YOU SIT IN YOUR OFFICE, TRYING DESPERATELY TO FOCUS on your computer screen. You squint to make out blurred characters and images. The computer screen is not malfunctioning. Something is wrong with your eyes.

Perhaps this is just the result of getting older? You pass it off as fatigue - jetlag, maybe. After all, you've recently returned from a 12-day diving trip to the Solomon Islands. You felt fine on the dive trip and had not noticed any significant eyesight problems. You'd done many long dives possible because of your rebreathers. You decide to get some rest. Two days later your eye problems are even worse. Everything around you appears very fuzzy. Your driver's license indicates that you are 47 years old and do not need corrective lenses to drive.

Now, you have extreme myopia (nearsightedness) and cannot read the digital clock on your microwave oven or your videocassette recorder unless you get within 50 cm. You can't see well enough to think about driving without eyeglasses. Could your sudden eye problems have something to do with your diving? You know that you do not have decompression sickness, and you are well aware that denial has been called the first symptom of that illness.

The answer to your problem turns out to be surprising, not least because it is thought very rare, though it could very well be a new danger for divers going increasingly deeper, with ever longer dive-time exposures. You contact the Divers Alert Network (DAN). They insist you go to the local hyperbaric facilities for a neurological checkup to determine if you might be suffering from decompression sickness. The diagnosis? You are not bent. The bill to confirm what you already know is US $550. Your eyesight is still terrible even after the medical exam. What could be wrong?

You recall better visual days, when you had read that underwater cinematographer and rebreather diver Howard Hall (see Immersed, Spring 2003) suffered this same malady only a few years ago. Howard had nearly gone blind after one of his expeditions. Could this be happening to you? Will your next best friend be a guide dog? You press your face very close to your telephone's keypad and punch in Howard and Michele Hall's telephone number. Michele graciously discusses your dive profiles and symptoms. You had conducted 46 dives within 12 days. She confers with Howard and together they confirm that your dive profiles, and your overall number of dives, were similar to Howard's. And then she gives you the bad news.



Central nervous system oxygen toxicity is a form of oxygen poisoning brought about by exposure to high oxygen partial pressure. The dosage of oxygen is a function of two factors: the exposure to oxygen as a function of partial pressure, and the time that exposure lasted. Symptoms of acute oxygen toxicity include tunnel vision, tinnitus (ringing in the ears), nausea, twitching muscles (primarily facial tics), irritability, dizziness and convulsions.


Nearsightedness occurring when the eye focuses images in front of the retina, which results in distant objects being out of focus.


Oxygen Tolerance Unit is a measure of the diver's exposure per minute to a given level of oxygen partial pressure. The higher the pressure, the more OTUs accumulate per minute. OTU limits are expressed as single dive maximums and multiple day maximum allotments.


Dalton's Law states that the pressure exerted by any gas components in a mixture of gases is equal to the fraction of that gas in the mix multiplied by the absolute pressure of the total gas. In diving the absolute pressure of the gas is the same as the absolute pressure at depth. Oxygen partial pressure is expressed as PO2.


This is also known as ‘whole body oxygen toxicity', and is the result of exposure to a high oxygen partial pressure over a long period of time at a lower oxygen partial pressure than is the case with CNS oxygen toxicity. (See above for definition of CNS oxygen toxicity).

The First Case

Howard Hall's was the first case of hyperoxic-induced myopia (HIM) in a technical diver reported in a scientific journal, something with profound implications for both technical and military divers.
Too high an oxygen exposure for too long a time had damaged Howard's eyes, according to the diagnosis that was published in the Undersea and Hyperbaric Medical Society journal. It is ironic that Howard had breathed oxygen at a lower partial pressure than is typical during a recompression chamber treatment, and he was within acceptable U.S. Navy oxygen-exposure standards. He was also within acceptable oxygen tolerance units (OTUs), a concept developed by Bill Hamilton, Ph.D. (see sidebar "REPEX Oxygen Exposure Tracking Method"). Immersion and exercise are known to be contributing factors to HIM, and these certainly applied to you and to Howard. Other factors did not apply. (For a list of factors that may bring in HIM, see sidebar. "Synopsis of First HIM Case Report")

While all of this is bad news, you decide to get more information from another diver you know who regularly engages in multiweek, deep diving trips with his rebreather. Icythologist Richard Pyle, at the Bishop Museum in Hawaii, has made as many as 80 dives within a two-week period at a slightly higher oxygen partial pressure than either you or Howard Hall had dived. Yet Richard has not suffered HIM. Perhaps that's because he's in his 30’s, which is a lot younger than Howard and yourself.

Now that you have limited vision, what do you do? You get glasses and regularly monitor your vision on an eye chart. In Howard's case, it took one month before his symptoms resolved. You decide to wait 10 weeks since your first symptoms of HIM, and then you feel that your eyes have healed enough for a cold water dive expedition to the Queen Charlotte Islands in northern British Columbia, Canada.

Within a 10-day period you make 16 dives, many of which last less than an hour. You adjust your oxygen partial pressure so that you have an exposure of roughly one-third what you experienced in the Solomon Islands. You make certain to stay well within oxygen toxicity and OTO tables (See REPEX and CNS charts on previous pages).

You should be safe, right? Three days after returning from this trip you again experience HIM. You are in disbelief. The return of the myopia is much more disheartening than your original experience. You don't have many opportunities for dive exposures such as you experienced in the Solomons, because your normal cold water diving environment just does not allow it.

Yet even after all of your precautions. HIM is back. You again have to stay out of the water and use glasses so that you can see properly. Your second bout with HIM has increased your myopia. You have trouble finding your glasses when you put them down. You can read without glasses, but bifocals would negate the need to put on glasses every time you look up from your book.

Overcoming the Princess's Dark Side

Oxygen has been called "The Princess of Gasses".

Today, more and more evidence is pointing to this Princess's dark side.

Synopsis of First HIM Case Report

A 48-year old male diver called the Divers Alert Network (DAN) after he experienced severe post-dive visual problems. The diver had experienced no symptoms of decompression sickness during the course of his dive trip.

Prior to his dive trip, the diver had 20/20 vision according to tests done by an optometrist.

After his dive trip, he needed eyeglasses to restore his perfect (predive trip) vision. It has been reported that patients undergoing hyperbaric oxygen therapy (recompression chamber treatment) have sometimes suffered progressive myopic changes. Reversal of the myopic shift after discontinuation of hyperbaric therapy usually occurs within 3-6 weeks, but may take as long as 6-12 months. The PO2 in these exposures typically varies from 2.0 to 3.0.

The diver had a "marked decrease in interocular pressures". This is highly unusual and is usually not seen unless there is trauma to the eye, inflammation of the eye or interocular pressure-lowering medications involved. A test for blood glucose level was done to rule out diabetes mellitus, and blood sugar was found to be normal.

The myopia experienced by this diver occurred despite his taking both a multivitamin supplement and an additional antioxidant vitamin supplement. This could mean that there is not a protective effect against oxygen toxicity from antioxidant supplements, or that the dose was not high enough. This is an area that should be researched.

Contributing Risk Factors for HIM

Risk factors for getting HIM include diabetes and too great exposure to increased oxygen partial pressure over a period of time.

Avoiding HIM

The best ways to avoid Him are to reduce your oxygen partial pressure exposure during the dive (change your breathing mix), keeping the PO2 well below 1.2, and longer surface interval times to let your body flush out the excess oxygen. Divers 40 years and older should stay well away from the OUT limits.

Source: F.K. Butler, Jr., E. White, and M. Twa. Hyperoxic, Myopia in a closed-circuit mixed-gas scuba diver. 1999. In Undersea & Hyperbaric Medicine 25 (1): 44-45.

While it has long been known that too high an oxygen partial pressure (PO2) can cause seizures, with a high likelihood of subsequently drowning, it has not been widely reported that divers exposed to high PO2s are subject to various post-dive maladies, including HIM and post-dive ear squeeze. (See "Oxygen and Your Ears" by Jeffrey Bozanic, Ph.D., Immersed, Winter 2002.) Current wisdom counsels you to be as conservative as possible and reduce your oxygen partial pressure exposure as much as possible.

For older divers especially, it is not enough to stay within the published oxygen exposure guidelines.
Allowing an even greater margin of oxygen-exposure safety would dictate that you do not come anywhere near the oxygen-exposure limits. Ways to back significantly off the oxygen exposure limits include shorter bottom times and spending more surface interval time between dives.

If you stay focused on the various hyperoxic-induced myopia risk factors and you plan your dives and post-dive-activities well, you should have a clear perspective well after you surface.


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