/nn.3045