Vagus Nerve/Sleep Apnea/Chiari

by Dr. Watson, Assistant Professor of Neurology at the University of Washington (UW) and Co-director of the UW Sleep Disorders Center at Harborview.

When I’m talking about sleep disordered breathing, I’m talking about sleep apnea mostly, which is a term that you all may be familiar with. Sleep apnea is basically having problems with breathing during sleep at night. There are multiple types of sleep apnea. There is obstructive sleep apnea - where there’s a blockage of tissue keeping you from breathing effectively. There’s central sleep apnea - where your brain is not telling the body to breath in your sleep. Then there are milder forms of obstructive sleep apnea which are similarly a result of blockages in breathing, what we call respiratory effort-related arousals. I’ll talk more about that and give you more background.

The relationship between Chiari Malformations and respirations is actually fairly complex. The way that we breathe is our brain samples carbon dioxide and oxygen from the blood as it goes through our brain stem and it adjusts our breathing level accordingly. When we’re awake we have a wakefulness drive to breathe, so we can remember to breath and we can cause ourselves to breathe when we’re awake. When we’re asleep our breathing is totally dependent on our brain stem measuring specific carbon dioxide levels and oxygen levels.

Chiari Malformation and RespirationThese areas here, [see Figure 4] you can see in the brain stem, the Dorsal and Ventral Respiratory Group and the Pontine Respiratory Group. These are the areas that have these cells or these chemo-receptors that take these measurements and then tell the body to breath. You can imagine that all the pressure that’s going on in this region in patients with Chiari Malformations that this affects the functioning of these cells. So that’s what happens. They don’t measure these blood gases as well as they should and therefore breathing is affected.

cranial nervesThere are some other things going on as well. [see Figure 5] Cranial nerves that are coming out of this region that sub-serve muscles of the tongue and the throat can be stretched and affected. That can cause your throat to become floppy and collapse in on itself also causing problems breathing in your sleep. Lastly there are nerves that send signals from the lung stretch receptors and also what’s called the carotid body and the aorta that are also monitoring these blood gases. So those nerves can be affected as they’re feeding information to this area. The signals that come out of here to the diaphragm and the other muscles that breathe can also be affected, particularly by people that have co-morbid syringomyelia. So it’s really a problem with blood gas sampling, it’s a problem with the output of these centers to control breathing and a problem of the sensory input that tells the brain what’s going on with the rest of the body as far as breathing is concerned.

This is a diagram [see Figure 5] that is showing you the nerves that would be involved that would be stretched and causing problems. We have 9, 10 and 12. The Glossopharyngeal nerve sends motor output to the upper pharyngeal muscles here and what’s called the Stylopharyngeus muscle which also helps keep the airway open. The Vagus nerve, the tenth nerve, sends motor output to the lungs so it helps control breathing and also the pharynx and larynx; so again here in the throat region. Then the Hypoglossal nerve, the twelfth nerve, controls motor activity in the tongue. That’s important because people with obstructive sleep apnea will often have a tongue that can fall back into your airway when you lie on your back and block air flow. Stretching and damage to these nerves can cause problems.

obstructive sleep apneaLet me talk a little bit more about the two different kinds of sleep apnea, the obstructive sleep apnea and the central sleep apnea. Obstructive sleep apnea as a sleep position is by far the most common type of sleep apnea that we see. What we have here is 120 seconds of an overnight polysomnogram, [see Figure 6] which is a sleep study which some of you may or may not have had. Just to go through these channels to let you know what we’re looking at in this colorful picture. This is measuring brain waves to tell you what stages of sleep you’ll be in during the night. Then you have this, measuring eye movements, which helps us stage sleep as well. A chin tone, which is muscular tone in the chin, helps us tell what sleep stage you’re in. We look at snoring; we look at chest and abdominal movement. This is very important because this is telling us whether or not a person is trying to breathe.

If you’re having central apnea, your chest and abdomen aren’t moving because your brain’s not telling your body to try to breathe. When you have an obstructive apnea, as a result of an obstruction in the throat and the nose and everything, your chest and abdomen are trying to suck in and create negative pressure to breathe. Itâ’s just that air can’t get through that area. Here you can see that this is the pressure flow transducer at the nose (which measures air flow at the nose) and we just have these repetitive apneas over and over again. There’s no air flow going on here, there’s a drop in the oxygen saturation level that’s pretty severe in this patient, in each one of these. That’s the amount of oxygen in the blood which is obviously important for your body functioning.

Then there’s an arousal - so there’s kind of a snort and a wake up here that opens the airway back up, allows a little bit of breathing, and then the patient falls back asleep and this happens again. This is very insidious, the patient doesn’t usually know that it’s happening unless their bed partner tells them about it or they’re noticing they’re sleepy during the day. Those are the reasons that these people end up coming to see me.

The important relationship between this and Chiari is that some people that have this can elevate because they’re not breathing well, they’re carbon dioxide levels can go up. When that happens, the blood vessels in the brain can dilate and so more blood can go into the blood. That has a tendency to increase intracranial pressure and increasing intracranial pressure is bad for people with Chiari Malformations. So it can make the Chiari Malformations worse so that’s a vicious cycle. The Chiari Malformation is making the sleep apnea worse, and then the sleep apnea with the CO2 and so that’s a concern.

The obstruction in obstructive sleep apnea can be multiple areas, large tonsils, large uvula, which is that thing that dangles down in the back of your throat, blockages in the nose, septal deviations and things like that. Large tongues can also contribute to this. And then also muscle tone in the throat, as I mentioned, those three nerves that can be stretched. When we all fall asleep we actually have about twenty muscles in our throat that have to increase their tone to keep your airway open. If they don’t do that very well you get a collapsible floppy airway which is a real problem for people.

For obstructive sleep apnea the prevalence is actually pretty high, it’s one of the most common diseases known to man. If you just took a random sampling of people off the street and did a sleep study on them and you said that those that had apneas or hyponeas (which are like apneas but not as severe) if they had more than five of those an hour you would diagnose them with this. You would find that about 9% of women and 24% of men would have an apnea/hyponea mix higher than 5. If you ask a second question where you say you also need to have symptoms in order for this to be a disorder; then it’s about 2-4% of the population that has this problem. That’s in the general population, not in Chiari folks and we’ll talk about Chiari specifically in a minute.

obstructive sleep apneaThe risk factors are, [see Figure 7] in the general population being a man, being older, being overweight, those are the most common ones. Smoking can contribute, having a large neck size. In men, if the neck size is 17 inches or greater you’re at risk for this. In women, if the neck size is 16 inches or greater. The treatments for it are really three - this is for the run of the mill patient, not the patient with the Chiari. CPAP, continuous positive airway pressure, which is a little mask, you can see this gentleman wearing it over his nose. It’s connected by a tube to a small box that pushes pressurized humidified room air. So there’s no additional oxygen or anything like that being put in there, and it’s usually between 5 and 20cm of water pressure and it works like a pneumatic splint to sort of pop the airway open and keep it open when you sleep. In people that have trouble with that or if they have really big anatomic obstructions, large tonsils for instance, we can do different types of surgeries. This is an example of a uvula palatal pharyngoplasty or a UPPP. We trim out the uvula which dangles down here and part of the soft palate. This is not usually curative but helpful and definitely helpful for snoring. Lastly, dental appliances that can be worn at night that move the lower jaw forward can pull the tongue forward and open up this air space.

central sleep apneacentral sleep apnea diagramCentral sleep apnea, which only makes up about 5% of the people with sleep apnea that we see [see Figure 8]is shown here with this polysomnogram fragment. This is similar to what I showed you before but what I want you to focus on down here is these chest and abdomen measurements of movement. You can see here, when we have these cessations in air flow right here, now we’re not having any respiratory effort here. This patient is just not trying to breathe when they’re asleep. This is often complicated as to what’s causing this. We divided it into whether or not CO2 levels are up or not. Some people have it, it’s called idiopathic, we don’t know why. Some people can have problems with this because they’re very, very obese. Other people that have neuromuscular diseases can have problems with this. Heart failure can often result in this type of picture. For those people that have central sleep apnea, here are all the different causes.