SSTI

Question 1

Epidemiology & Predisposing RF Cellulitis

Answer 1

Middle-aged and older individuals

Skin barrier disruption 2/2 trauma, skin inflammation (eczema, radiation), edema (impaired lymphatic drainage - surgery, venous insufficiency), obesity, immunosuppression, pre-existing skin infection, MRSA exposure

Question 2

MC Etiology Non-purulent Cellulitis

Answer 2

Beta-hemolytic strep - group A (S. pyogenes)

Purulent cellulitis = MRSA

Question 3

Epidemiology Erysipelas

Answer 3

Young children and older adults

Question 4

MC Etiology Erysipelas

Answer 4

Beta-hemolytic strep

Question 5

MCC Skin abscess

Answer 5

S. aureus (MSSA or MRSA) - 75%

Question 6

MRSA risk factors

Answer 6

Healthcare: Recent hospitalization, residence in long term care, recent surgery, hemodialysis

RF : HIV infection, IVDA, prior abx use, incarceration, military service, sharing sports equipment, sharing needles, razors, other sharp objects

Question 7

CP Cellulitis

Answer 7

Skin erythema, edema, warmth that develops 2/2 bacterial entry into breaches in skin barrier +/- fever (more local sx than erysipelas)

Almost ALWAYS unilateral
MC site = lower extremities

+/- regional LAD & lymphangitis
+/- dimpling of skin 2/2 edema = “peau d’orange”

Severe infection w/ severe systemic sx = investigation for additional underlying sources of infection - streptococcal toxic shock syndrome

Question 8

Causes of recurrent cellulitis

Answer 8

Inter-digit toe spaces w/ fissuring or maceration, tinea pedis, or chronic venous insufficiency for any reason

Question 9

Cellulitis involves which skin layers?

Answer 9

Deeper dermis & SC fat

Question 10

Erysipelas involves which layers of skin?

Answer 10

Upper dermis & superficial lymphatics

Question 11

CP Erysipelas

Answer 11

Acute onset sx w/ systemic manifestations - fever, chills, severe malaise, headache (sys sx can precede local inflammatory si/sx by hours)

CLEAR demarcation btwn involved & uninvolved tissue. Classic “butterfly” involvement of face. Involvement of ear ‘ classic “Milian’s ear sign” (ear has no deeper dermis or SC fat)

Question 12

Cellulitis w/ gangrenous appearance + crepitant is more likely 2/2

Answer 12

Clostridia & other anaerobes

Question 13

Skin abscesses involve which layers of skin?

Clinical presentation skin abscess

Answer 13

Collection of pus within dermis or SC space with or without surrounding cellulitis

Abscess can be 2/2 deep infection of hair follicle (furuncle/boil) which can coalesce into carbuncle - common areas = back of neck, face, axillae, buttocks

Fever, chills, and systemic toxicity are unusual - more likely if large carbuncle

Question 14

Complications of cellulitis & abscess

Answer 14

Bacteremia, endocarditis, osteomyelitis, sepsis, toxic shock syndrome

Question 15

Diagnosis of erysipelas, cellulitis, skin abscess

Answer 15

Clinical diagnosis based on clinical manifestations -

Cellulitis & Erysipelas: Erythema, edema, and warmth.

Erysipelas - clearly demarcated & SYSTEMIC SX
Cellulitis - no clear borders, systemic sx more rare (except possible fever)

Skin abscess - painful, fluctuant, erythematous nodule w/wo surrounding cellulitis

Question 16

Treatment abscess

Answer 16

Incision and drainage w/ culture and susceptibility testing

Blood cultures (BEFORE ABX) NOT necessary UNLESS:

• Systemic toxicity
• Extensive SSTI
• Persistent cellulitis
• Special exposures (animal bite, water-associated injury)
• Underlying co-morbidities (lymphedema, malignancy, neutropenia, immunodeficiency, splenectomy, DM)

Question 17

When to order imaging on abscess and what kind of imaging to order?

Answer 17

Reason: To determine if abscess if present - order ULTRASOUND

Question 18

What type of imaging to order to determine cellulitis vs osteomyelitis?

Answer 18

MRI

Question 19

Imaging cannot distinguish between cellulitis and _____ and imaging should not delay ______ intervention if there is clinical suspicion for the latter

Answer 19

Imaging cannot distinguish between cellulitis and NECROTIZING FASCIITIS/GANGRENE and should not delay SURGICAL intervention (debridement) if there is clinical suspicion for the latter

Question 20

Differential diagnosis cellulitis - three can’t miss dx

Answer 20

Cellulitis is often confused w/ other infectious or non-infectious illnesses -

Rapidly progressing signs of systemic toxicity - should prompt consideration of severe infection, including:

1. Necrotizing fasciitis -
   Deep infection = progressive destruction of muscle fascia - erythema, swollen, warm - key = PAIN OUT OF PROPORTION TO EXAM FINDINGS - dx established w/ visualization of fascial planes (surgical)
2. Toxic shock syndrome -
   Pain precedes physical findings. Si/sx regular SSTI…but THEN ecchymosis & sloughing of skin + fever. Go from normotensive to hypotensive.
3. Gas gangrene -
   Fever & severe pain in extremity AFTER recent surgery/trauma. + crepitus = clostridial infection - can be detected radiographically.

Question 21

Cellulitis differential diagnosis

Answer 21

Can’t miss: necrotizing fasciitis, TSS, gas gangrene

Other infectious dx:
Erythhema migrans, herpes zoster, septic arthritis, septic bursitis, osteomyelitis, mycotic aneurysm

Other non-infectious dx:
DVT, contact dermatitis, acute gout, drug reaction, vasculitis, insect bite, vaccination site reaction

Question 22

How to tell cellulitis/erysipela vs erythema migrans of Lyme disease

Answer 22

The rash of Lyme disease is not painful, has slow progression, and is associated with less marked fever than erysipelas.

A targetoid lesion with central clearing is sometimes present.

There may be a history of recent travel to endemic areas and a recent tick bite. Lyme exposure tends to occur in the spring or summer

Question 23

CP Cellulitis overlying a septic joint

Answer 23

Septic arthritis - cellulitis can overly a septic joint - manifests as joint pain, swelling, warmth and LIMITED ROM of joint - dx based on synovial fluid examination

Question 24

How to tell cellulitis vs osteomyelitis

Answer 24

Osteomyelitis may underlie an area of cellulitis

Cellulitis that’s not getting better w/ appropriate abx therapy - investigate further & get imaging to r/o osteo - MRI or plain film

Question 25

Cellulitis vs herpes zoster d/dx

Answer 25

Although less common, cellulitis can have vesicles & bullae

Herpes is along ONE dermatome. Erythematous papules that evolved into grouped vesicles. Dx via PCR.

Question 26

Irritant contact dermatitis vs cellulitis

Answer 26

Irritant contact dermatitis = PRURITIC & reaction is limited to site of contact & a/w burning, stinging

Question 27

Cellulitis vs acute gout

Answer 27

Acute gouty arthritis consists of severe pain, warmth, erythema, and swelling overlying a SINGLE joint, prior attacks etc

Dx: Synovial fluid analysis - Sodium urate crystals of gout (needle shaped, strong negative bifringence) or pseudogout (CPPD rhomboid, positive bifrinegence)

Question 28

Skin lesions that need to be distinguished from skin abscess

Answer 28

Epidermoid cyst
Folliculitis
Hidradenitis supprativa
Nodular lymphangitis