Epidemiology
The incidence of central nervous system toxicity among divers has decreased since the Second World War, as protocols have developed to limit exposure and partial pressure of oxygen inspired. In 1947, Donald recommended limiting the depth allowed for breathing pure oxygen to 7.6 m (25 ft), which equates to an oxygen partial pressure of 1.8 bar (180 kPa). Over time this limit has been reduced, until today a limit of 1.4 bar (140 kPa) during a recreational dive and 1.6 bar (160 kPa) during shallow decompression stops is generally recommended. Oxygen toxicity has now become a rare occurrence other than when caused by equipment malfunction and human error. Historically, the U.S. Navy has refined its Navy Diving Manual Tables to reduce oxygen toxicity incidents. Between 1995 and 1999, reports showed 405 surface-supported dives using the helium–oxygen tables; of these, oxygen toxicity symptoms were observed on 6 dives (1.5%). As a result, the U.S. Navy in 2000 modified the schedules and conducted field tests of 150 dives, none of which produced symptoms of oxygen toxicity. Revised tables were published in 2001.The variability in tolerance and other variable factors such as workload have resulted in the U.S. Navy abandoning screening for oxygen tolerance. Of the 6,250 oxygen-tolerance tests performed between 1976 and 1997, only 6 episodes of oxygen toxicity were observed (0.1%).Central nervous system oxygen toxicity among patients undergoing hyperbaric oxygen therapy is rare, and is influenced by a number of a factors: individual sensitivity and treatment protocol; and probably therapy indication and equipment used.
Therefore, current guidelines for patients on mechanical ventilation in intensive care recommends keeping oxygen concentration less than 60%. Likewise, divers who undergo treatment of decompression sickness are at increased risk of oxygen toxicity as treatment entails exposure to long periods of oxygen breathing under hyperbaric conditions, in addition to any oxygen exposure during the dive. Ocular toxicity
Prolonged exposure to high inspired fractions of oxygen causes damage to the retina. Damage to the developing eye of infants exposed to high oxygen fraction at normal pressure has a different mechanism and effect from the eye damage experienced by adult divers under hyperbaric conditions. Hyperoxia may be a contributing factor for the disorder called retrolental fibroplasia or retinopathy of prematurity (ROP) in infants. In preterm infants, the retina is often not fully vascularised. Retinopathy of prematurity occurs when the development of the retinal vasculature is arrested and then proceeds abnormally. Associated with the growth of these new vessels is fibrous tissue (scar tissue) that may contract to cause retinal detachment. Supplemental oxygen exposure, while a risk factor, is not the main risk factor for development of this disease. Restricting supplemental oxygen use does not necessarily reduce the rate of retinopathy of prematurity, and may raise the risk of hypoxia-related systemic complications.Hyperoxic myopia has occurred in closed circuit oxygen rebreather divers with prolonged exposures. It also occurs frequently in those undergoing repeated hyperbaric oxygen therapy.