Case Based Pediatrics For Medical Students and Residents
Department of Pediatrics, University of Hawaii John A. Burns School of Medicine
Chapter XIX.5. Septic Arthritis
Floyd S. Ota, MD
February 2002

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A 2 year old male presents with fever and refusal to walk for two days. He complains of pain and points to his right lower extremity. The pain has become increasingly worse, and he is unable to sleep at night. His appetite is decreased. There is a recent history of an upper respiratory tract infection about two weeks ago, but no recent trauma. The pain is not known to migrate. He has no past medical history but his immunizations are delayed (last immunizations at two months of age). There is no history of cough, headache, abdominal pain, vomiting, diarrhea, hematuria, or known tick exposure. Family history is negative for sickle cell disease and arthritis.

Exam: VS T 39.5, P 120, R 18, BP 100/50, oxygen saturation 100% in RA. Wt 10%ile, Ht 50%ile. He is thin appearing and refuses to walk. He is not fussy and nontoxic. HEENT exam is normal. His neck has good range of motion without pain. Heart, lungs, abdomen, and genital exams are normal. He is lying in a hospital bed with his right lower extremity externally rotated, abducted, and motionless. He has severe discomfort with minimal internal and external rotation of the right hip despite attempts to distract him. His other joints and neurological exam are normal. There are no notable skin lesions.

Laboratory studies show WBC 20,000, 80% segs, 10% bands, 8% lymphs, 2% monos, H/H 14/43, Platelet count 265,000. ESR 45. CRP 12. Serum glucose 95. UA SG 1.020, negative for blood. EKG normal sinus rhythm. Hip radiographs show widening of the acetabular space on the right.

An orthopedic surgeon is consulted. An arthrocentesis of the right hip is performed which shows WBC 110,000, glucose 35, gram stain shows many WBCs and few gram positive cocci. Surgical debridement of the right hip is performed. Empiric treatment with vancomycin and ceftriaxone is initiated after cultures are obtained. ASO, ANA and rheumatoid factors are negative. PPD and control is negative. Synovial fluid culture grows out Staph aureus sensitive to methicillin. Blood culture is also positive for methicillin sensitive Staph aureus. Vancomycin and ceftriaxone are discontinued and the patient is treated with oxacillin. Within three days of treatment onset, his fever declines and he slowly begins to ambulate. His appetite returns and he is eventually transitioned to high dose oral antibiotics to complete four weeks of treatment. He is discharged with home care physical therapy services.


Septic arthritis generally refers to bacterial infection of the joint space; however fungal and mycobacterium can also cause disease. Septic arthritis is a medical emergency and failure to provide prompt diagnosis and treatment may lead to severe morbidity and disability. The incidence is estimated to be 5.5 - 12 per 100,000 individuals (1). Septic arthritis is a disease primarily of young children in the first decade of life.

Diarthroidial joints have a synovial lining that separates the adjacent articular cartilages. This histologic lining is extremely vascular and lacks a basement membrane. This tissue produces synovial fluid, a viscous media that has an electrolyte and glucose concentration similar to that of plasma and acts as a lubricant to the adjacent cartilage. This fluid is normally sterile, but if invaded by bacteria, it provides a good environment for bacterial growth. The three main routes of joint infection are: 1) hematogenous (most common in children), 2) contiguous spread, and 3) direct inoculation from a procedure or trauma. The amount of blood flow to the synovium is high, equivalent to that of the brain. Thus, transient bacteremia can cause a large number of organisms to be delivered to this region. Bacteria normally cleared by synovial macrophages can be overwhelmed when presented with a large quantity of organisms. Proteolytic enzymes produced by bacteria and inflammatory cytokines incite damage to the articular cartilage. This process begins early in the infection, and its effects may render the articular surface susceptible to future degenerative joint disease. Furthermore, swelling of the joint capsule may predispose the femoral head to avascular necrosis due to ischemia of the capital femoral epiphysis. Dislocation or subluxation can also result from the increased intracapsular pressure (2). An important concept to emphasize is that the inflammatory process and tissue damage may progress despite the fact that the causative organisms have been eradicated.

Children with septic arthritis all present with one common feature, pain to the affected limb. This is due to stretching of the joint capsule from edema or an effusion. Joint pain may present as refusal to walk, to bear weight, or to utilize the affected limb. Often the children have fever and they can appear toxic to well appearing in their presentation. A history of trauma or upper respiratory infection in the weeks prior is sometimes elicited, which may mislead one from the true diagnosis of septic arthritis. Furthermore, septic arthritis may be a complication for patients with a history of recent surgery, urinary tract infection, and infection due to varicella zoster virus (due to secondary cutaneous infection of the lesions with Staph aureus or group A strep) (1).

On physical exam, swelling, tenderness, erythema, and warmth may be apparent to joints with little overlying tissue. However in a deep (well enclosed) joint such as the hip, these findings may be minimal to absent. Subtle findings such as a loss of natural body curvatures or normal skin creases may be all that is present. Thus, examination of the opposite side for symmetry is an important aspect of the physical exam. Range of motion is the most sensitive method to determine the presence of joint effusion (2). Children with septic arthritis often have significantly decreased and painful range of motion since any movement that stretches the joint capsule produces severe discomfort. In infants with septic arthritis of the hip, the classic physical finding is of a child lying motionless with his/her leg externally rotated and abducted. In septic arthritis of the axial skeleton and pelvis, direct compression of the joints may be the only way to produce clinical signs. It is important to examine all the joints of the lower extremities in a child with a limp, because the child may complain of knee pain, when in fact it is the hip that is affected. The most commonly affected joints are the knees and hips (67%). These are followed in incidence by the ankle, elbow, wrist and shoulders (1). Examination for signs of meningitis and performing a lumbar puncture when indicated is important in children who are susceptible to Haemophilus influenza, type B (but one would not necessarily know this until gram stain and/or culture information is available). One study found that 30% of children with septic arthritis due to this organism had concurrent meningitis (2). Haemophilus influenzae, type B (HiB) infections are currently almost nonexistent because of widespread effective HiB immunization. In the neonatal period septic arthritis often is present concurrently with acute osteomyelitis of the adjacent bone.

The differential diagnosis of a child with fever and joint pain includes: septic arthritis, transient synovitis, reactive arthritis, trauma, acute rheumatic fever, Henoch-Schonlein purpura, Kawasaki disease, serum sickness, lyme disease arthritis, inflammatory bowel disease, hematologic cancer, and connective tissue disease (i.e., juvenile rheumatoid arthritis, systemic lupus erythematosus, etc.). Toxic synovitis (also known as transient synovitis) of the hip is a viral or post infectious process causing acute arthritis that is important because it often causes a diagnostic dilemma for the clinician. Transient synovitis of the hip is often preceded by an upper respiratory tract infection or pharyngitis in previously healthy children. The peak incidence is 3-6 years of age (3). The etiology is unclear; however children with this condition may have a predisposition for hypersensitivity reactions. CBC, ESR and CRP studies are often (but not always) normal or only minimally elevated, and radiological studies often fail to show impressive changes. Rarely is joint aspiration performed, despite the presence of a hip effusion if the clinical findings and laboratory studies are suggestive of this diagnosis. Toxic synovitis is a diagnosis of exclusion, and treatment consists of non-steroidal anti-inflammatory medications and bed rest. Overall prognosis is usually good (about 70% of patients have resolution of their symptoms within two weeks) (3), but avascular necrosis may occur in some patients.

Laboratory studies are helpful in diagnosing septic arthritis. The most useful laboratory values are the acute phase reactants, the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP). These values can be elevated in the presence of acute septic arthritis, and are non-specific indicators of acute inflammation (Refer to the chapter on osteomyelitis for a discussion of CRP and ESR). A retrospective study found that patients with an ESR >20 combined with a fever >37.5, identified 97% of cases of septic arthritis and recommended joint aspiration if these values were present in a child with an irritable hip (4). The peripheral white blood cell (WBC) count is often obtained in the work up of septic arthritis. As a single value, it is unreliable to make the diagnosis of septic arthritis, or to rule it out. Despite this fact, a CBC can be helpful to rule out other diagnoses (such as leukemia), and thus it is an integral part of the workup. Joint aspiration is the most helpful test to make the diagnosis of septic arthritis. The synovial WBC count usually is greater than 80,000 with a predominance of segmented neutrophils. Glucose concentrations are decreased to about 30% of the serum glucose (2). This helps to differentiate septic arthritis from other etiologies of acute joint pain. Typically WBC count and glucose values are not as dramatically affected in cases of transient synovitis, reactive arthritis, and JRA. Bacterial culture and gram stain, when positive, are very helpful in the diagnosis and management of acute septic arthritis. Identification by gram staining is important because joint aspirates are sterile about 30% of the time in patients with septic arthritis (2). Identification of the offending organism and antibiotic sensitivities are an extremely important aspect to guide therapy. The most common organism isolated is Staphylococcus aureus (50%). This is followed by group A Streptococcus (25%), Streptococcus pneumonia (4%), and HiB (16%) (1). The percentage for HiB is probably substantially lower today because of widespread immunization. Neisseria gonorrhoeae (GC) should be considered in neonates and sexually active adolescents. GC septic arthritis may present with a polyarticular presentation, which is unusual for other causes of septic arthritis. Other pathogens to consider in the newborn period are group B Streptococci, and Escherichia coli. Recently, Kingella kingae has become a more recognized pathogen (1).

Imaging should start with plain film radiographs. When looking at the hip, AP and frog leg views should be obtained. The findings on plain radiographs that suggest septic arthritis are displacement of normal fats plains and widening of the joint space due to capsular swelling from an effusion. These signs are subtle and may not be present early on in the disease process. Ultrasound is a quick and noninvasive means of detecting the presence of a hip effusion. CT, MRI, and bone scans all have limited value in the diagnosis of straightforward septic arthritis. However, MRI and nuclear bone scan can help to differentiate between septic arthritis in the presence or absence of a concurrent osteomyelitis. Nuclear bone scans are also helpful when the foci of infection are unclear. All imaging modalities discussed above are able to detect the presence of a joint effusion, however none can differentiate between infectious and non-infectious causes of the effusion. Direct aspiration of the joint fluid is the most definitive means of diagnosis. Joint aspiration does not affect subsequent bone scan results (2).

Treatment of acute septic arthritis consists of surgical debridement and antibiotic treatment. The risk for poor prognosis is increased if any of the following factors are present: a delay in initiation of treatment, age less than six months, history of prematurity, the presence of S. aureus as the infectious etiology, and the presence of a concurrent osteomyelitis (5). Early intervention is required to minimize morbidity. Surgical arthrotomy for large joints is the rule; however this clinical intervention is not always indicated for involvement of smaller joints. The purpose of surgery is to produce an environment with minimal inflammatory products so that antimicrobial therapy is maximized. After surgical intervention, empiric parenteral antibiotic coverage for Staphylococcus aureus should be initiated. The drug of choice is vancomycin due to the high risk (currently about 30%) that the organism may be resistant to cephalosporins and methicillin (MRSA). Clindamycin is another choice that will cover most strains of MRSA, but it is not 100%, so vancomycin is preferred. A third generation cephalosporin should be added if the child is at risk for H. influenza or gonococcal disease. If CNS infection is suspected, meningitic doses should be implemented. The antibiotic regimen can then be narrowed once the cultures and sensitivities are received. As a rule, treatment duration is 3-4 weeks (6). Peripherally inserted central IV catheters can be placed and home antibiotics can be arranged with home care. Recently, oral antibiotics have become an accepted option to complete therapy. This can be done because antibiotic concentrations in the synovium are often higher than that of the serum due to slow reabsorption of the drugs (from the synovium). However, all of the following criteria must be met: organism identified and sensitivity to oral antibiotics is documented, the patient is able to take and keep down oral antibiotics, a clear response to parenteral treatment is demonstrated, and routine compliance is assured (2). Following the CBC, CRP and/or ESR at routine intervals may be helpful to monitor clinical progress and monitor iatrogenic side effects.

In conclusion, for straight forward cases of septic arthritis, the overall prognosis is good. Differentiating infectious from non-infectious etiologies of joint pain can be a clinical dilemma. The clinician must utilize a broad range of clinical tools to expeditiously diagnose and treat this condition so that outcomes are favorable.


Questions

1. True/False: Septic arthritis is a disease most commonly found in adolescent males.

2. True/False: In septic arthritis, the hips and knees are the most commonly affected joints.

3. True/False: In a child with septic arthritis of the hip, redness, swelling, and warmth are often detectable on physical exam.

4. True/False: Children with toxic synovitis never present with fever.

5. True/False: The ESR and CRP can usually distinguish between toxic synovitis and septic arthritis.

6. True/False: The most common bacterial etiology of septic arthritis is Staph aureus.

7. True/False: Haemophilus influenzae type B used to be a common cause of septic arthritis in young children, but this is very uncommon today.

8. True/False: Surgical arthrotomy is always warranted for cases of septic arthritis.


Related x-rays

Ankle septic arthritis: Young LL. Aspirating the Ankle Joint. In: Yamamoto LG, Inaba AS, DiMauro R. Radiology Cases In Pediatric Emergency Medicine, 1995, volume 3, case 6. Available online at: www.hawaii.edu/medicine/pediatrics/pemxray/v3c06.html

Septic arthritis of the hip: Rosen MH. Fever and Refusal to Walk in a 4-Year Old. In: Yamamoto LG, Inaba AS, DiMauro R. Radiology Cases In Pediatric Emergency Medicine, 1996, volume 4, case 17. Available online at: www.hawaii.edu/medicine/pediatrics/pemxray/v4c17.html

Hip effusion in a case of osteoid osteoma: Yamamoto LG. Osteoid Osteoma. In: Yamamoto LG, Inaba AS, DiMauro R. Radiology Cases In Pediatric Emergency Medicine, 1996, volume 4, case 15. Available online at: www.hawaii.edu/medicine/pediatrics/pemxray/v4c15.html


References

1. Krogstad P, Smith AL. Chapter 64: Osteomyelitis and Septic Arthritis. In: Feigin RD, Cherry JD (eds). Textbook of Pediatric Infectious Disease, 4th edition. 1998, Philadelphia: W.B. Saunders Co., pp. 698-703.

2. Morrissy RT. Chapter 13: Bone and Joint Sepsis. In: Morrissy RT, Weinstein SL (eds). Lovell and Winter Pediatric Orthopaedics, 5th edition. 2001, Philadelphia: Lippincott Williams and Wilkins, pp. 459-500.

3. Koop S, Quanbeck D, et al. Three Common Causes of Childhood Hip Pain. Pediatr Clin North Am 1996;43(5):1053-1066.

4. Del Baccaro MA, Champoux AN, Bockers T, Mendelman PM, et al. Septic Arthritis Versus Transient Synovitis of the Hip: The Value of Screening Laboratory Tests. Ann Emerg Med 1992;21(12):1418-1422.

5. Chapter 5: Pediatric Orthopaedic Infections. In: Richards SB (ed). Orthopaedic Knowledge Update; Pediatrics. 1996, Illinois: American Academy of Orthopaedic Surgeons, pp. 35-46.

6. Sonnon GM, Henery NK, et al. Pediatric Bone and Joint Infections. Pediatr Clin North Am 1996;43(4):933-947.


Answers to questions

1. False. It is a condition that usually affects younger children early in the first decade of life.

2. True

3. False. The hip joint is deep and has a significant amount of surrounding tissue, thus inflammation may not be easily detected on physical exam. Exam findings may be subtle, such as asymmetry or loss of function. Decreased and painful range of motion is the best way to detect an effusion by physical exam.

4. False. They also can present with fever. This is why differentiating between toxic synovitis and septic arthritis can be a difficult clinical problem.

5. No definite answer here. Low ESR and CRP values make septic arthritis unlikely. Very high ESR and CRP values make septic arthritis more likely. Intermediate ESR and CRP values are not very helpful in distinguishing toxic synovitis from early septic arthritis.

6. True

7. True

8. False. In larger joints surgical intervention is almost always performed. However in cases of septic arthritis of smaller joints, medical management can be carried out with good results. Orthopedic surgical consult should always be obtained expeditiously whenever the diagnosis is considered.


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