Limp can be a feature of inflammatory joint or muscle disease.
The limp may be acute, intermittent or insidious. There may be regression of acquired motor milestones. Where a limp is not resolving or intermittent, then there needs to be suspicion of inflammatory disease and referral to paediatric rheumatology is warranted.
The clinical assessment is key, noting that young children may not localise pain and non-verbal signs of discomfort need to be looked for.
Inflammatory joint disease - The commonest cause is Juvenile Idiopathic Arthritis - Making a diagnosis can be difficult as pain may be non-verbalised and blood tests (full blood count and acute phase) may be normal, and rheumatoid factor is usually absent. Radiographs are usually normal - so the diagnosis can be easily missed. Any suspicion should warrant referral to paediatric rheumatology.
Inflammatory muscle disease - The commonest cause is Juvenile Dermatomyositis (JDM). The skin changes may be subtle and easily missed. The child with JDM is proximally weak and Gowers sign is likely to be present (see pREMS - Hip). The school-aged child who cannot jump warrants concern about proximal weakness. Any suspicion should warrant referral to paediatrics, paediatric neurology or paediatric rheumatology.
Other muscle diseases can present with limp (e.g., inherited myopathies) but the history is usually of delayed milestones (walking or development).