Cyanosis is an abnormal bluish discoloration of the skin and mucous membranes, caused by high levels of deoxygenated (reduced) hemoglobin (or its derivatives) circulating within the superficial dermal capillaries and subpapillary venous plexus.

The appearance of cyanosis depends on dermal thickness, cutaneous pigmentation, and state of the cutaneous capillaries. For example, it may be not easily detected on darker skin. Therefore, cyanosis is best appreciated in areas of the body where the overlying epidermis is thin and the blood vessel supply abundant, such as the lips, malar prominences (nose and cheeks), ears, tongue (as in this case) and oral mucous membranes.

Cyanosis is classified as being either peripheral or central. As its name implies, in addition to the hands and feet, central cyanosis is apparent at the lips, tongue, and sublingual tissues. Peripheral cyanosis spares the oral mucosa.