Radiology Cases in Pediatric Emergency Medicine Volume 3, Case 6 Lynette L. Young, MD

A 2-year old male was playing indoors three hours ago. His mother noticed that he was not walking and he began to complain of right leg pain. There was no history of trauma. He did have a cold for the last week and had a fever. His highest temperature measured at home was 40.5 degrees. The patient was seen in his pediatrician's office earlier in the day for fever and was prescribed amoxicillin for an ear infection. The patient returned the pediatrician's office later in the afternoon because he was refusing to walk. There is no history of sore throat or rash. The patient was sent to the ED for further evaluation. His past medical history is unremarkable.

Exam: Vital signs T38.1 (tympanic), P193, R28, BP 127/85, O2sat 99%(RA), Wt13.4kg (75%ile). He is alert and is being carried by his mother. His left lower extremity is nontender with full range of motion of his left hip, knee, and ankle. His right hip, femur, knee, proximal tibia and fibula, and foot are nontender. There is normal range of motion of his right hip and knee. There is reproducible point tenderness over the right anterior ankle joint, possibly the talus. There is pain elicited with right ankle range of motion. There is no swelling, overlying skin lesion, erythema, or asymmetric warmth. Pulses are good. Toes are pink. Radiographs of his right hip, knee, leg, ankle and foot are obtained.

His hip, knee, and leg radiographs are normal. They are not shown here. His ankle and foot radiographs do not reveal any fractures, foreign bodies, or lytic lesions. There is no periosteal elevation noted. Laboratory results: CBC WBC 26.1, 2% bands, 75% segs, 19% lymphs, 4% monos, Hgb 11.8, Hct 34.1, platelet count 381,000. ESR 25. A blood culture is drawn.

Questions

1) What is your current diagnosis and management plan ? 2) Would you consider hospitalizing him at this point or starting him empirically on a different oral antibiotic and discharging him home ? In summary, this child has a high fever and is refusing to bear weight. He has tenderness on range of motion about the right ankle and reproducible point tenderness over the anterior aspect of the ankle. He has a high WBC, but only a modestly elevated sedimentation rate. 3) Specifically, would you consider performing a bone scan, an ankle arthrocentesis, or a bone aspiration at this point ?

This patient arrived in the E.D. in the evening. A bone scan may take several hours to perform. It is not easily performed in the evening in most small hospitals. In this case, it would have to wait for the morning. A right ankle arthrocentesis is performed. The patient is given IV midazolam and morphine. One mL of buffered 1% lidocaine is used for local anesthesia. The ankle is aspirated with an 18 guage needle.

Three mL of purulent yellow fluid is aspirated. The fluid is sent for culture, cell count, and gram stain. He is given oxacillin intravenously. Laboratory studies performed on the arthrocentesis fluid: WBC 22,100, 2% bands, 78% segs, 6% lymphs, 14% monos, RBC 373,000. Gram stain: many WBC's, no organisms. The patient is hospitalized for intravenous antibiotics for septic arthritis. The next day, the culture of the ankle joint aspirate is positive for Streptococcus pneumoniae. He has a bone scan which shows increased uptake of the right talus. He undergoes a right ankle arthrostomy, synovial biopsy, curettage and windowing of the talus for septic arthritis AND osteomyelitis. He has a Penrose drain placed. A repeat ESR is 40. The blood culture is negative. The right ankle surgical wound heals well without complications.

Aspirating the ankle joint:

Identify two landmarks, the medial malleolus and the thick anterior tibial tendon. The anterior tibial tendon can be palpated easily with dorsiflexion. It is located about 1 cm anterolateral to the medial malleolus.

The arrows on the adult's foot photographed here point to the anterior tibial tendon. In most patients, this may not be as easy to see as in this photo, however, locating this tendon by palpation is usually not difficult when asking the patient to DORSI flex against force. The anterior margin of the medial malleolus is also easy to locate. These landmarks are shown on our 2-year old patient's ankle as two vertical marks (right image). The lateral mark (lighter) is drawn over the anterior tibial tendon and the medial mark (darker) is drawn over the anterior margin of his medial malleolus. Have the patient supine with the foot in slight PLANTAR flexion. Plantar flexion opens the ankle joint space wider.

Notice how the joint space is relatively closed in dorsiflexion and more open in plantar flexion. Plantar flexion optimizes needle entry into the joint space.The puncture site is between the two landmarks. Prep the area with povidone-iodine solution, and anesthetize the skin with 1% lidocaine. To aspirate, have an 18 or 20 gauge needle with a 10 ml syringe. Insert the needle medial to the anterior tibial tendon and aim toward the anterior edge of the medial malleolus. Aspirate the fluid in the joint space.

• Septic arthritis (infection within a joint space) is often difficult to diagnose early in its course. It is frequently not diagnosed on the first visit. There must be a high index of suspicion to make the correct diagnosis. It is essential to recognize septic arthritis early because a delay in the diagnosis and treatment can lead to serious complications. Septic arthritis is a condition that urgently requires intravenous antibiotics and sometimes open drainage. Serious complications include joint destruction and long term disability.
  
  
• Septic arthritis is more common in children than in adults. Most of the cases occur in children less than 5 years old. The highest incidence occurs in children between 6 months and 24 months of age. Males outnumber females 2:1.
  
  
• Predisposing factors include significant trauma (in 30% to 45% of cases), preceding upper respiratory infection, and otitis media. Less frequently there could be an associated skin or soft tissue infection. Often there are no preceding factors identified.
  
  
• There are three mechanisms by which the organism can enter the joint space. The most common mechanism in children is hematogenous spread. Rarely there could be direct inoculation or extension from a contiguous site of infection. With the bacteria in the joint space there is a subsequent inflammatory response. There is neutrophil infiltration and the purulent material accumulates within the joint space. The direct pressure as well as proteolytic enzymes cause the symptoms of tenderness, swelling, and erythema.
  
  
• The organism most frequently found in septic arthritis varies with age. In the neonatal period (first 2 months of life), the most common organisms are group B Streptococcus and Staphylococcus aureus. There also may be gram negative enteric organisms. Beyond the neonatal period up to 2 years of age the most common organisms have been Hemophilus influenzae and Staph aureus. The frequency of H. influenzae has decreased because of the ubiquitous use of H. flu vaccine. In the 2 to 5 year old age range the most common organisms have been H. influenzae, S. aureus, group A Streptococcus, and Streptococcus pneumoniae. In children older than 5 years old the most frequent organism is S. aureus. In sexually active adolescents, Neisseria gonorrhea has also been implicated as an etiology of septic arthritis. Pseudomonas aeruginosa is associated with puncture wounds. In patients with sickle cell anemia, Salmonella species may be recovered in the culture. Frequently there is no organism recovered in patients diagnosed with septic arthritis (up to 30%).
  
  
• The most common joint involved in children is the hip, followed by the knee and then the ankle joint. Less frequently the site is a joint in the upper extremity elbow, shoulder, or wrist. Since a joint in the lower extremity is the more common site, the patient often presents with a limp or a refusal to walk. Occasionally more than one joint is involved (6-11%).
  
  
• Common symptoms include erythema, swelling, tenderness and lack of active motion at the affected joint. The signs are often subtle, especially in the neonates. Fever of greater than 38.3 is found in about 75% of the patients.
  
  
• The WBC count in septic arthritis is variable. In most patients there is a leukocytosis with a shift to the left, but about 50% of patients will have a WBC count less than 15,000/cu mm. A normal white count does not rule-out septic arthritis. The ESR and CRP are more frequently abnormal in patients with septic arthritis. The ESR is elevated in about 90% of the cases. Although this test is more sensitive than the WBC count, it is nonspecific. The ESR may be elevated in many other conditions. Blood cultures are positive in about 40% of the cases, more frequently if the organism is H. influenzae. Radiographs are recommended although they are usually normal in early septic arthritis. Late findings may include joint widening and bony destruction. There is no laboratory test or X-ray that can rule-out septic arthritis. The joint fluid must be analyzed if there is any suspicion of septic arthritis.
  
  
• The joint fluid should be sent for WBC count, gram stain and culture, and glucose. The WBC count is usually greater than 50,000 cells/cu mm, with a predominance of neutrophils (>75%). There have been cases of septic arthritis with the WBC count as low as 5,000 cells/cu mm. The glucose is usually less than 40 mg/dL. The joint fluid culture is positive in about 60% of cases. If an organism is recovered it is most frequently found in the joint fluid, therefore an arthrocentesis is very important.