Multiple sclerosis (MS) is the most common chronic disease of the central nervous system in young adults. No curative therapy is known. Currently, six drugs are available that can reduce the activity of MS. The first-line drugs can completely reduce the activity of the disease in nearly two-thirds of the patients. In the remainder, who suffer from breakthrough disease, the condition of the patient worsens, and second-line therapies must be used. The second-line drug natalizumab exhibits almost double efficacy of the first-line drugs, but also have less favourable adverse effects. As a severe side-effect for instance, natalizumab carries the risk of the development of progressive multifocal leucoencephalopathy (PML), caused by a polyoma virus, the JC virus. There are three major risk factors for PML: an anti-JCV antibody status, a long duration of natalizumab treatment and prior immunosuppressant therapy. The lowest-risk group (1:14 286) comprises of patients who are anti-JCV antibody-negative, in whom the prior immunosuppressant use and duration of natalizumab therapy do not influence the risk of PML. With no prior immunosuppressant treatment, the incidence of PML increases to 1 in 192 patients after 2 years among those who are anti-JCV antibody-positive. These data may lead the physician to decide to discontinue natalizumab treatment. The half-life of natalizumab is three months; during this time other therapies can not be administered and the patients encounter the rebound effect: as the patients receiving natalizumab therapy displayed a high disease activity before treatment, the rebound effect can lead to relapses. After the termination of natalizumab second-line disease-modifying therapy with fingolimod may be introduce; no PML cases occur in response to fingolimod treatment. In the large majority of patients taking natalizumab who do not develop PML, this drug is highly effective and can prevent the progression of MS. The benefit of therapy and the risk of PML must be considered on an individual basis, with regard to the disease activity, the progression and the MRI activity, before natalizumab therapy is implemented
