Obturator internus pyomyositis presenting as a pararectal abscess

Abstract

This report describes two children who presented with fever, hip pain and a limp, and were subsequently found to have a primary pyomyositis of the obturator internus muscle. A clinical diagnosis of septic arthritis of the hip was made initially, but in both children MRI showed a pararectal abscess, which required incision and drainage. Staphylococcus aureus was cultured from pus from the abscesses and both children made a full recovery subsequently. This report highlights the main features of this unusual entity and emphasises the need for early imaging in the child with an unexplained limp.

Figure 1

Axial post contrast PD fat saturation (A) and coronal STIR (B) MRI. The abscess is seen to deviate the rectum to the left. The high signal is consistent with oedema and inflammation involved the proximal adductor muscles on the coronal STIR image.

Figure 2

T2 coronal (A) and sagittal (B) MRI. Surrounding high signal on T2 and STIR sequences consistent with inflammatory change extend to involve the adductor and gluteal muscle groups. There is no bony involvement.