Sleep is a key part of our body’s healthy functioning and it is therefore not surprising that we nearly spend a quarter of our life asleep. However, for some people, their sleep gets disrupted by breathing problems, so-called obstructive sleep apnea, with previous reports suggesting that obstructive sleep apnea might increase our risk for Alzheimer’s disease. A new scientific study investigated this in more detail and found clear results*.
Sleep does not only allow us to get some well-deserved rest after a busy day but it also allows the body to ‘tidy up’ and restore many of our physiological functions. This is particularly true for the brain where sleep has key functions in clearing waste material from the brain.
Such ‘waste removal’ is especially important for Alzheimer’s disease. Alzheimer’s disease is caused by the accumulation of amyloid and tau proteins in and around the brain cells. The increasing accumulation of amyloid and tau leads them to become toxic to the brain cells, which in turn die and cause the symptoms of Alzheimer’s disease.
The body is trying to fight this accumulation of amyloid and tau but removing it as fast as it can from the body. Most of this removal happens during our sleep, with certain sleep phases being particularly important for this ‘waste removal’.
If our sleep is therefore interrupted regularly, it can effect the brain’s efficiency in removing the excessive amyloid and tau. The consequence of this is that amyloid and tau might accumulate faster, increasing our risk for Alzheimer’s disease. One common condition which can disrupt our sleep on a regular basis is obstructive sleep apnea.
What is obstructive sleep apnea?
Obstructive sleep apnea is a common sleep disorder, affecting up to 30% of the population. It causes people to start and stop breathing during their sleep. For obstructive sleep apnea, the obstruction or blockage occurs in our throat. When we sleep our soft palate – at the back of our throat – relaxes too much and blocks our airways which lie just behind it (see graphic below). It is, therefore, our own soft palate which causes the obstruction of our airways.
The apnea term means literally ‘lack of breath’ (from Greek a = ‘absence of’ and pnea = ‘breath’). Based on the term we know now that this is a condition which affects our sleep breathing when our soft palate is blocking our airways, in particular when we are lying on our back.
If breathing problems during sleep sound a bit scary, we should remind ourselves that we all at some time experience such mild breathing problems during sleep. But we call it simply snoring.
Snoring is nothing else than our soft palate slightly blocking our airways. When we breathe in or out during snoring, the soft palate ‘flaps’ like a rattling lid on our airways, causing these horrendous noises when we sleep. Usually, the more relaxed our soft palate is the more we snore, which explains why we often snore more after drinking too much alcohol, as alcohol is a muscle relaxant.
Snoring from time to time is completely normal but obstructive sleep apnea is something more serious. During obstructive sleep apnea, instead of the soft palate only ‘flapping’ on our airways, it actually completely blocks our airways. This blockage causes us literally to stop breathing for short periods before the air comes through again.
These short periods – usually only a few seconds – of no breathing cause a mild form of hypoxia in the body. Hypoxia means too little oxygen (from hypo = ‘little’ and oxia = ‘related to oxygen’) and signifies that the body does not receive enough oxygen to work properly.
Short periods of hypoxia are nothing to worry about, however longer or repeated short periods of hypoxia can create difficulties for the body. It is those repeated short periods of hypoxia which are caused by obstructive sleep apnea.
Imagine having these short periods of lack of breath and oxygen every night for many years. It will make an impact on your health and particularly your brain health.
Many previous studies have shown that people with obstructive sleep apnea, in particular those with the more severe forms, show changes in the brain and its function. Especially the hippocampus and its memory functions are affected by the hypoxia due to obstructive sleep apnea.
Since obstructive sleep apnea affects the hippocampus and Alzheimer’s disease also affects the same brain regions, there have been suggestions that obstructive sleep apnea increases the risk for Alzheimer’s disease.
The logic is that due to the obstructive sleep apnea, not only our sleep is disrupted affecting the clearance rate for excessive amyloid and tau but the hypoxia caused by obstructive sleep apnea ‘weaken’ the key region for Alzheimer’s disease, the hippocampus.
Such a double-hit of reduced clearance and weakened hippocampus might explain why people with obstructive sleep apnea are at increased risk for Alzheimer’s disease. However, to date, there has been no conclusive evidence whether obstructive sleep apnea actually causes increased amyloid or tau accumulation and therefore increases our risk for Alzheimer’s disease.
A new study investigated exactly this issue by analysing the post-mortem brains of people who had obstructive sleep apnea for a long time. They investigated whether those people showed higher levels of amyloid or tau in their hippocampus and whether this weakened the hippocampus.
The researchers found that indeed people with obstructive sleep apnea had increased amyloid levels in the hippocampus, in particular those with more severe obstructive sleep apnea. For tau, the results were more not conclusive and need to be further investigated but there was some indication that tau clearance is also affected by obstructive sleep apnea.
The authors conclude therefore that obstructive sleep apnea indeed increases the risk for Alzheimer’s disease by making the hippocampus more vulnerable to Alzheimer’s disease. It is the combination of the hypoxia, which weakens and shrinks the hippocampus, plus the reduced removal of amyloid (and potentially tau) because of the disrupted sleep, which explains the increased risk for Alzheimer’s disease in people with obstructive sleep apnea.
The good news is that we can reduce our risk for obstructive sleep apnea by staying within a healthy weight range, exercising regularly, stopping smoking and reducing our alcohol intake. These are all factors which do not only reduce our risk for obstructive sleep apnea but also cardiovascular disease, cancer and even Alzheimer’s diseases.
It is therefore worth emphasising how important a healthy lifestyle is for the prevention of Alzheimer’s disease, as current evidence suggested that adherence to a healthier lifestyle shows can reduce our risk of Alzheimer’s disease by up to 40%.
That certainly takes my breath away.
*Alzheimer’s disease neuropathology in the hippocampus and brainstem of people with obstructive sleep apnea. Owen JE, Benediktsdottir B, Cook E, Olafsson I, Gislason T, Robinson SR. Sleep. 2020 Sep 21:zsaa195. doi: 10.1093/sleep/zsaa195