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MSS > MSS07 Infection Of The Skin, Soft Tissues, Bones And Joints > Flashcards

MSS07 Infection Of The Skin, Soft Tissues, Bones And Joints Flashcards

1
Q

Normal skin flora

A

Most abundant:

  1. ***Staphylococcus (mostly coagulase -ve; 20-30% healthy population: Staph aureus carrier)
  2. Corynebacterium
  3. Propionibacterium acnes
  4. Gram -ve bacilli (non-fermenters e.g. Acinetobacter, occasional)
  5. Yeasts (Candida, Malassezia furfur)
  6. Micrococcus spp.

Varies with:

  • age
  • occupation
  • body location
  • hospitalisation
  • medications
  • diseases (local / systemic)
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2
Q

Normal defence of skin

A
  1. Normal ***integrity of skin
  2. ***Rapid cell turnover
  3. Normal ***flora of skin
  4. Antimicrobial effect of ***epidermal lipids (sebum-derived) of normal skin
  5. ***Mild acidity of normal skin (pH 5.5)
  6. ***Antimicrobial peptide
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3
Q

Pathogenesis of skin and soft tissue infections

A
  1. Breach of normal integrity of skin (trauma, surgery)
  2. Alteration of normal skin flora
  3. Changes in local environment of tissues e.g. presence of devitalised tissue, haematuria, foreign bodies etc.
  4. Introduction of pathogenic exogenous / endogenous microbial flora
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4
Q

Dermatophytosis / Ringworm (spread out like a ring, most active at peripheral) / Tinea
Derma: skin
Phytosis: fungal infection

A

Infection of **keratinised tissue by dermatophytic fungus that is **keratinolytic:

  1. Trichophyton spp.
  2. Microsporum spp.
  3. Epidermophyton floccosum

Diagnosis:

  • ***KOH wet mount
  • fungal culture

Treatment:
- Topical +/- Systemic anti-fungals

Dermatomycoses: fungal infections of skin caused by non-dermatophytic fungi (***Candida)

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5
Q

Paronychia

A

Superficial infection of the ***Nail fold

Acute paronychia:
- Staphylococcus aureus

Chronic paronychia:
- Candida spp. (Candida albicans) (wet and moist environment)

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6
Q

Pyoderma

A

Pus in the skin

Common types:

  1. Impetigo (strept/staph)
  2. Folliculitis (***staph)
  3. Abscesses (strept)
  4. Cellulitis (strept)
  5. Erysipelas (strept)
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7
Q

Impetigo

A

Superficial ***intra-epidermal unilocular vesicopustule

  • often in children
  • highly communicable
  • spread facilitated by overcrowding and poor hygiene

Pathogens:

  • Streptococcus pyogenes
  • Staphylococcus aureus
  • mixed
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8
Q

Folliculitis

A

Abscess formation around hair follicles

Pathogens:

  • Staphylococcus aureus (MOST)
  • other organisms
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9
Q

Abscesses

A
  1. Furuncle
    - ***subcutaneous abscesses (may complicate folliculitis)
  2. Carbuncle
    - large, contiguous groups of furuncles
    - common at back region of elderly, diabetic, immunocompromised
  3. Hydradenitis suppurativa
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10
Q

Cellulitis

A

An acute spreading infection of skin extending to involve the ***Subcutaneous tissues
- generally preceded by trauma / underlying skin lesion

Clinical features:

  • local signs of inflammation
  • ***ill-defined margin of inflammation (NO distinct borders)
  • +/- local abscess
  • fever, chills
  • bacteraemia

Pathogens:

  • Beta-haemolytic Streptococci (esp. Strept pyogenes)
  • Staphylococcus aureus (sometimes)
  • other bacteria (Vibrio (seawater), Enterobacteriacae, other Gram -ve bacilli)
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11
Q

Erysipelas

A

Infection of ***Dermis

Form of cellulitis presents with

  • very painful, red lesions
  • with ***distinct border and spreads rapidly
  • Marked ***subepidermal oedema with heavy infiltration of neutrophils

Pathogens:
- Beta-haemolytic Streptococci (mostly, e.g. Streptococcus pyogenes)

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12
Q

Treatment of pyoderma, cellulitis, erysipelas

A
  1. ***Drainage of collections whenever feasible
  2. ***Beta-lactam antibiotics

Methicillin-sensitive Staph. aureus (MSSA):

  • ***Cloxacillin
  • Beta-lactam + Beta-lactamase inhibitors
  • 1st gen cephalosporins

Methicillin-resistant Staph. aureus (MRSA):

  • ***Vancomycin
  • Other non-beta lactam (***Clindamycin, Linezolid etc. depending on antibiotic susceptibility pattern)

Beta-haemolytic streptococci:

  • Penicillin
  • Ampicillin
  • ***Amoxicillin

Mixed infection:
- depend on exposure history + flora causing infection

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13
Q

MRSA

A
  1. Conventional methicillin-resistant Staph. aureus (MRSA)
    - generally a nosocomial pathogen: hospital-acquired MRSA (HA-MRSA)
    - associated health care risk factors (frequent hospitalisation, use of antibiotics)
  2. Community-associated MRSA (CA-MRSA)
    - NO associated health care risk factors
    - important cause of skin and soft tissue infections
    - present as:
    —> Soft tissue abscesses (Furuncles)
    —> occasionally fulminant and rapidly fatal pneumonia (    Necrotizing pneumonia) in relatively young individuals
    - NOT multi-resistant towards antibiotics
    - Resistant to most beta-lactam but often susceptible to other agents (**Clindamycin etc.)
    - distinguished by special genetic elements —> possess **    Panton-Valentine leukocidin (toxin)
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14
Q

Necrotising soft tissue infections

A

Involves ***multiple tissue levels (Dermis, SC fat, Deep fascia)

  1. Nerve affected: ***Hypoaesthetic upon touch
  2. Blood supply cut off: pink (***dusky discolouration) / black (dead)
    —> high mortality and morbidity despite aggressive medical and surgical treatment

Different forms of necrotizing soft tissue infections clinically

  1. Necrotizing fasciitis (most frequent)
    - thrombosis of blood vessels perforating the fasciae envelope
    - extension of necrosis under the skin
  2. Myonecrosis (gas gangrene)
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15
Q

Necrotizing fasciitis Etiologies

A

Etiology:
1. Type I:
- **Anaerobes (e.g. Bacteroides, Clostridium)
- **Facultative Anaerobes (e.g. Streptococci, Enterobacteriaceae)
—> occurs after ***intra-abdominal / pelvic surgery
—> may arise de novo in perineal area (Fournier’s gangrene)

  1. Type II (Most common):
    - ***Streptococcus pyogenes
    - +/- other organisms
  2. Type III:
    - Vibrio spp. (esp. Vibrio **vulnificus)
    - occurs after exposure to water / consumption of **    seafood containing pathogens

Clinical features:

  1. Cyanotic skin —> dusky and black: full-thickness necrosis of skin
  2. Anaesthetic of central part (中間無感覺)
  3. Very tender esp. at the spreading edge of lesion (外面好痛)
  4. Fever, pain, oedema
  5. Septic shock

Management:

  1. Surgical emergency (early and aggressive surgical debridement)
  2. Antibiotics
  3. Supportive care for sepsis
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16
Q

Pyomyositis

A
  • Abscess in skeletal muscles (Psoas abscess)
  • uncommon

Pathogen:

  • Staphylococcus aureus
  • other bacteria
17
Q

Clostridial Myonecrosis (gas gangrene)

A

Pathogens:

  • ***Clostridium perfringens (85-90% cases)
  • other Clostridium spp.
  • can mix with facultative anaerobic bacteria (Enterobacteriaceae)

Pathogenesis:
- Muscle injury
—> Contamination with soil / other foreign materials containing **spores of Clostridium perfringens
—> **Coagulative necrosis of muscle fibres

Clinical features:

  1. Local tense oedema
  2. ***Serosanguineous discharge (contain both blood and serum)
  3. ***Foul odour of the wound
  4. Crepitus

Laboratory diagnosis:

  1. Gram smear of wound discharge: numerous bacteria but few leukocyte
  2. Aerobic and anaerobic culture

Treatment:

  1. Surgical emergency, urgent surgery
  2. Antibiotics (Penicillin, Clindamycin / Metronidazole for clostridia)
18
Q

Surgical site infection

A

Surgical wound vs Surgical site (organ spaces deep to skin and soft tissue, such as peritoneum and bone)

Etiology depend on type of operation:

  • Skin flora (orthopaedic / neurosurgery) (Staphylococcus aureus, Streptococcus spp.)
  • Intestinal flora (abdominal / pelvic surgery) (E. coli, Enterobacter, Proteus, Klebsiella)
  • Environmental flora (hospital-acquired) (Pseudomonas aeruginosa)

Presence of wound infection indicated by:

  1. ***Purulent drainage (pus) from the incision
  2. ***Pain, tenderness, localised swelling, redness, dehiscence of wound, fever
  3. Organisms isolated from an aseptically obtained culture of tissue / fluid from the wound
  4. Gram smear of the wound tissue / fluid / swab may reveal numerous leukocytes and bacteria

Wound class: (strong association with incidence of infection)

  • clean
  • clean contaminated
  • contaminated
  • dirty
19
Q

Osteomyelitis

A

An infectious process involving the various components of bone (Periosteum, Medullary cavity and Cortical bone)

Route of infection:
1. Acute osteomyelitis
—> **Haematogenous osteomyelitis
—> **Contiguous focus osteomyelitis
- clinical features: fever, chills, leukocytosis, pain, local swelling
- signs / symptoms may be minimal / non-specific in some patients e.g. infants, IV drug abusers

  1. Chronic osteomyelitis (resulting from untreated / inadequately treated acute osteomyelitis, haematogenous)
    - **Sequestrum (nidus of infected, ischaemic, dead bone)
    - Chronic infection —> local bone loss, persistent drainage, local abscess / adjacent soft tissue inflammation, **    sinus tract formation (tunneling wounds: dead space with potential for abscess formation)
    - Clinical features:
    —> Chronic pain + drainage
    —> may have low grade fever, sometimes symptomatology could be very mild
  2. Prosthesis-related infection:
    - early: during operation / from post-operative wound infection
    - late: haematogenous spread
    - important complication of joint replacement surgery
  • **Pathology:
    1. Acute inflammation —> obliteration of vascular channels —> ***Ischaemia / Necrosis (products of inflammation may also contribute to bone necrosis)
  1. Subperiosteal extension of infection —> lifting of periosteum away from bone (Infection由骨入面谷出黎) —> new bone formation —> ***Sinus tract
  2. Ischaemic segments of bone separated —> ***Sequestrum

Diagnosis:

  1. Radiographical procedures
    - Plain X rays: X ray changes lag >= 2 weeks behind the evolution of disease
    - CT / MRI
  2. Radionuclide imaging (bone scan / gallium scan)
  3. Microbiological diagnosis
    - blood culture
    - **bone biopsy for culture
    - in Chronic osteomyelitis, culture of sinus tract is **    NOT reliable for predicting the organism causing osteomyelitis

Treatment:

  1. ***Prolonged antibiotic course necessary
  2. Surgical debridement (may need)
  3. Chronic osteomyelitis: surgical debridement of ***sequestrum essential
20
Q

Acute haematogenous osteomyelitis

A

Infants and young children: ***monomicrobial infection usually

  1. Infants:
    - Staphylococcus aureus
    - Streptococcus agalactiae (neonates)
    - E. coli
  2. Children >1 year:
    - Staphylococcus aureus
    - Streptococcus pyogenes
    - Haemophilus influenzae (uncommon after 4 years of age)
  3. Adults:
    - ***Staphylococcus aureus (commonest)

Acute Vertebral Osteomyelitis:
- Haematogenous (usually)
—> Segmental arteries supplying vertebrae bifurcates to supply 2 adjacent bony segments
—> usually involves ***2 Adjacent vertebrae + Intervertebral disc

21
Q

Contiguous focus osteomyelitis

A 
Predisposing factors:
- trauma
- surgical operation of bones
- open fractures
- chronic soft tissue infections
- vascular insufficiency (e.g. diabetes mellitus): often ***Polymicrobial
—> Staphylococcus aureus
—> Gram -ve bacilli
—> Streptococci
—> Enterococci
—> Anaerobic organism
22
Q

Infective arthritis

A

Acute vs Chronic
Monoarthritis vs Polyarthritis
Native vs Diseased vs Prosthetic
Age and underlying disease

Route of infection:

  1. Haematogenous seeding
  2. Direct inoculation

Pathology:

  1. ***Bacterial seeding of synovium (susceptible since highly vascular and lacks basement membrane)
  2. ***Intra-articular inflammation —> destruction of articular cartilage

Clinical features:

  1. Pain, limitation of movement at the joint, joint swelling
  2. Fever
  3. Leukocytosis
  4. Usually affects ***larger joints (knee, hip, shoulder, ankle, elbow), but any joint can be involved

Infective causes of polyarthritis:

  1. ***Viruses
  2. ***Neisseria gonorrhoeae (part of disseminated gonococcal infection: polyarthralgia / polyarthritis, tenosynovitis)

(Non-infective arthritis: RA, SLE, DM, steroid therapy)

23
Q

Acute bacterial arthritis

A

Pathogens (depends on groups of patients)

  1. Infants <1 month:
    - Streptococcus agalactiae
    - aerobic Gram -ve bacilli (E. coli)
    - Staphylococcus aureus
  2. Children <2: Haemophilus influenzae type b
  3. Children >2: Staphylococcus aureus

***4. Adults: Staphylococcus aureus

  1. Sexually active: Neisseria gonorrhoeae
  2. IV drug abusers:
    - Staphylococcus aureus
    - Pseudomonas aeruginosa
  3. Elderly, Chronic debilitating disease, underlying chronic arthritis
    - Gram -ve Bacilli

Diagnosis:
1. Differential diagnoses
—> autoimmune diseases (RA)
—> crystal-induced arthritis (Gout: monosodium urate, Pseudogout: calcium pyrophosphate) (concurrent infective and crystal-induced arthritis can occur)
—> trauma, haemarthrosis, osteoarthritis, tumour

  1. Blood culture
  2. ***Diagnostic synovial fluid aspirate:
    - leukocyte count
    - crystals
    - Gram stain
    - culture
  3. Synovial biopsy

Treatment:

  1. Antibiotics
  2. Surgical drainage
24
Q

Skeletal mycobacterial infection

A

Pathogen:

  1. Mycobacterium tuberculosis (commonest)
    - usually from haematogenous spread during primary infection
    - occasional contiguous lymphadenitis
    - Adults: axial skeleton commonest (***TB spine)
  2. Mycobacterium marinum:
    - Skin lesions / Tenosynovitis (esp. in upper limbs)
    - recreational / occupational exposure to sea water

MSS (32 decks)

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