Image Source: en.wikipedia.org/wiki/Tunica_intima
CADASIL has several characteristics and features, of which arteriopathy (any disease that affects the arteries) is a main one.
The arteriopathy of CADASIL affects both small and medium arteries, particularly the deep penetrating arteries of the brain.
These arteries have vascular smooth muscle cells (hence the term muscular artery) that allow the artery to contract and dilate. This narrowing (contracting) and widening (dilating) adjusts the size of the lumen (the central ‘tunnel’ or ‘pipe’ through which blood flows within the artery) to control a regular flow of blood. In CADASIL, damage occurs to the smooth muscle cells, causing the artery wall to thicken and stiffen, thus compromising the artery’s ability to contract and dilate.
Note how thick the smooth muscle layer is compared to the rest of the artery wall.