Ocular migraine: the shimmer that scares you once

What do you see during an ocular migraine?

The script is remarkably recognisable. It starts with a small flickering or glittering spot, often just off the centre of vision — reading becomes awkward, a face 'misses' a piece. The spot slowly grows into an arc or horseshoe of trembling zigzag lines, often glittering like crystal or in rainbow colours, sometimes with a blurred or blind area inside. Over fifteen to thirty minutes the arc drifts to the edge of the visual field and dissolves. Afterwards vision is completely normal — at most you feel a little tired or 'off'.

Duration is the key feature: typically 15 to 30 minutes, almost always under an hour. And it happens in both eyes at once, though it often feels like one; covering one eye at a time during an aura shows the disturbance in both — and it stays visible with the eyes closed.

Where does it come from?

Despite the name, ocular migraine is not an eye phenomenon. The source lies in the visual cortex at the back of the head: a slow wave of altered nerve activity travels across the cortex (cortical spreading depression), and the brain translates that wave into shimmer at the corresponding spot of the visual field. The wave's speed exactly explains the slow growth of the image — and the aura's characteristic timeline.

If headache follows, it is called migraine with aura. But especially in middle age the aura often comes without noteworthy headache — a familiar, harmless pattern that causes a lot of unnecessary worry.

When should you get it checked anyway?

The first episode always deserves assessment by a GP, optometrist or ophthalmologist — not because it is probably serious, but because the diagnosis 'typical aura' may only be made once the pattern fits and nothing else is going on. Beyond that, some patterns do not fit a classic aura and call for examination: vision loss or a 'curtain' without shimmer (think retina or vessels — see our eye stroke article), an aura lasting longer than an hour, shimmer clearly confined to one eye (test by covering), accompanying neurological symptoms such as speech trouble, weakness or numbness, and a first aura over 50 or with known cardiovascular disease. Brief sparks at the edge in the dark are yet another story (a tugging vitreous) — covered in our floaters and flashes article.

What can you do about it?

During an attack: pause driving or precision work, and let it run its course — the aura resolves by itself and no remedy cuts it short. Structurally it pays to learn your triggers: irregular sleep, skipped meals, stress release (the weekend effect), bright or flickering light, sometimes alcohol or specific foods. For frequent attacks with headache there are good treatment options via your GP, from attack medication to prevention. And for peace of mind: once you know your own aura pattern, you will usually recognise the next episode within a minute — and stay surprisingly unbothered.