Septic Arthritis & Osteomyelitis

Septic Arthritis (infectious arthritis of a synovial joint), Osteomyelitis (infection of bone).

• These conditions are rare but can be life threatening (red flags).
• Early diagnosis and treatment are important.
• They are due to bacterial infection of the bone or joint and should be suspected with acute onset of: fever, unexplained limp, reluctance to use a limb or inability to weight bear, bone or joint pain with a hot and swollen joint, bone or joint tenderness and complete reluctance to move a joint or limb. The child with vertebral discitis can be difficult to diagnose and may be suggested by the child crying on spine flexion (e.g., nappy / diaper change).  
• These conditions may occur together, usually affect one joint but can be > 1 joint especially if the child is very young or immunosuppressed (by disease or treatment). Neonates and infants may not appear that unwell and may not always have a high fever. Careful examination and clinical suspicion are important. 

Septic Arthritis - most common in young children (50% < 2 years) and most commonly affects lower limb joints (knee, hip, ankle). Bacteria may reach the joint through the blood (most common route), through the skin, or by local spread from an adjacent infected site. In neonates and infants, bacterial infection can spread easily through blood vessels from bone into the joint as the metaphysis is intracapsular. In the adolescent with septic arthritis, and especially of the sternoclavicular joint or shoulder joint, IV drug use must be suspected. 

Osteomyelitis - is most common in infants and young children (< 2 years).  Infection tends to occur in the bone metaphysis. Most infections are blood spread from a primary site of infection (chest, ear, nose or throat, skin). Infection may also occur by direct inoculation (open wounds after trauma) or from local soft tissue infection.  Infection can be acute, subacute (over 2-3 weeks) or chronic. Abscesses may form in the bone. Chronic Recurrent Multifocal Osteomyelitis is a diagnosis of exclusion and is now thought to be an autoinflammatory condition. 

Tuberculosis (TB) needs to be suspected in endemic areas or when a child is unwell with a swollen joint, and especially if they are immunosuppressed due to disease or treatment. Inflammatory markers may be elevated and there can be anaemia of chronic disease. Atypical infection including Mycobacterial infection must be suspected in the immunosuppressed (disease or treatment) and may not present with a red hot joint but with a more indolent course. 

The Table below lists the most common pathogens for both Septic Arthritis & Osteomyelitis (although pathogens are not always detected and blood or synovial cultures can be sterile). 

Principles of management of septic arthritis and osteomyelitis 

• Prompt diagnosis, urgent washout and drainage of joint and rapid treatment with antibiotics.
• There is likely involvement of several clinical teams (paediatrics, orthopaedics, microbiology) to determine the management plan, whether to opt for bone aspiration / biopsy as well as joint aspiration, take the appropriate cultures and choice of antibiotics (given by intravenous route).
• Blood tests, cultures and imaging (radiographs, ultrasound and sometimes CT or MRI) are needed. Gram stain, cultures are required and tests for mycobacterial infection may also be needed.
• There are usually high inflammatory markers (ESR, CRP), high white cell count (neutrophils) and these guide clinical progress and response to treatment. CRP is a better predictor than ESR for acute infection.
• Radiographs may be normal or show fluid or soft tissue swelling or increased joint space. Bony changes may not be apparent for 2-3 weeks. Ultrasound may show joint effusion and is useful to aid aspiration. MRI is very sensitive to early changes and identify bone involvement. Isotope bone scan (increased uptake or hot spots) can be useful if MRI is not available.
• Antibiotics are usually given empirically whilst awaiting cultures.
• Antibiotics are usually continued for several weeks (IV) and then oral, pending clinical progress and blood tests (acute phase reactants - ESR, CRP - and white cell count).
• Analgesia and resting the joint / limb (splinting) are important.  

Pointers towards septic arthritis at the hip are given below and are useful in practice (Kocher's rules)- especially in the child with hip effusion and differentiating between irritable hip and sepsis.

Lyme disease - is an infection caused by the spirochaete Borrelia burgdorferi and transmitted by the bite of the Ixodes tick (host - deer) and can affect the skin, joints, nervous system, and heart. This infection is endemic in some parts of the world (North USA and central Europe) and most common cause of chronic arthritis (more common than Juvenile Idiopathic Arthritis [JIA]). 

It is important to distinguish the arthritis from Lyme disease (most commonly a monoarthritis affecting the knee) from JIA, septic arthritis, or mycobacterial infection (see Table below). A travel history is important. The patient may not know that they have been bitten and there may be no rash.  

Treatment is with antibiotics and in most cases the prognosis is excellent.

The clinical presentation has several phases - 

Early localized- Erythema chronicum migrans - is the most common in children, occurring several days and even several weeks after the tick bite. The rash often occurs with a flu like illness and there can be large lesions. 

Early disseminated - weeks after infection with neurological features (facial nerve palsy, meningitis and rarely meningoencephalitis), heart conduction defects and arthralgia.                        

Late disease - is mainly involving joints, most commonly a single knee with intermittent, relatively painless large effusions lasting days or even weeks. Small joints are not affected. 

Diagnosis is made by exclusion of other infection and by DNA analysis of the synovial fluid or by serology (albeit the serological tests have low sensitivity and specificity is variable).

 Differentiating between Lyme disease, septic arthritis and Juvenile Idiopathic Arthritis (JIA) 

 Further Reading 

Kocher MS, Zurakowski D, Kasser JR. Differentiating between septic arthritis and transient synovitis of the hip in children: an evidence-based clinical prediction algorithm Journal of Bone and Joint Surgery J Bone Joint Surg Am. 1999 Dec;81(12):1662-70.