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Micbi > Skin and Wound Infections > Flashcards

Skin and Wound Infections Flashcards

1
Q

Basic structure and function of the skin

A

Anatomical defence barrier against microorganisms

Epidermis

  • 95% keratinocytes
  • stratified squamous epithelium

Dermis

  • blood vessels, capillary beds, lymphatics, nerve endings, glands
  • connective tissue to cushion body

Subcutis (surrounded by superficial and deep fascia)
Muscle

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2
Q

Structures for defence in the skin

A

Skin surface

  • microbiota
  • acid mantle (dry, high salinity, acidic)
  • sebum
  • continual skin desquamation
  • antimicrobial peptides

Epidermis

  • CD8 T cells
  • specialised dendritic cells

Dermis

  • innate immune cells (macrophages, dendritic cells)
  • mast cells
  • innate lymphoid cells
  • CD4 and CD8
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3
Q

Mechanisms of defence

A 
Mechanical: oil film, keratin, dermis
Chemical: acidic pH of skin, fatty acids in oil fit
Biological: resident microbiome
Cellular: phagocytes, lymphocytes
Immunological: antibodies, lymphokines
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4
Q

Classification of skin and soft tissue infections

A

Cellulitis and superficial infections

  • primary pyodermas
  • cutaneous involvement in systemic bacterial and mycotic infections e.g. disseminated s. aureus (scalded skin syndrome), candidiasis, scarlet fever

Subcutaneous tissue infections
- necrotising fasciitis

Secondary to breached skin defence

  • decubitus/ diabetic ulcer
  • burns
  • surgical wounds/ bite wounds
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5
Q

Wound infections: Predisposing factors (host)

A

*Malnutrition, anaemia, weight loss
*Dehydration, shock
*Extreme age
Malignancy
Drugs (steroids, cytotoxic)
Alcoholism
*Diabetes, uraemia, cirrhosis
Infection in other areas

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6
Q

Wound infections: 5 Predisposing factors (local)

A 
Type of wound
Site of wound
Time lapse from injury to treatment 
Number and virulence of bacteria 
Presence of dead/ devitalised tissue
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7
Q

Types of wound

A

Based on degree of contamination

Clean
- only normal skin organisms involved; GI tract, respiratory tract and GU tract not involved

Clean-contaminated
- GIT/ GU and Resp tract entered but without unusual contamination (i.e. mucosal organisms) e.g. endoscopy

Contaminated
- Wound with spillage expected, sterile techniques can’t be maintained e.g. incision of the GIT/GU or Resp

Dirty
- perforated viscus e.g. trauma or open wound

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8
Q

Site of wound

A

Inadequate blood supply increases susceptibility to infection (as there is decrease in cellular and humoral defence cells due to less flow)

Extremities: DM, PVD

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9
Q

Presence of devitalised tissue

A

Devitalised tissue –> poor blood supply
Retained foreign bodies –> promote infection

Necrotic devitalised tissue should be debrided to achieve source control

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10
Q

Number and virulence of bacteria

A

Extent of infection is proportional to number of pathogens and their virulence

Most commonly:
- virulent: s. aureus, grp A/C/G streptococci

Avirulent bacteria cause wound infection if excess in number (minimised by aseptic techniques and sterilisation techniques)

Causative organisms can be exogenous (personnel, environment, surgical equipment) or endogenous (normal flora e.g. GN bacilli, enterococcus)

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11
Q

Wound infection: clinical presentations, diagnosis, treatment, prevention

A 
Redness, swelling, warmth, pain
Loss of function
Purulent discharge
Systemic e.g. fever, chills
Non-union, wound dehiscence

Diagnosis

  • often clinical
  • culture and gram stain: debrided tissue or biopsy at infected margin of wound
  • superficial wound swab not useful due to presence of colonisers (except in presence of genuine pus)

Treatment

  • incision and drainage of pus
  • remove devitalised tissue and foreign bodies
  • antibiotics targeting suspected org\

Prevention

  • control underlying illness e.g. DM
  • operation: bowel decontamination, antibiotic prophylaxis
  • aseptic technique
  • decrease operation time and tissue trauma/haematoma
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12
Q

Soft tissue infections causative organisms

A

Bacteria: S. aureus, Strep pyogenes
Virus: HSV (cold sores, genital infection), VZV, Coxsackie A (hand foot mouth)
Fungal: Candida
Parasite: Sarcoptes scabiei

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13
Q

Staphylococci

A 
Impetigo
Folliculitis --> Furuncles --> Carbuncles
Cellulitis
Cutaneous/ Subcutaneous Abscesses
Thrombophlebitis

Scalded skin syndrome (exfoliative toxin, very severe)
Toxic shock syndrome (intravaginal tampon contamination)

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14
Q

Streptococci

A

Mostly strep pyogenes (grp A), sometimes grp G

Impetigo (overlap)
Erysipelas
Cellulitis (overlap)
Necrotising fasciitis 
Myositis
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15
Q

Superficial skin and soft tissue infections

A

Impetigo (epidermis)
Erysipelas (dermis)
Cellulitis (subcutis)
Folliculitis/ furuncle/ carbuncle (hair follicle and subcutis)

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16
Q

Impetigo: organism, site of infection, clinical features, variations, treatment

A

Organism: Strep pyogenes, S. aureus
Infection of EPIDERMIS

Clinical features:

  • usually in children
  • hot, humid environment
  • in exposed areas
  • initially papules –> vesicular –> rupture to form golden “honey crust”

Highly contagious – scratching causing spreading

Variations:

  • bullous impetigo – specific group of s. aureus lead to formation of bulla (large vesicles)
  • ecthyma – penetrate through epidermis and form punch out lesions

Treatment:

  • local hygiene care
  • cloxacillin (staph) and ampicillin (strep)
17
Q

Erysipelas: organism, site of infection, distribution, clinical features, treatment

A

Organism: Strep pyogenes
Infection of DERMIS

Clinical features:

  • 80% lower extremities, 20% face
  • distinctive type of superficial cellulitis confined to dermis with *prominent lymphatic involvement
  • infants, young children and older adults
  • painful, bright red, oedematous, indurated (beau d’orange) appearance
  • SHARPLY DEMARCATED from adjacent normal skin
  • usually with fever

Treatment:
- cloxacillin

18
Q

Cellulitis: organism, site of infection, distribution, clinical features, treatment

A

Organism: Strep pyogenes, s. aureus
Infection of SUBCUTIS

Clinical features:

  • diffuse area of erythema and oedema with ILL-DEFINED edges
  • no fluctuation or pus
  • generally no fever
  • accompanied by lymphadenitis and lymphangitis

Treatment:

  • elevation, immobilisation
  • antibiotic (cloxacillin and ampicillin)
  • no incision and drainage unless pus present (same applies for other superficial skin infections)
19
Q

Necrotising fasciitis: organisms, course, site of infection, clinical features, Fournier’s gangrene, diagnosis, treatment

A

Organism: Strep pyogenes (MC), s. aureus, vibrio vulnificans (seawater), anaerobes
(frequently mixed aerobic and anaerobic infections)

  • following cuts, insect bites or surgery

RAPID PROGRESSIVE necrosis of subcutaneous tissue and superficial fascia (along FASCIAL PLANE)

  • overlying skin –> cellulitis +/- blister or cyanosis –> necrosis due to destruction of blood supply (within hours)
  • VERY HIGH MORTALITY

Clinical features:

  • “cellulitis” with fever, pain and swelling –> must have NF as DDx!!!
  • pain out of proportion to signs (beyond area of erythema)
  • haemorrhagic bullae or blisters
  • Fournier’s gangrene = involving sputum and perineum

Diagnosis:
- exudate and tissue for culture of aerobes and anaerobes

Treatment:

  • AGGRESSIVE surgical debridement
  • antibiotics
20
Q

Skin ulcers: primary vs secondary, organisms, diabetic and decubitus ulcers (pathogenesis, organisms, treatment, prevention)

A

Primary (uncommon e.g. cutaneous anthrax)
Secondary (secondary infection of an ulcer e.g. decubitus/ diabetic)

Organisms: staph, strep, GN bacilli, anaerobes, yeast

Diabetic foot ulcer

  • DM with vascular disease, neuropathy, sugary blood nutritious for bacterial growth
  • mixed aerobes and anaerobes
  • complication –> osteomyelitis
  • treatment: incision and drainage, antibiotics
  • prevention: optimise blood glucose control, foot hygiene

Decubitus ulcer

  • prolonged pressure –> tissue ischemia –> necrosis –> ulcer
  • mixed aerobes and anaerobes
  • treatment: antibiotics, debridement
  • prevention: movement and turning
21
Q

Bites: onset, organism, diagnosis, treatment

A

Infection common, usually within 24 hrs

Organism: mixed aerobes and anaerobes (normal oral flora of inflicting animal)

  • dog: s. aureus, pasteurella canis
  • cat: pasteurella multocida, bartonella species
  • human: streptococci, s, aureus

Diagnosis:
- culture of wound/ discharge

Treatment:
- surgical debridement and antibiotics (cefuroxime and metronidazole)
- tetanus toxoid +/- tetanus IG
+/- rabies vaccination (animal bites)

22
Q

Burns: injury, risks for infection, organisms, treatment

A

Destruction of cutaneous mechanical barrier

  • Avascularity of burn wounds
  • plasma proteins leak –> decrease plasma Ig and albumin –> increase risk of local and distal infections
  • neutrophil dysfunction

Colonisation of non-viable tissues then viable tissues
- s. aureus, candida, p. aeruginosa, enterobacteriaceae, coag neg staph (any org is possible!)

Treatment:

  • debridement
  • topical/ systemic antibiotics
23
Q

Soft tissue infections: General diagnosis/ specimens and management

A

Diagnosis:

  • clinical features
  • culture and gram stain of:
  • – pus or needle aspirate
  • – biopsy from advancing edge (cellulitis)
  • – deep tissue culture/ ulcer base (superficial not useful)
  • – blood culture (systemic infection)

Management:

  • removal of infected material by drainage and debridement
  • antibiotics
  • local wound care
  • prevention
24
Q

Others (furuncle/ carbuncle, paronychia, gas gangrene)

A

Furuncle (boil) and carbuncle

  • s. aureus
  • furuncle = intracutaneous abscess in hair follicle –> carbuncle if subcutaneous extension (usually at neck or upper back)
  • drainage, cloxacillin for carbuncle

Paronychia

  • s. aureus
  • infection of subcutaneous tissue around nails (usually due to biting of hangnail)

Gas gangrene

  • c. perfringens
  • following major trauma/ bowel surgery/ amputation
  • tissue degrading enzymes e.g. lecithinase causes tissue necrosis and forms anaerobic environment
  • rapidly spreading gangrene of muscle, blackening tissue, foul smelling discharge, crepitus, toxaemia
  • treatment: wide excision/ amputation, antibiotics, hyperbaric oxygen

Micbi (32 decks)

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