Pyogenic infections are those which cause the formation of pus. Many bacteria are pyogenic, including staphylococcus aureus, streptococci, e. coli, etc.

It’s difficult to know what they expect with this topic, but I’ll go with what Lee assumed should be included. Cellulitis is a disorder of infectology. Necrotising infections and abscesses belong to surgery.

Abscess

An abscess is an enclosed collection of pus within tissue, usually caused by bacterial infection. They may develop from local invasion from nearby infections, from haematogenous spreading, or from direct entry from outside (from trauma). Abscesses are more common in immunocompromised, diabetics, and IV drug users. There are many types of abscesses, as they can occur almost anywhere:

Skin abscess and perianal abscess
Intraabdominal abscess
Brain abscess
Lung abscess
Peritonsillar abscess
Liver abscess
Psoas abscess (in the iliopsoas muscle compartment)
Periappendiceal abscess
Etc.

Clinical features and diagnosis

Abscesses may cause symptoms of infection, like fever, malaise, and chills, as well as local pain. Labs may show elevated inflammatory parametres. For internal abscesses, CT is important for diagnosis and to guide drainage. Skin abscesses are diagnosed clinically.

Treatment

Most abscesses require both antibiotics and drainage to resolve. Drainage may be transcutaneous or surgical. Often, a drain is left in the abscess for a few days to allow for complete drainage. In most cases, the pus should be cultured to guide antibiotic therapy. One notable exception is lung abscess however, which often resolves with antibiotics alone.

Necrotising soft tissue infection

Necrotising soft tissue infections (NSTI) are necrotising bacterial infections of the soft tissues, which are life-threatening and must be treated surgically immediately. Depending on the affected soft tissue, this may be necrotising fasciitis, necrotising myositis, or necrotising cellulitis. It can progress rapidly and so early recognition is important. Treatment involves early broad-spectrum antibiotics and surgical debridement. Despite optimal therapy, the mortality is high.

Etiology

Necrotising fasciitis may be polymicrobial (type I necrotising fasciitis), or monomicrobial (type II). Monomicrobial necrotising fasciitis is usually caused by group A streptococci (GAS).

Type III necrotising fasciitis is also called gas gangrene or clostridial myonecrosis. It’s caused by clostridium perfringens.

There is usually a history of trauma to the overlying skin, which provides an entry point for bacteria. Immunosuppression, including diabetes, further increases the risk.

Clinical features

The infection usually progresses over hours or days. The clinical features include erythema, oedema, severe pain, and fever. The patient may be septic. In some cases, the infection is caused by gas-producing bacteria, in which case crepitus may be a finding. Crepitation is highly specific for NSTI but not always present. In the later stages, necrosis will occur.

When the perineum is affected, the condition is known as Fournier gangrene.

Diagnosis and evaluation

The diagnosis is clinical, and must be made urgently to prevent delay in management. Labs show elevated inflammatory parametres. Elevated AST or CK may be a sign of muscle or fascia affection. Blood cultures should be obtained.

Imaging may help in the diagnosis, but if the diagnosis can be made clinically, imaging should not be performed as it delays the treatment. CT is the most helpful in showing soft tissue necrosis.

Management

Surgical debridement is essential in the management; antibiotic treatment without surgery results in a 100% mortality rate. However, even with optimal management, the mortality rate may be 20% or higher. The goal of surgery is to remove all necrotic tissue, often requiring limb amputation. Inspection and debridement (if necrotic tissue is still present) should be repeated daily until no necrotic tissue is present.

Antibiotic therapy must be broad-spectrum and cover gram-positives, gram-negatives, and anaerobes. Several antibiotics are needed, one of which should be clindamycin, for its anti-toxin properties.

In case of confirmed or suspected streptococcal infection, administration of IVIG is indicated as it reduces mortality.