The acutely painful, swollen joint: a case-based approach

The case: a hot, swollen knee overnight

A middle-aged man wakes with a right knee that is swollen, red, hot and so painful he can barely bend it or put weight on it. It came on over a few hours. He feels a bit shivery. He has had gout before in his big toe, so he assumes this is the same thing. From the outside, a gout flare and a joint infection can look identical: both cause a single joint to become red, hot, swollen and exquisitely tender. That overlap is exactly why a clinician does not simply accept the man's own explanation. A knee that has become acutely inflamed, especially with any feeling of being unwell, must be assessed properly, because missing an infection can cost the joint.

Why septic arthritis is an emergency

Septic arthritis means bacteria have got into the joint, usually through the bloodstream or a nearby infection. Once inside, the infection and the body's response can destroy cartilage remarkably quickly — within days — leaving lasting damage or a stiff, painful joint. Severe infection can also spread and make someone seriously unwell. Because the joint can be permanently harmed so fast, septic arthritis is treated as an emergency: it needs urgent hospital assessment, not a wait-and-see approach. The risk is higher in people with existing joint disease such as rheumatoid arthritis, those with a joint replacement, people with diabetes or a weakened immune system, and injecting drug users. Any hot, swollen joint with fever should be assumed to be infection until proven otherwise.

Gout and pseudogout: the common mimics

Gout is caused by tiny crystals of uric acid forming in a joint, triggering sudden, intense inflammation. It classically hits the big toe but can strike the knee, ankle, midfoot or other joints, often overnight. A closely related condition, pseudogout, is caused by a different crystal and tends to affect older people and larger joints such as the knee. Both cause the same dramatic redness, heat and pain as infection, and both can occasionally cause a mild fever, which makes the picture even more confusing. A previous history of gout, like our patient's, makes another flare likely — but it does not rule out infection, because someone with gout can still develop septic arthritis, sometimes in the very same joint.

How doctors tell them apart

Because the outside appearance is unreliable, the key test is to take a sample of fluid from the joint with a needle, called joint aspiration. The fluid is examined for bacteria and for the crystals of gout or pseudogout, and its appearance and cell count give further clues. This is the single most important step when infection is possible, and it is usually done urgently before antibiotics are started. Blood tests for infection and inflammation, and blood cultures, add information but cannot replace looking at the joint fluid itself. Uric acid blood levels are not reliable during an acute attack and can even be normal in gout, so they do not settle the question. The examination, the fluid and the overall picture together guide the diagnosis.

Treatment and what happens next

If the joint fluid or the overall picture points to infection, treatment starts urgently in hospital: antibiotics, usually given into a vein at first, and often washing out or repeatedly draining the joint to remove pus. This is combined with pain relief and, later, physiotherapy to restore movement. If instead the tests confirm a crystal flare such as gout, treatment focuses on calming the inflammation with anti-inflammatory medicines and, once the attack settles, considering longer-term treatment to lower uric acid and prevent future attacks. The crucial message from this case is that you cannot safely tell infection from gout by looking. A joint that is suddenly hot, swollen and very painful — particularly with fever or feeling unwell — needs urgent medical assessment the same day.