Hematidrosis is an extremely rare condition in which a person appears to sweat blood. Although there is no clear scientific evidence concerning this condition, it has been reported to exist for several centuries. The condition has been most commonly reported during extreme bouts of fear or anxiety, such as is often experienced when a person is facing death. A small net-like structure of tiny blood vessels, known as capillaries, surrounds the sweat glands. It is possible for these blood vessels to rupture when a person is faced with an extremely stressful situation, causing blood to exit the body through the sweat glands.
Although there are other potential causes of hematidrosis, fear and anxiety seem to be the most commonly reported. The body's natural fight-or-flight reflexes may lead to capillary rupture, causing droplets of blood to enter the nearby sweat glands and exiting the body through perspiration. Excessive physical exertion, psychogenic purpura, and vicarious menstruation are other potential causes of hematidrosis.
Psychogenic purpura is a possible contributing factor to the development of hematidrosis. This is a rare condition in which painful lesions appear on the face and extremities. Psychogenic purpura is poorly understood and is thought to be related to mental illness, sometimes resulting in what has often been termed hysterical bleeding. Blood tests and other types of diagnostic testing do not lead to any abnormal results, contributing to the difficulty in scientifically understanding this phenomenon.
Vicarious menstruation has been listed as yet another possible cause for hematidrosis in some women. This condition is marked by bleeding from parts of the body other than the uterus at the normal time of menstruation. This is thought to occur due to the increased capillary permeability that naturally occurs during menstruation. The reasons for this occurring in some women but not in others is not clinically understood.
Despite this condition being extremely rare and having no scientific evidence to support the existence of hematidrosis, sweat mixed with blood has been reported for many centuries throughout the world. In almost all of the reported cases, the affected person has been facing bouts of extraordinary fear, often at anticipating death or other types of physical injury. The physical dehydration that occurs in moments of extreme stress, anxiety, and fear is thought to be a contributing factor in the development of this condition. Any instances of blood being present when perspiring should be reported to a doctor for further medical evaluation.