High-powered magnetic resonance imaging (MRI) scan showing a chronic active lesion in the brain (highlighted). Chronic active lesions are dark rimmed spots that show 'smouldering' inflammation. Scientists have discovered that patients with more chronic active lesions are more likely to have more aggressive multiple sclerosis. Multiple sclerosis is a condition affecting the myelin sheaths around the nerve fibres of the brain and spinal cord. Axons in the affected area can no longer conduct nerve impulses, resulting in symptoms ranging from tingling to paralysis. Multiple sclerosis is believed to be an autoimmune disorder in which the immune system attacks myelin.