Monthly Archives: December 2013

Piranha attacks

Attribution: Andrewself at en.wikipedia.org

Jaw of the piranha

70 people were injured in a piranha attack in Argentina on Christmas Day. While nobody was killed, one can imagine the panicked atmosphere present as bathers who were simply trying to cool off were suddenly involved in a fish biting frenzy. The question is, what sequence of events led to this happening? The media has implicated hot weather and fisherman waste as the inciting factors. Other authors have discussed similar attacks in the literature, and implicate damming of streams and rivers causing ideal spawning grounds for piranhas. These spawning grounds can then coexist with recreational areas, and this leads to the fish being in close proximity to people.

There is a large amount of folklore surrounding piranhas and their behavior. What is important to note is that the bites are typically defensive, and the fish are not feeding on humans (or other large mammals typically).  The documented cases of piranhas eating humans involve people dying of other causes, then being eaten by the fish. The defensive nature of piranha bites is evident in that the most are single bites that are of a warning behavior. That bite can pack a punch, as one study demonstrated that piranhas bite up to three times harder than alligators proportionally.

This bite is typically circular and crater-like in nature, and damages skin and underlying tissues. The majority of these bites are on the lower extremities, which is not surprising. Most need minimal first aid, but occasionally bites can amputate digits or cause severe bleeding. They do not seem to need prophylactic antibiotics, but can rarely become infected with pathogenic bacteria, so the wound needs monitoring.

As far as mass attacks go, they typically occur during the major spawning seasons for piranha. They also occur most often when large numbers of people are in the water, and at shallow dammed sites. Because dammed sites can allow spawning year round, and allow larger groups of people to recreate together the attacks can be more common and larger in size. Thus, it seems that the attacks are fairly predictable, and not something that should cause paranoia.

Piranha attacks in dammed streams used for human recreation in the State of São Paulo, Brazil
http://www.ncbi.nlm.nih.gov/pubmed/21085879
Mega-Bites: Extreme jaw forces of living and extinct piranhas (Serrasalmidae)
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3526859/
Piranha attacks on humans in southeast Brazil: epidemiology, natural history, and clinical treatment, with description of a bite outbreak.
http://www.ncbi.nlm.nih.gov/pubmed/14719860

How long can we delay recompression?

Decompression sickness is the clinical constellation of symptoms that result from gas bubbles that form after going from a high pressure environment to a lower pressure environment. Classically, it was discovered in caisson workers, but today most of the cases are due to diving. The bubbles are not from the oxygen that is required for the diver to breathe at depth, but from the inert gases mixed with it to prevent oxygen poisoning.

Neutral Buoyancy Laboratory

Source: Mike Renland

Symptoms can be varied, from muscle and joint pain, to itching, to fatigue, to neurologic deficits and more. Current teaching is to refer all patients with suspected decompression sickness to facilities with hyperbaric treatment facilities as soon as possible. With more and more people flying to distant locales to dive, people are presenting with decompression illness days later. This isn’t necessarily a new phenomenon, as this article is from 1984.

It’s a case series, and only 3 patients at that. They presented to hyperbaric centers at 7, 5, and 3 days after symptoms started, respectively. All had neurological involvement. All also had complete resolution of their symptoms after appropriate treatment, at least until their one year followup.

So is there a consensus on how to treat patients who have delayed presentation? Not that I could find. The authors recommend titrating according to patient response. That isn’t particularly helpful, but all of the patients in this series were treated with US Navy Treatment Table 6.  Per the article, there have been successful treatments as far as 10 days out from symptom onset. As always, you can contact the Diver’s Alert Network at +1-919-684-9111 for help with managing your patient.

Delayed treatment of serious decompression sickness
http://www.ncbi.nlm.nih.gov/pubmed/3977150

Denim as snake protectant?

Source: Wikimedia Commons

Denim pants, worn since 1873 by workers, and since the 50s by everyone else, have had a reduction in their use as outdoor clothing due to the emergence of performance hiking apparal. Denim jackets are markedly less popular recently. Now, what if it turns out denim protects you from envenomation? Would denim then come back as an overlander outfit?

Crotalus oreganus helleri

Source: Matthew Robinson

This article tried to ascertain whether denim has any ability to decrease the severity of snake bites, mainly be decreasing or eliminating envenomation. To study this, they used 17 different southern Pacific rattlesnakes divided into small (35-54cm) and large (66-102cm) groupings, and had them bite latex gloves that were either uncovered or covered with standard denim cloth. The paper has a very detailed methods section, including even the material in the snake cages when they were being stored!

The results were pretty surprising. There was a 60% reduction in venom for the small snakes, and 66% reduction in the large snakes. If it were simply a material thickness, a larger snake should be better at penetrating the material, but this was not borne out by the data. Since defensive strikes by nature are very fast (<0.33s), the reduction is likely from simple physics.

Thus, you can take home that denim can reduce the amount of venom you receive if a snake strikes you defensively. Sadly, the reduction is probably academic for big snakes, as the amount you receive can still be enough to kill you. Especially in the case of the snake species studied, a particularly nasty rattlesnake that has both hemorrhagic and neurotoxic venom components.

The bigger take home point from this article is that it is yet another nail in the coffin for the “small snakes are more dangerous” myth about defensive strikes. Bigger snakes will deliver more venom, regardless of what you are wearing. In this paper, they delivered 41 times the venom load of the smaller snakes, and 26 times the load when denim interfered. Please make sure and correct the next person you hear trying to propagate this myth.

And wear some denim if you’re going rattlesnake wrangling.

Denim clothing reduces venom expenditure by rattlesnakes striking defensively at model human limbs.
http://www.ncbi.nlm.nih.gov/pubmed/19942067

Treating serious frostbite

Frostbite isn’t just limited to climbers. Many people live in climates where the homeless, mentally ill, or alcoholic patients spend the night outside and have the possibility of developing frostbite. Failure to diagnose or appropriately treat frostbite can lead to increased morbidity and tissue loss. And judging by the chart below*, it won’t take long over most of the US this weekend.  (*Chart possibly not evidence based)

Source: National Weather Service

Then how do we treat serious frostbite? Well, we can use this protocol developed by plastic surgeons in Korea using only 17 patients. And it took them 3 years to accumulate these 17 patients. One can surmise that frostbite might not be a problem, at least not around this particular institution.
That being said, their protocol is based on other studies of frostbite, so it isn’t unsound. Basically it goes as follows:

  • Rewarm early to 40-42℃
  • NSAIDs during rewarming to prevent thrombosis
  • Prostaglandin E1 for 24 hours
  • Wait until necrotic margins are defined, then operate

The authors also give prophylactic antibiotics during dressing changes, but have no reference for that. They include tPA and hyperbaric oxygen in their discussion, but based on my reading none of their patients actually received it.

The majority (12/17) of the cases occurred in January, with the other 5 coming in December and February. Climbing was the highest risk factor in their case series. The good news was that all the second degree cases, and three of the five third degree cases resolved without operative management. Of course, conservative management of those third degree cases lasted longer than a month on average. As expected, all 4th degree frostbite cases required surgery, with the surgeries occurring 3 weeks after injury.

Certainly this is a reasonable plan to limit the damage of frostbite once it has been identified and the patient is in a hospital environment. The only treatments one could reasonably perform in the wilderness would be rewarming and NSAIDs. As always, don’t rewarm  if there is a chance the extremity could freeze again.

Proposed Treatment Protocol for Frostbite: A Retrospective Analysis of 17 Cases Based on a 3-Year Single-Institution Experience
http://e-aps.org/DOIx.php?id=10.5999/aps.2013.40.5.510