Childhood Habit Behaviors and Stereotypic Movement Disorder

Childhood habit behaviors appear in numerous different forms. Many people engage in some degree of habitlike behavior in their lifetime. For example, habits can range from seemingly benign behaviors, such as nail biting or foot tapping, to more noticeable physically damaging behaviors, such as teeth grinding (bruxism) and head banging.

Habit disorders, now subsumed under the diagnostic term stereotypic movement disorders, consist of repetitive, seemingly driven, and nonfunctional motor behaviors that interfere with normal activities or that result in bodily injury. Fortunately, many childhood habits are benign, are considered a normal part of development, and do not meet the criteria for a disorder; such habits typically remit without treatment.

Stereotypies are repetitive, purposeless actions that are most commonly seen in childhood. They do not have a clear definition, because of the wide range of possible stereotyped behaviors and the overlap with other movement or behavioral disorders. ^[3] Nevertheless, it is helpful to classify stereotypies into general categories so as to distinguish between the different movements. Stereotypies can first be divided into either primary or secondary stereotypies: ^[4]

• Primary stereotypies occur in otherwise developmentally normal children with no specific underlying cause, and are thought to be purely physiologic in nature.
• Secondary stereotypies occur in conjunction with a neurologic or behavior disorder, such as autism, intellectual disability, psychiatric conditions (ADHD, tic disorders, OCD), and some rare neurodevelopmental syndromes.

Primary stereotypies can be further broken down into subtypes: ^[2]

• Common stereotypies: These are by far the most frequent type and comprise habits such as nail biting, hair pulling, pencil tapping, and others.
• Complex motor stereotypies: These consist of various repetitive limb movements, and can include flapping, waving, opening and closing of a fist, finger wiggling, or wrist flexion and extension.
• Head nodding: This has been identified as a separate and unique primary stereotypy.

Other involuntary movements that may present similarly to stereotypies include tics and automatisms. A tic disorder is the most common misdiagnosis in patients with stereotypic movement disorder. Complex motor tics are also repetitive, involuntary actions; however, they usually have a later age of onset, are more variable in movements and rhythms, and are briefer. More important, stereotypies, unlike tics, are not associated with premonitory urges and subsequent relief from those urges.

Voluntary repetitive movements that mimic stereotypies include attention deficit hyperactivity disorder (ADHD), mannerisms, and compulsions. ADHD tends to be characterized by more generalized and restless actions than stereotypies are. Mannerisms are rarely continual and accompany a normal activity. Compulsions comprise repeated ritualistic movements to relieve anxiety or fear and may occur with tics and stereotypies.

This article also addresses various common childhood behaviors (eg, thumb sucking, nail biting, nose picking, breath holding, bruxism, head banging, and rocking or rhythmic movements). Mild forms of some of these behaviors do not interfere with normal daily activities and thus are not psychiatric disorders; however, they can progress to cause functional impairment or physical or psychological sequelae or stigmatization. When functional impairment is substantial, the diagnosis of stereotypic movement disorder should be considered.

Some childhood habits remain unnoticed and can persist if left untreated, even when they interfere with optimal functioning. Childhood habits can result in negative social interactions and avoidance by peers and family members. Some repetitive behaviors can cause damage. For example, bruxism (teeth grinding) can result in tooth damage. Occasional hair pulling can result in hair loss or evolve into a more severe disorder, trichotillomania.

In most children who are otherwise developing normally, however, few of these habits result in permanent physical damage. In some cases, treating a childhood habit before clear-cut dysfunction arises may prevent serious psychopathology and social dysfunction. Much of what is known about childhood habits derives from the literature about common habit behaviors in adults.

Diagnostic criteria (DSM-5)

The specific DSM-5 criteria for stereotypic movement disorder are as follows: ^[1]

• Repetitive, seemingly driven, and apparently purposeless motor behavior (eg, hand shaking or waving, body rocking, head banging, self-biting, or hitting one’s own body)
• The repetitive motor behavior interferes with social, academic, or other activities and may result in self-injury
• Onset is in the early developmental period
• The repetitive motor behavior is not attributable to the physiologic effects of a substance or neurologic condition and cannot be better explained by another neurodevelopmental or mental disorder (eg, trichotillomania or obsessive-compulsive disorder)

The following specifiers are used:

• With self-injurious behavior (or behavior that would be self-injurious if not prevented)
• Without self-injurious behavior

Whether the stereotypy is associated with a known medical or genetic condition, developmental disorder, or environmental factor is also specified, and an additional code is used to identify the associated element.

Severity is specified as follows:

• Mild - Symptoms are easily suppressed by sensory stimulus or distraction
• Moderate - Symptoms necessitate explicit protective measures and behavioral modification
• Severe - Continuous monitoring and protective measures are required to prevent serious injury

Research on the mechanisms of motor stereotypies has followed two different approaches. Psychogenic hypotheses aim to explain the cause of stereotypies from a behavioral standpoint. Neurobiologic hypotheses look at the structural and molecular basis for stereotypies.

Psychogenic hypotheses

Stereotyped behaviors are known to occur in animals, especially those that are caged or restrained. One hypothesis is that such animals experience a lack of external stimuli, and the stereotypic movements help maintain their state of arousal. In support of this view, stereotypies are more common in children with sensory deprivation due to blindness or deafness and in conditions where there is less interaction with the external environment (eg, autism). ^[5]

Another hypothesis is that stereotyped movements are a method of expending excess energy or attention. Attending to the movements may help to diminish other unwanted or unpleasant stimuli, as a form of negative reinforcement. From a related perspective, sensory stimulation in various stereotypies is proposed as automatic positive reinforcement that causes the actions to persist.

For example, vestibular stimulation may reinforce head banging. It is more common in infants with otitis media who have an unwanted stimulus (eg, ear pain). Sensory extinction has been shown to reduce the behaviors. ^[6] In the case of a child whose stereotypy consisted of spinning plates on a hard surface, placement of a carpet removed the sound of the spinning plate, leading to a decrease in the frequency of the behavior.

Neurobiologic hypotheses

Anatomically, the basal ganglia are implicated in stereotypic disorders. ^[7] In monkeys, stereotypies are correlated with neuronal activity in the striatum, especially the putamen. Intrastriatal injection of amphetamine, which increases dopamine levels, causes stereotypies in rodents. ^[8]

In humans, case reports of stereotyped movements have spontaneously emerged after lesions of the putamen, orbitofrontal cortex, or thalamus. A volumetric magnetic resonance imaging (MRI) study in children with complex motor stereotypies demonstrated a reduction in the size of the caudate nuclei and also in frontal white matter. ^[9] Precise localization within the basal ganglia and corticostriatal circuitry is yet to be achieved.

Dopaminergic pathways appear to mediate complex motor stereotypies. ^[10] In the treatment of Parkinson's disease, patients receiving high doses of levodopa sometimes perform repetitive, purposeless actions, known as punding. In addition, levels of plasma homovanillic acid, a dopamine metabolite, are reduced in adults with stereotypies. ^[11]

Studies in rodents have shown that administration of dopamine can produce repetitive behaviors such as sniffing or head bobbing. ^[3] From studies investigating the effect of selective dopamine agonists, dopamine D2 receptors are known to be important in enhancing stereotypies. ^[8]

Although the origins of most habit disorders are not well established or understood, associated biologic or environmental etiologic factors may exist. Some habit behaviors emerge from normal repetitive behaviors in infancy (eg, hand and thumb sucking) and are believed to represent intrinsic movement patterns generated by the developing nervous system. Certain behaviors (eg, nose picking, which may initially arise as a response to dry nasal membranes and nasal irritation or itching) can later develop into a cycle that is difficult to break.

Some behaviors may start spontaneously and then may be inadvertently reinforced or maintained by other factors, such as attention. Behaviors that produce arousal or modulate self-stimulation may also be influenced by the level of environmental stimulation; for example, some behaviors may be viewed as entertainment for a bored child or as a means of coping with overstimulation.

Repetitive or habitual behaviors may be associated with an underlying condition, such as a sensory impairment or developmental disorder, an unrecognized medical or neurologic condition, the side effect of a medication, or a psychiatric disorder. Certain genetic syndromes are associated with repetitive behaviors (eg, skin picking in Prader-Willi syndrome, hand flapping in fragile X syndrome).

Thumb sucking is initially a biologically driven reflexive behavior that is often documented in utero and in the young infant. In older infants and toddlers, it is reinforced as a form of self-stimulation or self-comfort and is most often seen when the child is sleepy, hungry, frustrated, or fatigued. Most children who suck their thumbs past infancy develop the habit before the age of 3-9 months. Children who have stopped thumb sucking may resume the behavior after an acute or chronic distressing event (eg, illness, hospitalization, or separation).

Nose picking occurs in children and adults. Older children and adults are most likely to pick their nose in private, whereas young children commonly do so in public view. The behavior may begin in association with rhinorrhea, nasal irritation, or nasal itching resulting from colds or allergies.

Breath-holding spells are generally observed in response to strong emotions or pain. In pallid breath-holding spells, an enhanced vagal response has been postulated to be a precursor to bradycardia or asystole.

Head banging can be associated with temper tantrums, tension, or stress. In some children, it can also develop as a sleep ritual. Neurologic or psychological precursors are rarely identified. Head banging is most commonly associated with developmental delay or autism. In some cases, the onset of head banging has been associated with teething or otitis media and has been hypothesized to serve a pain-relieving function.

Genetic factors

Some complex motor stereotypies appear to exhibit mendelian inheritance in animal and human studies. In a study of developmentally normal children, 25% had a family history of stereotypies, and a first-degree relative was affected in 17% of cases. ^[12] Nearly half of the children had a coexisting behavioral disorder.

To date, no specific genes have been identified. A causative role for the MECP2 gene on the X chromosome has been suggested. Those patients who have a detectable mutation in the gene show a greater number of stereotypies than those without a known mutation. ^[13]

United States and international statistics

Accurate prevalence figures for childhood habits are extremely difficult to estimate, because of the various classes of habits and the differing topographies of a child’s presenting habit. Data on the prevalence of stereotypies are limited, especially internationally, and tend to vary according to the exact definition used by the researchers. Nevertheless, habits or common stereotypies are thought to occur in as many as two thirds of infants and in 20-50% of children overall. ^[14]

Thumb and hand sucking are seen in 17-59% of children younger than 15 years. Thumb sucking is common in infancy and in as many as 25-50% of 2-year-old children; however, it is observed in only 15-20% of 5- to 6-year-old children.

Nail biting is mainly observed from preschool age to adolescence and is the most common stereotypy in school-age children and college students. The prevalence is as high as 45%-60%; in one study, it was found to exceed 60% in a group of 8-year-olds.

Few studies of nose picking have been performed. However, in one survey, as many as 91% of adults reported nose picking.

Breath-holding spells are common in as many as 4-5% of children younger than 8 years.

Bruxism is observed in 5-30% of children. The worldwide prevalence is 8%. ^[15]

Head banging can occur in approximately 10% of developmentally normal infants and in 3-19% of developmentally normal children younger than 3 years. It is more frequently observed in children with autism or developmental delay and in those living in institutional environments.

Body rocking and rhythmic movements occur in most infants aged 6-12 months and may be seen in 6-19% of children younger than 3 years. ^[16] The behavior is most often observed in children with developmental disabilities or sensory impairments; however, it persists beyond age 2 years in 3% of children with normal development.

Self-injury is less common; it is observed in 10-20% of individuals who are institutionalized and have intellectual disability, 1-3% of individuals who live in the community and have moderate intellectual disability, and up to 5% of individuals who live in the community and have intellectual disability and autism.

In a psychiatric journal, the rate was determined to be only 0.6% in college students when the patients were restricted into the group having related mental tension and relief. ^[17] Without such restrictions, the rate of hair pulling resulting in visible hair loss is 1.5% for males and 3.4% for females. ^[17]

The prevalence of primary complex motor stereotypies is unknown but may be as high as 3-4% of preschool children in the United States. ^[18] Secondary stereotypies are of course determined by the associated disorder. Autism occurs in 1 in 20,000 live births. Stereotypies are seen in more than 60% of autistic preschool children but in only 25% of nonautistic children with similar mental ability. ^[19]

Stereotypies are observed in 40-60% of individuals who are institutionalized and have profound intellectual disability, 8-10% of individuals who live in the community and have moderate intellectual disability, and up to 20% of individuals who live in the community and have intellectual disability and autism.

Age-related demographics

The age at which specific habits originate, peak, and remit is related to the individual habit behavior. The age of onset and resolution of habit behaviors may be delayed in children with developmental disabilities.

Thumb sucking may be observed in utero as early as 29 weeks’ gestation. It is common in infancy and is observed in as many as 25-50% of children aged 2 years. Rates of thumb sucking declines with increasing age; most children spontaneously stop at about age 4 years. It is observed in only 15-20% of children aged 5-6 years.

Nail biting rarely begins in children younger than 4 years. Its peak prevalence is between the ages of 8 and 11 years, but it persists in 28% of 18-year-olds. Rates as high as 45-60% are observed from preschool to adolescence.

Nose picking is common in adults and children.

Bruxism is observed in 5-20% of children. The frequency increases during childhood, peaking at age 7-10 years and decreasing after that. It is common in children and adults.

Hair twirling or pulling can be observed as early as preschool (ages 3-4 years) and is typically thought to be a fairly benign stereotypy, similar to thumb sucking or nail biting. It is important to distinguish this from trichotillomania, a psychiatric disorder closely related to obsessive compulsive disorder (OCD), which often has a later onset (ages 9 to 13). ^[20] Breath-holding spells are reported to occur in 4-5% of the pediatric population. They may begin as early as infancy, and their peak incidence is at age 2-3 years. Approximately 80-90% of preschoolers with breath-holding spells stop by age 6 years.

Head banging can occur in 3-19% of developmentally normal children younger than 3 years. It usually begins at around 9 months of age and may last up to 3 years. The incidence peaks between the ages of 18 months and 2 years, declining rapidly after that. Head banging continues in 1-3% of children older than 3 years. The behavior can recur at age 5-6 years. Head banging is more frequently observed in children with autism or developmental delay or in those living in an institutional environment.

Body rocking and rhythmic movements develop in the first year of life, usually as transient phenomena during gross motor development. They occur in most infants aged 6-12 months. The prevalence peaks in children aged 6-18 months and rapidly declines after that. The behavior persists beyond age 2 years in 3% of children with normal development. Body rocking and rhythmic movements are most often observed in children with developmental disabilities or sensory impairments and may be associated with a lack of environmental stimulation.

More than 80% of complex motor stereotypies begin before age 2 years, with a peak incidence at age 6 months. ^{[21, 12]} Another 10% begin in the following year. The outcome is variable, and approximately one third of patients show resolution or improvement of stereotypies. For the most part, this happens in the first year, but it may take 10 years or longer. Stereotypies stabilize in about 60% of patients and worsen in a minority.

Sex- and race-related demographics

Just as the overall prevalence rates of childhood habits are unknown, data on sex-based differences in prevalence are limited at best. In cases of self-injurious behavior, head banging is believed to occur 3 times more frequently in male individuals than in female individuals. Thumb sucking is suspected to occur slightly more often in girls than in boys. Nail biting also tends to be more common in females than in males. However, sex-based differences in nose picking are unknown. Breath-holding spells occur with equal frequency in boys and girls. No sex-based differences in rocking and rhythmic movements have been reported.

When the types of bruxism are separated into clenching and grinding, more females than males grind their teeth, but the number of males and females who engage in teeth clenching are equal. In addition, resulting dysfunction of the temporomandibular joint (TMJ) may be more common among female adolescents than among male adolescents.

Overall, complex motor stereotypies are nearly twice as likely to occur in males as in females. The increased prevalence of conditions such as autism or intellectual disability in males is a confounding factor, but even in primary stereotypies, males are more likely to be affected.

There are no known racial differences in the frequency of stereotypies.

Common stereotypies have an excellent prognosis in most cases. As a general rule, common stereotypies in infancy usually regress in later childhood. Many common stereotypies that are present in later childhood will persist into adulthood. However, the frequency and severity of the stereotypies are stable or improve with time in most children.

Because childhood habits take various forms, mortality and morbidity profiles vary widely. Mortality is extremely rare. All habits have the potential to produce social stigmatization and distress, depending on the environmental context in which they occur.

Some habit disorders, though not directly causing observable physical damage, result in impaired social functioning. Stigmatization resulting from the habit can cause the child considerable distress, humiliation, social rejection, academic problems, feelings of shame and guilt, discomfort in social activities, and depression or anxiety.

Although the range of physical sequelae varies greatly, serious medical complications are rare. A child with breath-holding spells does not have an increased risk of seizures but does have an increased risk of syncopal episodes as an adult. Data about habit behaviors as markers for increased emotional stress, anxiety, or behavior problems are inconsistent.

Self-injurious stereotypies are unusual in developmentally normal children, though they occur in as many as 40% of children with autism. ^[22] Such stereotypies include self-biting and self-hitting.

Most childhood habits that do not involve self-injury are benign and remit without intervention. When a habit persists and interferes with daily functioning, intervention is warranted. The prognosis for reducing and eliminating habit disorders is typically good. Treatment research shows that behavioral intervention can reduce the habit behavior by 90%.

Complex motor stereotypies are now known to persist in many more children than was previously thought. ^[23] Such stereotypies cease completely in 4% of children within 1 year of onset. In two thirds, the stereotypies last for more than 5 years from onset. ^[12] Head nodding has a better prognosis than other complex motor stereotypies. Half of children show regression of the behavior within 1 year of onset. ^[12]

Education of both parents and patients is an important part of management. Many Internet resources are available for common stereotypies, including the following:

• WebMD.com, Thumb sucking
• Babycenter.com, Head banging
• WebMD.com, Nail biting
• WebMD.com, Bruxism