INTRODUCTION
Attention deficit hyperactivity disorder (ADHD) affects 5% of children and 4% of adults. It is characterized by inattention, hyperactivity and impulsivity inconsistent with age. These characteristics result in dysfunctional social, academic and occupational environments[1-3]. Children are at risk for a range of problems including low self-esteem, peer rejection, physical injuries and poor adaptive functioning[2]. Similarly, adolescents with ADHD have significant difficulties with executive function and behavioral inhibition. They tend to make decisions impulsively and are prone to use alcohol, tobacco and other substances in comparison to adolescents without ADHD[4-6]. Furthermore, early onset of sexual activity, accidents, delinquency and reckless driving is heightened in adolescents with ADHD[7]. ADHD symptoms may also cause impairment in adult patients[8]. Similar to younger groups, adults with ADHD experience difficulties in academics, work, interpersonal areas and life skills, such as driving and parenting. They tend to have high rates of substance use, unemployment, divorce, antisocial behaviors and psychiatric comorbidities[9]. Overall, available data suggests that ADHD is a chronic handicapping condition associated with negative outcomes across the life span.
Recent advances in research have led to improved screening, diagnostic algorithms, as well as pharmacologic and psychosocial treatment for patients with ADHD[2,3,10]. In the United States, stimulants remain the first line pharmacotherapy and are associated with a remarkable response rate; however, despite the use of stimulants, impairing residual symptoms persist for most affected individuals[11]. Behavioral training for parents of affected children and cognitive behavioral therapy in older groups to augment the effects of medications has shown some promise[9]. Unfortunately, most studies in this area only evaluate the short-term outcomes of these therapies, and the long-term benefits remain unclear[5,10]. The interest in developing sustainable, non-pharmacological options to augment ADHD treatments has led to interest in mindfulness training (MT). MT is an intervention derived from Buddhist practices that may enhance attention and reduce impulsive responses[12]. The core aspects of mindfulness involve paying attention on purpose in the present moment and with nonjudgmental acceptance. Over the past few years MT has been adapted for use in clinical populations including adolescents and adults who have difficulties regulating their emotions and behaviors[13,14]. This article outlines the clinical presentation and neurobiology of ADHD and explores empirical evidence supporting the use of meditation for inattention in ADHD.
CLINICAL FEATURES OF ADHD
ADHD is a neurodevelopmental disorder that typically presents in childhood and is characterized by troubles with inattention, hyperactivity and/or impulsivity[1,15]. According to the DSM-5, the diagnosis requires at least six symptoms of either inattention or hyperactivity/impulsivity, several of which must be present prior to the onset of adolescence. For individuals over the age of seventeen, only five symptoms are necessary for diagnosis. These symptoms must be present for at least 6 mo, occur in at least two settings, and hinder social, academic and/or occupational functioning[15]. ADHD is often thought of as a disorder of childhood; however, the reality is that it often persists into adolescence and adulthood[16].
The symptom presentation of ADHD differs across the lifespan. As individuals reach adolescence, the hyperactivity tends to abate somewhat while the inattention persists and often becomes more disabling[16,17]. In adolescence, impulsivity often presents as recklessness, unpremeditated actions and impatience. Similarly, the characteristic rambunctiousness of children with ADHD recedes and is replaced by motor restlessness. Adolescents with ADHD report feeling on edge and often choose activities that are physical in nature in order to avoid sedentary tasks. Despite the significant presence of restlessness, a study conducted by Sibley et al[16] found the symptoms of inattention to far outweigh those of hyperactivity/impulsivity in adolescence. Inattention is manifested by distractibility, carelessness, forgetfulness and organizational difficulty. Adolescents with ADHD have difficulty focusing during lectures or while reading; they complain of their mind “wandering” and often have difficulty listening, following through on instructions or finishing tasks. They frequently lose or misplace items and characteristically work in cluttered and disorganized environments. Additionally, there is a penchant to procrastinate and avoid activities that require prolonged attention. The difficulty with sustaining attention leads to marked school difficulties. Struggles with studying or paying attention in class can lead to academic decline and failure[18]. Adults often run the risk of unemployment due to ADHD jeopardizing job performance and acceptable attendance records. Socially, relationships with others are threatened as inattention is seen as laziness, irresponsibility and unwillingness to cooperate[15]. While current pharmacological and behavioral treatments can reduce debilitating symptoms significantly, partial remission is common and residual difficulty with focus, impulse control and emotional regulation cause significant dysfunction for many adolescents and adults with ADHD.
NEUROBIOLOGY OF ADHD
Neurobiological research has tied ADHD symptoms to structural-functional brain abnormalities and delayed development of the neocortex[19]. As children develop, brain maturation progresses in a posteroanterior fashion. Initially, myelination takes place in the visual pathway and then progresses to anterior areas such as the prefrontal cortex (PFC). The brain development that occurs during adolescence primarily involves changes in the frontal and parietal cortices, the sites responsible for executive function. A peak in gray matter volume at puberty is followed by a gradual decline, as the cortex undergoes synaptic pruning in areas that play a role in impulse control, planning and emotion regulation. The evolutional path of the brain in children with ADHD follows a parallel course with controls, but always with significantly smaller gray matter volumes[20]. Traditional magnetic resonance imaging (MRI) studies in children with ADHD have shown reduced volume in the frontal cortex, anterior cingulate cortex (ACC), basal ganglia and cerebellum[21]. Similarly, structural MRI studies in adults with ADHD have found overall reductions in cortical gray matter volume in the ACC, orbitofrontal cortex (OFC), inferior frontal cortex, dorsolateral prefrontal cortex (DLPFC), temporoparietal, cerebellar and occipital regions[22-25]. While volumetric reductions in subcortical areas have also been reported in adults with ADHD, most research focused on the frontal striatum circuit including the ACC, OFC and DLPFC.
Early research implicates dysfunction of the PFC and fronto-striatal circuitry in ADHD, including the OFC, DLPFC and ACC. The ACC modulates the peripheral nervous system (PNS) and the central locus ceruleus (LC), both norepinephrine (NE)-driven systems. For the most part, the ACC is activated by novel or salient stimuli as part of a neural circuit that serves to regulate cognitive and emotional processing. By rapidly modulating the activity levels of both principal NE systems, the ACC is able to adapt the state of the whole organism to optimize attention and support complex behavior. It is also responsible for selective attention and conflict monitoring. Furthermore, the ACC is the primary structure responsible for salience and detection of executive PFC networks. These networks integrate input from the internal and external environments to maximize adaptive processes[26]. The PFC provides top-down (cerebral-to-limbic) regulation of attention and behavioral inhibition through connections with posterior cortical and subcortical structures[27]. The OFC is involved in decision-making and impulsivity. The function of the DLPFC is to sustain attention, problem solve and organize information[28]. It has also been implicated in activating the brain mechanisms necessary for working memory and task completion. Overall, these PFC structures act in concert as well as communicate with subcortical structures (e.g., basal ganglia, cerebellum) to optimize attention and executive function[29]. The catecholamine neurotransmitters, dopamine and NE, are critical for communication between the PFC and other brain structures[27]. Concentrations of these neurotransmitters in the PFC follow a bimodal curve: too much or too little disrupts its functioning. In persons afflicted with ADHD, poor connectivity among these brain areas may underlie the pathology associated with impaired goal setting, organization and planning.
Functional MRI (fMRI) technology has been used to map out brain-symptom correlations in ADHD. It assesses brain activity by measuring hemodynamic response in different regions of the brain under various cognitive tasks. In ADHD studies, fMRI findings have suggested that individuals with ADHD have hypoactivity in the PFC areas, superior parietal areas, caudate nucleus and thalamus[30]. These changes that result in impairment with tasks requiring attention and response inhibition have been show to normalize with appropriate treatment.
Overall efforts to delineate the neural systems underlying attention control in individuals with ADHD do not suggest a simple locus of neural dysfunction, but rather a deficit in integration of information across numerous brain areas[31]. Generally, the literature indicates that ADHD is characterized by multiple functional and structural neural network abnormalities beyond the classical fronto-striatal model, including fronto-parietal-temporal, fronto-cerebellar and even fronto-limbic networks.
MINDFULNESS MEDITATION
Mindfulness is a technique of focusing attention that is derived from Eastern meditation practices. It has been defined as “bringing one’s complete attention to experience the present moment”[32]. The basic elements of MT are intention, attitude and attention. The ability to direct one’s attention can be developed through the practice of intentional self-regulation of attention from moment to moment[13]. Mindfulness has been conceptualized as a fundamental way of being, and is a learned skill involving non-judgmental observation of thoughts, emotions and somatic sensations that arise in one’s awareness[33]. Over the past several years, researchers have been exploring the therapeutic potential of MT[34]. These efforts have included development of manuals that apply MT principles to Western psychotherapy. In the United States, the most frequently cited method of MT is mindfulness-based stress reduction, which was first used for the treatment of chronic pain[35]. The use of MT has since been extended to other clinical populations, and has garnered acceptance as a legitimate psychotherapy modality[36]. Although research in ADHD is limited, some studies have documented improvements in attention skills in adolescents, as well as adults with ADHD[13,37].
When considering the various types of practices, two types of meditation are broadly recognized: focused and receptive attention. As the name implies, focused or concentrative meditation entails focusing on a specific thought, such as an image or body sensation, while distracting events are disregarded[26]. This allows the meditator to concentrate on one thing at a time rather than allowing attention to split among tasks. One example is the mindfulness of breathing, where a participant is trained to focus on his/her breathing and sustain attention on this sensation for the practice duration. The training includes explicit instructions to notice mind wandering and respond by redirecting attention. If the participant notices his/her attention wander away from the breath, he or she must notice the drift in attention and return to the breath. This practice leads to less distractibility and better ability to stay on task.
Another type of meditation, called open monitoring or receptive attention, includes observing the content of one’s experience (e.g., sensations, thoughts and emotions) from moment to moment without reacting. In the open monitoring meditation, attention is extended to the whole field of awareness. It involves being alert to any stimuli that arise in the moment rather than a steady focus on one specific object. This type of meditation enhances attention switching, the ability to purposefully shift the attention focus between stimuli[38]. By directing attention to the experience of the moment, subjects learn to identify and dismiss unhelpful automatic reactions. This improvement in receptive attention can improve self-regulation and impulse control.
MT AND ATTENTION
By enhancing attention, MT can potentially improve several core symptoms of ADHD, namely task completion, self-regulation and impulse control. Given its low cost and sustainability, it is of interest to clinicians to empirically validate the positive effect of MT on attention. Early research has studied the benefits of concentrative meditation using the Posner model of attention. According to Posner[39], the attention system is supported by three components: alerting, orienting and conflict monitoring. The alerting network attains and sustains a state of vigilance and is subserved by the reticular activating system. The orienting subsystem involves the parietal lobe. Its function is to select relevant environmental information and enable one to react quickly to a situation. The conflict monitoring subsystem prioritizes competing stimuli to regulate thoughts, emotions and actions. The ACC plays an important role in conflict monitoring by anticipating what to do next in situations. It has been proposed that MT’s capacity to influence attention relates to its ability to act on these three attention processes[40].
In a study by Jha et al[41], the effects of focused MT on the elements of attention, including alerting, orienting, and conflict monitoring, were examined. Participants were divided into three groups - a novice group, who received 8 wk of training in MT, an experienced MT group and a meditation naïve control group. The novice participants were taught to focus attention, detect distraction, then disengage their attention from the source of distraction, and flexibly redirect and engage attention to an intended object. The experienced MT group participated in a 4-wk meditation retreat. On the first evaluation, the participants in the retreat group demonstrated improved conflict monitoring performance relative to those in the novice MT groups and controls. At follow up, those in the novice MT groups demonstrated significantly improved orienting in comparison with the other groups. These results suggest that MT may improve certain aspects of attention[41]. Another study by Tang et al[42], focused on the effects of short-term meditation on attention. Participants included forty Chinese college students randomly assigned to 5 d of either MT or relaxation. Results showed that there was an advantage of MT in conflict monitoring relative to the group that received relaxation. Apart from these studies, additional data supports that focused MT practice improves conflict monitoring and orienting[43,44].
A study by Chiesa et al[36] reviewed research evaluating the effects of MT on measures of cognitive function. This review supports the notion that focused MT may be associated with significant improvements in conflict monitoring. One proposed theory suggests that the mechanism underlying the effect of MT on conflict monitoring functions via activation of the attention-related cortices such as the PFC and ACC[36,45]. Efforts to elucidate the meditation-attention link have measured brain activation before and after variable periods of MT using fMRI[36]. One of the most frequently observed changes in response to meditation is the activation of the PFC and the ACC[26]. Activation of the ACC is typically accompanied by a widespread activation of other brain areas such as the LC and the autonomic nervous system. The LC is thought to optimize behavioral performance by modulating arousal and adaptively responding to environmental demands[46]. There is an inverse relationship between cortical arousal and peripheral sympathetic nervous system (SNS) arousal. It is believed that meditation acutely activates the ACC, which in turn inhibits the SNS, and decreases peripheral NE drives. The result is parasympathetic activation and relaxation promotion. At the same time, the ACC signals LC activation and an increase in cortical NE release, which results in enhanced attention processes.
Additional research has explored the effects of cognitive behavioral therapy and MT (CBT/MT) on attention in incarcerated adolescents. In this study, by Leonard et al[47], the investigators randomly assigned dormitories of incarcerated youth, ages 16-18, to a CBT/MT intervention (n = 147) or a control intervention (n = 117). Both arms received about 750 min of training in small groups over a 3-5 wk period. Youth in the CBT/MT arm also reported on the amount of homework practicing MT exercises. The Attention Network Test was employed to measure attention performance at baseline and at 4-mo follow-up. The results of the study found that multi-session CBT/MT interventions had a protective effect on the incarcerated youths’ attention capabilities[47]. Overall, there is emerging evidence that MT has an impact on attention capacity, which encourages its adaptation and application to patients with ADHD.
MT IN ADHD PATIENTS
The use of meditation with ADHD populations is in its nascence[48]. In 2008, Zylowska et al[32] conducted a feasibility study, which adapted MT for adolescents and adults with ADHD. Subjects consisted of 24 adults and eight adolescents with ADHD who were treated with 8 wk of MT along with homework assignments. The curriculum, called Mindful Awareness Program, adapted to the specific requirements of persons with ADHD and included education in practices that ameliorate self-regulation and MT exercises. Participants began by meditating for 5 min at a time and gradually increased to 20 min. Each session lasted 2.5 h and was supplemented with daily at-home practice (CD’s with guided meditations). The results were promising with 78% completing the study and 30% reporting a greater than 30% reduction in symptoms of ADHD (i.e., set shifting and conflict attention)[13]. The authors concluded that an 8-wk MT adapted for adolescents and adults with ADHD was feasible; however, without a control group it is not clear whether the improvement in attention was due to the MT or non-specific factors.
Mitchell et al[49] also completed a study of the feasibility and acceptability of mindfulness meditation for adults with ADHD[49]. They compared a sample of 11 adults against a control group of 11, and found MT without additional ADHD treatment modalities was both feasible and acceptable to the participants. Sixty-three point six percent in the treatment group also rated a ≥ 30% reduction in inattention and hyperactivity symptoms compared to 0% reduction in the control group in self-rating scales. These results paralleled the in the clinician rating assessments (inattention and hyperactivity reductions of 81.8% and 72.7% in the treatment group vs 0% and 11% reductions respectively in the control group). Overall, these studies show evidence of acceptability and feasibility in MT[50]. However, controlled trials exploring MT as an attention enhancing strategy in the treatment of adult ADHD are needed to strengthen the evidence for MT’s usefulness.
CONCLUSION
ADHD is characterized by inattention, hyperactivity and impulsivity. It is a common developmental disorder that can persist throughout life and cause significant impairment in social, academic and occupational functioning. Although various treatment approaches exist, including pharmacologic and psychosocial treatments, residual symptoms persist for most affected individuals. In recent years, MT has been explored in ADHD populations to address ongoing distractibility, emotional dysregulation and impulsivity. Evidence suggests that certain meditative practices improve attention and may ameliorate the symptoms of ADHD by activating brain regions implicated in both sustaining and directing attention. There is much to gain from an improved understanding of the possible role of MT in enhancing attention. Overall, these findings support the idea that deliberate cultivation of attention using MT may prove to be a useful strategy to ameliorate residual inattention after pharmacological treatment of ADHD. However, adequately powered prospective studies of the relationship between meditation and inattention in ADHD populations are needed to firmly establish efficacy.
P- Reviewer: Yew DT S- Editor: Ji FF L- Editor: A E- Editor: Jiao XK
