Central Sleep Apnoeas
by Dr J M Shneerson MA DM FRCP
Director of the Respiratory Support and Sleep Centre,
Papworth Hospital Cambridge
When people think of sleep apnoeas they normally think of obstructive sleep apnoeas. In some situations however, the irregularities in the breathing pattern are of a different type and are called central apnoeas. Central apnoeas are due to a problem either in the drive to breathe which is generated within the brain or to weakness of the breathing muscles. The result is that the patient does not appear to make any breathing movements and no air enters the lungs through either the mouth or the nose (figure 1). As a result the oxygen level in the bloodstream falls and carbon dioxide which is a waste product of the body's chemical processes, accumulates. After a while the changes in these two chemicals in the bloodstream stimulate breathing again and the patient starts to make respiratory efforts. This pattern is rather different to what is seen with obstructive sleep apnoeas where the drive to breathe is normal and the breathing muscles are perfectly strong. The problem in obstructive sleep apnoeas is that the muscles that normally hold the airway open in the throat over-relax during sleep so that it can flop closed. This leads to strenuous breathing efforts against the closed or partially closed airway with a variety of snoring and snorting sounds and eventually arousal from sleep when the intensity of the breathing efforts reaches a certain level.
Fig 1. Sleep study tracing showing 30 second central apnoea with no air flow and little thoracic or abdominal movement until the moment of arousal which is marked by movement of the patient's legs as well as respiratory movements and the return of air flow into the lungs.
The partner of the patient with central apnoeas observes that the patient appears to stop breathing and usually lies perfectly still until arousal from sleep, at the end of the apnoea, takes place. Most patients are only aware of arousing from a very small percentage of central apnoeas that take place during their sleep. Most of the awakenings are too brief to be registered mentally but they do fragment the sleep pattern and lead to a feeling of hardly having slept when the patient wakes in the morning. Excessive daytime sleepiness is probably as common with central as with obstructive sleep apnoeas and can lead to severe problems both at home and at work.
Conventionally an apnoea is defined as cessation of air flow into the lungs which lasts for more than 10 seconds, but this is rather an artificial definition and some apnoeas which last for more than 10 seconds during sleep are perfectly normal. These occur particularly during rapid eye movement (or dream sleep) during which the breathing pattern is always erratic. It is quite common for intervals between breaths to last for 10-20 seconds without there being any underlying sleep or respiratory disorder. Another common situation in which breathing may stop for more than 10 seconds is when the stage of sleep changes. The level of response to the oxygen and carbon dioxide levels in the blood varies according to the stage of sleep and in general the response is less in the deeper than the lighter stages of non-rapid eye movement sleep and even less in dream sleep. When, for instance, the patient moves from a lighter to a deeper stage of sleep, breathing may temporarily stop until the carbon dioxide level reaches the new level at which the respiratory centres in the brain begin to respond. If the patient switches sleep stages rapidly the pattern of breathing can fluctuate considerably. This type of unsteady sleep occurs particularly early in the night and often worries the patient's partner who is frequently still awake.
Occasionally this instability in the breathing pattern has a regular waxing and waning pattern in which case it is known as Cheyne-Stokes respiration after two doctors who first described the condition around 200 years ago. Cheyne-Stokes respiration can also occur in specific situations, such as at altitude or if the patient has heart disease which causes the circulation to slow down, but more commonly it is a perfectly normal event, particularly in older people. There is also a variety of normal reflexes which can temporarily stop or slow down the breathing pattern.
These tend to be reflexes which in some way protect the lungs. A well-recognised reflex is when the larynx (or voice box) is stimulated by acid which has regurgitated up from the stomach into the throat. During sleep this will cause the breathing to stop which in effect prevents the acid from being inhaled into the lungs. To the partner however it would appear simply as a central sleep apnoea. A similar reflex is recognised when the walls of the throat or pharynx close off. This occurs during obstructive sleep apnoeas and some patients with this condition, therefore develop a mixed picture with obstruction occurring first and a central apnoea following the obstruction. It is not known why only some people develop central apnoeas with these types of reflex stimulation, whereas in other people any changes in breathing pattern are much less obvious.
Occasionally central apnoeas can be a sign of a disorder of the breathing centres in the brain which are responsible for the drive to breathe. Normally these centres respond to a variety of reflexes and other nervous and chemical inputs, but in some people they are insensitive, particularly to a build up of carbon dioxide or a lack of oxygen in the blood. This insensitivity may be the result of a stroke or inflammation of the brain, but some people are born with a defect in the respiratory centres or the nerve pathways leading to or from these centres. This can cause central apnoeas early after birth, but more frequently the condition is first noticed when patients are in their twenties or thirties. A similar but usually much milder form of respiratory depression leading to slowing or stopping of breathing can occur either with severe sleep deprivation or with sedative drugs, including sleeping tablets such as benzodiazepines and alcohol. Another type of 'central' apnoea is one which is not due to under activity of the respiratory centres, but is a result of severe weakness of the breathing muscles. In this situation although the respiratory centres try to stimulate the muscles they are too weak to respond and the patient appears to be not taking any breaths. This type of 'central' apnoea is a feature of muscular dystrophies and other similar types of condition affecting the nervous system. In very obese subjects it may also be difficult to detect any breathing movements because of the fatty tissue overlying the chest, and central apnoeas may be suspected although more commonly the problem is purely obstructive apnoeas.
Some central apnoeas are perfectly normal
There are therefore many types of central apnoea some of which are perfectly normal while others can indicate a disorder either of the brain or the breathing muscles. Not surprisingly there is no single treatment which is effective in all these situations. The first step is usually to be sure what type of central apnoea is present. This is often difficult simply from watching the breathing pattern and a detailed sleep study is usually needed. The flow of air into the nose and mouth, the expansion of the chest and abdomen, recording of the oxygen and carbon dioxide levels in the blood and often measurement of the sleep stage throughout the night are required to assess the situation accurately. The facilities that are needed for this type of detailed sleep study are available in a small number of specialist centres.
The results of the study guide the approach to treatment. Quite often it is sufficient to reassure both the patient and the partner that there is nothing wrong. Occasionally drugs which alter the drive to breathe can help to stabilise the breathing pattern when the irregularity is due to rapid or frequent changes in sleep stage, but most patients with Cheyne-Stokes respiration do not need any specific treatment. When central apnoeas are the result of reflexes originating in the larynx or pharynx it is usually best to treat the underlying cause, for example the regurgitation of acid from the stomach into the throat. Sometimes nasal continuous positive airway pressure (CPAP) can be very effective in these situations and this treatment is identical to that which is used in obstructive sleep apnoeas. When the apnoeas are due to a disorder affecting the breathing centres in the brain or caused by severe weakness of the breathing muscles a more intensive type of breathing support equipment is usually needed. This uses the same type of nasal mask as with CPAP but instead of the pump producing a constant pressure, a more complex machine called a ventilator pushes air into the lungs intermittently and between these machine assisted breaths air is exhaled from the lungs. In this way the lack of breathing efforts is compensated for by the ventilator, and the oxygen and carbon dioxide levels in the blood are maintained at normal levels.
Central sleep apnoeas are less common than obstructive apnoeas, but when they do occur it is important to assess what is causing them and whether any treatment is needed. This assessment is often more complex than for patients with obstructive apnoeas, and the treatment has to be carefully matched to the patient's requirements, to obtain the best results.