Flashing lights in vision: causes, red flags and when to get it checked

The retina creates flashes when it is pulled or stimulated, such as when the vitreous gel inside the eye tugs on it, or when the visual cortex at the back of the brain is briefly overactive in a migraine aura. Knowing which eye, how long it lasts and what accompanies it distinguishes them.

• Migraine aura: shimmering, zig-zag or sparkling shapes in both eyes, spreading over 10 to 30 minutes and followed by headache, though not always.

• Posterior vitreous detachment: the gel shrinks and separates from the retina, causing brief flashes, usually in one eye, often with a new floater, commonly after the age of 50.

• Rubbing or pressure on the eye, or a blow to the head, causing a "seeing stars" effect.

• Retinal tear or detachment: persistent flashes with many new floaters, a shadow or curtain spreading across vision.

• Posterior uveitis or inflammation, and optic neuritis, which can cause flashes with pain on eye movement.

• Rarely, transient ischaemic attack or occipital lobe seizure, with brief visual phenomena.

New flashes in one eye, especially with a sudden shower of floaters, a dark shadow or curtain at the edge of vision, blurred or distorted central vision, loss of vision, or flashes after eye injury are red-flag features.

• Call 999: flashes with sudden vision loss, facial droop, speech difficulty, weakness or severe headache.

• Same-day emergency eye service, optometrist or A&E: new flashes with floaters or a shadow, curtain or reduced vision in one eye.

• Within days: flashes without visual loss, in someone with high myopia or previous eye surgery.

• Routine: typical migraine aura you have had before.

The clinician asks which eye, how long flashes last, the shape and colour, any headache, floaters, shadow or curtain, recent injury, short-sightedness, previous cataract or retinal surgery, and family history. They test visual acuity and fields, and examine the retina with dilating drops.

• Dilated retinal examination: the key test to find tears or detachment.

• Optical coherence tomography (OCT): detailed scan of the retina and macula.

• Ultrasound of the eye: if the view is blocked, for example by a bleed.

• MRI brain: for atypical aura or other neurological signs.

If flashes are in one eye with new floaters, do not wait for them to settle; arrange same-day assessment. A retinal tear is typically treated with a laser or freezing treatment, and a detachment needs surgery. If typical migraine aura, rest, hydration and your usual migraine plan apply. Avoid driving while visual symptoms persist.

On the NHS, an optometrist, eye casualty or A&E can assess; ophthalmology treats. Private ophthalmology can usually see you quickly for dilated examination and laser or surgery.