"So what causes AMS?"
Now that's a question that can leave anyone floundering! The answer, like so many things in life, starts simply but quickly becomes more complicated. If you want to know how I try to explain AMS in 5 minutes or less, please read on...
The easy part of the answer is that AMS, like all other high altitude illnesses is triggered by a lack of oxygen. As we ascend to altitude the pressure exerted by the gases in the atmosphere starts to fall. Oxygen isn't excluded! Unfortunately movement of any gas through the human body relies upon a decent amount of pressure. When this pressure is high, such as that found at sea level, oxygen molecules speed through the lungs, enter the blood stream and penetrate the tissues at a rapid rate of knots. But as we climb higher that pressure falls and oxygen molecules drag their heels and slowly meander through the body. The result? Oxygen doesn't get to where it's needed, cells stop functioning and any form of physical or mental activity gets much harder to do.
If you want to demonstrate the effect of pressure upon the movement of oxygen all you need is a party balloon. Blow it up as much as you can and let go. The air inside the balloon represents the pressure of oxygen at sea level. Let go and watch it zip around at great speed just like the oxygen entering the body and travelling towards the cells. But fill the balloon with only 1 or 2 breaths and you'll get a good idea of what happens at high altitude. With very little pressure inside the balloon there is no movement and the balloon drops slowly to your feet.
So, there you have it - it is the lack of oxygen that is to blame for high altitude illnesses! But that's not enough is it? We all experience a lack of oxygen at high altitude but only some of us get AMS. Why's that? Well, that's the harder question to answer. One thing that we do know is that AMS sufferers probably have a little less oxygen in their circulation than those who don't. This can be measured by either a blood test from an artery or something a lot less messy - a pulse oximeter probe placed on a warm finger tip! The pulse oximeter measures the percentage of red blood cells fully loaded with oxygen. As the pressure of oxygen decreases, the lower the oxygen saturation drops. At sea level oxygen saturation in healthy humans is rarely less than 96%, however at altitude this can plummet. Measurements taken near the summit of Mt Everest have occasionally been found to be in the 30's!
In the late 1980's a team of Swiss researchers studied a group of 66 mountaineers who climbed to the Margherita Hut (4559m) on the Monte Rosa massif. Measurements taken showed that AMS sufferers had oxygen saturations 9% lower than healthy volunteers (Bartsch et/al 1987)
If you're someone who has less oxygen in your circulation this is due to 1 of 2 reasons. Either you're using a lot more of it OR there's not enough getting into your body. The former may explain why AMS is commoner in those who move fast or carry heavy loads. It might also be one of the reasons obesity is now recognised as a risk factor for the condition. But these are just ideas and haven't been formally studied. Researchers have instead focused upon the second explanation - those with AMS are unable to get enough oxygen in. This has been put down to 2 reasons. Either their brains fail to send messages to the lungs to breathe faster and deeper when faced with a lack of oxygen, or there is some form of barrier in the lungs that limits the passage of gas itself. Thanks to the arrival of portable ultrasound devices there has been much made of the small volumes of fluid that accumulate in the lungs following ascent to altitude. These have the potential to flood the alveoli and prevent oxygen from crossing into the capillaries. Whilst small studies suggest that this as a possible explanation for lower oxygen saturations in AMS sufferers more research in this area is much needed!
An observational study of 18 healthy trekkers in Nepal revealed that those who developed AMS had lower oxygen saturations and more extensive pulmonary oedema than those who remained healthy (Pratali et/al 2010). Pulmonary oedema can be seen on ultrasound as B lines or "comet tails" - these sharp white lines stretch the full length of the lung and are only seen in pulmonary edema
So far so good? OK, but there's one big question left to answer - what connects this lack of oxygen with AMS symptoms like a splitting headache? Most of what's contained within the skull doesn't feel pain. In fact, only the outermost lining of the brain and blood vessels have pain receptors. So if you want to find an explanation for AMS these are the places to start.
Let's take the lining first. Historically, it was thought that AMS was due to swelling of the brain that caused an increase in pressure and a stretch of the brain lining. If you were immune to AMS you were widely believed to have plenty of space within your skull to accommodate this swelling and enjoy only a very small pressure rise. This explanation became known as the "tight fit" hypothesis. Historically, supportive evidence was limited. In general, MRI scanning in AMS sufferers revealed only a very modest increase in volume (<10mls) and measurements of pressure within the skull were no higher than those who were symptom free. However in the last few years studies are beginning to emerge showing possible links between swelling, pressure and AMS. More results are likely to emerge soon - watch this space!
Retinal blood vessels at sea level (A) and after 24 hours at 5300m (B)*. Enlargement of retinal veins is commonly seen at altitude and has been linked to AMS in a small number of trials
Now for the blood vessels. An increase in blood flow is seen in the arteries and veins of those who ascent to altitude. For many years the focus was on linking high flows through the arteries to AMS. Unfortunately, this was never clearly shown. More recently, attention has turned towards the passage of blood through the veins. This has shown that in some individuals with AMS the drainage of blood from the brain is limited by a series of anatomical variations. The resulting build up of pressure may have a large part to play in AMS and explain why symptoms are worse at night or occur when bending down. At this stage much still needs to be done to establish this link however it's likely to play a significant part in the AMS of some sufferers.
So there we have it - AMS is caused by a lack of oxygen, it is worst in those who have the least oxygen and the symptoms are triggered by pathways that we don't yet fully understand!
*For more detailed information on the pathophysiology of AMS I would strongly recommend Mark Wilson's article Intracranial Pressure At Altitude. Let me know if you need a copy. Andy Luks' brilliant overview of high altitude illnesses can be downloaded here.
See this post for an illustration of how difficult diagnosing AMS can be!