Medicine

How Does Deep Brain Stimulation Work?

This post originally appeared on Medical News Bulletin.

Sci-fi sounding brain implants like Elon’s much touted Neuralink are often in the news, but did you know brain implants are already here and helping people to control neurological symptoms? Read on to find out how.

What is Deep Brain Stimulation?

Deep Brain Stimulation (DBS) is an effective treatment for neurological disorders such as Parkinson’s disease, tremors or epilepsy. It requires brain surgery, during which electrodes are implanted into specific parts of the brain. The surgeon will decide where in the brain to place the electrodes, depending on the symptoms that the patient wants to control.

How does Deep Brain Stimulation work?

A nerve cell, or neuron. See the finger-like projections that reach out to other neurons? Source:openclipart-vectors, pixabay.
A synapse. The finger-like projection releases neurotransmitters that float across the synapse to another neuron or to the message’s target. Source: Clker-Free-Vector-Images, pixabay.com.

How Does Our Brain Send Messages Around Our Body?

The brain is a huge assemblage of nerve cells called neurons. The neurons bundle together in groups depending on their role in the brain. Neurons in each region of the brain are carefully arranged into sub-collections and pathways. These pathways then relay messages from one part of the brain to another. To make a pathway, neurons physically reach out to each other with finger-like structures to make connections. At the points where the fingers touch, one neuron will pass a set of molecules to another neuron to send a message. When a neuron relays a message, we say it is activated or ‘firing’. We call the connection point between the two fingers a ‘synapse’.

Neurons fire by converting an electrical current into a chemical signal. The nerve cell opens tiny channels at the synapse and spits out neurotransmitters. These neurotransmitters diffuse very quickly across the miniscule gap between the fingers (synapse) to the next neuron. That neuron then sucks up the neurotransmitter and translates the chemical signal back into an electrical one. Then that neuron will pass along the signal to the next neuron in the pathway, or to it’s final target where the message will be translated into an action. This action could be a movement, for example the message could tell a muscle to contract.

Why Would the Brain Need Stimulating?

A complicated network of neural connections develops over our lifetime to govern almost everything we think and do – even things we don’t know we are doing. Groups of neurons send exquisitely coordinated messages to control our movements, our vital body functions and our thoughts. The timing of these communication relays is very important. If the signals from the neurons are disrupted, mistimed, too slow, too fast or out of sync the message becomes distorted. This distortion is what causes some neurological symptoms. Sometimes the neurons stay activated for too long, sometimes they fire when they shouldn’t. This can lead to seizures in the case of epilepsy, or difficulty moving in the case of Parkinson’s disease. Deep brain stimulation can help to correct these timing errors.

What Stimulates a Neuron?

Because the neurons conduct electricity, placing an electrode nearby and giving an electric pulse can make them fire. During DBS, the electrodes activate neurons in specific parts of the brain. Instead of receiving signals from other neurons across their synaptic connections, the neurons are activated directly by the electrode. Gentle stimulation can help the neurons to calm down and rediscover the rhythm of things. It can assist the neurons to get back into synchrony or to wake them up when they have been slow. You could think of it as a bit like a reset or a pace maker.

Why Does It Work?

While scientists and doctors have proposed various ideas about how DBS operates, we still do not understand exactly why and how it works1. Somehow the right amount of stimulus provided by the implanted electrodes modulates the activity of neurons so that their proper function is restored. Each patient requires a unique stimulation setting because no two people’s brains are perfectly alike. Every network of neurons needs its own adjustment.

How is Deep Brain Stimulation Performed?

Deep brain stimulation itself is very simple. A little control panel fitted under your skin can be programmed in advance. The doctor enters settings that will tell the device when to stimulate, for how long and at which intensity. The settings are easily adjusted, the idea is that once installed, you can go about your life without thinking much about it. In fact, you might not even notice when the stimulation is happening.

Brain Surgery

While the Deep Brain Stimulation itself is simple, getting the device in place is not so easy. A brain surgeon will implant an electrode into the patient’s brain. This is a complicated surgery performed by a specialized neurosurgeon2. The doctor will drill small holes into the skull, through which they will insert electrodes into the brain. This is very difficult surgery, the surgeon must be perfectly precise, accurate to the millimetre, when they place the electrodes. Usually the surgery is conducted under local anaesthetic – yes, you might be awake! Since brain tissue does not contain touch or pain receptors, we don’t feel anything when an electrode goes in. Don’t worry the anaesthesiologist will numb the site of the surgery, so you won’t notice the drilling. The advantage to being awake is that you can help the surgeon to find the perfect spot for your electrode. During the procedure, the doctor will switch on the electrode and see how the patient responds. This direct feedback allows the surgeon to position the electrode exactly where it is needed.

Implant Control

After the electrode is placed, a match box sized pulse generator is implanted. Usually the pulse generator is inserted below the collarbone or the ribcage. The control box is then connected to the electrodes with a thin wire that runs under the skin. This pulse generator – similar to a pacemaker for a heart – sets the frequency and amplitude of the stimulation. In the following weeks to months, the patient, together with a physician, will experiment with the settings to find the best programme to treat their symptoms. This fine-tuning requires several lengthy appointments but is vital to experiencing good results long-term.

In principle, DBS is a reversible surgery, meaning that electrodes, pacemaker and wires can be removed again.

Which disorders can DBS treat?

DBS is a versatile treatment. As of 2024, DBS is FDA-approved for four conditions, essential tremor, Parkinson’s Disease, dystonia, epilepsy.

Essential Tremor (FDA approved in 1997)

This condition causes a rhythmic, involuntary trembling, most common in hands and head. It occurs during voluntary movements, making daily tasks harder to complete. DBS to treat essential Tremor targets the ventral intermediate nucleus of the thalamus and reduces symptoms up to 78%.2

Parkinson’s disease (FDA approved in 2002)

Parkinson’s primarily affects movement control. The three main symptoms are:

  • Tremor: rhythmic trembling that occurs during rest (unlike in Essential Tremor when it happens during movement)
  • Rigidity: muscle stiffness
  • Bradykinesia: slowness of movements.

Every Parkinson’s patient has their own needs, but the most commonly stimulated areas are the subthalamic nucleus and the globus pallidus
internus. 75% of patients report that DBS markedly improved their motor symptoms and
Ninety-five percent of patients would recommend the treatment3 . Patients said they found the most benefit in treating tremors, but they also felt considerable improvement with rigidity and bradykinesia.4

Dystonia (FDA approved in 2003)

Dystonia is also a motoric disorder that causes muscle spasms. Patients suffer from involuntary muscle contractions leading to twist and jerks while trying to move. Dystonia can cause difficulties in communication as well as movement. Fairly often it affects jaw, tongue or vocal cords, which makes it difficult to speak loudly and clearly. DBS to treat dystonia targets the globus pallidus internus and leads to symptom reduction of up to 75%.5

Epilepsy (FDA approved in 2018)

Epilepsy is characterized by recurring seizures. The seizures are caused by abnormal firing by a group of neurons. Doctors will consider DBS for patients who find medication doesn’t help. The stimulations help to regulate groups of overactive neurons getting them back in sync. The doctor will place the electrode according to what form of epilepsy the recipient has. However, the anterior thalamic nucleus is the most common target, reducing seizures by 60%.6

New Applications

Although DBS is FDA approved solely for these four conditions, it is currently being tested in clinical trials for a variety of illnesses including:2,7

  • Obsessive-Compulsive disorder
  • Tourette’s Syndrome
  • Chronic pain
  • Opioid-related disorder
  • Alzheimer’s disease
  • Major depression
  • Obesity
  • Multiple Sclerosis

Could regulating the satisfaction-related parts of the brain help with compulsive disorders like addiction?

We’ll have to wait and see.

Who Can Get Deep Brain Stimulation?

Because Deep Brain Stimulation involves an invasive, expensive surgery and a lengthy adjustment period, it is not usually the first line of treatment for diseases. Although DBS has the potential to slow disease progression in early phases, it is usually only considered once medication fails.2 The results of DBS can vary considerably among individual
patients, as yet physicians are still gathering data on when and how it works best. As more people try DBS, doctors will be better able to predict which patients are candidates with a high chance of success.

Like any therapy, also DBS involves side effects. For example, Parkinson’s patients treated with DBS can develop problems with their long-term memory, may have difficulties finding the right words during speaking or they feel their thinking has slowed down.8 The surgery itself poses certain risks, for example damage of blood vessels or infections. The decision to undergo DBS, therefore requires careful consideration of the pros and cons.

Where does Deep Brain Stimulation Come From?

The technology was not the result of a single ‘eureka’ event, but gradually emerged from decades of brain surgery and a growing understanding of the nervous system. In the 1930s, doctors began treating epilepsy by surgically removing portions of the brain.9 The first step of the surgery was to locate the part that caused the abnormal activity. They used electrodes to stimulate those areas, often very deep inside the brain, while the patient was awake. Based on the patient’s response, they could identify the epileptic region.

Mind Mapping

While homing-in on the area of interest, surgeons stimulated different parts of the brain. Because patients were usually awake, surgeons could witness their reactions. Removal of apparently malfunctioning brain tissue became a common operation for otherwise untreatable brain conditions, not just epilepsy but also movement disorders or psychiatric ailments. With increasing numbers of surgeries, physicians could create a map of the brain and link certain areas to certain functions. Moreover, during stimulation doctors sometimes realized that the symptoms of the patients disappeared, especially in patients with chronic pain or movement disorders. The idea to continue stimulating after the surgery was completed, had practical limitations. The pulse generators to activate the electrodes were chunky and their batteries were short-lived. Thus, it was impossible to implant them in the body together with the electrodes.

Shrinking Technology

A technological breakthrough in another area solved this problem: the first cardiac pacemaker was developed and implanted in 1958. The improved battery technology and ever-shrinking microprocessors enabled the creation of small, implantable neurostimulators, too. With this device at hand, specialized hospitals began to treat conditions like dystonia or epilepsy with chronic (deep) brain stimulation. The breakthrough for this technique came in 1987 when Dr. Benabid and his team of neurosurgeons in Grenoble, France used this method successfully to treat patients with Parkinson’s disease, establishing DBS as a serious therapy.10 Since then, over 100,000 patients have undergone surgery for DBS.1

Ingenuity

Deep Brain Stimulation is an approach that exemplifies the ingenuity of humankind. It started with the observation that electrical stimulation of the brain changes patients’ symptoms. Step by step, DBS was transformed through mindful research and thoroughly conducted clinical trials into an approved standard treatment for several brain diseases. Now, skilled neurosurgeons regularly arrange precision electrodes deep inside delicate brain tissue. Deep brain stimulation is an extraordinary example of the medical progress made in the 20th century.

References:

This post originally appeared on Medical News Bulletin.