4.3.1

Vagal Nerve Stimulation

Vagal nerve stimulation (VNS) is a medical treatment that delivers electrical impulses to the vagus nerve. Several systems on the market allow patients to treat themselves (auricular stimulation with removable electrodes). Here are some essential aspects of VNS:

  1. Medical uses: VNS is primarily used to treat certain types of epilepsy and depression. For epilepsy, it's typically considered when seizures are not well controlled by medication. For depression, it's used in cases that haven't responded to other treatments.

  2. How it works: The device used for VNS is similar to a pacemaker. It's usually implanted under the skin in the chest, and a wire runs from the device to the vagus nerve in the neck. The device sends regular, mild pulses of electrical energy to the brain through the vagus nerve, which can help regulate abnormal brain activity.

  3. Effectiveness: For epilepsy, VNS typically doesn't eliminate seizures, but it can reduce their frequency and intensity. For depression, results may vary, and it may take several months to see improvement.

  4. Side effects: Common side effects include voice changes, sore throat, cough, and shortness of breath, especially when the device is active. Most side effects are mild and tend to diminish over time.

  5. Procedure and Maintenance: Implantation of the VNS device is a surgical procedure. A physician may adjust the device settings externally to optimize treatment. The battery life of the device is limited and may require replacement surgery.

  6. Mechanism of Action: The exact mechanism by which VNS works is not fully understood. It is thought to affect various neurotransmitters and pathways in the brain, affecting mood and seizure activity.

  7. Recent developments: Ongoing research explores the use of VNS for conditions such as chronic pain, heart failure, and certain types of headaches.

  8. Mechanisms of Action (MOA)
    Because stimulation is applied to the vagal nerve without distinguishing between the various fibers, researchers still don't know exactly what happens. 80% of the fibers are sensory, ascending to the NTS (part of the dorsal), and 20% are divided between the ventral and dorsal branches of the parasympathetic motor fibers. The Polyvagal Theory can't help us here. But one thing is sure: adhering to the dichotomous polyvagal view — a vagal system divided into a good ventral and a bad dorsal — is counterproductive. It creates a massive obstacle to understanding the healing vagal circuits that include all the afferent and efferent branches of the vagal nerve.

Further reading on mechanisms of action (MOA)

In Vagus nerve stimulation as a tool for enhancing extinction in exposure-based therapies (Noble, 2019), the authors describe how to implant the VNS to treat fears. Let us quote a passage from the article linking the VNS to the PVT:

In this sense, stimulation of the vagus nerve results in upstream activation of brain areas involved in learning, while it reduces heart rate and controls tachycardia (…) providing evidence for what Porges describes as the polyvagal theory (Porges 1992). According to the polyvagal theory, low vagal tone is associated with the inability to overcome stress (Porges 1992; 2009) while higher vagal tone correlates with active forms of coping and responsivity to environment challenges (Porges 2009).

We have a truism here: when the calming system is active, we are calm, and when the calming system is depressed, we cannot calm ourselves. That doesn't explain the mechanism of action. Quoting Porges doesn't help.

Even with intraneural electrodes (Marmerstein 2021), researchers can't identify which fibers produce the desired effect (in depression, epilepsy, heart damage, etc.): afferent sensory or efferent motor fibers. At best, they can measure the global activity of the vagal nerve (vagal tone), which is not quite identical to HRV (ibid, 2021). As scientists often state, more research needs to be done.

Vagus nerve stimulation: An update on a novel treatment for treatment-resistant depression (Kamel, 2022). This review provides details on key clinical trials investigating the effects of VNS in treatment-resistant depression (TRD), discusses the neuroanatomy of the vagus nerve, and the mechanism of action of VNS in TRD.

Vagus Nerve Stimulation for Depression: A Systematic Review (Lv, 2019). New non-invasive VNS (nVNS) devices have been developed to allow external vagus nerve stimulation. Brain imaging, blood biomarkers, and wearable sensing devices can measure their effects on patients' physiology. nVNS affects autonomic tone, cardiovascular function, inflammatory responses, and central brain areas involved in emotion modulation. This makes it particularly applicable to patients with stress-related psychiatric disorders (e.g., PTSD and depression). However, the authors conclude: “The efficacy and safety of VNS for depression is still unclear. Further randomized controlled trials are needed to confirm the efficacy and safety of VNS.”

Non-invasive Vagal nerve stimulation effects on hyperarousal and autonomic state in Patients with Posttraumatic stress Disorder and history of Mild Traumatic Brain injury: Preliminary evidence (D. G. Lamb, E. C. Porges, G. F. Lewis, and J. B. Williamson, 2017). With Eric Porges and J.B. Williamson on the team, we expect some polyvagal quotes. And there it is, at the end of the discussion: “These effects suggest that tVNS may modulate emotional state as reflected by downregulating fight-or-flight and upregulating a physiological state conducive to positive social engagement (Porges, 2007).”

Untangling the Vagus Nerve, (Pincott Jena), Scientific American, January 2025, Vol. 332.

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