When questioned, the patient usually says that these are probably present all the time but are only noticeable in the dark. They seem to be especially apparent before going to sleep at night. The flashes are usually seen in the peripheral part of the visual field. They must be distinguished from the flashes seen in migraine, which are quite different and are usually followed by headache. The migrainous subject tends to see zig-zag lines,which spread out from the centre of the field and last for about 10min. Elderly patients with a defective vertebrobasilar circulation may describe another type of pho-topsia in which the flashing lights tend to occur only with neck movements or after bending.
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