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As a rule they are not serious.
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Floaters = normal ageing, esp. >55 yrs; posterior vitreous detachments, haemorrhage or choroiditis
Flashing lights = normal ageing, esp. >55 yrs; posterior vitreous detachment or indicates traction on retina (?retinal detachment).
Reassure patient and suggest specialist review. Appearance of a fresh onset is of concern re. retinal detachment, refer to an ophthalmologist.