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Original Report |
1 Department of Radiology, University of Michigan Health System, C.S. Mott
Children's Hospital, 1500 E. Medical Center Dr., Ann Arbor, MI
48109-0252.
2 Department of Orthopedics, University of Michigan Health System, C.S. Mott
Children's Hospital, Ann Arbor, MI 48109-0252.
Received March 11, 2002;
accepted after revision May 7, 2002.
Address correspondence to R. J. Hernandez.
Abstract
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CONCLUSION. In the absence of joint effusion on sonography, MR imaging should be considered in pediatric patients who present with a febrile illness and incapacitating pelvic pain.
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The clinical presentation of focal pyomyositis involving the pelvic muscles surrounding the sciatic nerve is similar to other pelvic inflammatory processes such as toxic synovitis, sepsis in the hip, or sacroiliitis. The purposes of this report are to describe the role of MR imaging in the diagnosis of focal pyomyositis surrounding the sciatic nerve and explain how to differentiate this entity from other conditions.
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Sonographic evaluation of the hip was performed with a variety of equipment. Linear array transducers were used. Imaging was optimized to visualize the hip joint and exclude the presence of joint fluid.
Three-phase (radionuclide angiogram, blood pool, and delayed) radionuclide bone scintigrams were obtained after the IV administration of 99mTc methylene diphosphonate. The dosage of 99mTc methylene diphosphonate was adjusted based on patient weight and approximate body surface area (vs weight and body surface area of an adult) to a maximum of 25 mCi (925 MBq). Delayed whole-body images were obtained approximately 4 hr after injection. Spot images of the pelvis and proximal femurs were also obtained.
MR imaging was performed on a 1.5-T unit (Signa; General Electric Medical Systems, Milwaukee, WI). T1-weighted spin-echo and fat-suppressed T2-weighted fast spin-echo MR images were obtained in the axial plane. Coronal short tau inversion recovery (STIR) images were obtained of the pelvis and proximal thighs. After IV injection of gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ) at a dose of 0.1 mmol/kg of body weight, T1-weighted spin-echo MR imaging with fat suppression was performed. TR and TE times varied slightly. A 256x128 acquisition matrix and 2 excitations were used. Either a body coil (n = 1) or a phased array coil (n = 3) was used.
Results
Patient demographics, clinical presentation, and laboratory findings are
summarized in Table 1. The
laboratory findings were consistent with an inflammatory process, although the
severity of the changes varied among the four patients. Three of the children
had an elevated erythrocyte sedimentation rate. The WBC was elevated in two of
the patients, and all four patients had an increase in the percentage of
neutrophils. The serum C-reactive protein level was markedly higher than
normal in the three patients for whom this laboratory value was determined.
Blood cultures were positive for Staphylococcus aureus in two
children and negative in the other two children.
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Routine radiographic and sonographic examinations of the hip joint revealed normal findings in all patients. Delayed phase images of the 99mTc methylene diphosphonate—enhanced bone scintigrams showed normal findings in three patients. The fourth patient did not undergo radionuclide bone scintigraphy. In one patient, slightly increased activity in the region of soft-tissue infection was seen on the radionuclide bone scintigrams that were obtained during the angiogram and blood pool phases. MR imaging findings consisted of high signal intensity on T2-weighted and STIR sequences and enhancement with gadolinium. The muscles affected were the piriformis, obturator internus, and gemelli (Figs. 1A,1B,2A,2B,2C,3A,3B,3C,3D). The piriformis muscle was involved in three patients, one of whom had additional involvement of the obturator internus and superior gemellus muscles. The fourth patient had involvement of the obturator internus and the superior gemellus muscles.
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The first patient of this series who was evaluated on MR imaging (Fig. 2A,2B,2C) underwent surgical exploration of the affected muscles and samples were obtained for culture. The surgical specimens grew S. aureus in culture. This patient was one of the two with blood culture results positive for S. aureus. All four patients were treated with a 3-week course of IV antibiotics and completely recovered. Although two of the four patients did not have positive results from blood cultures or positive findings at surgery, presentation with a febrile illness and pelvic pain and the prompt, complete response to IV antibiotic therapy were considered confirmatory that the abnormality seen on MR imaging represented pyomyositis.
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Focal pyomyositis of the muscles surrounding the sciatic nerve may simulate an intraarticular hip abnormality or osteomyelitis of the pelvic bones. Our patients had a relatively short history (days) of fever, severe pain, and impaired ambulation. The severity of the pain required narcotic medication in two children and antiinflammatory agents in the other two. The pain and inability to walk are attributable to the proximity of the inflamed muscles (piriformis, obturator internus, and gemelli) to the sciatic plexus and nerve. The sciatic plexus courses anteriorly to the piriformis muscle, and the sciatic nerve courses posteriorly to the gemelli muscles and the quadratus femoris muscle.
An ordered diagnostic approach for patients with a musculoskeletal inflammatory process affecting the pelvis usually consists of radiography of the pelvis and sonography of the hip followed by radionuclide bone scintigraphy, or, occasionally, CT. Radiographs of the pelvis are insensitive to soft-tissue infection and early stage osteomyelitis. Sonographic evaluation of the hip is useful in determining whether joint fluid is present and, hence, whether septic arthritis is the cause. Although 99mTc methylene diphosphonate—enhanced radionuclide bone scintigraphy is highly sensitive for osseous processes including osteomyelitis, soft-tissue inflammatory processes including myositis are not detected or are poorly defined.
The MR imaging findings of focal pyomyositis consist of high signal intensity on T2- weighted and STIR sequences and enhancement of the affected muscles with gadolinium. Animal and clinical studies have shown gadolinium enhancement in inflamed soft tissues, intramuscular abscesses, and areas affected by cellulitis [6, 7].
Pyomyositis is an acute infection of skeletal muscle. Although initially thought to occur most commonly in tropical regions, pyomyositis is increasingly recognized in areas located in other climates, including the United States and Europe. However, this entity is mainly associated with diabetes mellitus, malignancies, and AIDS [8, 9]. Pyomyositis involves skeletal muscle, predominantly those in the thighs (quadriceps), calves, and buttocks (gluteal muscles), although it may occur in any part of the body and may be multifocal. The infection is associated with hematogenous seeding; however, blood culture results are positive for bacterium in only 31% of patients at the time of clinical presentation [10]. The cultured bacterium is usually S. aureus. Less commonly, streptococci or gram-negative micro-organisms are seen.
The clinical presentation of pyomyositis is subacute with symptoms including fever, muscle pain, and localized swelling. After 1-3 weeks, the infection enters a second stage during which a painful induration of muscle and diffuse swelling occur; inflammatory signs are obvious locally and at biologic examination. Because of the nonspecific symptoms and radiographic findings, pyomyositis may be misdiagnosed and cause morbidity or death as a result of the delay in antibiotic treatment.
During the early phase of pyomyositis, MR imaging shows an area of hyperintense signal on T2-weighted MR images obtained with fatsaturation sequences. In most patients, edema in the adjacent subcutaneous adipose tissue can be seen. In some patients, an abscess may develop. The abscess shows high signal intensity on T2-weighted and STIR images. The peripheral rim is usually slightly hyperintense on T1-weighted MR images and hypointense on T2-weighted MR images and enhances after IV injection of a gadolinium chelate [1, 2].
The MR imaging signal abnormalities of the muscles are not specific for focal pyomyositis and can have various causes including trauma, neoplastic processes, and other inflammatory processes. Clinical presentation, history, and physical examination will aid in differentiating cases of trauma and neoplastic disease from pyomyositis. Other inflammatory processes of muscles such as dermatomyositis or focal myositis can be differentiated from pyomyositis on the basis of clinical findings and MR imaging appearance. The clinical presentation and distribution of the affected muscles are thus helpful in differentiating features of dermatomyositis from focal pyomyositis [11]. Dermatomyositis is idiopathic, as opposed to having an infectious cause, and it also differs from pyomyositis in that fever is rarely observed. Focal myositis is a rare clinical— pathologic entity characterized by the location of the myositic process in a single muscle [12]. Patients with focal myositis usually present with an enlarging painful mass within muscle. The diagnosis of focal myositis can be established after exclusion of not only local muscle diseases, notably pyomyositis, but also malignant neoplasms and inflammatory pseudotumors of skeletal muscle—namely, myositis ossificans.
In summary, we describe the role of MR imaging in the diagnosis of focal pyomyositis surrounding the sciatic nerve in children. Although the MR imaging findings are not specific for this entity, in the appropriate clinical context MR imaging findings may suggest the diagnosis of pyomyositis. In the absence of a joint effusion on sonography, MR imaging should be performed in children who present with a febrile illness and severe pelvic pain because MR imaging can depict both osseous and soft-tissue infections. MR imaging can clearly show the muscles affected by pyomyositis so that appropriate therapy can be initiated.
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  S. Park, J. B. Shatsky, B. R. Pawel, and L. Wells Atraumatic Compartment Syndrome: A Manifestation of Toxic Shock and Infectious Pyomyositis in a Child. A Case Report J. Bone Joint Surg. Am., June 1, 2007; 89(6): 1337 - 1342. [Full Text] [PDF]
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