Images: Benign myoclonus of sleep associated with K-complexes on electroencephalography

Abstract

In this brief case report on paroxysmal sleep-related movements, we describe an adolescent patient’s presentation of brief jerking movements during sleep and the accompanying differential diagnosis. In examining the patient’s overnight electroencephalogram we use hallmark sleep architecture to provide reassurance to the patient and her family.

Citation:

Silverman A, Miglis MG, Gallentine W. Images: Benign myoclonus of sleep associated with K-complexes on electroencephalography. J Clin Sleep Med. 2024;20(1):183–184.

INTRODUCTION

Abnormal movements during sleep are commonly encountered early in life, affecting roughly 15–20% of children and 4% of adults.¹ Understanding these nocturnal events is paramount in the approach to patients who may have an underlying treatable disorder or neurological abnormality, as opposed to a benign, age-appropriate movement during sleep.^2,3 Using an exemplary case involving electroencephalography (EEG) neuromonitoring, we build a framework and differential diagnosis for classifying simple, paroxysmal sleep-related movements.

REPORT OF CASE

A healthy 14-year-old female with jerk-like truncal flexion movements during sleep is admitted for overnight EEG monitoring. The patient is unaware of the movements, continues to sleep well, and feels rested in the morning. Her family has noticed the movements over the past several months and is worried about a possible seizure disorder. The patient, however, has no overt seizure risk factors (ie, she was born full-term without any perinatal complications and met her developmental milestones appropriately; she has never had a central nervous system infection, traumatic brain injury, or neurosurgery; she has no family history of seizures; she does not drink alcohol or use any substances). She has no significant past medical or surgical history and takes no medications or supplements. We accordingly approach this case as isolated, brief movements during sleep, the differential diagnosis for which includes epileptic myoclonus, hypnagogic/hypnopompic jerks, propriospinal myoclonus, excessive fragmentary myoclonus, and benign myoclonus of sleep.

VIDEO

In this continuous video EEG (longitudinal anterior–posterior bipolar montage), a torso jerk is time-locked with a K-complex and a nurse knocking on the door (Video 1 ^{(1.5MB, mp4)} in the supplemental material). In more detail, the jerk reflects sleep myoclonus (brief truncal flexion) time-locked with the highlighted normal, biphasic K-complex followed by sleep spindle in stage II sleep. This myoclonic truncal flexion occurs at the same time as a nurse entering the room and recurs 4 distinct times throughout the overnight recording, each time coincidental with an auditory disturbance. Despite the disturbances, the patient does not awaken and progresses normally throughout subsequent sleep stages. In the morning, she does not recall the nurse entering the room or her torso jerks.

DISCUSSION

The primary clinical concern in this case is differentiating epileptic vs nonepileptic movements during sleep. Because the patient’s movement is not associated with epileptiform activity on EEG, the movements most likely represent benign stimulus-induced myoclonus of sleep. To provide reassurance to patients and families, it is critical to differentiate normal K-complexes from epileptiform K-complexes with overriding spike–wave activity.^4–6 Epileptiform K-complexes, a form of dyshormia (defined as abnormal or deviant arousal), may be seen in genetic generalized epilepsies and constitute discrete EEG abnormalities.⁵ Our patient shows no evidence of spike–wave activity preceding or superimposed on K-complex formations or sleep spindles. Furthermore, in considering her normal awake and asleep EEG together with her lack of epilepsy risk factors, the likelihood of a generalized seizure disorder is highly unlikely. Finally, it is worth considering the possibility of movement artifact contaminating the EEG signal; however, the biphasic, large-amplitude deflection followed by a fast activity spindle is classic for K-complex formation, as opposed to movement artifact.

With respect to the remainder of the differential diagnosis, it is important to categorize the patient’s movements as simple, brief, nonrhythmic muscle contractions. This framework helps rule out both rhythmic and complex movements such as periodic limb movements of sleep and various parasomnias. Table 1 delineates and contrasts key features of simple sleep-related movement disorders most germane to our patient.

Table 1.

Definitions of simple sleep-related movement disorders.^2,3,7,8

	Epileptic Myoclonus	Hypnagogic and Hypnopompic Jerks	Propriospinal Myoclonus	Excessive Fragmentary Myoclonus
Description of movement	Sudden jerks of the limbs that occur with waking or soon after waking in patients with juvenile myoclonic epilepsy	Also known as sleep starts (hypnic jerks); common, nonpathological phenomena comprising brief myoclonic jerks, typically involving the whole body during sleep–wake transition	Quasi-periodic jerks involving axial musculature during relaxed wakefulness, or at sleep onset, possibly suppressed by mental and sensory stimulation	Arrhythmic, asynchronous, asymmetric twitches that do not cause large movements, usually throughout the night; involves small movements of the corners of the mouth, fingers, or toes

Although the semiology of our patient’s truncal flexion might reflect that of propriospinal myoclonus, the latter classically ceases with the emergence of K-complexes and spindles.⁷ Instead, our patient’s axial contraction represents a simple, benign myoclonus of sleep that neatly coincides with K-complexes throughout her overnight recording, an association noted by sleep researchers for decades and yet, to our knowledge, not published in video case reports.⁹ Similar to benign sleep myoclonus of infancy, a condition typically seen in infants and toddlers up to 3 years of age, our patient’s myoclonus ceases in wakefulness.³ In both our case and infants with benign sleep myoclonus the EEG is normal.

First described in 1938, the K-complex is a hallmark of stage II sleep, characterized by high-amplitude biphasic waveforms with an initial negative phase followed by a slow positive phase, often followed by sleep spindle. It is thought that “K” signifies “knock,” as the complex may arise with auditory stimuli, such as the nurse entering the patient’s room.^9–11 Debate continues as to whether K-complexes reflect excitatory arousal phenomena, inhibitory sleep-promoting responses to arousing stimuli, or cerebral epiphenomena of sleep-specific thalamocortical oscillations.²

This case highlights key clinical considerations in simple, paroxysmal movements during sleep, the utility of overnight video EEG in making a diagnosis, and a review of the K-complex formation. We also emphasize the importance of differentiating normal K-complexes from those with overriding spike–wave activity to rule out a generalized seizure disorder. Fortunately, we were able to provide reassurance to the patient and her family regarding the benign nature of her movements.

DISCLOSURE STATEMENT

All authors have seen and approved the manuscript. The authors report no conflicts of interest.

ABBREVIATIONS

EEG

electroencephalography