Alkaptonuria (Ochronosis)

Definition

Rare hereditary disorder
Deficiency of Homogentistic Acid Oxidase » accumulation of Homogentistic Acid
Memory aid “ONC = HOG”

Characterised by

Homogentistic acid in urine
Dark pigmentation of connective tissues - Ochronosis
Calcification of hyaline & fibrocartilage - Chondrocalcinosis

Epidemiology

Autosomal recessive disorder
Usually presents with symptoms in 4th decade

Pathogenesis

Deletion of Homogentistic Acid Oxidase in the pathway concerned with metabolism of the amino acids Phenylalanine & Tyrosine
- Accumulation of Homogentistic Acid occurs
- Excreted in urine
- Oxidisation (when urine left standing) & Alkalinisation (hence name “alkaptonuria”) of Homogentistic Acid turns urine black
Homogentistic Acid retained in body is deposited in
- Articular & fibrocartilage
- Skin
- Sclerae
Pigment causes the collagen fibres to loose normal resilience
Becomes brittle & fibrillated
Degenerative changes in joints follow
May have pyrophosphate crystals in joint
Loose body formation not uncommon

Clinical features

The first complaint is back pain & stiffness with deposition of pigment in the intervertebral discs & later calcification
May resemble Anklosing Spondylitis but sacroiliac joints spared
Then large joint disease follows with classic degenerative changes & loose bodies frequent
Chondrocalcinosis common
Deposition of pigment in sclerae, pinna of ear & dark sweat (staining clothes) is commonly seen

Radiographically

See calcification & narrowing of intervertebral discs & surrounding osteoporosis
Severe osteoarthritis in peripheral joints with chondrocalcinosis

Treatment

No treatment for the metabolic disorder but may need surgery for arthritic joints