The term Ochronosis was coined by Virchow who described yellow hue of cartilage [Ochre- yellow] under the microscope.
Ochronosis is the bluish black discoloration of certain tissues, such as the ear cartilage and the ocular tissue, seen primarily with alkaptonuria, a metabolic disorder.
Ochronosis can sometimes occur from exposure to substances like
- Phenol
- Trinitrophenol
- Resorcinol
- Mercury
- Picric acid
- Benzene
- Hydroquinone
- Antimalarials.
Alkaptonuria causes accummulation and deposition of homogentisic acid cartilage and resulting in causing the characteristic diffuse bluish black pigmentation. The accumulation of homogentisic acid inhibits collagen cross-linking by affecting a crucial enzyme in collagen synthesis, leading to a diminution of structural collagen integrity
Because the affected connective tissue becomes weak and brittle with time, the cartilage undergoes chronic inflammation, degeneration, and osteoarthritis.
This results in ochronotic arthropathy in their fourth and fifth decades of age affecting larger joints more
Presentation
Alkaptonuria is often diagnosed by discoloration of the diapers but up to 25% of patients with alkaptonuria do not have the characteristic dark urine staining. This subgroup of patients may remain undiagnosed till adulthood.
Alkaptonuria is often recognized at birth when parents note discoloration of the urine.
Pigmented patches in the sclera develop around 30-39 years of age between the corneal margin and the inner canthus followed by thickening and blue-black or gray-blue discoloration of the ear cartilage in next decade.
Eyelids, the forehead, the cheeks, the axillae, the genital region, the nail beds, the buccal mucosa, the larynx, the tympanic eardrum, and the tendons are other places to look for the pigmentation and thickening.
Ochronotic arthropathy occurs in later years leading to narrowing of the joint spaces and calcifications of disc.
Accumulation of this pigment in cardiac tissue is said to lead to inflammatory reaction and progressive heart valve dysfunction.
Lab Studies
Urine analysis would reveal Increased urinary levels of homogentisic acid in alkaptonuria.
Elevated levels of same are also found in blood.
Imaging
Patients with ochronotic arthropathy show narrowing of articular space, calcifications, osteophytosis, and reactive sclerosis of the articular surfaces.
Dermoscopy or epiluminescence microscopy reveals amorphous densely pigmented structures obliterating some follicular openings.
Skin biopsy shows pigmented bodies in the dermis that represent altered widened elastic fibers,
Treatment
There is no medical treatment available for alkaptonuria at present. Diets low in tyrosine and phenylalanine are advised.
Ochronotic arthropathy is treated with physiotherapy, analgesics rest or joint replacement where necessary
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