SPD defined

Sensory processing is defined as the brain’s ability to register, organize, and make sense of information received from one’s senses. Although most children with autism exhibit symptoms of SPD, this condition is also common in children who have other developmental disabilities and may occur in typically developing children. When sensory processing is dysfunctional, the individual may struggle with behaving in line with the demands of the environment, contributing to broader functional difficulties. A variety of terms have been used to describe SPDs. Box 1 provides a glossary for commonly used terminology.

To increase diagnostic precision for research and treatment purposes, a unified nosology has been proposed for the discussion of sensory processing dysfunction. In this nosology, several categories of SPD have been suggested to describe different patterns of behavioral dysfunction: sensory-based motor disorder, sensory discrimination disorder (SDD), and sensory modulation disorder. Their hierarchical relationships are illustrated in Fig. 1 . Sensory-based motor disorder is described as poor motor planning and/or postural instability resulting from ineffectual processing of information from the senses and can present as discoordinated or immature movement patterns. SDD is described as the inability to interpret differences and similarities between information received from the senses. For example, difficulty discriminating visual information may present with inability to distinguish between letters, making acquisition of reading skills a challenge. Sensory modulation disorders are described later and are of particular relevance to a discussion on children with autism.

SPD hierarchy.

( From Miller LJ, Anzalone ME, Lane SJ, et al. Concept evolution in sensory integration: a proposed nosology of diagnosis. Am J Occup Ther 2007;61(2):137; with permission.)

Sensory modulation disorder constitutes a cluster of disorders characterized by impairment in the intensity and nature of behavior in response to sensory input. As a result, the response to sensation is out of synchrony with the demands of the environment, and attentional and emotional regulation is affected. Three subcategories of sensory modulation disorder have been hypothesized: sensory overresponsivity, sensory underresponsivity, and sensory seeking. Although these subcategories are currently hypothetical, physiologic research is emerging to support these distinctions.

Sensory overresponsivity is the subtype of sensory modulation disorder that is characterized by fast, intense, sustained reaction to sensory stimuli that is out of proportion with the situation. People with sensory overresponsivity are hypothesized to have an unusually low threshold for sensation in any or all of the sensory systems (tactile, vestibular, proprioceptive, visual, olfactory, gustatory, and/or auditory). As a result, people with sensory overresponsivity may overreact to sensations that are typically not perceived as threatening or even noticed by most people. Symptoms may include inappropriate behavioral outbursts that may be triggered by the feeling of textures on skin (clothing, socks, food on fingertips), movement activities (swinging, riding in the car), and loud or unexpected noises (bell ringing at school, toilet flushing). Often, children with sensory overresponsivity seem to be excessively cautious, become upset with changes in routine, and have difficulty with transitions between activities. These automatic and inappropriate responses prevent effective functioning in daily life tasks. Sensory overresponsivity is often most evident when the person is faced with unexpected or novel situations or during transitions from one activity to another. Insistence on sameness and difficulty in discontinuing or initiating an activity are common behaviors in children with autism that may be related to dysfunctional sensory processing.

Sensory underresponsivity is a subtype of sensory modulation disorder that is believed to result from an unusually high threshold for sensation. Sustained, loud, bright, fast, or otherwise intense input is required to get the attention of someone with sensory underresponsivity. Symptoms might include not responding when name is called, extreme preference for sedentary activities, and failing to respond to pain. The lack of ability to register sensation may lead to a child being described as unresponsive or unaware of their surroundings, both common concerns in children with autism.

The sensory seeker subtype of sensory modulation disorder is characterized by the appearance of an excessive insatiable drive for sensory experiences. Children with this subtype may climb to unsafe heights, mouth nonfood items, or touch people and/or food objects to the point of annoying others. Children with sensory seeking may also engage in dangerous movement activities not in line with their motor skill level or demonstrate disruptive behavior that limits their ability to integrate into classroom or group situations. Sensory-seeking behavior has the potential to cause injury and disrupt the development of meaningful routines and relationships. This could be particularly problematic in children with autism who already have difficulty with relationship formation.

Sensory modulation disorders of any type can be disruptive to a child’s development. Children with ASD demonstrate high levels of sensory modulation disorders across ages and severity of autism. The presentation of these symptoms is heterogeneous, with highest incidence of sensory underresponsivity followed by overresponsivity and sensory seeking. Links between sensory modulation disorders and psychological dysfunction may contribute to autism symptom severity and deficits in everyday functioning, in effect, compounding the severity of autism symptoms.

Investigation into the underlying neurologic causes of sensory modulation disorders are ongoing, with significant discoveries emerging. Technological advances have made it possible to measure physiologic functioning related to the behavior observed in response to sensory input. The research into the physiologic correlates of sensory modulation disorders can provide insight into this comorbid condition.

Physiologic evidence for distinction of sensory modulation disorders

In the 1970s, Jean Ayres was one of the first to describe what seemed to be a sympathetic nervous system fight-or-flight reaction to sensation for children demonstrating difficulty modulating their arousal level. Physiologic measurement tools are now available that make it possible to explore the neurologic correlates of sensory modulation disorder. Using tools such as electrodermal sensors, research is beginning to capture the physiologic response of individuals with behavioral abnormalities to sensory information, implicating the autonomic nervous system in this dysfunction. Two key studies point to differences in sympathetic and parasympathetic response to sensation for children with clinically identified sensory modulation disorder compared with children developing typically.

In a study of the physiologic responses to sensation in children clinically identified with sensory modulation disorder, McIntosh and colleagues demonstrated connections between different patterns of sensory processing, electrodermal responses (EDR), and functional behavior. The researchers chose EDR as the objective physiologic measure of sympathetic nervous system activity based on prior research that had shown associations of EDR activation with startling or threatening stimuli and during positive and negative emotional events. EDR changes were measured for the study in the context of an innovative space ship–themed protocol, dubbed the Sensory Challenge Protocol, which was designed to present sensory stimulus in a nonthreatening manner. Each of the sensory modalities was stimulated during the protocol (eg, tactile stimulus, feather touched to the skin; vestibular stimulus, child tipped back in the chair; olfactory, smell of wintergreen under nose; auditory and visual, series of bright lights and sounds). The physiologic responses of children who had been identified clinically as having sensory modulation disorder were compared with those of children without such a diagnosis.

The study revealed that there were significantly more children in the sensory modulation disorder group who failed to exhibit EDR response to stimuli than in the typically developing group. This finding tentatively points to a decrease in sympathetic nervous system response in children with sensory underresponsivity. The children who did not respond to the stimulus were then removed from the next analysis. Physiologic evidence for the overresponsivity pattern of dysfunction was provided with the greater response and higher number of responses to sensation in the remaining sensory modulation disorder group. These findings provided physiologic support for the distinction between sensory underresponsivity and sensory overresponsivity. Specifically, as expected, there was both a decrease and an increase in EDR response to sensory stimuli in children with sensory modulation disorder consistent with the clinical presentation of sensory underresponsivity and overresponsivity compared with typically developing children. These data provided support for the unique physiologic pattern of processing sensory information in children clinically diagnosed with sensory modulation disorder and served as the basis for additional research into the nature and causes of sensory modulation disorder.

A subsequent study provided evidence for indirect implication of the sympathetic nervous system and involvement of the parasympathetic system. Schaaf and colleagues conducted a pilot study of differences in parasympathetic functioning of children with clinical symptoms of sensory modulation disorder that were age matched to children functioning typically. They used the cardiac vagal tone index as the primary measure of parasympathetic activity based on its demonstrated validity in a variety of clinical populations and age groups for this purpose. Data on cardiac vagal tone were collected during the Sensory Challenge Protocol described earlier.

The results indicated that children with sensory modulation disorder had significantly lower cardiac vagal tone measurements, suggesting that these children have less effective parasympathetic functioning compared with typical controls. The investigators discussed the implications of sensory modulation disorder on functioning, suggesting that a compromised parasympathetic system may disrupt the child’s ability to maintain a calm focused state in the face of sensations encountered in everyday life.

Physiologic evidence for distinction of sensory modulation disorders

In the 1970s, Jean Ayres was one of the first to describe what seemed to be a sympathetic nervous system fight-or-flight reaction to sensation for children demonstrating difficulty modulating their arousal level. Physiologic measurement tools are now available that make it possible to explore the neurologic correlates of sensory modulation disorder. Using tools such as electrodermal sensors, research is beginning to capture the physiologic response of individuals with behavioral abnormalities to sensory information, implicating the autonomic nervous system in this dysfunction. Two key studies point to differences in sympathetic and parasympathetic response to sensation for children with clinically identified sensory modulation disorder compared with children developing typically.

In a study of the physiologic responses to sensation in children clinically identified with sensory modulation disorder, McIntosh and colleagues demonstrated connections between different patterns of sensory processing, electrodermal responses (EDR), and functional behavior. The researchers chose EDR as the objective physiologic measure of sympathetic nervous system activity based on prior research that had shown associations of EDR activation with startling or threatening stimuli and during positive and negative emotional events. EDR changes were measured for the study in the context of an innovative space ship–themed protocol, dubbed the Sensory Challenge Protocol, which was designed to present sensory stimulus in a nonthreatening manner. Each of the sensory modalities was stimulated during the protocol (eg, tactile stimulus, feather touched to the skin; vestibular stimulus, child tipped back in the chair; olfactory, smell of wintergreen under nose; auditory and visual, series of bright lights and sounds). The physiologic responses of children who had been identified clinically as having sensory modulation disorder were compared with those of children without such a diagnosis.

The study revealed that there were significantly more children in the sensory modulation disorder group who failed to exhibit EDR response to stimuli than in the typically developing group. This finding tentatively points to a decrease in sympathetic nervous system response in children with sensory underresponsivity. The children who did not respond to the stimulus were then removed from the next analysis. Physiologic evidence for the overresponsivity pattern of dysfunction was provided with the greater response and higher number of responses to sensation in the remaining sensory modulation disorder group. These findings provided physiologic support for the distinction between sensory underresponsivity and sensory overresponsivity. Specifically, as expected, there was both a decrease and an increase in EDR response to sensory stimuli in children with sensory modulation disorder consistent with the clinical presentation of sensory underresponsivity and overresponsivity compared with typically developing children. These data provided support for the unique physiologic pattern of processing sensory information in children clinically diagnosed with sensory modulation disorder and served as the basis for additional research into the nature and causes of sensory modulation disorder.

A subsequent study provided evidence for indirect implication of the sympathetic nervous system and involvement of the parasympathetic system. Schaaf and colleagues conducted a pilot study of differences in parasympathetic functioning of children with clinical symptoms of sensory modulation disorder that were age matched to children functioning typically. They used the cardiac vagal tone index as the primary measure of parasympathetic activity based on its demonstrated validity in a variety of clinical populations and age groups for this purpose. Data on cardiac vagal tone were collected during the Sensory Challenge Protocol described earlier.

The results indicated that children with sensory modulation disorder had significantly lower cardiac vagal tone measurements, suggesting that these children have less effective parasympathetic functioning compared with typical controls. The investigators discussed the implications of sensory modulation disorder on functioning, suggesting that a compromised parasympathetic system may disrupt the child’s ability to maintain a calm focused state in the face of sensations encountered in everyday life.