Fig. (3) Virtual graph (*) of the possible effects of a treatment that modifies the evolution of the disease. *: In reality, the natural evolution of impairment is not lineal and varies according to the stage of the disease; however, the lineal diagram facilitates the conceptual understanding of the action of the drugs. 1: Cognitive decline related to ageing. 2: Cognitive and functional loss due to an untreated progressive disease. 3: Effect of a curative treatment (reversible disease). 4: Action of a treatment that modifies impairment progression speed, either by recovering the physiological slope (optimum situation, 4a) or by reducing progression speed (4b). 5: A treatment may have both symptomatic and modifying effects on the evolution of the disease, so that, after achieving an initial symptomatic improvement, the patient shows a diverging progression line in comparison to that of untreated patients, regaining (5a) or not (5b) the physiological slope (1).