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.
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.
Henoch Schonlein Purpura (HSP) - Arthritis and arthralgias occur in 50 to 80% of children with HSP and this may be the presenting feature in 25% of cases. The typical rash on the extensor surfaces is characteristic. There may also be renal involvement and gut involvement.
Acute Rheumatic Fever (ARF) - Arthritis in rheumatic fever is seen in about 30-50% children and more often in older children. Rheumatic arthritis is an asymmetric, migratory / fleeting polyarthritis with predominant involvement of large joints of lower limbs and migrates to the upper limb.
Inflammatory muscle disease - the commonest cause is Juvenile Dermatomyositis (JDM). The skin changes may be subtle and easily missed. The child is proximally weak (ask if they can jump) or perform Gowers test (see pREMS - Hip). Diagnosis of JDM rests on elevated muscle enzymes which are key but can be normal. Any suspicion should warrant referral to paediatrics, paediatric neurology or paediatric rheumatology.
Other muscle diseases can present with limp (eg inherited myopathies) but the history is usually of delayed milestones (walking).