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Skin and soft tissue infections: Clinical practice

Skin and soft tissue infections: Clinical practice


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USMLE® Step 2 style questions USMLE

31 questions

A 10-year-old boy comes to the emergency department with progressive right periorbital swelling for the past two days. He also complains of fever, nasal congestion, and blurry vision. On physical examination, his right eye is swollen closed. He also has proptosis, conjunctival injection, and is unable to abduct his right eye. The left eye is normal. Which of the following is the most likely diagnosis in this patient?


Content Reviewers:

Rishi Desai, MD, MPH

Focal skin and soft tissue infections are often due to bacteria infections, and include conditions like impetigo, folliculitis, cellulitis, erysipelas, furuncles and carbuncles, and necrotizing fasciitis.

The first step is getting a full history.

In impetigo, there’s usually no pain, whereas in necrotizing fasciitis the pain is severe.

Most skin infections tend to be localized and around a particular anatomic structure.

For example, folliculitis, furuncles, and carbuncles involve the hair follicle.

Also, a superficial infection like impetigo or folliculitis causes itchiness.

In erysipelas, the fever is high and abrupt in onset, and in necrotizing fasciitis the fever usually persists.

Some individuals have had contact with other individuals with skin infections, and this is commonly the case with community-acquired methicillin-resistant S. aureus, which causes cellulitis and furuncles.

On physical exam, there may be signs of systemic illness like fever and chills, and signs of toxicity like lethargy, tachycardia, and hypotension.

Additionally, there may be adenopathy, which can occur in non-bullous impetigo and cellulitis, bullae which can be seen in bullous impetigo, and crepitus with edema that exceeds the rash border, which can be seen in necrotizing fasciitis.

The rash may be papular as in folliculitis, vesicular or pustular like in impetigo, or there may be macular erythema like in cellulitis.

Additional labwork should be done when there are signs and symptoms of systemic toxicity.

A Gram’s stain, skin and swab cultures can help identify a specific pathogen like community-acquired methicillin-resistant S. aureus.

Blood cultures are unlikely to be positive in simple localized infections like impetigo and folliculitis, but should be taken when there’s deep tissue involvement like necrotizing fasciitis or erysipelas.

Other tests include a CBC, C-reactive protein level, and liver and kidney function tests.

An x-ray can be done when there’s a deep infection and bone involvement is suspected.

Ultrasound, on the other hand, may be used to look for evidence of an abscess below an area of cellulitis or to assess crepitus when necrotizing fasciitis is suspected.

In necrotizing fasciitis, a CT scan is often done as well, although diagnosis is always made through exploratory surgery.

Head CT scans are specifically helpful in orbital cellulitis, which can cause neurological deficits, proptosis or protrusion of the eyeball, deteriorating vision, bilateral ocular edema or ophthalmoplegia with diplopia.

Finally a skin biopsy isn’t routine but can be done when other tests are inconclusive, often to look for non-infectious causes.

Impetigo can be classified as non-bullous impetigo, which is usually caused by S. aureus or S. pyogenes, and bullous impetigo, which is caused by toxin-producing S aureus.

In non-bullous impetigo, the lesions develop on previously damaged skin by things like insect bites or abrasions, and they start as vesicles or pustules which evolve over about a week into gold-crusted plaques, that are often about 2 centimeters in diameter.

Some are pruritic and there may be regional adenopathy.

The lesions usually affect the face and extremities and heal without scarring.

Bullous impetigo is characterized by flaccid, fluid-filled vesicles and blisters or bullae.

These are painful, spread rapidly, and there are usually systemic symptoms like fever, chills, and malaise.

There are usually multiple lesions, particularly around the nose and mouth, buttocks and trunk, and in body folds.

The bullae rupture easily, leaving a rim of dry skin that surrounds a shallow wet erosion.

Unlike non-bullous impetigo, the lesions develop on intact skin and there’s usually no surrounding erythema and regional adenopathy.

A Gram stain and culture of pus or exudate should be obtained in case it’s caused by methicillin-resistant S aureus infection.

Empiric topical treatment is usually given and common choices include mupirocin, applied three times daily and retapamulin, applied twice daily, both for five days.

Oral therapy is given to those with numerous lesions. Usually a 7 day course of cephalexin or dicloxacillin is used.

Second is folliculitis which is an infection of the hair follicle, and it’s most often caused by S. aureus.

Superficial folliculitis is when there’s a tender or painless pustule with the hair shaft in its center.

Usually multiple lesions develop anywhere there’s hair, like the scalp, arm pit, or groin area.

Other symptoms include pruritus or tenderness, and the pustules tend to heal without any scarring or follicle loss.

A gram stain and culture should be done to identify the causative organism.

Treatment is not always necessary because mild folliculitis with few pustules often resolves spontaneously.

If there are numerous papules or pustules involving a large body area then topical antibiotic therapy like clindamycin 1% lotion or solution can be used twice a day.

Severe cases may require a systemic antibiotic like dicloxacillin or cephalexin for a week, and chronic recurrent folliculitis, may require daily application of a benzoyl peroxide 5% gel or wash.

Third is cellulitis which is an infection of the connective tissue, and it’s usually caused by S. pyogenes and S. aureus.

In cellulitis the margins are hardly noticeable because the process is deep underneath the skin.

Adenopathy and symptoms like fever and chills are common.

Although cellulitis occurs mostly on the extremities, it’s particularly worrisome when it affects the eyes.

Orbital cellulitis involves the contents of the orbit, mostly the fat and ocular muscles, and it causes pain with eye movements, proptosis, and in some cases ophthalmoplegia with diplopia.

Preseptal or periorbital cellulitis affects the anterior portion of the eyelid without involving the eye, and it tends to be milder.

Generally speaking, cellulitis can also be worrisome if it extends into deeper structures - it can lead to thrombophlebitis, osteomyelitis, and septic arthritis - as well as the development of an abscess.

Additional tests are recommended when the individual is febrile and appears toxic.