Page 960 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 74: Soft Tissue Infections 691

to facilitate objective daily assessments of the extent of spread. Frequent subcutaneous infection that surface to produce a gangrenous ulcer on
inspection of the involved area is necessary to detect the development the skin. Pathologically, the process is usually limited to the upper third
of any areas of crepitus or suppuration, which may require surgical of the subcutaneous fat, but occasionally it extends down to fascia. The
drainage. Abscesses of the subcutaneous tissue are not infrequent after lesion was originally thought to be caused by a synergistic interaction
extensive cellulitis; judicious use of repeated needle aspiration may be between microaerophilic streptococci and S aureus, but other micro-
necessary. Failure to achieve defervescence and a decrease in systemic organisms, including Proteus species and other gram-negative enteric
toxicity within 48 to 72 hours after institution of appropriate antimicro- bacilli, have been implicated.
bial therapy should arouse suspicion of suppuration or a more virulent
soft tissue infection, such as necrotizing fasciitis or myonecrosis. Management: Drainage from the wound or site of local injury should
be sent for immediate Gram stain and culture. A simple method for
■ ANAEROBIC CELLULITIS obtaining anaerobic specimens for culture is to use a needle and syringe
Pathogenesis: The term anaerobic cellulitis is not properly descrip- to aseptically aspirate the crepitant area at a site removed from the
wound. All air should be carefully expressed from the syringe. If a swab
tive, and many terms are used for this process including gas abscess, is used, contact with normal flora should be avoided, and a commercial
gangrenous cellulitis, localized gas gangrene, necrotizing cellulitis, and anaerobic transport medium should be used. Blood cultures should
epifascial gangrene. The process usually represents infection of already also be obtained. Imaging studies with ultrasonography, computed
devitalized subcutaneous tissue without involvement of the deep tomography, and magnetic resonance imaging can be very valuable to
fascia or underlying muscle. Microorganisms are introduced into the assess the presence and extent of the soft tissue involvement. 23,24
subcutaneous tissues from an operative or traumatic wound or from Initial antimicrobial selection is guided by the Gram stain of the
a preexisting local infection. The subcutaneous tissues are devital- purulent drainage. If only large “boxcar-shaped” gram-positive bacilli
ized owing to a local injury, an inadequately debrided wound, or a are present, the causative microorganism is Clostridium, and moderate
metabolic disturbance that compromises vascular supply (eg, diabetes to large doses of parenteral penicillin G (10-20 million U/day in six to
mellitus). Usually, the infectious process is not invasive but instead eight divided doses) are indicated. If multiple organisms of different
remains localized in the area of devitalized tissue. 21,22 Extensive gas morphologies are present on the Gram stain, then one may assume that
formation and suppuration, usually limited to the area of devitalized the process is polymicrobial, and an empirical broad-spectrum antimi-
tissue, are present. crobial regimen should be instituted. An aminoglycoside (gentamicin or
Etiology: Anaerobic cellulitis may be clostridial or nonclostridial. tobramycin, 3-5 mg/kg per day in three divided doses or, alternatively,
Clostridium perfringens is the most commonly isolated clostridial 5-7 mg/kg as a single daily dose) and clindamycin (1200-2400 mg/day in
species, followed by Clostridium septicum. Gram-negative rods, three or four divided doses), with or without penicillin G (10-20 million
staphylococci, or streptococci are occasionally present but are not the U/day in six to eight divided doses), would be appropriate. In patients
predominant isolates. The nonclostridial form of anaerobic cellulitis with impaired or changing renal function, a third-generation cephalo-
is essentially the same process as clostridial cellulitis, but has a differ- sporin, such as cefotaxime, ceftriaxone, or ceftazidime, or a fluoroqui-
ent microbiologic etiology. Obligate anaerobes are the predominant nolone can be used in place of the aminoglycoside for gram-negative
isolates, with Bacteroides fragilis, other Bacteroides species, Prevotella coverage. Alternately, a carbapenem such as imipenem, meropenem, or
species, Porphyromonas species, Peptostreptococcus and Peptococcus doripenem may be used as a single agent.
species encountered most frequently. Other bacteria that may be The major conditions to be differentiated from anaerobic cellulitis are
5,25
present include the gram-negative enteric bacilli (Escherichia coli and necrotizing fasciitis and the myonecrotic syndromes. Distinguishing
Klebsiella species), staphylococci, and streptococci. between clostridial myonecrosis and anaerobic cellulitis is necessary
to avoid unnecessary extensive debridement and imaging studies may
Presentation: The clinical pictures of clostridial and nonclostridial be very valuable in this regard. 23,24 The distinction is made definitively
anaerobic cellulitis are very similar and may be discussed together. at the time of surgery, however, which is mandatory to establish the
Because this infection represents the local invasion of already devital- diagnosis. The involved soft tissue must be laid open widely; devitalized
ized tissue, the process does not generally have a virulent progressive tissue must be debrided; suppurative foci should be drained; and all
course. The onset is gradual, with mild to moderate local pain and involved fascial planes should be opened. The deep fascia and muscle
only mild to moderate tissue swelling. Constitutional symptoms are must be carefully examined; if they are healthy, no further surgery is
not prominent; the relative paucity of symptoms is helpful in dis- necessary. Further debridement may be necessary, depending on the
tinguishing this entity from myonecrotic infections. A thin, dark, amount of devitalized tissue present. The management of Meleney gan-
malodorous discharge from the wound or inoculation site, sometimes grene includes wide excision of the lesion plus antimicrobials as dictated
containing fat globules, with extensive and prominent gas formation, by the culture results.
is characteristic. A dusky erythema may be present, and there may be
sive beyond the area of devitalized tissue, the condition must not be ■ NECROTIZING FASCIITIS
extensive crepitus in the involved area. Although not initially inva-
considered benign. If it is inadequately managed, the infection will Pathogenesis: Necrotizing fasciitis is an uncommon but severe infec-
eventually spread and lead to a rapid and extensive undermining of tion involving the subcutaneous tissues and the deep fascia. It spreads
the skin similar to that seen in necrotizing fasciitis, with correspond- rapidly in the fascial cleft but spares the overlying skin until later stages
ing systemic toxicity. in the process. Extensive undermining of the skin is the hallmark of this
A distinctive variant of gangrenous cellulitis was described and infection. It affects persons of all ages but is most common in middle-
named by Meleney several decades ago. It has been called progres- age and elderly adults. However, with an increase in the occurrence of
9
sive bacterial synergistic gangrene, postoperative progressive gangrene, group A Streptococcus and the emergence of community-associated
Meleney gangrene, and—if associated with burrowing necrotic tracts methicillin-resistant S aureus (CA-MRSA), predominantly of the USA
producing distant lesions—Meleney ulcer. The process usually begins 300 pulsotype and containing the Panton-Valentine leukocidin, as a
postoperatively, particularly after abdominal or thoracic procedures, cause of soft tissue infections, the incidence of necrotizing fasciitis
with a slowly developing shaggy ulcer with a gangrenous center sur- in previously healthy young adults has increased. 7,26-28 The infections
rounded by an inner zone of purple discoloration, which in turn is may occur anywhere, but infections in the perineal region and in the
surrounded by an outer zone of erythema. Without treatment, the course extremities are most commonly reported.
is one of relentless indolent extension, but without significant systemic The most common initiating injury leading to infection is minor
toxicity. Satellite lesions may occur, which represent tracts of burrowing trauma (~80% of reported cases); operative wounds and decubitus ulcers

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