Ochronosis (or alkaptonuria) is a rare multisystem autosomal recessive metabolic disorder resulting in excessive build up of homogentisic acid (HGA) due to lack of homogentisic oxidase. The excess homogentisic acid (HGA) binds to collagen in connective tissue. A defective gene has been mapped to chromosome 3q21–q23 5

Some authorities refer to musculoskeletal manifestations of alkaptonuria as ochronosis 3.

Epidemiology

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

Clinical presentation

Patients often have pigmentation of auricular cartilages and sclera. Urine colour tends to be dark. A progressive arthropathy as well as other tissue damage develop due to accumulation of by products of HGA.

Radiographic features

Plain film

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

Vertebral

* multi level intervertebral disc calcification : tends to be widespread (and involves nucleus pulposus)
* syndesmophytic formation
* multi level disc space narrowing

Joints

* symmetrical or assymetrical joint space loss
* sub-chondral sclerosis
* chondrocalcinosis

Complications

* spontaneous tendon ruptures have been reported 3
* aortic stenosis 4 - Deposition of polymerized HGA occurs in the aortic intima, the aortic and mitral valves, the coronary arteries, the subendocardium, and the pericardium. The exact incidence of cardiovascular disease in patients with ochronosis is not clear; a number of reports suggest a high prevalence of aortic valve stenosis in such patients who are more than 50 years old.7,8

Article Author : Dr Yuranga Weerakkody, Radiopaedia

References

1. Jacobson JA, Girish G, Jiang Y et-al. Radiographic evaluation of arthritis: degenerative joint disease and variations. Radiology. 2008;248 (3): 737-47. doi:10.1148/radiol.2483062112 - Pubmed citation
2. Bangert BA, Modic MT, Ross JS et-al. Hyperintense disks on T1-weighted MR images: correlation with calcification. Radiology. 1995;195 (2): 437-43. Radiology (abstract) - Pubmed citation
3. Manoj kumar RV, Rajasekaran S. Spontaneous tendon ruptures in alkaptonuria. J Bone Joint Surg Br. 2003;85 (6): 883-6. J Bone Joint Surg Br (link) - Pubmed citation
4. Ffolkes LV, Brull D, Krywawych S et-al. Aortic stenosis in cardiovascular ochronosis. J. Clin. Pathol. 2007;60 (1): 92-3. doi:10.1136/jcp.2005.032417 - Free text at pubmed - Pubmed citation
5. Roser M, Möller J, Komoda T et-al. Alkaptonuric aortic stenosis. Eur. Heart J. 2008;29 (4): 444. doi:10.1093/eurheartj/ehm406 - Pubmed citation
6. Kruithof E, Baeten D, Veys EM et-al. Case Number 29: Ochronosis: synovial histopathological characteristics. Ann. Rheum. Dis. 2004;63 (2): 130. doi:10.1136/ard.2003.013912 - Free text at pubmed - Pubmed citation
7. Van Offel JF, De Clerck LS, Francx LM, Stevens WJ. The clinical manifestations of ochronosis: a review. Acta Clin Belg 1995;50:358–62. [PubMed]
8. Phornphutkul C, Introne WJ, Perry MB, Bernardini I, Murphey MD, Fitzpatrick DL, et al. Natural history of alkaptonuria. N Engl J Med 2002;347:2111–21. [PubMed]