A Differential Diagnosis of Painful Hip

Introduction: Obturator internus muscle abscess is an uncommon clinical entity and should be considered as a differential diagnosis in the child who presents with a painful hip.

The clinical presentation can be similar to bacterial infection of the hip joint and it generally presents as fever, hip pain with decreased range of motion, and limp.

Clinical Case: We report the case of a 10-year-old boy, previously healthy, that resorted to the emergency department because of right hip pain, limp and a three-day fever. In the physical examination, there was pain over the right thigh area which aggravated with hip flexion and rotation.

He played football, but there was no history of recent trauma.

In the laboratorial tests performed, there was an increase of the erythrocyte sedimentation rate and C-reactive protein (ESR 23 mm/h, CRP 99,5 mg/L). Blood cultures were negative. Ultrasound of the soft tissues revealed hip joint effusion. On suspicion of septic arthritis, surgical drainage of the joint was performed, and this hypothesis was not confirmed. A magnetic resonance imaging (MRI) of the pelvis revealed an abscess of the right internal obturator muscle and acetabular osteitis.

The child completed 6 weeks of cefuroxime antibiotic therapy with favorable clinical and analytical evolution. Six months after discharge, the patient is asymptomatic and without changes to the objective examination.

Conclusion: The diagnosis of obturator internus muscle abscess is frequently late because of its rarity and is often mistaken for septic arthritis of the hip.

Small collections may not be detected on ultrasound, especially when they affect the obturator internus muscle, because of its deep location. MRI is a very sensitive examination for the diagnosis.

Appropriate antibiotic therapy initiated early is critical in the resolution of this pathology.