21.1 Musculoskeletal & Soft-Tissue Infections: Osteomyelitis, Septic & Reactive Arthritis, Pyomyositis & Gas Gangrene, Cellulitis, Necrotizing Fasciitis and Bite Wounds

Why this area matters

The TOS musculoskeletal area lists:

• osteomyelitis;
• arthritis;
• myositis;
• cellulitis.

These infections are common in Philippine practice, especially:

• after trauma;
• in diabetic patients with foot ulcers;
• in children with acute hematogenous bone and joint infection;
• in the tropical form of pyomyositis.

Questions give a host clue (sickle cell disease, a nail through a sneaker, a dog bite, a fisherman's wound) and ask for the organism, or describe a red, painful limb and ask you to recognize a surgical emergency.

Osteomyelitis

• Pathogenesis: in children, hematogenous infection settles in the metaphysis of long bones, where blood flow is slow in looping capillaries.
• Pathology:
  • pus raises the periosteum and dead bone forms a sequestrum;
  • new periosteal bone around it forms an involucrum;
  • a Brodie abscess is a walled-off subacute abscess;
  • long-standing draining sinuses can develop squamous cell carcinoma (Marjolin ulcer).
• Diagnosis:
  • X-ray changes appear only after 10 to 14 days;
  • MRI is the most sensitive imaging test;
  • raised ESR and CRP are used to follow treatment;
  • bone biopsy culture identifies the organism.
• Treatment: at least 4 to 6 weeks of antibiotics, with surgical debridement of dead bone in chronic disease.

Septic arthritis

• Organisms:
  • Staphylococcus aureus is the most common cause at all ages.
  • Neisseria gonorrhoeae is common in sexually active young adults. Disseminated gonococcal infection causes migratory polyarthralgia, tenosynovitis of the wrists and hands, and sparse pustular skin lesions, or a purulent monoarthritis, usually of the knee.
  • Kingella kingae affects toddlers.
  • Streptococci and Gram-negatives affect older and immunocompromised patients.
• Clinical picture: a hot, swollen, painful joint with limited movement. The knee is the most common site. In infants, septic arthritis of the hip is an emergency because of the risk of avascular necrosis of the femoral head.
• Synovial fluid: more than 50,000 white cells/uL, mostly neutrophils, low glucose, and a positive Gram stain or culture. Crystals do not exclude infection.
• Management: prompt joint drainage (aspiration, arthroscopy or arthrotomy) and IV antibiotics.
• Prosthetic joint infection: coagulase-negative staphylococci and S. aureus form biofilms, so the prosthesis often has to be removed or exchanged.

Other infectious and post-infectious arthritis:

• Reactive arthritis:
  • follows urethritis (Chlamydia) or dysentery (Shigella, Salmonella, Campylobacter, Yersinia), usually in HLA-B27-positive people;
  • an asymmetric arthritis of large lower-limb joints, often with conjunctivitis and urethritis;
  • joint cultures are sterile.
• Viral arthritis: parvovirus B19 and rubella cause a symmetric small-joint arthritis in adults. Chikungunya causes severe, sometimes chronic, polyarthralgia. Hepatitis B causes an immune-complex arthritis.
• Tuberculous arthritis is a chronic monoarthritis of the hip or knee.

Myositis

• Pyomyositis (tropical pyomyositis):
  • a primary abscess of skeletal muscle, usually S. aureus;
  • common in tropical countries such as the Philippines, in children and young adults, and in HIV and diabetes;
  • affects the large muscles of the thigh, buttock and trunk;
  • begins with cramping pain, followed by a firm "woody" swelling and then an abscess with fever;
  • diagnosed by ultrasound or MRI and treated with drainage and anti-staphylococcal antibiotics.
• Clostridial myonecrosis (gas gangrene):
  • Clostridium perfringens alpha toxin (lecithinase) destroys muscle in contaminated deep wounds;
  • severe pain, swelling, crepitus, bronze skin with hemorrhagic bullae, and shock;
  • Gram-positive rods with few neutrophils on Gram stain;
  • needs emergency surgical debridement (sometimes amputation) with high-dose penicillin plus clindamycin, which suppresses toxin production.
• Viral myositis:
  • Influenza B causes benign acute childhood myositis, with calf pain and toe walking.
  • Coxsackie B causes pleurodynia (Bornholm disease).
  • Leptospirosis causes marked calf myalgia with high CK.
• Parasitic myositis:
  • Trichinellosis, from undercooked pork, causes myalgia, periorbital edema and marked eosinophilia.
  • Cysticercosis leaves calcified "cigar-shaped" cysts in muscle.

Cellulitis and deeper soft-tissue infection

• Erysipelas:
  • an infection of the upper dermis and superficial lymphatics, usually group A streptococcus;
  • a bright red, raised, sharply demarcated plaque, often on the face or leg.
• Cellulitis:
  • involves the deeper dermis and subcutaneous fat, with ill-defined borders;
  • nonpurulent cellulitis is mostly streptococcal;
  • purulent cellulitis or abscess suggests S. aureus, including MRSA, and is drained.
• Necrotizing fasciitis:
  • Type I is polymicrobial and occurs in diabetics and after surgery, including Fournier gangrene of the perineum.
  • Type II is caused by group A streptococcus, with or without S. aureus.
  • Warning signs are pain out of proportion to skin findings, rapid spread, tense edema, bullae, skin anesthesia, crepitus and shock.
  • Surgical exploration shows gray, "dishwater" fluid and fascia that separates easily.
  • Management is emergency debridement plus broad-spectrum antibiotics including clindamycin.
• Water-related wounds:
  • Vibrio vulnificus (seawater or raw shellfish) causes hemorrhagic bullae and sepsis in cirrhosis.
  • Aeromonas (fresh water) and Mycobacterium marinum (fish tanks) are other causes.
• Bites:
  • Dog and cat bites carry Pasteurella multocida, which causes cellulitis within 24 hours. Cat bites are deep and often infected.
  • Human bites and clenched-fist injuries carry Eikenella corrodens and oral anaerobes.
  • Treatment and prophylaxis are amoxicillin-clavulanate, together with rabies and tetanus prophylaxis as indicated.