Phenotypes within sensory modulation dysfunction
Introduction
Occupational Therapists have studied sensory processing since the early 1960s when Dr A. Jean Ayres [1], [2], [3] wrote the first scholarly articles in the field. However, questions remain about the validity of the diagnosis of sensory processing disorder (SPD), formerly referred to as sensory integration dysfunction by Ayres [3] and others [4]. Recently, a new diagnostic nosology was postulated to aid researchers in selecting homogenous samples and ultimately furthering the specificity of discussion related to theory, diagnosis, and intervention of sensory processing difficulties [5]. The new nosology differentiates 3 patterns of SPD: sensory modulation disorder (SMD), sensory discrimination disorder, and sensory-based motor disorder with subtypes noted within each pattern.
This study explores the diagnostic specificity within 1 of the 3 major patterns, SMD. Sensory modulation is the ability to regulate and organize the intensity and nature of responses to sensory input so that responses can be appropriately graded to the constantly changing sensory experiences of daily life. Sensory modulation disorder results in difficulty achieving and maintaining a developmentally appropriate range of emotional, attentional, and motoric responses to sensory stimuli [6], [7], [8], resulting in difficulty adapting to challenges encountered in daily life [8].
The clinical presentation of SMD varies with considerable heterogeneity in symptomatology [9]. One or more of the 7 sensory systems may be involved: tactile, vestibular, proprioceptive, visual, auditory, olfactory, and/or gustatory. Symptomatology includes sensory overresponsivity, sensory underresponsivity, sensory seeking/craving, or a combination of symptoms from the 3 subtypes. Atypical behaviors that result from SMD can range from severe to mild. Thus, clinical heterogeneity in SMD occurs in severity, number, manner, and which sensory systems are involved [4], [5], [7], [10]. Given this heterogeneity, empirical evaluation of whether symptoms can be classified into subtypes is crucial for both assessment and intervention. Clinical diagnostic specificity related to types of SMD permits clinicians to better define and describe the child's sensory processing needs and guides appropriate intervention strategies.
Recent taxonomic efforts of SMD-type behaviors include the diagnostic manuals of 0 to 3 organization (DC:0-3) [11], which proposed 3 subtypes of SPD of regulation, and the Interdisciplinary Council of Developmental and Learning Disorders, which also proposed 3 subtypes of regulatory SPD [4].
The categories of SMD proposed by the diagnostic manuals of 0 to 3 organization and Interdisciplinary Council of Developmental and Learning Disorders were synthesized into 3 groupings within SMD by a recently proposed nosology [5]:
- 1.
Sensory overresponsivity: a greater than typical response to sensory stimuli; responses to sensations are more intense, quicker in onset or longer lasting than those typically observed; the individual exhibits “fight, flight, or freeze” behaviors to sensation, for example, impulsive, aggressive, or withdrawn reactions.
- 2.
Sensory underresponsivity: a disregard or passive response to sensory stimuli; responses are less intense or slower in onset than those typically observed; the individual is difficult to engage, lethargic, self-absorbed, and seems unaware of sensation, lacking an inner drive to explore sensory materials and environments.
- 3.
Sensory seeking/craving: an intense, insatiable desire for sensory input; input is less than needed for the individual to feel satiated; individuals energetically engage in actions geared to adding more intense sensation, constantly moving, touching, watching moving objects, and/or seeking loud sounds or unusual olfactory or gustatory experiences.
Although these proposed patterns of SMD are clinically useful, there are limited empirical data related to the accuracy of the clinical categorization schemes that have previously been proposed for SMD. Ayres' original work [3] suggested one SMD pattern characterized by difficulty modulating tactile input, which she labeled tactile defensiveness. Later, Dunn [12], [13] conducted a factor analysis of behaviors from her parent-report measure, the Sensory Profile, and proposed a quadrant classification scheme accounting for high versus low neurologic thresholds to sensory stimuli in combination with either a either passive versus active regulatory strategies. The 4 categories she proposed were sensation seeking, low registration, sensory avoiding, and sensory sensitivity. Miller et al [14] discussed a more complex model with multiple subtypes based on their ecologic model of sensory modulation that accounts for both internal and external factors affecting the ability to achieve homeostasis.
These models have face validity; however, empirical research defining subtypes is needed to clarify the variability in children with SMD and to determine if natural boundaries exist between subtypes. This clarification of subtypes will improve selection of homogenous samples in all applied SMD research, improve treatment planning for specific clinical cases, and decrease sample variation, thus increasing power in effectiveness research.
Many developmental and behavioral disorders are diagnosed based on the presence of a cluster of symptoms, for example, attention-deficit/hyperactivity disorder (ADHD), anxiety disorders, and autism spectrum disorders. Cluster analysis is a statistical technique commonly used to empirically define diagnostic taxonomies of complex disorders. Cluster analysis has been used to define subtypes of depression [15], to categorize suicidal patients into 3 subtypes [16], and to identify subtypes of eating disorders [17]. More recently, cluster analysis has been used to decipher subtypes and patterns in ADHD [18], [19], [20], bipolar disorder and other manic diseases [21], [22], [23], and the full spectrum of psychopathologic symptoms [24].
Cluster analysis is appropriate as an exploratory tool to define structure within data and reflect more homogenous patterns within groups and more diverse patterns across groups. This study uses cluster analysis to group behaviors in children with SMD based on the hypothesis that SMD clusters into meaningful subtypes (sensory overresponsivity, sensory underresponsivity, and sensory seeking/craving) based on behavioral characteristics of attention, sensation, and emotion as reported clinically in the literature [4], [14], [24], [25], [26], [27].
Section snippets
Participants
Participants were children referred from the Occupational Therapy (OT) Department at The Children's Hospital in Denver, CO, with a clinical diagnosis of SMD. Children were referred to OT before being recruited for this study. Referrals to OT were made by physicians, teachers, and parents based on aggressive or withdrawn behavior, sensory or motor problems, inattention and impulsivity, and other behaviors disrupting activities of daily living. The identification of SMD was proffered after
Cluster identification
With hyperactivity and movement sensitivity as the explanatory variables, the strongest agreement between the 3 clustering statistics occurred with 2 clusters: cluster 1 (n = 72) and cluster 2 (n = 22). The former reflected problems with hyperactivity, and the latter reflected problems with movement sensitivity.
Fig. 1 display the hyperactivity and movement sensitivity range standardized mean subtest scores for each cluster. Values closer to 1 represent more typical (normal) performance, and
Discussion
The main finding from this study is that 2 distinct clusters of SMD exist in this sample, potentially representing 2 subtypes of SMD [5]. In this cohort of children with SMD cluster 1, the first subtype includes 75% of the sample (n = 72) and is marked by the explanatory variable hyperactivity. Additional defining characteristics include sensory seeking/craving, impulsivity, delinquent attributes, aggressiveness, poor socialization, poor adaptation, impaired cognitive/social, and many
Limitations
Limitations exist in this study. First, the cohort was patients referred to an OT clinic and may not generalize to children not referred to occupational therapy clinics, including those children with SMD that go undiagnosed. Children referred to other OT clinics could have characteristics that are not the same as this sample; for example,., they may have fewer or more sensory impairments, have more comorbidities, be of different socioeconomic status, or have environmental factors that
Conclusions
The findings of the 2 clusters of SMD in the sample studied suggest that SMD subtypes include at least 2 distinct subtypes: sensory seeking/craving as determined by cluster 1 (subtype 1) and sensory underresponsivity as determined by cluster 2 (subtype 2). This is the first empirical study to differentiate sensory seeking and sensory underresponsivity, which are typically thought to be the same clinically, due in part to the Dunn model that identifies these 2 subtypes as falling on the same
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