Gout

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The most important crystal arthropathy is caused by deposition of uric acid, causing true gout.

Urate gout is characterized by deposition of urate crystals in joints and soft tissues, which is caused by the presence of hyperuricaemia. Uric acid is normally derived from the breakdown of purines and is excreted in the urine. Urate gout is largely confined to men, although some women develop it after the menopause. It can present at any time between the ages 20 and 60 years, most cases presenting with an acute attack of arthritis, often affecting the big toe.

There are two main reasons for hyperuricaemia:

Underexcretion of uric acid.} Seen in the majority of patients with urate gout, but of uncertain origin. It is clinically associated with hyperlipidaemia, renal failure, alcohol consumption, and some drugs (diuretics).

Overproduction of uric acid.} High-cell turnover (leukaemia and chemotherapy for tumours). Rare congenital enzyme defects of purine metabolism.

In the majority of patients with urate gout, disease is probably the result of polygenic factors leading to underexcretion of uric acid (primary gout). Only a small proportion of patients develop urate gout as a result of overproduction of uric acid.

Urate crystals are deposited in certain joints, particularly the metatarsophalangeal joint of the big toe, stimulating an acute inflammatory reaction leading to a painful acute arthritis. Uric acid crystals are also deposited in the soft tissues around joints, where their presence excites a foreign-body giant-cell reaction. These soft tissue masses may enlarge to produce a palpable mass composed of white chalky material (tophi).

In the joint the urate crystals are deposited on the surface of the articular cartilage to form a white powdery deposit, beneath which the cartilage shows degenerative changes. Attacks of acute gouty arthritis are intermittent and may be precipitated by dietary indiscretion. 

Urate crystals deposited in the kidney may lead to an interstitial nephritis and to renal calculi composed of uric acid. Precipitation of urates in renal tubules may produce acute tubular necrosis and acute renal failure in leukaemic patients with massive purine release after chemotherapy.

Hyperuricaemic gout has a familial tendency and is believed to be polygenically inherited. It is associated with an increased predisposition to hypertension and coronary artery disease.

INFECTIVE ARTHRITIS

Infective arthritis is mainly due to pyogenic bacteria and mycobacteria

Pyogenic bacteria may gain access to a joint either by bloodstream spread or, more commonly, by local trauma or spread from adjacent infective foci. Infective arthritis is a well-recognized complication of prosthetic surgery at the knee and hip (the most common sites for infective arthritis of all types and causes).

A wide range of bacteria may be responsible, but Staphylococcus aureus, streptococci and Haemophilus are the most important. Blood-borne infection by the gonococcus is an important cause in teenagers and young adults.

Tuberculous arthritis is now rare, and is the result of bloodstream spread from pulmonary TB. In adults the vertebral column is most often affected; tuberculous osteomyelitis leads to collapse of affected vertebrae, particularly in the lumbar and lower thoracic spine (Pott's disease), and paravertebral collection of tuberculous caseous material occasionally tracks down the psoas muscle (psoas abscess) to point in the groin. In children, tuberculous arthritis mainly affects the hip and knee. Synovial biopsy shows caseating tuberculous granulomas. 

Infective arthritis in syphilis and brucellosis is now rare. Arthritis due to the spirochaete Borrelia burgdorferi (Lyme disease) and to some fungal infections such as blastomycosis occurs in outbreaks in the USA and Europe.

Seronegative spondylarthritides

The seronegative spondylarthritides are a set of inflammatory arthritides that involve peripheral joints, as well as the sacroiliac joint and spine; they are distinguished from rheumatoid disease by the absence of circulating rheumatoid factor. The cause and pathogenesis of these diseases are not known, but an autoimmune reaction is suggested by a high incidence of the antigen HLA B27. The most important conditions are:

ankylosing spondylitis

psoriatic arthropathy

enteropathic arthropathy

reactive arthritis.

 Intervertebral disc disease
Intervertebral disc disease is a common and important cause of back pain and neurological symptoms

The intervertebral discs not only permit a limited degree of movement between adjacent vertebrae, they also act as shock absorbers, and are constantly exposed to vertical compressive forces. Each disc is composed of cartilaginous end-plates, with densely collagenous annulus fibrosus surrounding a central slightly gelatinous nucleus pulposus. The narrow cartilaginous end-plate is most prominent in the young. 

Abnormalities of the disc occur with increasing age; in the elderly the disc becomes shrunken and has a less gelatinous nucleus pulposus.
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