Ochronosis is the bluish black discoloration of certain tissues, such as the ear cartilage and the ocular tissue, seen with alkaptonuria, a metabolic disorder. Additionally, ochronosis can occasionally occur from exposure to various substances such as phenol, trinitrophenol, resorcinol, mercury, picric acid, benzene, hydroquinone, and antimalarials.
Ochronosis was defined by Virchow who histologically described the connective tissue in alkaptonuria, given the cartilage's ochre, or yellow, hue under the microscope.
Alkaptonuria is a rare autosomal recessive metabolic disorder caused by deficiency of homogentisic acid oxidase, the only enzyme capable of catabolizing homogentisic acid (HGA). Alkaptonuria features a defect in the biochemical pathway by which phenylalanine and tyrosine are normally degraded into fumaric and acetoacetic acid. The genetic defect is autosomal recessive and is mapped to the HGO gene on arm 3q1, and 18 genetic missense mutations are known to cause homogentisic acid oxidase aberrations.  This deficiency results in accumulation and deposition of HGA in cartilage, causing the characteristic diffuse bluish black pigmentation. These affected connective tissue become weak and brittle with time, leading to chronic inflammation, degeneration, and osteoarthritis.
Exogenous ochronosis, in which bluish black pigmentation of cartilage is noted iatrogenically by exogenous agents, has been seen after exposure to noxious substances, including phenol, trinitrophenol, benzene, and hydroquinone.
Alkaptonuria is a rare autosomal recessive disease with a prevalence of 1 case per 1 million population.
Alkaptonuria occurs worldwide, with the highest frequency seen in Slovakia and Dominican Republic, in which the prevalence approaches 1 case per 19,000 inhabitants. 
Alkaptonuria is seen in persons of all races.
The incidence of alkaptonuria is equal in both sexes.
Alkaptonuria is present at birth and is often diagnosed by discoloration of the diapers. Up to 25% of patients with alkaptonuria do not have the characteristic dark urine staining, and many patients remain undiagnosed until adulthood.
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