Gout occurs in people who have levels of urate in their blood that are persistently above a critical level which allows urate crystals to form. About two thirds of the urate in our bodies comes from the breakdown of purines which are naturally present in the cells of our bodies. The other third comes from the breakdown of purines in some foods and drinks.
Having urate in your blood doesn’t mean you’ll definitely develop gout – it’s normal and healthy to have some urate in the bloodstream. When urate levels start to build up, your body usually gets rid of any excess urate through your kidneys into your urine; however, if your body is making too much urate or your kidneys are unable to remove enough urate, then urate levels start to rise. If the level goes above a certain point (the saturation point), it’s possible for urate to form crystals of sodium urate.
These crystals mainly form in and around joint tissues, especially joints at the ends of your legs and arms, such as your finger and toe joints.
The diagram above shows a joint with urate crystals. The crystals gradually build up in the cartilage and other joint tissues over years. You will not know this is happening. When there are a lot of crystals in your joints some of them can spill out into the joint cavity (the space between the bones). This process is called crystal shedding. The hard, needle-shaped crystals touch the soft lining of the joint (the synovium) and make it very inflamed very quickly.
The inflammation process breaks down the crystals that have become loose inside the joint, and the attack gradually settles over a few days or weeks, depending on how many crystals spilled out.
Apart from causing sudden attacks of inflammation, a build-up of crystals can eventually lead to tophi forming in and around your joints. These hard tophi can grow and cause pressure damage to your cartilage and bone. This is just like the damage caused by osteoarthritis and can cause more regular, daily pain when you use the affected joints. At this stage the condition is often called chronic tophaceous gout. Some tophi may be seen and easily felt under the skin, but by this time the unseen part of the tophi in your joints and deeper tissues are usually quite extensive.
Several factors can affect the level of urate in your body:
- The genes you’ve inherited may make it more likely that your kidneys don’t flush urate out of your body as well as they should, even though your kidneys are otherwise completely normal and healthy. This is the most common cause, especially when there are several family members affected.
- The bigger the body the more urate is produced each day, so if you’re overweight or obese it could cause your body to make more urate than your kidneys can get rid of.
- If you have high levels of cholesterol and fats in your blood (a condition called hyperlipidaemia), high blood pressure or late-onset (type 2) diabetes, your kidneys won’t be able to get rid of urate as well as they should, so all these conditions tend to be linked with raised urate levels. This combination of problems is often called metabolic syndrome.
- Kidney disease may mean that your kidneys aren’t able to process urate as well as they should.
- Some tablets such as diuretics (water tablets) reduce your kidneys’ ability to get rid of urate effectively.
- Rarely, if you have a chronic blood disorder that causes your body to produce too many blood cells, the level of urate produced by the breakdown of those cells may be higher than your kidneys can cope with.
Where a particular cause can be identified (such as kidney disease or regular use of diuretics), the condition is called secondary gout. However, most gout is primary and is usually due to a combination of factors, for example, through having inherited kidneys that aren’t very good at getting rid of urate and then becoming overweight.
If you’re prone to gout and have urate crystals in your joints, several things can encourage urate crystals to shake loose from your cartilage and trigger an acute attack. These can include:
a knock or injury to the affected joint
- an illness, such as pneumonia or flu, that makes you feverish
an operation – this also puts your temperature up a little
overeating and drinking too much alcohol
Similar attacks can be caused by a condition called acute calcium pyrophosphate crystal arthritis (acute CPP crystal arthritis), which was sometimes previously called ‘pseudogout’. In this type of arthritis it’s calcium crystals that are deposited in joint cartilage rather than urate crystals. Acute CPP crystal arthritis affects the knee and other joints more than the big toe and is most common in people with osteoarthritis.
You may be more at risk of gout attacks if any of your parents or grandparents had it.