Pyogenic Sacroiliitis and Pyomyositis Caused by Streptococcus pneumoniae

A fully vaccinated 5-year-old girl with a history of molluscum contagiosum presented to the emergency department with acute onset of left hip, buttocks, and leg pain associated with an inability to bear weight.

History. The patient’s initial pain was sudden and began 24 hours prior to presentation. She refused to bear weight or sit upright. No other joints were painful and she had no history of fever. She had been treated with cefoxitin of an unknown dose for a urinary tract infection several weeks before presentation and had a presumed upper respiratory infection during that time, which self-resolved. Several days prior to pain onset, she had an incident in which another child fell on top of her while playing but at the time she seemed uninjured. She had no past medical history of recurrent sinusitis, otitis media, pneumonia, or skin infections.

In the emergency department, she was febrile to 100.9°F (38.3°C), tachycardic (heart rate of 138 beats/min), and hypertensive (blood pressure of 127/77 mm Hg). Upon physical examination, she was lying in the bed with her left leg everted and flexed at 90°. Any movement of her left hip produced pain in her buttocks, left hip, and left upper thigh. Palpation of the lumbosacral spine produced left hip pain, but the patient did not feel pain with left knee flexion or extension. Skin examination was notable for a minor healing abrasion on the left knee and numerous umbilicated lesions on the buttocks, consistent with molluscum contagiosum. There was no surrounding erythema or drainage.

Diagnostic testing. The patient’s laboratory workup was notable for leukocytosis (white blood cell count [WBC] of 19.55 K/μL), neutrophilia (absolute neutrophil count of 17.36 K/μL), elevated c-reactive protein (7.6 mg/dL), and elevated erythrocyte sedimentation rate (87 mm/hr). A hip ultrasound showed no joint effusion. Magnetic resonance imaging (MRI) of her left hip was performed (Figures 1 and 2), which showed septic left sacroiliitis with an associated intramuscular abscess and myositis involving the left iliacus and psoas muscles. The day following admission, a blood culture was obtained that grew Streptococcus pneumoniae (S. pneumoniae) at 12 hours.

Figure 1. Magnetic resonance imaging of the pelvis showing extensive soft tissue edema involving the left iliacus muscle and tracking superiorly into the left psoas muscle. There was also asymmetric enhancement and fluid signal within the left sacroiliac joint with some asymmetric joint space widening.

Figure 2. Magnetic resonance imaging of the pelvis showing a fluid collection (1.6 cm x 0.7 cm) demonstrating restricted diffusion and peripheral enhancement within the left iliacus muscle, which appeared continuous with the left sacroiliac joint. Overall, the imaging findings were consistent with septic left sacroiliitis with an associated intramuscular abscess and myositis involving the left iliacus and psoas muscles. There was no evidence of bone marrow involvement or osseous erosion.

Based on the patient’s laboratory findings, a pediatric orthopedic surgeon and interventional radiologist were consulted to determine the possibility of draining the abscess and fluid collection, but both specialists believed the difficulty and risks of accessing the region outweighed the clinical benefit. Ultimately, the patient’s MRI findings and positive blood culture led to a final diagnosis of S. pneumoniae pyogenic sacroiliitis and pyomyositis (involving the iliacus and psoas muscles).

Differential diagnoses. On presentation to the emergency department, the differential diagnosis included septic arthritis and transient synovitis. However, an infectious process was heavily favored given the patient’s inability to bear weight and elevated inflammatory markers, such as c-reactive protein and erythrocyte sedimentation rate. As such, an MRI of the hip was obtained to rule out a periarticular or subperiosteal abscess.

Treatment and management. The patient was started on intravenous (IV) ceftriaxone (75 mg/kg daily) and vancomycin (15 mg/kg every 6 hours). A repeat blood culture 24 hours after starting antibiotics was negative. Vancomycin was discontinued, and clindamycin 7.5 mg/kg every 6 hours was started in the off chance that the patient’s S. pneumoniae was not fully susceptible to ceftriaxone. Once the susceptibility testing confirmed the organism was susceptible to ceftriaxone, clindamycin was discontinued.

Outcome and follow-up. Clinically, the patient had significant pain relief 24 hours after starting on antibiotics. Repeat laboratory tests after 3 days of hospitalization were notable for resolved leukocytosis (WBC of 8.27 K/μL). She was discharged home after 1 week of hospitalization on oral amoxicillin 1000 mg every 12 hours.

Discussion. Pyogenic sacroiliitis and pyomyositis are rare entities that can be challenging to diagnose in pediatric patients. Pyogenic sacroiliitis is a rare form of septic arthritis that makes up approximately 1% to 2% of all osteoarticular infections within the pediatric population.¹ Infection at this anatomical site can be difficult to diagnose as patients may present with nonspecific signs and symptoms.^2,3 Fever, unilateral hip pain, and reduced mobility are often key features of the history that can help lead to the correct diagnosis.⁴ However, diagnosis is often delayed as these symptoms can mimic more common conditions, such as acute appendicitis, septic arthritis of the hip, sickle cell crisis, or osteomyelitis.^5,6 Notably, these symptoms can also be found in pyomyositis, which is defined as infection or abscess formation within striated muscle.⁷ For both of these entities, Staphylococcus aureus (S. aureus) is the causative organism in the majority of cases.^1,8,9 Fortunately, the patient in this case presented to the emergency room within 24 hours of symptom onset and had an MRI within 18 hours of arrival. This led to a relatively swift diagnosis and prompt initiation of IV antibiotics.

Patients who have been identified as having an increased risk for pyogenic sacroiliitis include those with sickle cell disease and those who are immunosuppressed.⁴ Other predisposing factors include recent pelvic trauma, atopic dermatitis, folliculitis, insect bites with severe cutaneous reaction, and acute furunculosis.^2,10 In our case, the patient had previously been healthy without recurrent infections or other signs to suggest immunosuppression. She did, however, have lesions consistent with molluscum contagiosum on the buttocks directly overlying the area where she initially presented with pain. While the skin lesions did not appear overtly infected on physical examination, infected molluscum could have been a potential nidus of infection given that the patient had no other signs for another source of infection (eg, respiratory tract infection or meningitis).

For both pyogenic sacroiliitis and pyomyositis, S. aureus is by far the most common causative organism with few reported cases associated with S. pneumoniae. One study reported a case of pyogenic sacroiliitis in a 9-year-old girl who presented with fever, sudden-onset hip pain, and inability to walk.¹ S. pneumoniae was identified on blood cultures collected on admission, and pelvic MRI was used to confirm the diagnosis. A slide agglutination reaction test was subsequently performed, which confirmed the S. pneumoniae serotype as 16F, which is not included in any current vaccine.¹

A single-center, retrospective review of eight cases of primary pelvic pyomyositis in children was done in 2011.⁸ In the majority of cases (n = 5), the obturator internus was the site of infection, whereas the gluteus, iliopsoas, and iliacus were involved in one case each. They found that all patients presented with moderate-to-severe hip pain and elevated acute phase reactants, and blood cultures were positive in 87.5% of cases, consistent with the presentation in this case. Within that cohort, methicillin-susceptible S. aureus was the most common causative pathogen (n = 4), followed by methicillin-resistant S. aureus (n = 2), and group A Streptococcus (n = 1). Only one patient in this cohort did not have a causative pathogen identified by blood culture.⁸

To date, a total of 10 pediatric patients from six different countries have been reported to have pyomyositis caused by S. pneumoniae, including our case (Table).^{5,6,9,11–16}

Abbreviations: F = female, M = male, IV = intravenous.

The patients ranged from 3 months to 13 years old, and both boys and girls were affected equally. The majority of patients (n = 7) had positive blood cultures, one patient had sterile blood cultures, and two patients did not have blood culture results reported. A number of different muscles were involved, mainly in the pelvis or lower extremities, although there was one case involving the biceps brachialis.⁹ IV antibiotics were the mainstay of treatment for most patients, with surgical debridement and/or abscess drainage used in select cases only.⁸ Of the cases that reported patient outcome, there was one patient with pneumococcal pyomyositis who ultimately died from associated meningitis, sepsis, disseminated intravascular coagulation, and cerebral edema.¹⁵ Of note, that patient had multiple congenital intramuscular hemangiomas that developed pyomyositis associated with pneumococcal meningitis. As a result, the authors postulate that intramuscular hemangiomas may be another risk factor for the development of pyomyositis.

Conclusion. Clinicians should consider pyogenic sacroiliitis and pyomyositis when diagnosing patients that present with fever and hip, buttock, or low back pain. We present this case of pyogenic sacroiliitis and pyomyositis to highlight these two rare entities caused by an unusual organism: S. pneumoniae.