Plasticity of the motor system in multiple sclerosis

Multiple sclerosis (MS) is a chronic neurological disease characterized by inflammation and degeneration within the CNS. Over the course of the disease, most MS patients successively accumulate inflammatory lesions, axonal damage, and a rather diffuse CNS pathology, along with an increasing degree of disability. Pharmacological treatment options which are currently approved for MS aim at limiting inflammation and decreasing the relapse rate, or at simply relieving symptoms. Established disease-modifying and immunosuppressive treatments are unable to prevent the accumulation of pathology in most patients over long-term. Therefore, therapies promoting the innate ability of the CNS to compensate for dysfunction resulting from brain injury might be highly beneficial in MS. As a precondition, however, development of such strategies requires well-grounded knowledge about the extent to which central plasticity is intact and accessible in MS patients, and whether it is functionally relevant at all. This review will focus on plasticity of the motor system in patients with MS. A number of functional imaging studies have assessed patterns of brain activation during simple motor tasks in MS patients and their relationship with CNS damage and motor function. Deeper insights about causal and functional relationships were gained by neurophysiological techniques, predominantly by transcranial magnetic stimulation. In addition, and probably closest to rehabilitative approaches, practice-induced plasticity has been probed in a few studies. Altogether, there is growing evidence for a preservation of rapid-onset motor plasticity and for functionally relevant chronic reorganization processes, which might be limited by high CNS injury in advanced stages of the disease. Clinical implications of these findings with regard to the development and optimization of rehabilitative treatments in MS are discussed, as well as open questions which need to be addressed by future studies.