Page 961 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

692 PART 5: Infectious Disorders

account for most remaining cases. The presentation is usually acute Local injury
or subacute, ranging from 3 to 14 days after the injury. In some cases,
particularly those associated with group A Streptococcus or CA-MRSA,
the onset is very sudden; the condition may progress dramatically from Anoxia
a tiny abrasion to septic shock, with massive subcutaneous necrosis,
within 24 hours. 7,26,28 Many patients have underlying chronic illnesses, 27,29
with diabetes present in 20% to 50% of patients, severe arteriosclerosis Bacterial
in 20% to 33%, and cardiovascular or renal disease in 50%. Nutritional inoculation
status is also an important consideration, with marked obesity or
marked wasting noted in many cases. With infection due to group A Bacterial Infection
Streptococcus, more than 50% of patients have no underlying illness synergy
and were previously in good health. Similarly, necrotizing fasciitis due Impaired host
to CA-MRSA has also been often associated with individuals who have defenses
been previously in good health.
After the initial bacterial invasion, the infection spreads rapidly along
fascial planes and subcutaneous fat, with ischemic tissue facilitating Ischemia and necrosis of
spread of the necrotizing process. At an early stage, histologic examina- fat, fascia ± muscle
tion of full-thickness skin biopsies shows no abnormality. However, the
subcutaneous fat and fascia show a contiguous nonspecific inflammatory
reaction, with fibrinoid arteriolitis and thrombosis of vessels, with angio- Rapid spread along
thrombotic microbial invasion and subsequent liquefactive necrosis. If fascial planes
31
the condition is left untreated, the overlying skin becomes extensively FIGURE 74-2. The pathogenic process in necrotizing fasciitis.
necrotic because of thrombotic occlusion of the venules and arterioles
supplying it.
It has been shown that traumatic surgical and vascular injuries gen- the buttocks, trunk, neck, external genitalia, and inguinal areas. In most
5
erate areas of relative tissue anoxia, with the result that carbohydrate cases of polymicrobial origin, multiple organisms are present, with an
and protein metabolism proceed anaerobically, generating lactic acid. average of three or four isolates per patient. Some investigators distin-
Buffer systems become depleted and acidosis develops, which causes guish acute group A streptococcal or CA-MRSA necrotizing fasciitis
lysosomal disruption and, hence, local autolysis and destruction. This as a separate entity. Vibrio vulnificus and Aeromonas hydrophila have
environment provides an ideal milieu for anaerobic growth. Whether also been reported to cause a particularly virulent form of necrotizing
actual infection evolves is determined by several factors, including the fasciitis.
means of inoculation and the size of the inoculum, altered host defense
mechanisms, and the virulence of the bacteria. Altered host defenses Presentation: With necrotizing fasciitis, there is often a trivial injury
play an important role in propagation of the infection. For example, followed, after several hours or days, by the onset of pain and swelling
high blood alcohol levels, steroids in large doses, and metabolic aci- accompanied by chills and fever. The pain is progressive, relentless,
dosis inhibit adherence of phagocytes, and patients with cirrhosis and and severe and is often out of proportion to the severity of the physical
metastatic carcinoma have poor phagocyte chemotaxis. The virulence findings. There may be considerable pale erythema in the involved area;
of the bacteria is determined, to some extent, by their capacity to pro- brown-to-bluish skin discoloration is not uncommon later in the course
duce various enzymes (hemolysins, fibrinolysin, hyaluronidase, and of the illness (Fig. 74-3). If the condition is left to progress, frank cuta-
collagenase). In addition, for S pyogenes, the presence of M protein on neous gangrene may be seen. Pain is gradually replaced by numbness or
the surface of the organism has an anticomplement effect and may func- analgesia as a result of compression and destruction of cutaneous nerves.
tion as a superantigen, leading to a massive release of potent vasoactive Hypesthesia of the affected area may be a useful sign of the extensive
mediators such as tumor necrosis factor, interleukin 1, and myocardial undermining that occurs. Edema is present in most patients. Crepitation
depressant factor. The streptococcal pyrogenic exotoxins A, B, and C is not usual, but it may be found in patients seen later in the course of
or other unknown antigens may also function as superantigens and the illness. Fluid-filled vesicles may appear in the area of erythema, often
have been found to share DNA sequence homology with staphylococcal
toxic shock syndrome toxin. Functioning as superantigens, these toxins
share the ability to mediate nonspecific binding to antigen-presenting
macrophages and T-helper cells, leading to polyclonal activation of large
numbers of these lymphocytes. The cytokine release associated with
this activation is responsible for the severe toxic shock–like syndrome
associated with S pyogenes infections. Synergistic activity of different
bacterial species has also been postulated on the basis of evidence
from clinical experience and from experimental infections in animals.
32
It is commonly assumed that aerobic organisms assist the growth of
anaerobes by using oxygen, diminishing redox potential, and supply-
ing catalase. Local ischemia and reduced host defense mechanisms in
the presence of virulent pathogens combine to produce a milieu that is
responsible for the alarmingly rapid spread (Fig. 74-2).
Etiology: Necrotizing fasciitis may be due to a synergistic polymicrobial
bacterial infection in which at least one anaerobic organism (usually a
Bacteroides, Prevotella, Porphyromonas, Peptostreptococcus, or Peptococcus
species) is isolated in combination with one or more facultative organisms
(usually streptococci, E coli, Klebsiella or Proteus species, or S aureus),
5,33
or it may be due to a single organism, either S pyogenes or CA-MRSA. FIGURE 74-3. Necrotizing fasciitis of the lower leg. Dusky erythema is present, with
It has been reported that the highest recovery rate of anaerobes was in blistering and small patches of dermal gangrene.

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