Gout (Monosodium Urate Deposition)

  • Gout affects 3% of the population
  • Predominantly males (5% females)
  • Common in men over 40yrs and post
    menopausal females
  • High prevalence in certain racial groups (eg Maori)
  • Commonly associated with hyperuricaemia
  • Commonly affects the foot

Aetiology

systemic disease resulting from the population of monosodium urate crystals into soft tissues including synovial membranes where they cause gouty arthritis

  • Tends to target previously damaged joints
  • The 1st MPJ is often damaged by OA
  • Also involves temperature ‐ uric acid is less soluble in cooler temps

Pathophysiology

  • Genetic factors ‐ play a role
  • Uric acid is an end product of purine metabolism
  • Purine uric acid (excreted by kidneys)

2 mechanisms cause high uric acid concentration in the blood

  • 1) Overproduction (idiopathic; metabolic or enzyme defect)
  • 2) underexcretion (renal disease; thiazide drugs; salicylates; alcohol; diabetes; starvation diets)

Hyperuricaemia (>7mg/100ml) causes crystals to be deposited in the synovium. This leads to an inflammatory response primarily involving neutrophils (PMNL’s). Phagocytosis of the crystals leads to rapid destruction of the phagosome membrane (within the neutrophil). The lysosome contents plus enzymes are released into the joint fluid which is capable of destroying the joint surface.

Possible reasons for first metatarsophalangeal joint being predominantly affected – (more than 90% eventually have the first MPJ affected):

  • Higher prevalence of osteoarthritis (maybe subclinical) in this joint – as exposed to trauma of weightbearing
  • Lower temperature of foot joints (approximately 29°C) compared to body core body temperature (37°C) – uric acid is saturated at 32°C – . As most attacks occur at night, the temperature of the foot may lower more than the rest of the body or more rapidly than the rest of the body.
  • Relatively lower pH

Differential Diagnosis

  • Septic arthritis
  • Reactive Arthritis/Reiters syndrome
  • RA
  • OA
  • Neuropathic arthritis
  • Pseudogout

Treatment

  • Cold/ice packs can be used during an acute attack to reduce pain and inflammation.
  • Splints to immobilise painful joint will help to reduce pain.
  • Local nerve block may be considered.
  • Management of co‐existent and associated diseases (eg renal impairment, diabetes, obesity)