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SKIN AND SOFT TISSUE INFECTIONS Cheat Sheet (DRAFT) by

Bacterial skin and soft tissue infections

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Skin and soft tissue infections

A group of conditions that affect the skin (epide­rmis, dermis, subcut­aneous tissue or superf­icial fascia)
2 types:
1. Uncomp­licated infections - mostly caused by gram positive pathogens (Staphy­loc­occus, Strept­ococcus) that infiltrate the skin after minor injuries (e.g. insect bites, scratches)
2. Compli­cated infections - have a higher tendency to be polymi­cro­bial.
SSTIs primarily present with painful, warm, erythe­matous (a superf­icial redness secondary to the dilation of capill­aries, may appear a deeper shade of purple or brown on darker skin) skin lesions and may lead to pus collection and/or necrosis (an irreve­rsible tissue injury that results in cellular death and inflam­mation) of the affected tissue.
Systemic symptoms like fever are a sign of a more severe infection.

Overview of skin and soft tissue infections

1. Impetigo
Highly contagious infection that affects the upper layers of the epidermis
Most common skin infection in children.
MOST COMMON PATHOGENS
a. Staphy­loc­occus aureus (bullous and non bullous)
b. Strept­ococcus pyogenes (non bullous)
TISSUE INVOLV­EMENT - Superf­icial epidermis
CLINICAL FEATURES
-highly contagious infection with honey-­yellow crust
-bullae formation
bulla- cutaneous fluid-­filled blister >1cm in diameter
2. Staphy­loc­occal scalded skin syndrome (gener­alized form of impetigo)
Caused by exotoxins of S.aureus following a local infection (e.g. pharyn­gitis, bullous impetigo).
MOST COMMON PATHOGENS - Exotoxin of S. aureus
TISSUE INVOLV­EMENT -Super­ficial epidermis
CLINICAL FEATURES
- erythe­matous rash
- fever
- heals completely
3. Nonpur­ulent SSTIs
A. Erysipelas
superf­icial skin infection of the upper dermis and superf­icial lympha­tics.
Most commonly affects the lower limbs
MOST COMMON PATHOGENS - Group A Strept­ococcus (S. pyogenes)
TISSUE INVOLV­EMENT - Superf­icial dermis, Lymphatic vessels
CLINICAL FEATURES
- Sharply demarcated erythe­matous skin lesion
B. Cellulitis
Rapidly spreading local infection of the deep dermis and subcut­aneous tissue.
Commonly appears in areas with broken skin (e.g. due to trauma or another infection.
MOST COMMON PATHOGENS
- Group A Strept­ococcus (nonpu­rulent)
S. aureus (purulent)
TISSUE INVOLV­EMENT - Deep dermis, Subcut­aneous tissue
CLINICAL FEATURES
- Rapidly spreading infection
- erythe­matous skin lesions with indistinct margins
- with or without pus
4. Purulent SSTIs
A. Skin abscess
An accumu­lation of white-­yellow pus predom­inantly consisting of proteins and neutro­phils in the dermis or subcut­aneous tissue.
MOST COMMON PATHOGENS - S. aureus (often MRSA)
TISSUE INVOLV­EMENT - deeper layers of skin
CLINICAL FEATURES
- walled-off infection with a collection of pus
B. Follic­ulitis, Furuncle, Carbuncle
Follic­ulitis- a localized inflam­mation of the hair follicle or sebaceous glands that is primarily caused by S.aureus. Charac­terized by erythema, tender papules, and/or pustules at the site of the hair follicles
Furuncle - a well-d­ema­rcated, firm, painful, purulent nodule that can develop after follic­ulitis. Extension into surrou­nding tissue can cause abscess and/or cellulitis
Carbuncle - a mix of multiple inflamed follicles (i.e., a mix of furuncles) with drainage of pus. Commonly caused by S. aureus and S. pyogenes
MOST COMMON PATHOGENS
- S. aureus
- Pseudo­monas aeruginosa
TISSUE INVOLV­EMENT - Hair follicle
CLINICAL FEATURES
- localized papules and pustules on the base of the hair follicle
- usually containing a central hair
5. Necrot­izing soft tissue infections
A group of life threat­ening infections charac­terized by soft tissue destru­ction and systemic toxicity.
Subtypes include: necrot­izing fasciitis, cellul­itis, myositis, and clostr­idial myonec­rosis or gas gangrene
MOST COMMON PATHOGENS
- MIXED INFECTION: Group A Strept­ococci + anaerobic bacteria
- Clostr­idium spp.: Clostr­idial myonec­rosis
TISSUE INVOLV­EMENT
- Necrot­izing cellul­itis: deep dermis and subcut­aneous tissue
- Necrot­izing fasciitis: superf­icial fascia
- Necrot­izing myositis: muscular tissue
CLINICAL FEATURES
- severe, rapidly progre­ssing infection with necrosis
- bullae and skin discol­oration
- high risk of systemic compli­cat­ions, high mortality
Tissue involv­ement of SSTI (from superf­icial to deep): impetigo (super­ficial epider­mis), erysipelas (super­ficial dermis and lymphatic vessels), cellulitis (deep dermis and subcut­aneous tissue), necrot­izing fasciitis (subcu­taneous tissue including superf­icial and deep fascia)

Risk factors for skin and soft tissue infections

LOCAL FACTORS
SYSTEMIC FACTORS
INCREASED EXPOSURE TO PATHOGENS
1. Chromic lymphedema (edema associated with lymphatic obstru­ction and reduced fluid clearance)
1. Diabetes mellitus
1. Prolonged hospit­ali­zation, surgery (nosoc­omial pathogens)
2. Local skin defects (e.g. tinea pedis)
2. Immuno­sup­pre­ssion (HIV, asplenia, chemot­herapy)
2. Water exposure (sea water, hot tubs)
3. Circul­atory disorders: arteri­ovenous insuff­ici­ency, chronic edema, stasis
3. CKD (leads to chronic edema)
3. Long term IV devices
4. Peripheral neuropathy
4. Obesity, poor nutrit­ional status
4. Trauma (open wounds, exposed fractures)
 
5. Drug or alcohol use disorder
 
6. Older and younger age

Compli­cations

1. Local spread of infection
2. Systemic involv­ement with fever and possible sepsis
3. Spread of infection to distant sites (Staph­ylo­coccal infect­ions)