Couperosis is a dilation of small superficial skin vessels in the superficial dermis. The medical term corresponding to this vascular dilation is telangiectasias.
The network of superficial vessels forms a "capillary system" made of reddish linear lines on the face. Some of this network’s parameters may vary, depending on the person and his/her skin type, the age of the process, and the internal or external temperature or changes in temperature. The vessels are more or less wide and their colour ranges from bright red to purple or even bluish.
The topography varies but is usually relatively symmetrical. Sometimes only the nostrils are affected, but the cheekbones or the cheeks, more generally, are the most common area affected. However, the whole face may be affected.
When the tree branches in telangiectasias are difficult to see with the naked eye and redness dominates, this is called erythrosis.