MRI evidence for multiple sclerosis as a diffuse disease of the central nervous system.

The classical view of MS as a chronic inflammatory demyelinating disease leading to the formation of focal central nervous system (CNS) white matter (WM) lesions has been recently challenged by pathological studies and by the extensive application of modern MRI-based techniques. There is now overwhelming evidence supporting the following statements: MS causes widespread tissue damage in the normal-appearing white matter (NAWM) of the brain and spinal cord, whose extent and severity is more strictly associated to the clinical manifestations of the disease than the extent of focal pathology. Discrete, macroscopic lesions are just the tip of the iceberg of MS pathology. Grey matter (GM) damage is a consistent feature of all MS phenotypes, which is progressive from the start of the relapsing-remitting phase of the disease. As is the case for WM, GM damage is also a mixture of focal lesions and diffuse pathology. High-field strength MR scanners are improving our ability to image focal GM lesions and modern MR-based techniques are enabling us to quantify in vivo the extent and severity of GM pathology, which have been shown to correlate only moderately with the amount of WM changes. At least part of GM pathology in MS is not secondary to retrograde degeneration of fibers traversing WM lesions. The neurodegenerative component of the disease is not a late phenomenon and it is not completely driven by inflammatory demyelination. In fact, neurodegeneration occurs very early in the course of MS and the correlation between MRI measures of inflammation and neurodegeneration is weak in all disease phases. The interplay of inflammation and neurodegeneration is a complex and still poorly understood phenomenon. At least part of MS-related neurodegeneration is not directly driven by Wallerian degeneration. Functional cortical changes can be seen in virtually all MS patients and are likely to play a central role in the ability of the MS brain to respond to tissue injury and, hence, limit the functional consequences of structural damage. MS disability is not just the result of tissue destruction but rather a balance between tissue destruction, tissue repair and adaptive cortical reorganization. All of this calls for the concept of MS as a focal, inflammatory demyelinating, WM disease to be reexamined and to start viewing MS as a diffuse CNS disease with an important neurodegenerative component. This is central for identifying novel and effective treatment strategies.