Approach to skin and soft tissue infections: Clinical sciences

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Introduction0:00–0:42

Skin and soft tissue infections, or SSTIs for short, are infections that affect the skin and the underlying soft tissues like the fat, fascia, muscles, ligaments, and tendons.
They are typically caused by bacteria but can also occur from viruses or fungi. In severe cases, these infections can cause life-threatening complications, like necrotizing fasciitis, and toxic shock syndrome that can quickly lead to septic shock.
So, timely diagnosis is key to treat these infections promptly to prevent major complications. When a patient presents with a chief concern suggesting a skin and soft tissue infection, your first step is to perform an ABCDE assessment to determine if they are stable or unstable.

Unstable patient0:42–3:30

If the patient is unstable, stabilize their airway, breathing, and circulation right away. Establish IV access and start IV fluids for resuscitation.
You should also obtain blood and wound cultures, and then initiate broad-spectrum IV antibiotics. Make sure to continuously monitor vital signs like pulse oximetry, BP and heart rate, as the patient's condition can quickly deteriorate and require admission to the ICU.
Once you've initiated acute management, your next step is to obtain a focused history and physical examination. Additionally, order labs like CBC, CMP, lactate, and check the previously obtained blood and wound cultures.
Typically, unstable patients will have a history of fever and severe pain around the affected area. On exam, you might find signs of shock like altered mental status, tachycardia, or hypotension.
Local examination will reveal erythema, edema, and sometimes an associated rash on palpation. The affected area will be tender with crepitus in severe cases.
Crepitus feels like a crunching of small air pockets underneath the skin and is a red flag for necrotizing soft tissue infection.
In addition, you might see bully. Grayish or dark discoloration and purulent drainage, which is indicative of necrotic tissue.
Now, labs will typically reveal leukocytosis, electrolyte abnormalities, and elevated serum lactate. In most cases, blood and wound cultures will be positive.
If these are your findings, consider a life threatening SSTI. Now, if your patient has been stabilized and you are concerned for a deeper infection, your next step is to obtain a CT scan of the affected area.
CT might reveal fat stranding, inflammatory changes, or gas bubbles within the deep tissue layers. These characteristics strongly support your diagnosis of life-threatening SSTI, such as necrotizing fasciitis, toxic shock syndrome, and gas gangrene.
Here's a clinical pearl. Crepitus and pockets of gas along the fascia are strongly diagnostic of necrotizing fasciitis, which is a surgical emergency.
Patients need to be taken to the operating room emergently for debridement of the necrotic tissue and washout. Any delay in treatment can quickly lead to death.

Stable patient3:30–4:22

Now that unstable patients are taken care of, let's switch gears and talk about the stable ones. For stable patients, your first step is to obtain a focused history and physical exam.
Here, your goal is to differentiate between a purulent lesion and a non-purulent lesion. Let's begin with purulent lesions.
On history, your patient will typically report fever, malaise, and a lump with drainage, and they may have a history of diabetes mellitus.
Keep in mind that patients with a history of diabetes with suboptimal glucose control are at high risk for developing this kind of infection.
Expect your exam to reveal erythema, edema, and tenderness around a fluctuating mass containing pus. With these findings, consider a purulent lesion.
OK, let's begin with the most common type of purulent lesion, a subcutaneous abscess, which is a walled-off collection of pus underneath the skin.

Abscess4:22–4:58

These occur when bacteria seeds within the soft tissue through a cut in the skin. Patients typically report focal swelling with pain, and they might notice murky, malodorous drainage.
On exam. You can expect to see erythema, tenderness, and induration around a fluctuent mass with purulent drainage.
With these findings, you're dealing with a subcutaneous abscess. All right, let's talk about another type of pearlent SSTI called hydradenitis suppurativa.

Hidradenitis suppurativa4:58–5:42

This is a chronic condition where painful, inflamed nodules or abscesses form in the subcutaneous tissues of the hairy areas or skin folds, where there is constant rubbing.
Patients usually report painful lumps around the axilla, groin, and inframammary area. Your patient might have risk factors like cigarette smoking or obesity.
Exam typically shows multiple tender erythematous nodules, sometimes with visible sinus tracts and drainage. These findings support your diagnosis of hydradenitis suppurativa.

Folliculitis, furuncles, and carbuncles5:42–6:32

Moving on to our last type of purulent lesion, let's discuss folliculitis, furuncles, and carbuncles. Folliculitis is an infection of the hair follicle.
Furuncles are a collection of folliculitis, and carbuncles are a cluster of furuncles. Patients who frequently remove hair through shaving or waxing are at an increased risk, as well as those with diabetes and poor hygiene.
Patients often report localized pain around the affected area, and sometimes even fever and malaise. On exam you will likely find erythema, edema, and tenderness around hair follicles, and you may observe a single or cluster of boils.

Non-purulent infections6:32–7:07

If you see these findings, diagnose folliculitis, furuncles, or carbuncles. All right, now that we went over the purulent lesions, let's go back and talk about the non-purulent ones.
Non-purulent SSTIs generally referred to infections without a collection of puss. Although history entails non-specific symptoms like fever, malaise, and localized pain.
The physical exam will likely show an erythematous rash with localized tenderness but without drainage or fluctuent mass.

Cellulitis7:07–7:57

These signs should lead you to consider nonpurulent lesions. Let's dive in and talk about different types of non-purulent SSTIs starting with cellulitis, which is most commonly caused by streptococcus species.
Patients with cellulitis usually report a history of a minor cut or other skin wound and possibly systemic symptoms like fever and chills.
Exam often reveals blanching erythema with associated edema and warmth beyond the area of the injury. In some cases, you might even feel enlarged and tender local lymph nodes.
If these are your findings, you're dealing with cellulitis. Here's a clinical pearl.

Erysipelas7:57–8:26

Make sure to mark the edges of the erythema with a pen to track the progression of the infection. Regression of the rash and reduction of erythema with antibiotic treatment indicates improvement.
Another type of nonpurulent SSTI is erysyphilis, which involves the upper layers of the dermis and is most commonly caused by Group A streptococcus.
Patients typically report skin redness with mild swelling, but no systemic symptoms. On exam, you can expect to see a rash with sharply demarcated margins, often raised with warmth, tenderness, and induration.

Impetigo8:26–9:05

These characteristics strongly support your diagnosis of erysyphilis. Next, let's talk about Impetigo.
A highly contagious infection common in children, it's usually caused by Staphylococcus aureus or Group A streptococcus.
History is often non-specific, with fever, malaise, or mild pain or possibly itching around the affected area. Physical exam typically shows a classic rash characterized by erythematous maculopapapular lesions that progress to vesicles or bullae, which easily rupture with ooze, forming a honey-colored crust.

Tick-related infection/Lyme disease9:05–10:26

If these are your findings, your diagnosis is impetigo. Lastly, let's discuss a more rare type of SSTI associated with Lyme disease.
History often reveals tick bites or exposure to an endemic area like the eastern coast of the US. Patients also report flu-like symptoms like fever, headache, fatigue, malaise, or myalgias, as well as a non-painful rash at the site of a tick bite.
On exam, you will likely see the characteristic erythema migrans, which is a red circular rash with a central clearing resembling a bull's eye.
Now, erythema migrans is caused by the bacteria Borrelia bergdorferi, which is transmitted through deer ticks. So these findings should lead you to consider a tick-related infection.
Keep in mind the rash is self-limiting, but a small percentage of patients may develop longer term complications like migratory arthritis, neurological and cardiac symptoms.
To confirm the diagnosis, first order an enzyme-linked immunosorbent assay, also called EISA, which detects Borrelia antibodies.

Herpes zoster infection10:26–11:30

You'll also need to order a Western blot for confirmation. If the immunological tests are positive for serum Borrelia antibodies, diagnose Lyme disease.
Now, let's move on and talk about non-bacterial SSTI's, like viral infections, such as herpes, zoster, or shingles. This is caused by the varicella zoster virus, which is the same virus that causes chickenpox.
So, history almost always reveals an episode of chickenpox, often many years ago. Additionally, patients typically report a painful, itchy, or tingly rash, sometimes with a prodrome of malaise, headache, fatigue, and low grade fever.
On exam, you can expect to see an eruption of a tender maculopapular vesicular rash along a dermatome that does not cross the midline.
With these clinical features, consider a herpes zoster infection. Next, order a zinc smear and a PCR.

Fungal infection11:30–12:22

If the smear comes back positive for multinucleated giant cells and a positive PCR for HZV, your diagnosis of a herpes zoster infection is confirmed.
Finally, let's talk about fungal infections like tinapetis, corporis, or cruris. These typically occur in patients with diabetes or those who are immunocompromised.
On exam, you might see skin maceration, erythematous plaques, or pruritis. Which should raise your suspicion of a fungal dermatophyte infection.
To confirm the diagnosis, you can perform a KOH preparation test, wood light test, or possibly order a fungal culture. The KOH test will reveal branching hyphae, and the wood light test will show blue-green fluorescence.

Review 12:22–13:08

Additionally, the fungal culture will show positive growth, which can also specify the type of fungus. These findings will confirm your diagnosis.
All right, as a quick recap, skin and soft tissue infections or SSTIs include bacterial, viral, or fungal infections that can be purulent or non-purulent.
Purulent SSTIs include abscesses, hidradenitis suppurativa, folliculitis, furuncles, and carbuncles. Non-virulent infections include cellulitis, erysyphilis, impetigo, and tick-related infections, as well as viral and fungal infections.