Hereditary coproporphyria (HCP) is an acute (hepatic) porphyria in which the acute symptoms are neurovisceral and occur in discrete episodes. Attacks typically start in the abdomen with low-grade pain that slowly increases over a period of days (not hours) with nausea progressing to vomiting. In some individuals, the pain is predominantly in the back or extremities. When an acute attack is untreated, a motor neuropathy may develop over a period of days or a few weeks. The neuropathy first appears as weakness proximally in the arms and legs, then progresses distally to involve the hands and feet. Some individuals experience respiratory insufficiency due to loss of innervation of the diaphragm and muscles of respiration. Acute attacks are associated commonly with use of certain medications, caloric deprivation, and changes in female reproductive hormones. About 20% of those with an acute attack also experience photosensitivity associated with bullae and skin fragility.

An acute porphyria attack is characterized by a urine porphobilinogen (PBG)-to-creatinine ratio =10 times the upper limit of normal (ULN) and the presence of =2 porphyria manifestations (involving the visceral, peripheral, autonomic, and/or central nervous systems) persisting for >24 hours in the absence of other likely explanations. Onset of acute attacks typically occurs in the second or third decade of life. Acute attacks are more common in women than men. Although attacks in most individuals are typically caused by exposure to certain endogenous or exogenous factors, often no precipitating factor can be identified. The course of acute porphyria attacks is highly variable in an individual and between individuals. Recovery from acute porphyria attacks may occur within days; however, recovery from severe attacks that are not promptly recognized and treated may take weeks or months. The five categories of acute intermittent porphyria (AIP), caused by a heterozygous HMBS pathogenic variant, are based on the urine PBG-to-creatinine ratio and occurrence of acute attacks. Active (symptomatic) AIP: An individual who has experienced at least one acute attack within the last two years. Symptomatic high excreter: Urine PBG-to-creatinine ratio =4 times ULN and no acute attacks in the last two years but chronic long-standing manifestations of acute porphyria. Asymptomatic high excreter: Urine PBG-to-creatinine ratio =4 times ULN and no acute attacks in the last two years and no porphyria-related manifestations. Asymptomatic AIP: Urine PBG-to-creatinine ratio <4 times ULN and no acute attacks in the last two years but has had =1 acute attack in the past. Latent (inactive) AIP: Urine PBG-to-creatinine ratio <4 times ULN and no acute porphyria-related manifestations to date.