Patients with no skull are a window on brain activity

I’ve just clocked a stunning experiment, shortly to be published in the Journal of Cognitive Neuroscience, that recorded brain activity from patients who had part of their skull surgically removed for several months and had only flaps of skin between their brain and the outside world.

The operation is called a hemicraniectomy and is often used when the brain swells or the pressure builds up inside the skull to the point where it is damaging the brain.

Neurosurgeons will sometimes remove a portion of the skull (see the scans on the left) and just leave the scalp protecting the brain until the swelling subsides, before replacing the skull flap some months later.

As an aside, sometimes the surgeons will surgically insert the piece of skull into the abdomen so the bone marrow doesn’t die and it can be replaced ‘alive’ when the time comes. There’s a great description of this here.

The patients normally wear helmets, for obvious reasons, but they are unique in having such a thin covering of the brain.

A team of researchers, led neuroscientist Bradley Voytek, realised this provided a unique opportunity to examine the exactly how the skull affects EEG, one of the most common techniques for measuring the electrical activity of the brain.

EEG records brain activity from electrodes on the skull, but the signal gets ‘smeared’ as the electrical charge passes through the bone and so the source of the activity can’t be located very precisely to specific brain areas.

By working with the hemicraniectomy patients, the researchers could compare electrical activity on one side of the brain – recorded through just the skin, and the other, where recordings were made normally – through electrodes on the skull.

The researchers found that the non-skull signals were richer, were less subject to interference, were more closely tied to specific tasks and could be better linked to specific brain areas.

On the right is a comparison of the signal coming from a listening task, where participants are suddenly presented with an ‘oddball’ noise in the midst of a bunch of otherwise identical sounds. The brain reacts strongly to the change and this is reliably reflected in a positive spike in the electrical activity at about 50 milliseconds (consequently, the wave is called the ‘P50′ signal).

You can see that the activity on the craniectomy side is much stronger, tighter and cleaner whereas on the skull side it is quite indistinct. The team found similar results in several other tasks.

This not only helps us better understand EEG results on people with intact skulls, but it also meshes with brain activity recordings that are taken from electrodes implanted directly in the brains of patients undergoing neurosurgery.

Link to PubMed entry for study.
pdf of scientific article.
Link to Bradley Voytek’s blog post about his work.

8 Comments

  1. Posted January 17, 2010 at 8:50 pm | Permalink

    The applications you’re reporting are fascinating, but, in fact, you’re wrong to say that ‘the patients normally wear helmets, for obvious reasons,’ – most patients, in reality, don’t need to wear helmets. A simple skin flap, in time, hardens and becomes as much a protective barrier as the skull. Either that or, in many cases, a muscle graft is carried out – the muscle atrophying and forming a barrier just as tough as a skull. The idea, of course, is for surgeons and plastics to give the patient a natural replacement, and not have to rely upon any prosthetic, or external protection.

  2. Posted January 17, 2010 at 9:04 pm | Permalink

    Almost all published reports of hemicraniectomy report helmet use:
    http://scholar.google.com.co/scholar?q=hemicraniectomy+helmet
    Perhaps this is less common in decompressive hemicraniectomy where smaller bone flaps are more typically removed, although I note that some case reports still note helmet use:
    http://scholar.google.com.co/scholar?hl=es&q=decompressive+craniectomy+helmet

  3. Natalie
    Posted September 13, 2010 at 12:55 am | Permalink

    What type of helmet would you suggest for someone who has the skull flap removed? My 17 year old is close to returning to school & we need something. I would prefer an altered/reinforced baseball helmet or something similar that won’t make him look terribly odd.

  4. jean delaverriere
    Posted November 2, 2010 at 8:13 pm | Permalink

    Please help me,
    For my own sanity I wish to know the name of the space where the 3 parts of the skull join, Like a Y

  5. Jean delaverriere
    Posted November 2, 2010 at 8:18 pm | Permalink

    I was not precise enough, I need the name of the spot on the top back of a skull where the three lines meet.

    Thank you

    J de la Verriere

  6. Belle
    Posted January 9, 2011 at 9:26 pm | Permalink

    I actually had a hemicraniectomy when I was 22 after a massive stroke. My bone flap was harvested in my abdomen for a year. During that time, I wore a helmet for the year until the bone was replaced. My doctor instructed me not to take the chance going anywhere without it on. I wore a regular white hockey helmet because anything smaller was too tight and very uncomfortable.

  7. Storm
    Posted March 8, 2011 at 5:21 pm | Permalink

    Wel a good friend of mine is 22 now had a huge chunk of his skull removed last year in July 2010, it was placed in his abdomon but it went septic unfortunately . He wears no helmet and is at a sub acute now still comatose. When you touch the area without skullbone he freaks out and looks very confused so saying that it gets hard sounds weird since Marc responds so strangly.

  8. Shae
    Posted February 2, 2012 at 8:25 pm | Permalink

    I had a craniotomy for AVM Disection in my prefrontal cortex and originally had my bone plate put back in place after my AVM’s were removed. Unfortunately, about a month post-op the bone plate became infected requiring a second surgery to have it removed. Now, three years later I still don’t have a plate covering the hole and at the time when I asked my surgeons about a helmet for safety, they told me to be to just be careful. Needless to say, I’m not impressed…

    I would like to add, that I completely concur with Storm. It took about two years before contact to the area stopped being absurdly loud and caused me to become dazed. Now it takes an accidental “tap” to the area to cause that effect. Those who think we are fine without plates, imagine spending your days trying to avoid being hit in the head by objects…. You’d be surprised how often it happens.


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