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Uncovering the Link Between PFO and Inner Ear DCS

What is the causal relationship between a patent foramen ovale (PFO) and increased inner-ear decompression sickness? Diver Alert Network’s geeky risk mitigator Reilly Fogarty explains the latest science and emerging consensus regarding what we think’s going on.

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By Reilly Fogarty

Our knowledge of decompression stress is a little like a human heart; it’s pretty well understood on a functional level but full of unsolved mysteries and an occasional hole. The link between conditions like patent foramen ovale (PFO) and decompression sickness (DCS) has been troubling for divers and researchers alike for decades, and while the “black box” model of input and expected outcome has been pretty well squared away, we know very little about some of the fundamental mechanics or quantifiable risk factors.

It’s only been in the past few years that we’ve been able to draw any real conclusions as a result of some long-running projects led by a few of the best and brightest in the hyperbaric community. Simon Mitchell, Ph.D., has been at the forefront of much of the push to understand the relationship between PFO and DCS, and his recent presentation at the South Pacific Underwater Medical Society (SPUMS) brings together the best of what we know about the relationship between PFO and DCS, and inner-ear DCS (IEDCS) in particular.

Quick refresher: A PFO is an atrial defect that occurs in about 30 percent of adults. It’s the result of a fetal circulatory mechanism that doesn’t close as the person grows, resulting in a small hole in the wall between the atria. These rarely pose a health hazard and frequently go undetected, but it’s possible for divers to shunt bubbles directly from the right to the left side of the heart without passing them through the pulmonary filter. The result can be serious and sometimes unexpected cases of DCS.

IEDCS is DCS resulting from inadequate gas elimination from the tissues of the inner-ear that can lead to many of the common symptoms of DCS as well as tinnitus, hearing loss, vertigo or dizziness, nausea, vomiting, and impaired balance. The condition is more common in divers using mixed gas or conducting long technical dives because of the interactions between helium, nitrogen, and oxygen in the varyingly permeable tissues of the ear.

Hampered a little by a lack of definitive mechanism (we have yet to prove definitive causation) Mitchell managed the near miraculous feat of pulling together an expert consensus on the relationship between PFO and IEDCS. While the skeptics among the group will still argue that a PFO is extremely common (about 1 in 3 adults may have one), and DCS occurs in only a few of those adults (and IEDCS in even fewer), Mitchell has responded with a compelling argument.  

Mitchell’s model explains the disconnect between PFO occurrence and IEDCS symptoms by digging into the anatomy of the ear. During rapid ascents from even fairly shallow dives, the fluid-filled inner ear spaces have been shown to remain supersaturated with nitrogen for about 30 minutes post-dive, leading to potential DCS risk even without a PFO. With a PFO, some bubbles may shunt to an artery feeding the ear and then enter and perhaps combine with other bubbles exiting the fluid-filled inner-ear. The result is damage to inner-ear structures and the known symptoms of IEDCS. Current models show that the majority of bubbles passing through PFO end up in the brain and rarely cause injury, but this secondary model illuminates a likely model for the observed relationship between IEDCS and PFO.

Photo by Andreas Hagberg.

If you’re having trouble putting it all together, Mitchell’s model can be summarized in the following four steps:

  1. The diver must have a PFO of sufficient size to shunt blood through the atrial wall.
  2. There must be enough supersaturation for venous gas bubbles to form.
  3. Something must provoke the opening of the PFO and blood shunting.
  4. Embolization of the inner-ear tissues must occur while they are still supersaturated.

That’s a lot to parse at once, but it’s another step toward understanding how our physiology and behavior combine to put us at risk or keep us safe. Interested in reading more? Access the published position statement here.

Header Image: Annika Persson


Reilly Fogarty is a team leader for its risk mitigation initiatives at Divers Alert Network (DAN). When not working on safety programs for DAN, he can be found running technical charters and teaching rebreather diving in Gloucester, MA. Reilly is a USCG licensed captain whose professional background also includes surgical and wilderness emergency medicine as well as dive shop management.


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My Deep Dive Into The Dunning-Kruger Effect

Tech diver Brendan Lund shares his personal diving journey from summitting Mount Stupid and descending into the depths of Despair on trimix, before finally beginning his ascent on the Slope of Enlightenment. No Kool-Aid was involved in the making of this story.

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by Brendan Lund
Images courtesy of Brendan Lund

Lund diving in Socorro, Mexico,

I started diving in 1996 as a poor student in South Africa. I absolutely fell in love with diving, and haven’t stopped since. After moving to the UK in 2001, I finally started earning money and was able to dive more frequently. At this time, the Red Sea was starting to boom, and I was able to book a full week of diving—including flight and accommodation—for as little as 350 GBP! Many trips later, I became interested in tech diving, as it was the happening thing, and in 2004 I decided to begin my tech journey with a leading agency. This also signals the start of my journey with the Dunning-Kruger Effect!

Grokking Dunning-Kruger 

I first saw the Dunning-Kruger effect graph a year or so ago and couldn’t stop thinking about its relationship to my diving. 

What is the Dunning-Kruger Effect? Here’s what Wikipedia has to say:

In the field of psychology, the Dunning–Kruger effect is a cognitive bias in which people with low ability at a task overestimate their ability. It is related to the cognitive bias of illusory superiority and comes from the inability of people to recognise their lack of ability.

As described by social psychologists David Dunning and Justin Kruger, the bias results from an internal illusion in people of low ability and from an external misperception in people of high ability; that is, ‘the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others’.

The graph below simplifies this concept—again, this is taken from the internet, but the wording pretty much sums up what I have been feeling throughout  this journey:

The less knowledge you have, the more confident you feel. The more you learn, the less confident you feel. 

The View from Mount Stupid

In 2005, I progressed from an advanced nitrox diver to a fully-certified advanced trimix diver. My instructor was a well-known deep diver at the time, and I was super impressed with the courses. I was at the point of Mount Stupid on the graph. These courses were not a pass/fail style of course; you just had to prove you could complete the skill once, and bam! You’re qualified! 

“Wow, I’m the man! I’ve passed my Advanced Trimix Course and dived to 100m.”

I dug out the photo below from a box in the shed. We had someone take an underwater photo of us celebrating our successful completion of our deco procedure course. As you may notice, trim was not a requirement at the time (I’m now hiding my face with embarrassment)! Apologies to my buddies in the photo; it was a while ago, and the instructor is the only one in reasonable trim!

Class celebrating the completion of a deco procedures course.

To say I was chuffed is an understatement! I immediately went tech diving as much as possible. In 2008, I decided to travel the world for a year—diving of course. I met many amazing people and dived everywhere; I also became an instructor with a well-known recreational agency. I was at the top of my game (or so I thought), although I still don’t think I had any trim! I mean, who needs trim, right?

On one of my adventures, I met a guy who was really interested in checking out a new agency that he had heard of. This would be the first time that I heard about Global Underwater Explorers (GUE). After much research and reading internet forums for GUE in 2000, I believe the Dunning-Kruger realisation phase of my journey began: I thought, “There is definitely a lot more to this!”

The Slope of Despair

I’m not going to lie; the more I looked into GUE, the more nervous I got, and the more I slid down the slope of despair. It took me a good six years to build up the courage to sign up for a Fundamentals class, and I showed up on that day feeling very confident in my brand new drysuit and my horseshoe wing. Wow, did I learn that I was way out of my depth! I was in trouble. It was so much harder than I had ever imagined. I don’t think it was so much the course that was hard, but that it was hard for me to overcome my ego and overconfidence.



I received a provisional “Tech Pass” [i.e., I qualified to take GUE’s Tech 1 or Cave 1], and it felt like I had failed! I was at the bottom of the Dunning-Kruger slope of despair.  Was I ever going to get it? I questioned whether I should go back to get my pass, and I have to thank my buddies Nikky and Darren for encouraging me to do so. After lots of practice and some gear tweaks, I felt like I smashed it, and achieved my Tech Pass! Was this the beginning of the slope of enlightenment? Things were starting to make sense!

The Slope of Enlightenment

I have now completed two more GUE classes—DPV1 and Cave 1—and am signed up for Tech 1 next year. The future looks bright! I dive as often as I can with like-minded divers, and I realize that there is always more to learn and areas where I can improve. I am hopeful that I am now on the Slope of Enlightenment; it certainly feels that way.

I highly recommend GUE to any divers that would like to work on themselves. It has definitely helped me on my diving journey, and I now look forward to many more years of diving and learning. I can’t wait to get involved in more projects and extend my skill set.

Additional Resources

It’s Never Too Late To Tackle Fundamentals


Brendan is an events manager based in London and (currently) dives mainly around the southwest coast of England. He started diving in 1996 in South Africa and has dived all around the world. He found a passion for cave diving a few years agohis favourite place to do this is Tulum, Mexicobut still loves wreck diving. Brendan is looking forward to the next challenge, with project work planned with Project Baseline, and Tech 1 is in his diary for next year in Croatia, since Covid-19 postponed it this year.

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