Teaching Case

Right buttock pain and fever a week after a blunt trauma to the gluteal region.  The patient complained difficulty in walking and intolerance to sitting; temperature of 37.8 °C, tenderness to palpation of the greater sciatic notch, positive Lasegue’s sign, Freiberg’s sign, and Pace’s sign; leukocyte 12000/mm³, CPR 164 mg/L.

Pelvis x-ray in anteroposterior position was normal. Ultrasound showed a small hypoechoic area in the pelvis, between the iliac muscle (IM) and the maximum gluteus (MG) (Fig.1).

Magnetic Resonance (MRI) of the pelvis showed an enlargement of the right piriform muscle compressing the sciatic nerve (fig. 2a, arrow) with a hyperintense lesion at T1 and T2-weighted image (fig.2a-b, asterisks). Axial fat-suppressed T1-weighted (fig.2c) image obtained after intravenous administration of gadolinium showed a peripheral rim enhancement of the mass and inflammatory modifications in the surrounding tissues. At coronal fat-suppressed T1-weighted image, the inflammatory enhancement extended to the sacroiliac joint (fig.2d, arrow).

Posttraumatic piriform syndrome (PPS) is a neuromuscular disorder caused by the compression of the sciatic nerve [1]. The term “piriformis syndrome”, as reported by Robinson [2], has six key characteristics: 1) history of trauma to the buttock, 2) gluteal or sacroiliac pain radiating down the leg, 3) acute exacerbation of the pain with bending or lifting, 4) palpable sausage-shaped mass, tender to palpation, over the piriformis muscle on the affected side, 5) positive Lasegue sign, and 6) possible gluteal atrophy.

Piriform syndrome may be caused by intrinsic factors such as anatomical variations and abnormal bundles of the piriformis [3], or by extrinsic factors such as a trauma to the pelvis or to the buttock. In approximately 50% of the cases of PPS, there is a history of no dramatic trauma to sacroiliac and/or gluteal regions [4]. The inflammation of the piriformis muscle may compress the sciatic nerve between the tendinous portion of the piriformis muscle and the bony pelvis [5]. Benzon et al. [6] reported 15 cases of PPS due to a blunt trauma to the buttock that were treated by surgery. The diagnosis is challenging as the symptoms are often not specific, and the electrodiagnostic tests are difficult to perform due to deep location of the nerve [7].

In this case, PPS was due to the compression of the sciatic nerve by an abscess of the piriformis muscle secondary to a trauma to the buttock after a football match.

In emergency, pelvis x-ray and US excluded a bone fracture and find a collection in the gluteal region, giving a suspect diagnosis of abscess. The following MRI revealed the presence of an abscess of the muscle and the involvement of the sacroiliac joint.

If MRI is performed before the abscess is formed, the only finding may be muscle oedema. Abscess characteristically has a thick, irregular vascular rim associated with surrounding soft-tissue oedema. The central portion typically displays a low signal on T1-weighted images and high signal on T2-weighted images, but an intermediate or high signal on T1-weighted images may occur if the pus has a hematic or high protein content. Following intravenous contrast, there is avid enhancement of the wall and, to a lesser extent, the surrounding soft tissue, but there should be no enhancement of the central portion [8].