Multiple Sclerosis and biological definitions in neurodegenerative diseases.

Recently, biological definitions in Alzheimer´s disease (AD) and Parkinson´s disease (PD) have been proposed, where clinical descriptors such as "dementia" or "parkinsonism" lost the spotlight. Similar changes are in the horizon in Multiple Sclerosis (MS). However, in MS there is no single molecule (like amyloid) to call the main driver of MS pathogenesis. In fact, there has not been a historically steady candidate. Decades ago T-cells were thought to be paramount, then brain atrophy, and recently the heterogeneous concept of "smoldering disease". There are no established minimal necessary and sufficient conditions for disease pathogenesis in MS. Ethical issues will be important. Technology for biological/biomarker assessments is not universally available and there is risk of overmedicalization. Groups such as the Movement Disorders Society have expressed reservations about pure biological definitions for PD. In MS, we are just in time to tackle these issues in a critical and constructive way.