Furuncles and Carbuncles

Introduction
Folliculitis may progress to form subcutaneous inflammatory abscesses known as furuncles, or boils, which usually drain and resolve spontaneously; however, they may coalesce to form more extensive
collections involving multiple hair follicles called carbuncles.
Clinical Features
 A furuncle presents as an erythematous, painful, and firm nodule in hair bearing skin, especially those areas exposed to friction or minor trauma. The incidence tends to increase after puberty, with  S. aureus  being the most common causative agent. The lesion may progress into a fluctuant mass that will eventually rupture into the skins surface. This drainage of the purulent content diminishes the pain. If multiple or recurrent furuncles (furunculosis) are present, one should suspect chronic  S.aureus colonization. Constitutional symptoms in furunculosis are rare, in contrast to carbuncles.  Carbuncles present clinically as tender, erythematous, edematous, and multiple draining sinus tracts. They extend deep into the subcutaneous tissue. These lesions occur most
often in areas where the dermis is thick such as the nape of the neck, lateral thighs, and back. Malaise, chills, and fevers are usually present. Severe infections can result in extensive scarring and are more likely to develop complications such as cellulitis or septicemia.
Etiology
 As mentioned above, conditions compromising the integrity of the skin are portals for the entry of  S. aureus  thus predispose to the formation of furuncles and subsequently carbuncles. These are most commonly associated with systemic conditions such as diabetes mellitus, eczema, obesity,
alcoholism, malnutrition, and immunodeficiency states (Hyper IgE syndrome). Nonetheless, healthy individuals with no risk factors can also develop these infections.
Diagnosis
 Cultures of pus isolates, gram stains, and antibiotic sensitivities all support the clinical diagnosis and aid in management.  They are generally obtained in cases of recurrent abscesses, therapy response failure, systemic toxicity, immunocompromised patients, gas-containing abscesses, and involvement
of the face, muscle, or fascia.
Pathology
 The furuncle is a pyogenic infection with its origin at a hair  follicle extending into the deep dermis and possibly to sub-cutaneous tissue. The carbuncle is visualized as a furuncle with additional loculated abscesses.
Differential Diagnosis
 Among the differential diagnosis of furuncles and carbuncles the most common are hidradenitis suppurativa, ruptured epidermoid or pilar cysts and soft tissue infections.
Complications 
 Most cases resolve after treatment, but some cases are complicated by seeding to the bones, heart valves, or other organs as a result of bacteremia.