Osteomyelitis - did you get what was causing this child's limp?

In the latest in our series, Dr David Capehorn asked what the likely diagnosis is in this case of a child presenting repeatedly with a limp. Answer revealed below!

Note that details of the case have been altered to protect individuals’ identities

A four-year-old boy initially presents to clinic with a limp. His mother explains that it had started ‘the morning after a trampoline party,’ and that she ‘thought it was just a twist from falling’. She had taken him the following day to the local Minor Injuries Unit. An X-ray of his lower leg and ankle showed no fracture. The report reads: Diagnosis: Minor soft tissue injury. Reassure, advise rest and paracetamol

The child has mild developmental delay – he’s chatty, but clearly behind in speech. He walks with a subtle limp, but doesn’t appear to be in pain. On examination, there’s no swelling or bruising. Hip, knee and ankle all move freely, and there’s no tenderness. With nothing obvious to find, the GP reassures the family and advises them to return if things don’t settle.

A few days later, the child presents again. He’s still limping, and his mother says it’s getting worse. At times, he refuses to put any weight on the leg at all. He seems more tired and hasn’t been eating as well – though he hasn’t had a fever, and your observations are within normal limits: HR 100, Temp 36.9°C.

The child occasionally mutters ‘ow’ when moving his leg, but there’s no obvious focus. Again, the GP examines him thoroughly: still no swelling or warmth, no joint restriction. They suspect the soft tissue injury is simply taking time to settle. The patient is reassured again and safety-netted.

At a third appointment a week later, the child is noticeably quieter and less animated than before. The GP decides to ask the opinion of a senior colleague, who takes a fresh history. The mother casually mentions something new: ‘Actually, now I think of it… he wasn’t quite right even before the party. He was a bit off and maybe limping slightly then too. We just assumed he was being clumsy. He’s waiting for an OT assessment anyway.’

The senior GP conducts a very careful examination. There is still no hip or ankle tenderness, or obvious swelling, but when the lower thigh is gently palpated, the child winces – just slightly – when pressure is applied above the knee. This subtle reaction had not been elicited in previous examinations.

What do you think the diagnosis might be? How would you proceed?

Answer: Subacute osteomyelitis of the distal femur

The GP seeing the child at the third appointment refers the child to paediatrics. Blood tests reveal a raised CRP and white cell count. An MRI confirms the diagnosis: subacute osteomyelitis of the distal femur. He is admitted, treated promptly, and goes on to make a full recovery.

What helped to arrive at the diagnosis?

Reviewing the history: The trampoline party was a ‘red herring’ that shaped everyone’s thinking – parent and clinician alike. But the history was incomplete. It’s worth revisiting early assumptions when a child isn’t improving as expected.

Subtle signs: There was no swelling or redness, and joint movements remained full. But a faint wince on palpating the distal femur was the clue – and the key. Notably, there was no hip involvement, which is a common source of limp in this age group.

Behavioural change: The child’s withdrawal was significant. Children don’t always verbalise pain – behavioural cues often speak louder than words.

Systemic features: The child was off his food, low in energy, and intermittently refusing to bear weight. None of these alone were diagnostic – but together, they signalled something more serious than a sprain.

Consistency, curiosity and caution: Consistently being seen in the same practice set antennae twitching, due to attendance about the same problem; curiosity on the part of the GP about what was going on, and the caution of knowing something was amiss, meant a second, more experienced, opinion was sought.

A few clinical pearls

Osteomyelitis in young children is uncommon, with an incidence is estimated at 1-13 per 100,000 children per year, with higher rates in the under-fives. Boys are more commonly affected.

In children, infection often localises to the metaphysis of long bones due to their rich and slow-flowing blood supply. The distal femur, proximal tibia and proximal humerus are frequent sites. In young children, the metaphyseal region remains in continuity with the epiphysis – increasing the risk of spread to adjacent joints, particularly the knee.