August 31, 2018

The science behind TMS

In a recent article I talked about magnetic fields stimulating the brain using a device call the ‘God Helmet’. I’m not going to open that can of worms again, but I did promise to delve into TMS and how it uses magnetic fields to stimulate the brain.

TMS stands for transcranial magnetic stimulation – it uses magnetic fields to stimulate your brain  through your skull. It’s a ‘noninvasive’ procedure meaning you don’t have to be cut open or have anything injected or stuck into your body for it to work. They just wave a little infinity- or doughnut-shaped baton over your head where they’re trying to stimulate.

We’ve been stimulating nerves, muscles, and later the brain, since Luigi Galvani discovered it was possible in the late 1700’s. But for over 200 years the majority of the science in this field used electric stimulation. With muscles and nerves, attaching an electrode to the surface of the skin is generally sufficient to elicit a response with relatively low levels of electricity (think those cool muscle stimulation apparatuses that athletes use). You can also insert electrodes into flesh relatively easily (think acupuncture with a little electricity). With brains, it’s not so easy.

In the days when electric stimulation was the gold standard, you had two main options with stimulating the brain: 1) open it up and go in 2) apply electrodes to the scalp.

Option 1 is generally reserved for really severe cases to this day – we don’t just open people’s skulls and play around with their brains for fun. However, this is the most direct way to do it. A neurosurgeon can go in and stimulate the brain very precisely, resulting in very different results. If you haven’t seen it yet, you should watch the Heritage Minute youtube clip about Dr. Wilder Penfield who pioneered brain mapping and advanced neuroscience in leaps and bounds in the early-mid 1900’s (he’s a little bit of a neuroscience celebrity and we watched the heritage minute clip in no less than 3 of my courses studying neuroscience at McGill, which is where he did his research. Take a look at the heritage minute here and learn more about him here). So all in all, opening skulls is very useful and has its place, but also very dangerous and not something you’ll try unless it’s your last option.

Option 2 sounds great, but there’s one little hiccup. You have skin (and nerves and sensory receptors) on your skull that also pick up on electric currents. So not only does the electricity impact your skin (literally an electric shock wherever the electrodes are touching) but the ‘strength’ of the electric pulse also diminishes as it travels through your skin and skull. The amount of electricity you need to have an effect on the brain means for a very large  jolt to the head. It’s not very pleasant, so you’re generally given anesthesia before they do it.

Nurses at Winwick Hospital, near Liverpool, UK, in 1957, preparing to shock a patient with Option 2.

This is where magnetic fields come in. In very simple terms, moving a magnet (or changing the strength of a magnet) in proximity to something that can conduct current (think a wire or a neuron) will induce a current (for neurons, this is the ‘stimulation’ we talk about). We have Michael Faraday to thank for that one, and the law of physics named after him (for more info about him and his law check here and here). The brilliance of using magnetic fields instead of electricity is magnetic fields can travel through the skull more or less unhindered. You can get greater stimulation with next to no discomfort for the participant.

There was some suggestion by Faraday and others playing around with magnetic fields to stimulate the brain in the 1800’s (around the time he was doing experiments that would lead to Faraday’s law) but TMS in a controlled and relatively predictable form didn’t come to be until 1985 when Anthony Barker created and tested the first TMS coil as we know it today. He used the coil to stimulate the motor cortex, eliciting a twitch in the subject’s thumb.

That twitching thumb leads us to the first and least disputed main outcome of TMS – the use of pulses to elicit a muscular response. This is generally used as a calibrating step before moving forward with other applications. The coil is placed over the motor cortex (running approximately straight up from your ear near the top half of your head) and the strength of the magnetic field is slowly increased until your fingers twitch reliably with the pulse. This makes sure the strength of the magnetic field is high enough to work in your brain while avoiding over-stimulating.

TMS is used in stroke and brain injury rehabilitation (see a recent review here) to stimulate muscles that have lost their ability to communicate effectively with the brain. The stimulation helps with the brain’s natural healing process and helps get that communication back online sooner rather than later. However, as the review mentions, there isn’t a handbook “stimulating the brain post stroke” that will give you the protocols and reliably expected results – this research is still very new and still very divergent in procedures and effectiveness.

Epilepsy is another field where TMS has been used with a fair amount of success. Epilepsy is characterized by uncoordinated and excessive excitability of the brain. TMS has been shown to help with the over-excitability of the brain and bring that back down to less problematic levels. As with stroke rehabilitation, there is more work that needs to be done, but the results are promising. There’s a good review on the history of TMS in treating epilepsy here.

Now we come to TMS and mental health. There are a lot of sites and research hospitals looking into TMS as a means to ‘cure’ depression, obsessive compulsive disorder, schizophrenia, and other mental health concerns. In about 50% of cases, patients have fewer symptoms after treatment. This happens most often in cases that are difficult (think resistant to medication). Here’s the thing, with physical rehabilitation and epilepsy, we have a general idea of what’s wrong (under- or over-activation of the brain respectively) and in what area (motor cortex and wherever the seizures start). In most cases, we don’t have a strong enough understanding of what is actually going on in the brain to be able to say ‘patient X with mental health concern Y has problematic brain activity in brain area Z’. If that was the case, you could use TMS to stimulate brain area Z to either increase or decrease baseline activity as needed.

A US air force researcher demonstrates TMS treatment. It is our assumption that having two dudes watch you while you get TMS is not actually necessary for the treatment. 

Remember the God Helmet we were talking about and the power of suggestion? That happens in medical treatment as well – the placebo effect. The placebo effect is the improvement you see in the population as a result of the suggestion of a treatment – you give people a pill or an injection or put a TMS coil over their head without them knowing that the pill is just sugar, the injection is saline, and the TMS coil is not plugged in (meaning there’s no actual medical benefit to the treatment itself). It’s not necessarily a bad thing – in fact, in some cases you get some of the benefit of treatment without having to undergo treatment. However, the general rule of thumb is that if you can get the same benefit from going through the motions as doing the actual treatment, you shouldn’t subject people to the treatment. (There’s a good article about the placebo effect here). The majority of studies (see a review here) do not find a significant difference between the benefits of TMS and the benefits of ‘sham’ TMS (the same physical process of TMS but without the stimulation).

I am not a medical expert, but overall, the benefits of TMS (even if it is just a placebo effect) seem to outweigh the cons for most people at face value. You might end up with a bit of a headache or small muscle twitches (my eye twitched for a few hours after doing a TMS research study… it was kinda entertaining) but both wear off within a few hours. HOWEVER, this is unlikely to be a one-stop-shop solution, so it could be harmful to progress if you look at it like one. As with most procedures, the at-home work is just as important as the treatment itself (if not more). You wouldn’t go to physio for your knee, not do any of the prescribed exercises, and expect to get better (or you might, but that would be silly). So you shouldn’t get TMS and ignore meditation, mindfulness, or therapy.

Generally speaking, if you have epilepsy or are at risk for seizures it’s not a good idea unless you’re getting TMS calibrated to help with that. So is TMS some sort of miracle fixer-of-all-things? No. Can it be useful? Yes. We just need to be careful about what we claim it can do.

 

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