Cutaneous and Ocular Ramifications of Ultraviolet Radiation

Psoralen plus ultraviolet A does not increase the risk of cataracts: A 25-year prospective study

BACKGROUND

In some animal species, exposure of the unprotected eye to psoralen plus ultraviolet A (PUVA) therapy induces lens opacities. The relevance of these animal findings to human beings is not established. However, some case reports suggest that PUVA in human beings may increase the risk of lens abnormalities.

OBJECTIVE

Our aim was to evaluate any possible associations between exposure to PUVA and increased risk of ocular lens abnormalities.

METHODS

Since 1977 the PUVA follow-up study has periodically monitored the ocular status of 1237 cohort members with psoriasis using structured eye examinations. In our previous report we presented data results of the first 10 years of prospective study. This report includes data from two additional cycles of eye examinations that span an additional 14 years of follow-up.

RESULTS

Based on our data from the last pre-1993 to final eye examination (2004), compared with that observed for the earlier period (first ever to last pre-1993 eye examination), the age-adjusted incidence of cataract did not increase significantly (incidence rate ratio = 1.04, 95% confidence interval = 0.82-1.31). In both the univariate and multivariate analyses increasing exposure to PUVA was not associated with a higher risk of cataract.

LIMITATIONS

Our cohort principally enrolled middle-aged or older patients so our data do not permit us to assess the effects of PUVA on the eyes of younger persons.

CONCLUSIONS

Increasing exposure to PUVA does not increase cataract risk among persons using eye protection at the rates used in our cohort.

Phototoxicity to the retina: mechanisms of damage

Light damage to the retina occurs through three general mechanisms involving thermal, mechanical, or photochemical effects. The particular mechanism activated depends on the wavelength and exposure duration of the injuring light. The transitions between the various light damage mechanism may overlap to some extent. Energy confinement is a key concept in understanding or predicting the type of damage mechanism produced by a given light exposure. As light energy (either from a laser or an incoherent source) is deposited in the retina, its penetration through, and its absorption in, various tissue compartments is determined by its wavelength. Strongly absorbing tissue components will tend to "concentrate" the light energy. The effect of absorbed light energy largely depends on the rate of energy deposition, which is correlated with the exposure duration. If the rate of energy deposition is too low to produce an appreciable temperature increase in the tissue, then any resulting tissue damage necessarily occurs because of chemical (oxidative) reactions induced by absorption of energetic photons (photochemical damage). If the rate of energy deposition is faster than the rate of thermal diffusion (thermal confinement), then the temperature of the exposed tissue rises. If a critical temperature is reached (typically about 10 degrees C above basal), then thermal damage occurs. If the light energy is deposited faster than mechanical relaxation can occur (stress confinement), then a thermoelastic pressure wave is produced, and tissue is disrupted by shear forces or by cavitation-nonlinear effects. Very recent evidence suggests that ultrashort laser pulses can produce tissue damage through nonlinear and photochemical mechanisms; the latter because of two-photon excitation of cellular chromophores. In addition to tissue damage caused directly by light absorption, light toxicity can be produced by the presence of photosensitizing agents. Drugs excited to reactive states by ultraviolet (UV) or visible light produce damage by type I (free radical) and type II (oxygen dependent) mechanisms. Some commonly used drugs, such as certain antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), and psychotherapeutic agents, as well as some popular herbal medicines, can produce ocular phototoxicity. Specific cellular effects and damage end points characteristic of light damage mechanisms are described.