Flashes and floaters often indicate changes are occurring at the back of the eye (retina and vitreous). The most serious condition to exclude is retinal detachment. Posterior vitreous detachment is more common, and is a normal change that occurs with age – but in some cases can lead on to retinal detachment.
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Retinal detachment is an emergency. Tissue at the back of the eye pulls away from a layer of blood vessels that provide necessary oxygen and nourishment. This is often seen as a shadow or a curtain coming down over the vision, and part of the peripheral vision missing. There may have been flashing lights experienced before this happens. Retinal detachment requires urgent surgical repair.
Posterior Vitreous Detachment
Posterior vitreous detachment (PVD) is a normal change in your eye which happens to everyone at some point. It is very common, and although it can be associated with troubling symptoms in the vast majority of cases it does not cause any problems with the vision. It is due to the shrinkage of the vitreous gel, and when the back of the vitreous pulls away from the retina at the back of the eye this causes visual symptoms (usually flashes followed by the development of a floater). It is important to have a thorough eye examination when this occurs to detect any retinal breaks or tears, and make sure there is no sign of retinal detachment.
Further useful information on this commonly encountered condition: