Gout

Gout is a crystal arthropathy due to deposition of monosodium urate crystals in and around the joints. 

Epidemiology

Typically occurs in those above 40 years. There is a strong male predilection of 20 to 1.

Pathology

Characterised by monosodium urate crystals (negatively birefringent) deposition in periarticular soft tissues. The synovial fluid is generally a poor solvent for monosodium urate and therefore causes crystallisation at low temperatures.

The crystals are chemotactic and activate compliment.

There are five recognised stages of gout :

1. asymptomatic hyperuricaemia
2. acute gouty arthritis
3. intercritical gout (between acute attacks)
4. chronic tophaceous gout
5. gouty nephropathy

Risk factors

• obesity
• hyperuricaemia : only a small proportion of hyper uricaemic patients develop gout and often takes 20 - 30 years to develop
• alcohol intake
• myeloproliferative disorders
• chemotherapy
• Lesch-Nyhan syndrome
• medications
  • thiazides
• hyperparathyroidism

Location 

Usually has an asymmetrical polyarticular distribution

• joints : 1^st MTP joint most common ; hands and feet are also common.
• less common : bones, tendon, bursa.

Radiographic features

Most radiographic findings involve the skeletal system.

Plain film

Characteristic radiologic changes occur in the chronic stage, though not all patients progress to this. There is a predilection for the small joints of the hands and feet. Chondrocalcinosis is present in ~ 5%.  

Joints

• joint effusion (earliest sign)
• preservation of joint space until late stages of disease
• absence of periarticular osteopaenia.
• eccentric erosions
• the typical appearance is the presence of well-defined “punched-out” erosions with sclerotic margins in a marginal and juxta-articular distribution, with overhanging edges.

Bone

• punched out lytic bone lesions
• overhanging sclerotic margins  
• avascular necrosis
• mineralisation is normal

Surrounding soft tissues

• tophi : pathognomonic
• periarticular soft tissue swelling due to crystal deposition in tophi around the joints is common
• the soft tissue swelling may be hyperdense due to the crystals, and the tophi can calcify (uncommon in absence of renal disease).

MRI

Tophi

Signal characteristics of gouty tophi are usually

• T1 : iso intense
• T2 : variable ⁴ with the majority of lesions being heterogeneously hypointense
• T1 C+ (Gd) : the tophus usually enhances