Cataract is the clouding of the lens of the eye that results in loss of visual acuity, often progressing to incapacitating blindness. Nuclear cataracts of old age (affecting the central part of the lens) are the most common type of cataract. Hyperbaric oxygen therapy (HBOT) is the administration of 100% oxygen at pressures greater than one-atmosphere absolute pressure (1 ATA) for a therapeutic purpose. HBOT thus involves a patient being confined within an air-tight vessel and compressed to pressures above ambient and the administration of 100% oxygen to breathe. Oxygen may be supplied through compression in a 100% oxygen environment (most often in a monoplace chamber) or the use of an oxygen delivery system (usually a mask or a hood) delivered in a larger multiplace chamber. There are potential side-effects of HBOT, one of which is the development of cataracts. This is a rare event, but may be both unrecognized and under-reported. The cause of cataract remains to be fully elucidated, but evidence is emerging that points to one major precipitating factor: the lifetime exposure to oxygen, particularly the increased partial pressure of oxygen that reaches the lens as the vitreous humor deteriorates with increasing age.
The development of both nuclear cataracts and a reversible myopic shift in hyperbaric patients strongly suggests that oxidative damage to lens proteins is responsible. In an experiment using guinea pigs exposed to hyperbaric oxygen, the average apparent diameter of proteins in the nucleus of lenses in HBO-treated animals was nearly twice that of the control animals and was similar to the of aggregates found in human nuclear cataracts. The authors concluded that molecular oxygen in vivo could induce the cross-linking of lens nuclear crystallins into large disulfide-bonded aggregates capable of scattering light. A similar process may be involved in the formation of human nuclear cataracts, and may be the end-stage of the much more commonly described "myopic shift" that is detectable in the majority of hyperbaric patients who have a course of 20 to 30 treatments.