Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired (not inherited) disorder that leads to the premature death and impaired production of blood cells. The disorder affects red blood cells (erythrocytes), which carry oxygen; white blood cells (leukocytes), which protect the body from infections; and platelets (thrombocytes), which are involved in blood clotting. PNH can occur at any age, although it is most often diagnosed in young adulthood.\n\nPeople with PNH have sudden, recurring episodes of symptoms (paroxysmal symptoms), which may be triggered by stresses on the body, such as infections or physical exertion. During these episodes, red blood cells are broken down earlier than they should be (hemolysis). Affected individuals may pass dark-colored urine because of the presence of hemoglobin, the oxygen-carrying protein in blood. The abnormal presence of hemoglobin in the urine is called hemoglobinuria. In many, but not all cases, hemoglobinuria is most noticeable early in the morning, upon passing urine that has accumulated in the bladder during the night (nocturnal).\n\nThe premature breakdown of red blood cells results in a shortage of these cells in the blood (hemolytic anemia), which can cause signs and symptoms such as fatigue, weakness, abnormally pale skin (pallor), shortness of breath, and an increased heart rate (tachycardia). People with PNH may also be prone to infections because of a shortage of white blood cells (leukopenia).\n\nAbnormal platelets associated with PNH can cause problems in the blood clotting process. As a result, people with this disorder may experience abnormal blood clotting (thrombosis), especially in large abdominal veins; or, less often, episodes of severe bleeding (hemorrhage).\n\nIndividuals with PNH are at increased risk of developing cancer in blood-forming cells (leukemia). In some cases, people who have or have been treated for another blood disease called aplastic anemia may develop PNH. In a small number of affected individuals, the signs and symptoms of PNH disappear on their own.\n\nA very rare form of PNH involves abnormal inflammation in addition to the typical features described above. Inflammation is a normal immune system response to injury and foreign invaders (such as bacteria). In people with this rare form of PNH, the immune response is turned on (activated) abnormally and can cause recurrent aseptic meningitis (which is inflammation of the membranes surrounding the brain and spinal cord that is not related to infection); a red, itchy rash (known as hives or urticaria); joint pain (arthralgia); or inflammatory bowel disease. The inflammatory disorders usually begin earlier than the blood cell problems.