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THE EPILEPSY REPORT OCTOBER 2008
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THE EPILEPSY REPORT OCTOBER 2008
epilepsy and light
combination of flicker and pattern
would be sunlight through the trees as
seen from a moving car. Escalators and
occasionally fly-screens can also be an
issue for some people. Brighter lights
and patterns with greater contrast are
more likely to induce photosensitive
seizures than those that are dim or faint.
In addition, the more of the field of
vision taken up by a pattern or flickering
light, the more likely it is that a seizure
will be triggered.
The earliest descriptions of
photosensitivity focused on seizures
triggered by sunlight, usually shining
off the water or filtered into a pattern
by trees. Sunlight is quite difficult to
avoid but sunglasses, particularly those
with polarised lenses, can be of help.
Thankfully, sunlight is a problem for
only a few people, even among those
who suffer photosensitive epilepsy.
Television and video games
In the last decades the focus has
switched to television and video games.
This was brought to public attention
in 1997 when an episode of the cartoon
“Pokemon” was aired in Japan. This
episode contained a brief sequence
where the whole screen flashed at 12Hz
for a period of four seconds. Over 700
people nation-wide, mostly children,
suffered a seizure provoked by this
cartoon. There was a similar but less
wide-spread problem caused by an
advertisement in the United Kingdom.
There are now careful guidelines in
place to make sure that programs shown
do not have the kinds of flicker or
patterns that can produce seizures.
Despite the sensible design of
programs, television is still the
commonest cause for photosensitive
seizures due to how the picture is
produced.
Televisions are divided into hundreds
of vertical lines, known as the raster.
Although in standard definition
broadcasts the screen is said to refresh
at 50Hz (50 times a second), in fact,
only every second line is changed each
time. This produces a 25Hz flicker
when viewed from up close. This is
within the range of frequencies that is
commonly a problem and, when viewed
from a short distance, a television is
usually both bright and filling the field
of vision. Console games, usually played
via a television screen, have the same
problems.
Computer screens do not have a
raster and usually refresh at 60Hz or
more, therefore not flickering at the
critical frequency. Although computers,
particularly when playing games, are not
completely without risk, the absence of
the hardware-related flicker makes them
less of a trouble then television.
The risk from television can usually be
T
he tendency to suffer seizures
triggered by seeing flicker or
patterns is known as photosensitivity.
Although flicker is a widely known
cause of seizures the proportion
of people with epilepsy who are
photosensitive is reasonably small
(5%). This is mainly among those with
idiopathic generalised epilepsy.
Photosensitivity can be relatively
easily seen on an electroencephalogram
(EEG) as bursts of epileptic activity
triggered by patterns or strobe light. The
ability to test what is and is not likely to
cause seizures in the laboratory means
much is known about the things that
are a risk to people with photosensitive
epilepsy. This is important as although
medication can help a great deal, for
some people avoidance of flicker is still
required.
Flicker and patterns
Flicker frequency, usually measured
in flashes per second or Hertz (Hz), has
the strongest effect on whether a sight is
likely to cause a seizure. The frequencies
most associated with seizures are 10Hz
to 25Hz. Faster and slower frequencies
are a problem in occasional people.
Patterns can also precipitate
photosensitive seizures. The main issue
is fine patterns, particularly stripes
and checkers, or the combination of
pattern and flicker. The most common
What is photosensitivity? What does flicker-rate mean? Can television and computers
trigger seizures? In this article Dr Saul Mullen, from Austin Health in Melbourne
explains the relationship between epilepsy and light and suggests some strategies
that can help reduce the risk of triggering seizures.
reduced by sitting further back, making
the raster invisible and eliminating the
flicker. If approaching the screen, to
insert a DVD for instance, covering one
eye with a hand will usually reduce the
risk. Unfortunately, closing both eyes
can make things worse as the act of eye
closure while looking at flicker can itself
trigger photosensitive seizures. More
modern televisions, particularly LCD
or plasma screens, with high definition
pictures and fast (100Hz) refresh rates
are less likely to cause photosensitive
seizures.
Electric lighting
Electric light, particularly fluorescent
light, is a cause of much concern.
Incandescent bulbs, those that heat a
filament to produce light, are being
phased out and replaced by small,
energy-saving fluorescent bulbs. Older
fluorescent tubes do have a subtle flicker.
This is at 50Hz or 100Hz and is due to
the alternating current used in mains
electricity. This is just perceptible by
the human eye but outside the critical
frequency range for photosensitive
seizures. Modern, energy-saver bulbs
are designed to eliminate this flicker.
Even with the older tubes, although
many people find them unpleasant, the
light does not produce EEG changes
as are seen with the types of flicker
described above.
Strobe lighting
Other sources of flicker are
comparatively uncommon. The strobe
lights used in night clubs occasionally
precipitate seizures but they are usually
relatively slow (<5Hz). It is also often
difficult to separate the risks posed by
alcohol and sleep deprivation from that
of the strobe light.
Emergency services also use relatively
slow strobe lights but, when multiple
vehicles are parked together, these can
combine together to produce frequencies
in the critical range.
Managing photosensitivity
Photosensitivity is uncommon but
for those who suffer it can be very
troublesome. For many, medication is
sufficient but for those who need to avoid
lights and patterns the main offender is
television. Keeping at least two metres
from the set as well as considering
a modern screen can be helpful.
When presented with flicker that is
uncomfortable looking away or covering
one eye is most useful. Closing both eyes
can worsen the situation, particularly
with very bright lights.
For those who have suffered
photosensitive seizures it is best to
discuss these issues with your treating
specialist. There is great variability
between people with this condition and
individual advice is very important.
Saul Mullen
Austin Health, Melbourne Victoria
Practical ways
to reduce risk
Sit at least 2
metres from TV
screen
Cover one eye
when approaching
the TV screen eg
to insert a DVD, or
when unexpectedly
confronted with
flicker that is
uncomfortable
Wear sunglasses
with polarised
lenses