“Flesh-eating bacteria!”

It’s honestly a bit impressive how much we hear about “flesh-eating bacteria” in the news.  Truth is, in the United States in a given year there are fewer than 3000 cases; our estimates aren’t very good because Necrotizing Soft Tissue Infections (NSTIs) are not a common disease.  NSTIs are just common enough that almost every physician will see a case at some point in their career, but they are not common enough for most physicians to develop true familiarity.  This lack of familiarity represents a true diagnostic challenge for many physicians when they evaluate a patient with cellulitis or a complex soft tissue problem that is not an NSTI- while they don’t want to inappropriately ask for help from the surgeon-specialist, they also don’t want to miss a disorder that is life-threatening in the face of inadequate management.  We do know that the reported incidence of NSTI has increased over the last 20 years.

So, what is an NSTI?

An NSTI is an aggressive soft-tissue infection that results in the skin and tissue below it dying.  This infection may be cause by a single type of bacteria or by multiple types of bacteria.  NSTIs are often life-threatening, and they require early and extensive removal of the infected tissue.  Patients with NSTIs also require post-operative care that can be quite complicated and usually have wounds from the tissue removal that require specialized care.

Who gets NSTIs?

Sometimes we can’t predict this; I have taken care of completely healthy women in their 30s who have an NSTI.  However, most patients who get an NSTI have some known health problem.  Our group’s study using NSQIP data showed that obesity, hypertension, and diabetes are all more common in patients who develop an NSTI.  Similar findings were generated by analyzing the NIS, which showed that obesity, diabetes, and liver disease were associated with NSTIs.

What does an NSTI look like?

Common signs and symptoms include redness, swelling, and tenderness of the infected area.  Unfortunately, these signs and symptoms are essentially identical to the clinical findings with cellulitis, which is not a surgical disease- it simply needs treatment with antibiotics for a full recovery.  A patient with an NSTI may present with an elevated white blood cell count and a decreased sodium level, but this is not always true.  The LRINEC score has been used to estimate the likelihood that a patient has an NSTI by giving point values for various laboratory values, with a score >6 having a 92% positive predictive value and a 96% negative predictive value.

What are the risk factors for mortality with NSTIs?

First, it’s important to understand that mortality from NSTI seems to be decreasing; two different recent analyses showed mortality rates of 9% and 12%.  This is likely attributable to better recognition of the disease, allowing for more aggressive early intervention, as well as improvement in critical care and wound care.

Patient age is clearly related to mortality, but severity of illness at presentation also plays a role.  Two different studies using NSQIP data have shown that emergent surgery is a risk factor for morality.  Last year, our group created a risk calculator for mortality consequent to an NSTI; the biggest flaw in our calculator is simply that it caps at an 85% probability of mortality.

Should I worry about my patient having an NSTI?

Usually, no.  These are rare infections.  I do say that with some hesitation because I know a disproportionate number of NSTI survivors…but that’s a product of what I do in my work.  Distinguishing the patient with an NSTI from a patient with “just” cellulitis is a genuine diagnostic challenge, particularly for those who don’t care for these patients often.  The important thing is that if the idea that something might be an NSTI enters your head, it’s worth it to talk to an expert.  If they do have an NSTI, the sooner they get to surgery, the better they are likely to do clinically.

Bonus information: Our SlideShare about the mortality calculator from the 2012 Western Surgical Association meeting.