Infections of the skin and soft tissue commonly follow traumatic injury to the epithelium but occasionally may be blood-borne. Interruption of the integrity of the skin allows ingress of microorganisms to produce superficial, localized infections which on occasion may become more deep-seated and spread rapidly through tissues. Skin trauma complicates surgical incisions and accidents, including burns.
Infections of the skin and soft tissue commonly
follow traumatic injury to the epithelium but occasionally may be
blood-borne. Interruption of the integrity of the skin allows ingress of microorganisms to produce
superficial, localized infections which on occasion
may become more deep-seated and spread
rapidly through tissues.
Skin trauma complicates surgical incisions and accidents, including burns. Similarly,
prolonged immobilization can result in pressure damage to skin from impaired blood flow. It is most
commonly seen in patients who are
unconscious.
Microbes responsible for
skin infection often arise from the normal skin flora,
which includes Staph. aureus. In addition Strep. pyogenes, Ps. aeruginosa and anaerobic bacteria are other recognized pathogens.
Viruses also affect the skin and mucosal
surfaces, either as a result
of generalized infection or localized disease
as in the case of herpes simplex. The latter is amenable to antiviral therapy in selected patients, although
for the majority
of patients, virus infections of the skin are self-limiting.
Strep. pyogenes is responsible for a range of skin infections: impetigo is a superficial infection of the epidermis
which is common in childhood and is highly
contagious; cellulitis is a more deep-seated infection which spreads rapidly through the tissues to involve
the lymphatics and occasionally the bloodstream; erysipelas is a
rapidly spreading cellulitis commonly involving the face,
which characteristically has a raised leading
edge due to lymphatic involvement. Necrotizing fasciitis is a more serious, rapidly progressive infection of the skin
and subcutaneous structures including the fascia
and musculature. Despite early diagnosis and high-dose intravenous
antibiotics, this
condition is often
life-threatening and may require extensive surgical
debridement of devitalized tissue and even limb amputation to ensure survival. A fatal outcome is usually the result of profound toxaemia and bloodstream spread. Penicillin is the drug of choice
for all these infections, usually in combination with other agents such as an aminoglycoside and metronidazole in the case of necrotizing fasciitis; in severe
instances parenteral
administration is appropriate. The use of topical agents, such as tetracycline, to treat impetigo
may fail as drug resistance is now recognized.
Staph. aureus is responsible for a variety
of skin infections which require therapeutic
approaches different from those of streptococcal infections. Staphylococcal cellulitis is indistinguishable clinically from streptococcal cellulitis and responds to flucloxacillin, but generally fails to respond to penicillin
owing to penicillinase (β- lactamase) production. In hospital-acquired infection, and occasionally in community practice, MRSA must
be considered as a possibility, particularly where the patient
is
known to be colonized. Staph. aureus
is an important cause of superficial, localized skin sepsis which
varies from small pustules
to boils and occasionally to a more deeply invasive, suppurative skin abscess known
as a carbuncle. Antibiotics are generally not indicated for
these conditions. Pustules and boils settle with antiseptic soaps or
creams and often discharge spontaneously, whereas carbuncles frequently require surgical drainage. Staph. aureus may also cause postoperative wound infections, sometimes associated with retained
suture material, and settles once
the stitch is removed. Antibiotics are only appropriate in this situation if there is extensive accompanying soft tissue invasion. Rarely, strains of Staph. aureus may express a toxin
complex known as Panton-Valentine Leukocidin (PVL); these strains can cause
severe sepsis and an often
fatal necrotizing pneumonia in young, otherwise fit, patients. The
treatment for such infections usually
aims to minimize toxin production using protein synthesis inhibitors such as clindamycin plus rifampicin, in combination with linezolid.
Anaerobic bacteria are
characteristically associated with foul-smelling wounds.
They are found in association with surgical incisions
following intra-abdominal procedures and pressure
sores, which are usually located
over the buttocks
and hips where
they become infected
with faecal flora. These infections are frequently mixed and
include Gram-negative enteric
bacilli, which may mask
the presence
of underlying anaerobic
bacteria. The principles of treating
anaerobic soft tissue
infection again emphasize the need for removal of all foreign
and devitalized material. Antibiotics such as metronidazole or clindamycin should be considered where
tissue invasion has occurred.
The treatment of
infected burn wounds presents a number of peculiar facets. Burns are initially
sterile, especially
when they involve all layers of the skin. However, they rapidly become colonized with bacteria
whose growth is supported by the protein-rich exudate. Staphylococci, Strep. pyogenes and, particularly, Ps. aeruginosa frequently colonize burns and may jeopardize survival of skin grafts and occasionally, and more seriously, result in bloodstream invasion. Treatment of invasive Ps.
aeruginosa infections requires
combined therapy with an
aminoglycoside, such as gentamicin or tobramycin, and an anti-pseudomonal agent, such as ceftazidime or piperacillin. This produces high therapeutic
concentrations which generally act in a synergistic manner.
The use of aminoglycosides in patients with serious burns
requires careful monitoring of serum concentrations to ensure that they are therapeutic yet non-toxic, as renal function is often impaired in the days
immediately following a serious burn.
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