A real public health challenge, Alzheimer’s disease has origins that are still poorly understood and to date does not benefit from any curative treatment. Hence the importance of being able to identify people at risk as early as possible. Sleep disorders appearing long before the diagnosis of Alzheimer’s disease was made, scientists wanted to know if they were caused by the disease itself or if they participated on the contrary in neurodegeneration. They then plunged into the secrets of REM sleep…

Alzheimer’s disease and sleep disorders: what is the link?

Although it affects nearly a million people in France, Alzheimer’s disease remains a disease whose origins are still poorly understood and whose curative treatment proves to be non-existent. In practice, it is the accumulation in the brain of neurotoxic deposits of amyloid protein that causes the disease. It is then most often manifested by memory problems.

But people with Alzheimer’s disease can also suffer from sleep disturbances that often appear long before the onset of dementia. Therefore, scientists wanted to know if the sleep disorders were caused by the disease itself or if they were on the contrary a risk factor. In this second case, would sleep disorders promote the neurotoxic deposits observed in the brain?

In an attempt to elucidate these questions, a team of Inserm scientists have delved into the secrets of REM sleep in the elderly. Conducive to dreams, REM sleep is a phase of sleep that particularly interested them for two reasons:

• REM sleep involves neurons that are affected early in Alzheimer’s disease.
• Changes in paradoxical sleep are observed in the early stages of the disease, before those affecting slow-wave sleep.

To know ! A night’s sleep consists of successive cycles themselves structured by an alternation between “slow” sleep and “paradoxical” sleep. During REM sleep, conducive to dreams, brain activity is close to that of the waking phase.

Identifying people at risk through REM sleep?

To carry out their work, the scientists therefore analyzed the architecture of paradoxical sleep in a cohort of elderly people with no cognitive disorders, as well as the presence of neurotoxic deposits in their brains. They were then able to draw the following conclusions:

• The power of the theta brain waves characterizing REM sleep was all the lower as the amyloid deposits in the brain were numerous. This correlation might not be established with any other waves characteristic of slow, light or deep sleep.
• The decrease in theta brain wave power was accompanied by an increase in blood flow to several regions of the brain (frontal and parietal areas).

To know ! The cerebral electricity that is measured on the surface of the scalp is in the form of brain waves. There are four main types of waves according to their frequency: alpha waves (characteristics of calm wakefulness), beta waves (active wakefulness and sleep), theta waves (onset of sleep) and delta waves (sleep slow and deep).

The second finding is all the more surprising for scientists since a brain area whose activity is reduced should logically need less oxygen and therefore less blood flow. The team of researchers then put forward the hypothesis that the increase in blood flow in the frontal and parietal areas might reflect a compensatory mechanism. This transient mechanism contributing to maintaining normal neuronal activity might nevertheless prove to be deleterious in the longer term and lead to cognitive decline in the patient.

Next step for researchers? Confirm that sleep disorders have a predictive value for progression to Alzheimer’s disease. For this, it will be necessary to study in these same volunteers the evolution over time of sleep disorders, cognitive disorders and amyloid deposits. The researchers also aim to set up studies on a larger scale. The objective is to be able to identify people at risk as early as possible and to offer them preventive approaches.

Déborah L., Doctor of Pharmacy

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