The vagus nerve, the cranial base and heart rate variability: what is established and what is not

This is the subject I get asked about the most, and the one I read the most nonsense about. The vagus nerve has become a password. It gets credited with sleep, digestion and anxiety, and apparently you only need to “release” it at the base of the skull for everything to fall back into place.

It is not that simple. But there is something real in it, and that real part deserves to be stated properly.

A system with no on-off switch

The autonomic nervous system runs what you do not decide: heart rate, the calibre of blood vessels, digestion, sweating. It gets presented as two opposing camps, the sympathetic that accelerates and the parasympathetic that brakes. That image is convenient and slightly false.

Both work all the time, together, in proportions that shift by the minute. No one is “in sympathetic mode”. You have a balance that leans, and that should be able to lean back.

So the problem is not being on alert. It is no longer knowing how to come out of it.

What makes the cranial base particular

The interest of this area is not an osteopath’s idea. It is anatomy.

The vagus nerve leaves the skull through the jugular foramen, an opening between the occipital bone and the temporal bone. It passes there alongside the glossopharyngeal nerve and the accessory nerve. Just in front sits the canal of the hypoglossal nerve. All around are the suboccipital muscles, among the most densely supplied with position receptors in the whole body.

About four fifths of the vagus nerve fibres do not run down towards the organs: they run up towards the brain. The vagus is first of all a nerve that informs, not a nerve that commands. Berthoud and Neuhuber documented this in 2000, and it changes how its role should be understood.

So much for the facts. Now the point where the reasoning goes off the rails.

What was inferred from it, and is not there

From a bony opening crossed by a nerve, people concluded that the nerve could get “trapped” and that a manual technique would free it.

A foramen is not a joint. It is a hole in bone, and its dimensions do not change between one session and the next. The suture between the occipital and temporal bones does exist, and it keeps a degree of micromobility in adults, but no one has shown that a practitioner can assess its state reproducibly. The systematic review by Guillaud and colleagues, published in PLoS ONE in 2016, is clear on this: agreement between two examiners on cranial tests is poor, including between experienced practitioners.

I say this plainly to my patients. When I work on that region, I do not claim to free a trapped nerve. I work on muscles, on fascia and on upper cervical mobility, which are accessible and which I can assess.

Heart rate variability, and what it really measures

HRV measures the differences in duration between two heartbeats. A healthy heart does not beat like a metronome. That fine irregularity partly reflects vagal activity, and it is the best validated non-invasive marker of autonomic tone. The measurement standards date from 1996, published jointly by the European Society of Cardiology and the North American society of pacing and electrophysiology.

It is a good indicator. It is not a score for your health.

HRV falls after a short night, after a coffee, during digestion, at the end of the day, and rises again the other way round. It varies a great deal between people according to age. Comparing your value with someone else’s means nothing. Following your own trend across several weeks means something.

On osteopathy and HRV, the systematic review by Rechberger, Biberschick and Porthun, published in PeerJ in 2019, gathered the available trials. Autonomic changes are reported after manual treatment, but the samples are small, the protocols heterogeneous and the risk of bias high. The honest conclusion fits in one line: encouraging and insufficient.

And a measurement taken ten minutes after a session says nothing lasting. Lying down quietly for twenty minutes raises HRV. In everyone.

Hormonal stress

Cortisol follows a precise cycle. It climbs sharply in the half hour after waking, which is called the awakening response, then declines until the evening. Under prolonged stress, that profile flattens.

What is established is the link between chronic stress and the distortion of that profile. What is not established is that a session of manipulation corrects it. I have no argument for asserting that, so I do not assert it.

Neurotransmitters, where the shortcut is worst

You regularly read that a manual technique “releases serotonin”.

Between 90 and 95 per cent of the serotonin in the human body is produced in the gut, where it regulates digestive motility. It does not cross the blood brain barrier. Circulating serotonin and brain serotonin are two separate stories, and measuring the first tells you nothing about mood.

When someone explains that a technique applied to your neck will act on your brain neurotransmitters, ask for the measurement. It does not exist.

What I actually work on

A body kept on alert eventually shows it. Neck and trapezius under constant tension. Jaws clenched, at night without knowing it. High, short breathing, a chest that barely moves, a diaphragm that no longer travels far. Tension headaches at the end of the day.

These are musculoskeletal signs. They are within my scope, I can assess them and work on them. That is not the same as claiming to fix autonomic regulation itself, and the distinction is not a matter of vocabulary.

There is one solidly documented autonomic lever, and it does not belong to me: slow breathing. At around six cycles per minute, with an out breath longer than the in breath, you enter resonance with the baroreflex and heart rate variability rises clearly and reproducibly. It is measurable, it is free, and you can do it again at home on your own.

I show it at the end of a session. Five minutes in the evening is worth more than an hour every fortnight in my practice.

When you should see a doctor first

Some situations are not for me, and I would rather say so before an appointment is booked.

Palpitations, fainting, unusual breathlessness on exertion. Unintended weight loss, night sweats, a fever that drags on. A tremor together with weight loss and heat intolerance, which points towards a thyroid workup. Snoring with daytime sleepiness, which calls for a sleep assessment. Anxiety that stops you working or going out, and dark thoughts.

In all of these, your doctor first. I remain available afterwards, and quite readily alongside.

What I answer when I am asked whether it works

That I do not know how to regulate a nervous system. That I do know how to recognise and ease the tensions that a lasting state of alert installs in a body, which is not nothing when you have lived with it for months. And that the most effective part of what I offer is the part you take away with you.

It sells less well than a promise of rebalancing. It is what I can hold to.

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Sources

• Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation, and clinical use. Circulation, 1996.
• Rechberger V, Biberschick M, Porthun J. Effectiveness of an osteopathic treatment on the autonomic nervous system: a systematic review of the literature. PeerJ, 2019.
• Guillaud A, Darbois N, Monvoisin R, Pinsault N. Reliability of diagnosis and clinical efficacy of cranial osteopathy: a systematic review. PLoS ONE, 2016.
• Berthoud HR, Neuhuber WL. Functional and chemical anatomy of the afferent vagal system. Autonomic Neuroscience, 2000.
• Lehrer PM, Gevirtz R. Heart rate variability biofeedback: how and why does it work ? Frontiers in Psychology, 2014.

This subject overlaps with jaw tension and difficult sleep. I write about it more widely on the page stress, sleep and the nervous system.