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HYPERBARIC oxygen therapy (HBOT) can cause permanent myopic changes that appear to be linked with changes in the natural lens, reported researchers at the annual meeting of the Canadian Ophthalmological Society. Baseer Khan, MD and colleagues compared visual outcomes in a prospective study of 40 patients with phakic and pseudophakic eyes who underwent a series of hyperbaric oxygen therapy treatments. A total of 79 eyes from 40 patients were studied. Of these, 69 were phakic and 10 were pseudophakic. The average age of patients was 58.6 years. The researchers performed assessments prior to treatment, midway through the regimen and after HBOT concluded. Exclusion criteria included visual acuity less than 20/60 or concurrent eye disease. HBOT sessions lasted 90 minutes each with continuous exposure at 2.0 to 2.4 atmospheres. Patients underwent treatments five days per week for six to eight weeks. A total of 36 (52%) of the patients with phakic eyes had significant refractive changes, averaging a reduction of 1.50 D. Thirty-three phakic eyes had no significant changes. Patients with peripheral vascular disease were more likely than patients with other conditions to have persistent myopic changes. However, none of the 10 pseudophakic eyes had noticeable refractive changes. "Changes appeared only in patients who had their normal lenses. It appears the cause is in the lens of the eye not some other part of the eye. It's an interesting question why these changes occur. We're looking at further mechanisms to try to answer the question of why some patients develop myopic changes with this treatment and some don't," Dr Khan said. The researchers will continue the project, and are planning to do ultrasound biomicroscopy to investigate lens morphology in HBOT patients. This study isn't the first to suggest HBOT leads to myopic changes, said David Beebe, PhD, from Washington University. However, comparing phakic and pseudophakic eyes "is a novel way of corroborating that the lens is the ocular tissue most affected by HBOT," he told EuroTimes. The possible link between HBOT, myopia and peripheral vascular disease is of interest, but needs more study to sort out the reason why patients with peripheral vascular disease were more susceptible to the effects of oxygen treatment. Dr. Beebe knows of five studies in the medical literature on HBOT and eyes. Some have included ultrasound evaluations of axial length and corneal curvature. A recent study showed myopic shifts in workers who spent time in a pressurised underwater vessel building bridge supports. Myopia isn't the only problem associated with oxygen, noted Dr Beebe. His own research suggest that exposure of the lens to oxygen is the primary cause of age-related nuclear cataracts. People who develop age-related nuclear cataracts first get a myopic shift. "We believe the effect of oxygen on the lens is to harden the lens. This increases its refractive power, causing a myopic shift," he explained. HBOT is indicated for a number of medical conditions, including treatment for peripheral vascular disease, non-healing ulcers in diabetes, and other conditions in which increased blood oxygen is required. Patients often require 20 to 40 treatments. Overall, about half of patients who undergo HBOT experience myopic changes. Of those, half have transient changes that resolve within a month, while the remainder have permanent changes. Studies in the literature have noted myopic shifts from 1.6 D to 5.5 D. Baseer Khan, MD David Beebe, PhD,
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