College Answer

a) Necrotising fasciitis.
b) Clostridial species.

Discussion

This question is essentially identical to Question 25.1 from the first paper of 2009, the sicussion section from that SAQ is reproduced here with no modification:

According to an authoritative source, "postoperative necrotizing fasciitis of the abdominal wall is usually caused by peritonitis in patients who have undergone multiple procedures for complications of emergency laparotomy" (Casali et al, 1980).

So, one might ask: what features of this SAQ history makes necrotising fasciitis the most likely diagnosis, rather than an uncomplicated wound infection with surrounding cellulitis? In summary, there is virtually nothing. All history we get can be broken down into component parts:

• Old age (74 year old woman)
• Malignancy
• Perforated colon
• Extensive faecal soiling ("widespread peritoneal contamination")
• Extensive surgical intervention (laparotomy, peritoneal washout, colonic resection and a defunctioning  ileostomy) - what sounds like primary closure (no mention of an open abdomen)
• Infection is taking place on Day 6
• Clinically, it looks like abdominal wall cellulitis with oedema
• The blood culture grew Gram positive bacilli.

Apart from the suspicious bacilli, this whole thing looks very much like a bog-standard wound infection.  The history barely resembles the case series presented by Casali et al (1980). The authors present twelve cases of necrotising fasciitis of which the majority were in young people, recovering from abdominal gunshot wounds (none of the right age for this SAQ, and none with malignant perforation). S.aureus and E.coli were the dominant organisms.  Digging around in the pile of literature, one may come across an article or two which describe a situation which resembles the college's scenario. Miyoshi et al (2008) present a review of the clinical features to be expected from post-operative necrotising fasciitis, and Huljev et al (2005) reviewed some historical data in their case report. Mixing the data from these authors, the following features are common to patients who develop post-operative necrotising fasciitis:

• Most underwent colorectal surgery
• In most, the timeframe was within 7 days of operation
• Cellulitis-like features ("rubor of the skin and tenderness") were present in most (90%)
• Abdominal wall oedema was present in 80%
• Most were of old age
• A malfunctioning immune system seems to be a predisposing factor (diabetes, AIDS, splenectomy and malnutrition were listed)

Other classical features (skin bullae, insensate skin, crepitations) were absent from the college history, making it difficult to guess what the examiners were thinking.

Now; of the Gram positive bacilli we know to be common pathogens, which are likely to be responsible for this wound infection? Let's review them and consider whether they are likely to be in that blood culture.

• Corynebacterium diphtheriae (no)
• Proprionibacterium sp. (very unlikely)
• Nocardia asteroides (no)
• Listeria monocytogenes (no)
• Bacillus anthracis (hell no)
• Clostridium sp. (the only one left)

The typical case will present as a polymicrobial zoo, and whereas Clostridium species will likely flourish in the smelly pockets of avascular fat necrosis, it is unlikely that they will be found in the blood culture, particularly as the blood is so well oxygenated (much of the time). It is more likely that Clostridium perfringens would the sole organism in the cultures of a patient with gas gangrene of the abdominal wall. If the college mentioned subcutaneous emphysema of the abdominal wall, there would be no guesswork involved in this question. This is supported by an article from 1966 (back in the day when surgeons actually palpated people's abdomens instead of scanning them). It reports on ten patients; nine had proper crackly gas gangrene due to C.perfringens or C.multifermentans. One patient with a C.tertium infection only had abdominal wall cellulitis, just like in the college question.