Occurs in two forms.

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare condition in which hemolysis occurs due to defective hematopoietic stem cells. Results in hemoglobin excretion in the urine (hemoglobinuria). The main sign is dark or red-colored urine, which is most prominent in the first urination after the hemoglobin is able to concentrate overnight. While it is most obvious after long periods of sleep, PNH occurs constantly, not just at night. Common findings include fatigue, dyspnea, abdominal pain, erectile dysfunction, and chest pain. Affected patients are at increased risk of developing reoccurring thromboses that may be life-threatening. Occurs due to acquired mutation of PIGA gene.

Paroxysmal cold hemoglobinuria (PCH) is a rare condition in which healthy red blood cells are mistakenly destroyed by autoantibodies when the affected patient moves from cold to warm temperatures. Hemoglobinuria is observed within a few hours. PCH only occurs after cold exposure. Primarily seen in patients with comorbidities such as syphilis, autoimmune disorders, and after childhood infections such as varicella, measles, mumps, Epstein-Barr, and adenovirus. Other findings include back pain, leg pain, abdominal cramps, fever, and chills. Most cases are self-limiting. Avoidance of cold is recommended.