Dr Oli: Hyperbaric ulcer treatment risks

Q: This is not a diving-related question, but I am a diver, so forgive me for picking your brains but I hope you can help. I'm diabetic and due to start treatment in a hyperbaric chamber for an ulcer on my heel that just won't heal. The doctors treating me have mentioned fits as a possible side effect of the treatment, which naturally worries me slightly! How common is this, and what increases the risk of it happening? Are there any early warning signs of oxygen toxicity, or ways to reduce it? Obviously I want to try anything that will help my foot but don't want to become epileptic in the process.

A: I think I can reassure you that the risk is very, very low. Convulsions are the most dramatic end-result of central nervous system oxygen toxicity, and thankfully doing the funky chicken in the chamber is a rarity in modern day diving and hyperbaric practice. An imminent seizure will generally be preceded by other signs such as muscle twitches or spasms (often in the lips), anxiety, irritability, nausea or (my favourite) "a sense of impending doom" (think Private Frazer, the dour wild-eyed Scotsman in Dad's Army). At this stage, further toxicity can be avoided simply by removing or reducing the inspired oxygen fraction. During hyperbaric treatment (for divers and non-divers alike), "air breaks" are used for precisely this reason: at 18 metres, 5 minute periods of breathing normal air will reduce the body's oxygen content to non-toxic levels. Clearly longer exposures to higher oxygen partial pressures will increase risk, as will exercise. Drugs that mimic the sympathetic nervous system are thought to enhance toxicity, so for this reason Sudafed and similar medications are not advisable when diving near the oxygen limit. Finally, there's no evidence that an oxygen-related convulsion leads to epilepsy; much more evidence exists that hyperbaric oxygen will help your ulcer, so I'd encourage you to crack on with the treatment. Good luck.

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