What Is Cortical Spreading Depression And What Does It Have To Do With Migraine?

If you have read about migraine aura, you may have come across the term cortical spreading depression. The name gives you little help picturing what happens.

What spreads? Why is it called “depression”? And how could it explain a visual symptom that changes over several minutes?

Cortical spreading depression, or CSD, is a slowly spreading wave of electrical change in the brain’s outer layer. Researchers consider it the likely mechanism behind migraine aura. [1, 3]

What happens as the wave passes

Brain cells use differences in electrical charge across their membranes to send signals. During CSD, cells undergo a large shift in that electrical state, called depolarization. This change spreads into neighbouring tissue, followed by a temporary reduction in the affected cells’ usual electrical activity. [1]

Picture three neighbouring patches of tissue. The change reaches each patch in turn, with a quieter period following behind. This simplified illustration describes a process spreading across a region, rather than affecting the whole brain at once.

The name describes its parts:

• Cortical refers to the cortex, the brain’s outer layer.
• Spreading describes movement through neighbouring tissue.
• Depression refers to the reduced electrical activity that follows. It does not refer to mood. [1]

Why visual aura can seem to move

The visual cortex processes what you see. Different areas correspond to different parts of your visual field, so a spreading change in this tissue can help explain why a visual symptom gradually moves or expands.

For example, a shimmering line may widen, or an area of missing vision may grow. These changes can occur during visual aura, but a new visual symptom should not automatically be assumed to be migraine. [2, 3]

In a small study of three people experiencing visual aura, researchers used functional MRI to observe changes spreading through the visual cortex alongside the progression of the participants’ symptoms. [2]

The scans measured changes in blood oxygenation, an indirect indicator of brain activity. They did not directly record the electrical wave. The findings supported CSD as the mechanism behind visual aura, though the small study could not establish the connection on its own. [2]

How does CSD connect with head pain?

A process that helps explain aura does not automatically explain the headache that may accompany it. Aura can begin before head pain, overlap with it, or happen without a headache. [3]

Researchers have investigated whether CSD can activate pain pathways. In experiments in rats, inducing CSD activated pain-sensing nerves supplying the membranes around the brain. This suggests a possible route from a change in the cortex to migraine pain. [4]

However, inducing CSD in a rat is different from observing a naturally occurring migraine attack in a person. The experiment supports a possible biological connection; it does not establish how that connection works in every human attack.

CSD is most clearly linked to aura. It is not the established explanation for all migraine attacks, including attacks without aura. [1, 3, 4]

What researchers are still investigating

Scientists can induce CSD experimentally, but what starts the wave during a spontaneous migraine attack remains uncertain. Understanding an experimental trigger does not fully explain why the process begins at a particular moment in someone living with migraine. [1]

The relationship with pain also leaves questions. Why does aura sometimes occur without headache? How does the wave interact with the other systems involved in migraine? [3, 4]

CSD gives researchers a way to understand the gradual progression of aura. It cannot, by itself, tell you what caused an individual attack or which treatment will suit you.

Describing your own experience

You do not need to recognise an electrical process to describe your symptoms. For aura your healthcare professional has already assessed, focus on what you noticed and the order in which it happened.

When it is safe and manageable, note:

• What appeared first.
• How it changed.
• Approximately how long it lasted.
• Whether head pain occurred before, during or afterwards.

A simple sketch may help describe a visual change. You can keep these details in Migraine Buddy to bring to an appointment. Recording the timing and progression of symptoms can help your healthcare professional assess your experience; it cannot confirm that CSD occurred. [3, 5]

When to seek medical help

Seek prompt medical advice for first-time or changed aura-like symptoms. [5]

Call your local emergency number immediately for sudden trouble seeing or speaking, new weakness, or a sudden severe headache. Get help even if the symptoms disappear. Do not wait to record them or assume they are migraine. [6]

References

[1] Vitale M, Tottene A, Zarin Zadeh M, Brennan KC, Pietrobon D. Mechanisms of initiation of cortical spreading depression. The Journal of Headache and Pain. 2023;24:105. Read the study.

[2] Hadjikhani N, Sanchez del Rio M, Wu O, et al. Mechanisms of migraine aura revealed by functional MRI in human visual cortex. Proceedings of the National Academy of Sciences. 2001;98(8):4687–4692. Read the study.

[3] Headache Classification Committee of the International Headache Society. The International Classification of Headache Disorders, 3rd edition. Section 1.2: Migraine with aura. Read the classification.

[4] Zhang X, Levy D, Noseda R, Kainz V, Jakubowski M, Burstein R. Activation of meningeal nociceptors by cortical spreading depression: implications for migraine with aura. Journal of Neuroscience. 2010;30(26):8807–8814. Read the study.

[5] National Institute for Health and Care Excellence. Headaches in over 12s: diagnosis and management. CG150. Read the recommendations.

[6] National Institute of Neurological Disorders and Stroke. Signs and Symptoms of Stroke. Read the guidance.