Ochronosis (Alkaptonuria Arthropathy)

Alkaptonuria (AKU) is a rare autosomal recessive inborn error of tyrosine metabolism. The connective-tissue disease it causes downstream is ochronosis, and the joint disease that results is ochronotic arthropathy. Examiners expect the defining triad recited: dark urine, ochronotic pigmentation of connective tissue, and a progressive arthropathy.

The time course. The enzyme block is present from birth, so the urine darkens on standing from infancy. The pigment that damages tissue takes decades to accumulate, and ochronosis and arthropathy are usually silent until the fourth decade of life. Patients therefore often reach orthopaedics with "early, severe, spine-dominant osteoarthritis" long before the metabolic diagnosis is made.

Why it is examined. A single enzyme defect leads to a recognisable arthropathy, a classic spine radiograph, a named disease-modifying drug (nitisinone) and several spot-diagnosis signs: scleral and ear pigment, black urine, and dark cartilage seen intra-operatively. The topic rewards the candidate who can move smoothly from biochemistry to imaging to management.

This page uses "ochronosis" as a synonym for alkaptonuria, but the word actually describes the blue-black tissue pigmentation itself, which has two quite different causes. An examiner who asks "is all ochronosis alkaptonuria?" wants this distinction.

Endogenous (metabolic) ochronosis is alkaptonuria: a systemic, autosomal recessive HGD deficiency in which homogentisic acid polymerises and deposits in cartilage, discs, sclera, valves and tendons, producing the systemic ochronotic arthropathy.

Exogenous (acquired) ochronosis is a localised, skin-limited pigmentation with normal HGA and no arthropathy. It is caused by chronic external exposure to certain chemicals, most importantly long-term topical hydroquinone in skin-lightening or bleaching creams, and historically phenol, resorcinol, picric acid and some antimalarials. The chemical inhibits local homogentisic acid oxidase in the dermis, so HGA polymerises and deposits as identical-looking pigment, but only in the treated skin.

For the orthopaedic surgeon the practical point runs the other way. The systemic, arthropathy-causing ochronosis met at the operating table is essentially always the endogenous, alkaptonuric form, but knowing that a skin-limited, HGA-negative exogenous variant exists is the higher-mark answer.

There is no single named classification of ochronosis. Examiners expect you to organise the disease by system involvement and by the typical sequence in which sites are affected.

In childhood. Dark-staining urine, noticed on nappies and underwear, is often the only sign and is frequently missed.

From the fourth decade. The axial features are insidious, chronic low back pain and stiffness, reduced spinal mobility, loss of lumbar lordosis and progressive stiffening towards ankylosis. Progressive large-joint pain follows as osteoarthritis sets in, the knee most commonly, with effusions and mechanical symptoms from cartilage fragments. Embrittled tendons can rupture spontaneously, classically the Achilles.

Spot signs. Look for the pigment in four places:

• Sclera - slate-blue pigment between the cornea and the canthus
• Pinna - blue-grey, thickened, leathery ear cartilage, with dark cerumen
• Skin - blue-grey discolouration over sweat-gland-rich areas
• Articular cartilage - jet-black at arthroplasty

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Beyond the joint. The heart shows aortic valve pigmentation and stenosis, aortic root involvement, and coronary and valvular calcification. Renal and prostatic calculi are the genitourinary manifestation.

Do not stop at "not ankylosing spondylitis". Each cardinal sign has its own differential, and the unifying discriminator for ochronosis is a raised urinary homogentisic acid: everything else fails that test.

Management has two arms: disease-modifying metabolic therapy to reduce HGA, and supportive and orthopaedic care for the connective-tissue damage that pigment has already caused.

The disease accumulates complications across the spine, joints, heart, tendons and urinary tract, and its treatment adds one of its own:

• Axial disability - progressive disc calcification, narrowing and ankylosis cause chronic back pain, loss of spinal mobility and, rarely, neurological compromise
• Peripheral arthropathy - premature, severe osteoarthritis of the knee, hip and shoulder, often requiring multiple arthroplasties
• Cardiac disease - aortic valve and root involvement, with valvular and vascular calcification needing surveillance and sometimes valve replacement
• Tendon rupture - classically the Achilles
• Stone disease - renal and prostatic calculi related to HGA, occasionally the presenting feature
• Treatment-related - nitisinone-induced hypertyrosinaemia can cause corneal keratopathy and, with very high tyrosine, neurocognitive concerns, hence monitoring

Ochronosis is a rare disease, so practice is shaped by specialist centres and rare-disease frameworks rather than mainstream arthroplasty guidelines. The biology is universal; access to diagnosis and to nitisinone is not.

Global Epidemiology

• Alkaptonuria is rare worldwide, with an estimated birth prevalence of roughly 1 in 250,000 to 1 in 1,000,000 in most populations.
• Geographic clusters of much higher prevalence exist, notably in Slovakia and parts of the Dominican Republic, reflecting founder effects in the HGD gene.
• Many distinct pathogenic HGD variants have been catalogued internationally (for example, large variant cohorts described in Russia), underpinning genetic confirmation and family screening.

Therapy Access and Regulatory Status

There is broad consensus that HGA-lowering with nitisinone plus structured monitoring is the rational disease-modifying approach, and that end-stage arthropathy is treated with arthroplasty using conventional principles adapted for stiff joints and friable tissue.

Registries & Surveillance

• No large arthroplasty registry isolates ochronotic arthropathy, but national joint registries (NJR, AJRR, AOANJRR, SHAR, NZJR) capture implant survival data that inform implant choice when these patients require replacement.
• Rare-disease registries and dedicated AKU cohorts (linked to the SONIA programme and patient organisations such as the AKU Society) are the principal source of natural-history and treatment-outcome data.

High- vs Limited-Resource Practice Variation

• High-resource settings: access to genetic confirmation, nitisinone with tyrosine/ophthalmic monitoring, dietetics and elective multi-joint arthroplasty.
• Limited-resource settings: diagnosis is often delayed to the arthroplasty table; management relies on analgesia, physiotherapy and replacement when feasible, with limited access to nitisinone.

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2. Ranganath LR, Milan AM, Hughes AT, et al. Suitability Of Nitisinone In Alkaptonuria 1 (SONIA 1): a randomised, open-label, dose-response study. Ann Rheum Dis. 2016;75(2):362-7. PMID 25475116.

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