Leg Pain in a 10-year-old Girl

A 10-year-old obese girl presents to the emergency department with right thigh pain for 6 days and temperature up to 102°F (38.9°C) for 3 days. She fell while carrying her dog a day before symptom onset, with no reported abrasions, and developed bilateral leg pain, which then localized to the right thigh area. Her pain became progressively worse, leading to limping, difficulty climbing a flight of stairs at her school, and intermittent refusal to get out of bed because of pain. She has a mild cough and sore throat; her parents deny other complaints, recent insect bites, or animal exposures other than their dog at home. She is taking acetaminophen and ibuprofen, without much relief.She is alert; her temperature is initially 99.9°F (37.7°C) and then increases to 102.9°F (39.4°C) after an hour, her heart rate is 133 beats/min, her blood pressure is 128/78 mm Hg, and she is breathing comfortably at 19 breaths/min. Her weight is 143.3 lb (65 kg), with a body mass index of 31. Her ears, nose, throat, and neck are normal, with no palpable adenopathy. Her lungs are clear, with adequate air entry, and her heart sounds are normal with no audible murmurs. Her abdomen is soft, with no tenderness, distention, or organomegaly. Her skin is well perfused and intact, without erythema, lacerations, or ecchymoses; there is acanthosis over her neck. She can bear weight when asked and can walk but with an antalgic gait. There is tenderness to touch along the right proximal medial thigh and inguinal region, with no adenopathy or induration appreciated. She can straight leg raise and sit without difficulty. When supine, her leg is held straight, and passive external and internal rotation of the hip to the ends of its range of motion are normal but slightly painful. Assessment of her right leg adduction and abduction is limited because of pain. Her knees, ankles, and spine have full and painless range of motion, with no deformities or asymmetries; pulses are strong, sensation intact.Her white blood cell (WBC) count is 16,000/μL (16 × 109/L) with 73% neutrophils, hemoglobin level is 12 g/dL (120 g/L), and platelet count is 330 × 103/μL (330 × 109/L). Her fasting blood glucose level is 86 mg/dL (4.77 mmol/L), and her creatinine level is 0.4 mg/dL (35.36 µmol/L). Her erythrocyte sedimentation rate (ESR) is 103 mm/hr (reference range, <20 mm/hr), C-reactive protein level is 34 mg/dL (340 mg/L) (reference range, <0.9 mg/dL [<9.0 mg/L]), creatine phosphokinase level is 1,302 U/L (21.74 µkat/L) (reference range, 24–205 U/L [0.40–3.42 µkat/L]), and myoglobin level is 185 ng/mL (10.57 nmol/L) (reference range, 7–57 ng/mL [0.4–3.26 nmol/L]). Respiratory viral panel is negative. Blood culture is obtained. Point-of-care vascular ultrasonographic images as well as femur, hip, and knee radiographs are normal.Because the differential diagnosis for a febrile child with a limp because of pain is broad, careful assessment of information helps determine appropriate testing and management.Bacterial or septic arthritis of the hip often presents with acute fever, limited mobility of the hip, and pain. Examination typically reveals an ill-appearing child, commonly with limitation of movement of the hip; subtle swelling may be apparent in younger children. Kocher criteria, which include temperature higher than 101.3°F (>38.5°C), WBC count greater than 12,000/μL (12 × 109/L), ESR greater than 40 mm/hr, and inability to bear weight, aim to distinguish septic arthritis from transient synovitis, but their use is limited by suboptimal performance in validation studies. (1)(2) C-reactive protein level greater than 2 mg/dL (>20 mg/L) was later added to the criteria as another predictor of septic arthritis. With transient synovitis, presentation is less acute; viral infection often precedes symptom onset. (3) Radiography and ultrasonography can help confirm joint effusion.Osteomyelitis of the femur often presents with fever and worsening limp over several days; examination may reveal swelling and/or local tenderness and painful motion. Elevated WBC count and inflammatory markers support but are not specific for diagnosis. Early on, plain radiographs are normal, making magnetic resonance imaging (MRI) the preferred modality for diagnosis. (3)Deep soft tissue infections and abscesses may also present with hip pain and/or limp, especially when underlying muscles of the lower limbs or of the pelvic wall are affected. Findings may include warmth, erythema, swelling, and tenderness; however, local signs may not be apparent when the muscle is deeply situated. Suspicion warrants an MRI for diagnosis. (4)Another entity in an obese, limping older child is slipped capital femoral epiphysis, which presents with vague groin, hip, or knee pain, or acute hip pain with inability to walk. Radiographs show displacement of the femoral epiphysis. (5) Furthermore, tumors and trauma, including spinal injury, bone fracture, and muscle tears or hematomas, should be excluded in a child with lower extremity pain and/or limp.Right upper leg MRI with gadolinium reveals multiple small intramuscular abscesses with ring enhancement in the proximal thigh and in the obturator externus muscle, with extensive surrounding muscular edema involving the posterior and medial compartment muscles of the proximal thigh. Tendons and musculotendinous junctions of the muscle groups of the anterior and posterior compartments of the thigh as well as the hip joint and femur are normal.Pyomyositis can be categorized into primary, occurring due to hematogenous bacterial spread, and secondary, a result of direct extension from a contiguous infectious process. (6) Primary pyomyositis, formerly myositis tropicans, is a subacute, purulent, striated muscle infection that often presents as a local abscess but may also manifest as a phlegmonous or a rapidly progressing necrotic process. (7) Earlier literature described most cases occurring in tropical countries, and in individuals who were malnourished or immunocompromised, including individuals with diabetes mellitus. (8) These associations are not found in children with pyomyositis, as many reports are from temperate regions, (9) where the condition is described in immunocompetent children, and where there has been an increased incidence in the past couple of decades. (10)(11)(12)The affected age range is wide, with a trend toward older children. The highest incidence is in large muscle groups, such as the quadriceps and other thigh muscles, the gluteal and iliopsoas muscles; there are also reports involving obturators, hip adductors, and other smaller pelvic muscles. (7)(10)(13)(14) There may be an antecedent apparently minor trauma of affected or surrounding muscle(s), as pathogenesis points to muscle susceptibility to damage and seeding in the setting of transient bacteremia. Disease progresses from diffuse inflammation characterized by fever and nonspecific complaints to localized tenderness with abscess formation. (7) Pain is almost always universal; fever and limp are very common. Diagnosis may be delayed if local signs are not apparent on examination, potentially resulting in complications such as compartment syndrome, dissemination to adjacent joint or bone, or sepsis.Diagnosis of pyomyositis of pelvic musculature may also be delayed because of its rarity and similarities of presentation to skeletal infections. Pain is usually referred to the hip. In a series of 41 cases, obturator internus was the most commonly affected muscle. (15) In this series, 85% fulfilled the Kocher criteria for septic hip arthritis, often resulting in delay in diagnosis. Multifocal involvement is not uncommon.Staphylococcus aureus (S aureus) is the most common causative bacteria. Others include β-hemolytic streptococci, and less commonly Enterobacteriaceae. Bacterial isolation rates from blood culture vary widely in the published literature, ranging from 5% to more than 50%. (6)(7)(8)(9)(10)(11)(13)(14)(15) Although point-of-care ultrasonography can be used as a screening tool, MRI remains the preferred imaging modality for diagnosis. (16)Treatment includes drainage and culturing of abscesses combined with appropriate antibiotic therapy. During the early stages, antibiotics alone can be effective; empirical coverage should be directed against S aureus, including methicillin-resistant strains as guided by local susceptibility patterns, as well as β-hemolytic streptococci. Enteric gram-negative rod coverage should be included for immunocompromised children. (17) Narrowing coverage to target the isolated organism is imperative for avoiding resistance and toxicity from unnecessary antibiotics. Duration of antibiotic depends on the presence of complications, duration of bacteremia, and underlying medical conditions, entailing a minimum of 3 weeks in uncomplicated cases. Drainage can be performed by open procedure or by image-guided needle aspiration. (9)(13)The patient is started on vancomycin (15 mg/kg per dose every 6 hours) because the abscesses are deemed to be too small for drainage. Blood culture from the day of presentation grows methicillin-susceptible S aureus, after which the antibiotic is changed to cefazolin. She is bacteremic for 4 consecutive days, prompting adding gentamicin from the third through seventh days of hospitalization. Echocardiography reveals a structurally normal heart with no intracardiac vegetations or thrombi. She defervesces by day 4 but with only mild reduction in thigh pain. A repeated MRI reveals only a slight decrease in the size of the abscesses. Surgical irrigation of the medial proximal thigh muscle on day 6 demonstrates edema with no purulence or fluid collections. Intraoperative cultures have no growth. Several hours after her surgical procedure, she develops fever, and a blood culture obtained grows methicillin-susceptible S aureus after 4 days. Another echocardiogram is normal, and a subsequent blood culture has no growth. She defervesces 2 days after surgery, and her pain and ambulation gradually improve. She walks at her baseline, with normal inflammatory markers and a normal hemoglobin A1c level of 4.8%, at discharge on day 12. She completes 3 weeks of cephalexin therapy and does well on subsequent follow-up visits.Primary pyomyositis is an infection of the muscles, most commonly affecting muscles of the thigh, gluteus, hip, and pelvis.Because pyomyositis is uncommon and mimics skeletal infections, clinicians should maintain a high index of suspicion in a febrile child with a limp, or leg or hip pain.A careful history and examination, including range of motion of affected limb, and presence of local tenderness or warmth, are important in attempting to differentiate joint/bone from muscle pathology.Blood cultures and early imaging, preferably with magnetic resonance imaging, should be obtained in all suspected cases.Prompt recognition, with initiation of antibiotics, and surgical intervention when necessary are imperative to prevent complications.