Limp and Night Pain

A 15 year old boy presents with a poorly localizing pain in the region of the right lower thigh and knee of 2 months duration. The father recalls a fall in school a year ago and the pain started some time after the fall. The pain is generally constant but often wakes the boy at night.

Repeated investigations have shown normal full [complete] blood counts, acute phase reactants (ESR), X-rays and MRI of the knee. He has received various diagnostic labels before presentation including being a possible malingerer with school avoidance.

Examination reveals an alert but clearly uncomfortable teenager who walks in with an antalgic gait (limp). He has mild wasting of the muscles of the mid-thigh on the right side which is confirmed on measurement of mid-thigh circumference on both sides. Joint examination is normal. MRI of the hip reveals an osteoid osteoma near the upper end of the femur. An orthopaedic referral and subsequent excision provides long term relief.

This case illustrates several important learning points:

• Many cases of limp or limb pain are attributed to trauma which may be insignificant or eventually irrelevant to the final diagnosis.
• Nocturnal bone pain (a red flag symptom) should alert to the possibility of osteoid osteoma or malignancy.  Pain that persistently wakes a child from sleep at night excludes functional causes. Typically the pain from osteoid osteomas respond to NSAIDs. Subtle physical findings, in this case the mild wasting of the mid-thigh, can give valuable clues to the diagnosis.
• Hip examination is important and reduced internal rotation is the first movement to be restricted with hip disease irrespective of the cause.
• Pain originating in the region of the hip may be referred to the knee.
• The presentation is not consistent with growing pains – the rules of growing pains are useful and this patient ‘breaks’ several of the rules. Targeted investigation after localizing the origin of the pain easily reveals the diagnosis in most cases – radiographs may show subtle changes which can be missed. Further imaging such as MRI will often give the diagnosis. 
• The assessment of a limping child is ideally performed by a specialist with experience in paediatric musculoskeletal medicine – rheumatology or orthopaedics. It is important that a rheumatology opinion is sought before embarking on invasive investigations (such as arthroscopy or synovial biopsy) or a long wait for an MRI (as these may not be warranted). Many children with limp are referred to orthopaedics initially. Referral to rheumatology is warranted if there is concern about an inflammatory cause or the limp is not resolving.