Shakes Before Falling Asleep: Myoclonic Spasms

Why is there this feeling of falling into the void that wakes us up suddenly?

It’s three o’clock at night. You wake up abruptly, practically jumping into bed, with the sensation of falling off an eight-story building. You notice how your guts are still reacting with a certain panic.

Your partner wakes you up at three in the morning, slightly surprised and upset. It tells you that while you were sleeping you have kicked her several times. These two small fragments reflect the existence of a phenomenon that occurs with great frequency in most of the population: the realization of small sudden and involuntary movements  during sleep.

These movements are called nocturnal myoclonic spasms.

What is myoclonus?

When speaking of myoclonic spasms, it refers to a series of sudden and brief muscular contractions, totally involuntary, that cause a displacement of the body or a part of it. They are usually caused by sudden muscle contractions or muscle relaxation.

Although these types of spasms can be found in some disorders such as epilepsy, there are also so-called benign myoclonus. These as a general rule are not considered pathological, being considered normal in people without associated pathology. In fact, such a common phenomenon as hiccups would be a valid example of benign myoclonic spasm.

These spasms can appear both in the waking state and during sleep, with this article focusing on the latter.

Nocturnal myoclonic spasms

Although the general definition of myoclonus reflects the type of phenomenon we are talking about, those that occur during sleep have a peculiarity: as with hypnagogic and hypnopompic hallucinations, they occur in an altered state of consciousness: sleep or the transition between this and wakefulness. Myoclonic spasms would in this case be a type of parasomnia, phenomena or episodic disorders that occur during sleep and are characterized by the presence of vegetative or motor symptoms.

It is a generally non-pathological phenomenon with a high prevalence in the population. It is estimated that around 70% of the population have had a myoclonic spasm at some point during sleep. Now, if the symptoms occur repeatedly and constantly, it would be advisable to go to a medical consultation, because if they occur persistently it  could indicate the presence of a disorder.

Keep in mind that it is possible to confuse this type of alteration, not dangerous, with an epileptic seizure. In this regard, one of the few ways to differentiate them is by electroencephalogram, myoclonic spasms not assuming the same type of alterations that are seen in cases of epilepsy.

Neurological causes of myoclonic spasms during sleep

The reason for these spasms during sleep has a neuroscientific explanation. 

The appearance of nocturnal myoclonus is due to the presence of a lack of coordination, to the maintenance at the same time of the activity, of two specific brain areas. Specifically, the reticular formation or reticular activation system (SAR) and the ventrolateral preoptic nucleus.

Reticular activation system

This system located in the brain stem is the main one in charge of keeping us alive, since it is the brain system that directs unconscious processes such as  respiration, digestion or heart rate. Apart from these physiological processes, it also participates in maintaining alertness and focusing attention, maintaining the state of wakefulness.

Ventrolateral preoptic nucleus

The ventrolateral preoptic nucleus can be found in the  anterior hypothalamus, close to and in contact with the occipital lobe . This nucleus is responsible for “turning off consciousness” inducing the state of sleep, as well as protecting the body during sleep causing body paralysis that prevents us from moving and damaging during deep sleep.

When do myoclonic spasms happen

To understand the appearance of spasms, it must be borne in mind that although it reduces its functioning during sleep, the SAR does not cease its functioning (since this would cause the death of the affected person).

Thus, this system still has a certain activation that can sometimes contradict the functioning of the ventrolateral preoptic nucleus during sleep that causes us to sleep.

This contradiction, of which the cause is still unknown, can partially cause motor reactions typical of wakefulness during sleep. In other words, it is the origin of myoclonic spasms during sleep.

Types of nocturnal myoclonus

Myoclonic spasms during sleep are not uniform and homogeneous, but there are three basic types.

A first type is found in repetitive movements during sleep. Similar to the typical movements of epileptic seizures, these movements appear during non-paradoxical sleep, being repetitive movements of short duration. Although treatment is not usually required, very severe forms can be treated pharmacologically

A second type of myoclonic spasm that occurs during sleep is nocturnal jerks or startle myoclonus. The clearest example of this type of spasm is the typical movement that is made when waking up from a dream in which we have the sensation of falling. They usually occur in superficial sleep, that is, in the first two phases of sleep, causing the sufferer to wake up with a certain suddenness. They are usually massive shaking of the whole body, especially of the lower extremities.

Finally, some spasms can be found at the time of transition between wakefulness and sleep. This type of myoclonus, classified as nonspecific, acts on the muscles of the face and extremities.

Bibliographic references:

  • Ferber, R. & Kryger, M. (1995). Principles and Practice of Sleep Medicine in the Child. WBSaunders Company.
  • Besag, FMC (1995). Myoclonus and Infantile Spasms. In: Robertson MM, Eapen V, eds. Movement and allied disorders in childhood. Chichester: John Wiley and Sons, Ltd .; p. 149-76.
  • Fejerman, N .; Medina, CS & Caraballo, RN (1997). Paroxysmal disorders and non-epileptic episodic symptoms. In: Fejerman N, Fernández-Álvarez E, eds. Pediatric Neurology. 2nd ed. Madrid: Editorial Médica Panamericana SA; p. 584-99.
  • Fernández-Álvarez, E. & Aicardi, J. (2001). Movement disorders in childhood. London: Mac Keith Press.
  • Morairty, S .; Rainnie, D .; McCarley, R. & Greene, R. (2004). Disinhibition of ventrolateral preoptic area sleep-active neurons by adenosine: a new mechanism for sleep promotion. Neuroscience; 123: 451-7
  • Svorad, D. (1957). “Reticular activating system of brain stem and animal hypnosis”. Science 125 (3239): 156-156.
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