Last revised by Yaïr Glick on 9 Oct 2023

Ochronosis, or alkaptonuria (AKU), is a rare multisystem autosomal recessive metabolic disorder.

On imaging, the most particular presentation is on the spine, with osteoporotic bones and dense disc calcifications. 

The term ochronosis usually refers to the bluish-black discolouration of certain tissues, such as the ear cartilage and the ocular tissue, seen with alkaptonuria. Some authorities, however, refer to musculoskeletal manifestations of alkaptonuria as ochronosis 3. The term ochronosis originates from ‘Ocher / ώχρα’ in Greek meaning a dark yellow color, but can progress to bluish grey and black.

The estimated incidence is around 1:250,000-1,000,000 3,5.

Patients often have pigmentation of auricular cartilages and sclera. Urine color tends to be dark (at birth sometimes diagnosed by discolouration of diapers). Up to 25% of patients with alkaptonuria do not have the characteristic dark urine staining; many patients remain undiagnosed until adulthood.

The condition results due to excessive build-up of homogentisic acid (HGA) from a lack of homogentisic oxidase (a defect in the biochemical pathway for the degradation of amino acids phenylalanine and tyrosine). The excess HGA binds to collagen in connective tissue. The affected connective tissues become weak and brittle with time, leading to chronic inflammation, degeneration, and osteoarthrosis (progressive arthropathy). 

 A defective gene has been mapped to chromosome 3q21–q23 5.

The most well-described features are those involving the skeletal system. 

There is currently (as of April 2023) no effective treatment for alkaptonuria. A low-protein diet is recommended, thus minimizing phenylalanine and tyrosine intake. Vitamin C has been shown to slow down conversion of homogentisate to polymer deposits in the skin. Finally, nitisinone has been shown to halt ochronosis in mice 7.

  • aortic stenosis 4

  • spontaneous tendon ruptures have been reported 3

  • deposition in heart and kidneys can lead to cardiac and renal failure

Ochronosis was defined by Virchow, who histologically described the connective tissue in alkaptonuria, given the cartilage's ocher (yellow) hue under the microscope.

For skeletal radiographic manifestations consider:

There is a broad differential for intervertebral disc calcification.

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