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Migraine: Visual Aura
Typical visual auraA translucent semicircle with a jagged edge starts to the left of center in the visual field as a barely noticeable obscuration with a shimmery, zigzag left margin.
Within 10 minutes, it has enlarged to occupy nearly half of the left hemifield.
By 20 minutes, the entire left hemifield is obscured.
This is the "scintillating scotoma" ("fortification scotoma"), the most common and most specific aura (or prodrome) that precedes the headache of migraine. Here is what the visual aura of migraine looks like to the patient.
More about this phenomenonA visual cortical phenomenon, it is present in the hemifields of both eyes, although most patients believe it is only seen by the eye whose temporal field is affected (unless they went to medical school and learned about hemifield representation of vision!).
Current thinking is that aminergic pathways from the brain stem to the visual cortex set off a wave of excitation followed by depression that causes first the shimmering, then blindness. The jagged leading edge probably results from the way the wave travels across the visual cortex.
The jagged edge and buildup across the hemifield in 20 minutes are unique to migraine, usually a benign phenomenon. In a small minority, migraine is triggered by underlying metabolic or structural disease.
The buildup of a scintillating scotoma occurs in only 25% of migraineurs. Others report stationary shining spots or lines or just blurred vision. These latter hallucinations are not exclusive to migraine, however, and could reflect the visual hallucinations of many processes, including ischemia, intoxication, inflammation, dementia, psychosis, or tumor.
What to do?The diagnosis of migraine depends entirely on history. Remember that migraine is a "reaction pattern" in the central nervous system. Most of the time it is benign, but serious diseases can imitate it.
In order not to overlook these serious diseases, be sure that the patient's description hits enough characteristic features to make migraine a solid bet. Otherwise, order brain imaging and evaluate for cerebrovascular and meningeal disorders.