ULTRAVIOLET RADIATION AND THE EYE - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 253

temperature. Both sunlight and ambient temperature have been cited as potential etiological
factors in several age-related ocular diseases 5 . However, the combined role of these
physical factors and their possible synergisms generally has not been carefully examined.
These environmental co-factors, together with geometrical factors, have recently been
examined to give some suggestions of the etiology of these ocular diseases 15 .
4. Animal studies
Injury thresholds for acute UVR injury to the cornea, conjunctiva, lens and retina
in experimental animals have been corroborated for the human eye from laboratory and
accidental injury data 13-20 . Guidelines for human health and safety limits for exposure to
UVR are based upon this knowledge. 2,17-25
There are several types of hazards to the eye from intense UVR exposure, and
the dosimetric concepts applicable to the anterior segment differs from quantities
applicable to retinal exposure. The following effects are considered in current safety
standards, which depend upon the spectrum of the UVR exposure and the temporal and
geometrical characteristics of the exposure. 1,2,13,17,23-25
5. Photokeratoconjunctivitis
Ultraviolet photokeratoconjunctivitis (more simply, “photokeratitis”, also known
as "welder's flash" or "snow-blindness" - 180 nm to 400 nm) is a temporary
photochemical injury of the cornea (photokeratitis) and conjunctiva
(photoconjunctivitis) that appears several hours after the acute exposure; symptoms
generally last for only 24 to 48 hours. 19-22 Because of the very superficial nature of the
corneal injury and the rapid turnover of surface epithelial cells of the cornea (about
48-hour cycle), the effect is transient.
The symptoms of photokeratitis are severe pain and, in some cases,
blepharospasm (uncontrolled blinking). The onset of symptoms, like erythema, is
delayed for several hours after the UVR exposure. The signs and symptoms last for a
day or two, and the haze appears in the region of the cornea (the superficial surface
layer of the eye) within the palpebral fissure (i.e., within the lid opening). The condition
is almost always reversible and it has generally been accepted that the condition is
without sequelae. However, the laboratory studies of Ringvold 21 show damage of both
the keratinocytes and endothelial cells of the cornea, and it could be- speculated that
repeated, severe episodes of snowblindness may well increase the risk of delayed
corneal pathologies such as pterygium and droplet keratopathies.
6. Lenticular opacities
Although not directly related to the safety assessment of the light adjustable lens
(LAL), some research studies related to UVR and cataract are of interest, since
exposures of all other anterior structures would also be revealed in such research, and
all animal studies showed that the cornea was injured at lower exposures than the lens.
Ultraviolet cataract can be produced in animals from acute exposures to UVR of
wavelengths between 295 nm and 325 nm (and perhaps to 400 nm). Action spectra can