The clinical picture of children with autism spectrum disorder (ASD) is characterized by deficits of social interaction and communication, as well as by repetitive interests and activities, according to the criteria of the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5).1 Sensory abnormalities are a very frequent feature that often go unnoticed due to the communication difficulties of these patients. According to the DSM-5 criteria, this type of symptomatology is constituted by increased or decreased reactivity to sensory input, or by an unusual interest in sensory aspects of the environment. The following are some of the examples cited by the DSM-5: visual fascination with lights or spinning objects; adverse response to specific sounds or textures; excessive smelling or touching of objects; apparent indifference to pain, heat, or cold.1 Almost any sensory channel may be involved, either in the sense of reduced responsiveness to stimulation or in the sense of excessive responsiveness to stimulation. There may be several types of sensory abnormalities in the same person over the course of life, or even at the same time. Table 1 describes several possible examples of behaviors related to sensory abnormalities in children with ASD. Sensory dysfunction is probably related to an impaired modulation occurring in the central nervous system, which regulates neural messages concerning sensory stimuli.2 Already in Sukhareva's pioneering description of children with autism (1926), sensory reactivity disorders were unequivocally mentioned with foresight.3 Later, hypo-reactivity as well as hyper-reactivity to sensory stimuli were reported in the classical descriptions of Kanner in 19434 and Asperger in 1944.5 In the following decades, the attention to these features has fluctuated, although the clinical experience with ASD individuals has always suggested the importance of these aspects. In 1980, the DSM-III considered sensory hypo-responsiveness or hyper-responsiveness as infantile autism-associated features,6 while the following two editions, DSM-III-R7 and DSM-IV,8 did not include sensory abnormalities as definite diagnostic criteria. Finally, the DSM-5 (2013) included “hyper- or hypo-reactivity to sensory input” as well as “unusual interests in sensory aspects of environment” within the main ASD criterion concerning restricted interests and repetitive behaviors.1

Sensory abnormalities of ASD children may also impair their behavior in family daily activities, including eating, sleeping, and bathtime and bedtime routines; and outside the home, these abnormalities may create problems, for example when taking vacations or participating in community events. Consequently, interventions for autism should include also specific strategies for managing sensory-related behaviors in order to improve daily family activities and participation in community events.9

This narrative review summarizes the main features of abnormal sensory reactivity in ASD patients and their implications for the interpretation of various signs and symptoms of autism, and thus for treatment. The authors considered the most relevant papers (preferably concerning children) among those published on this topic over the period from January 1, 2007 to July 31, 2017 available on PubMed (United States National Library of Medicine), including reviews; the following keywords were used: “autism,” “sensory reactivity,” “sensory abnormalities.”

Table 2 summarizes the most important results of this review.10–19 Three main sensory patterns have been described in patients with ASD: hypo-responsiveness, hyper-responsiveness, and sensory seeking; to these, some authors have added a fourth pattern: enhanced perception.13 Among the findings emerging from Table 2, there is also the importance of associating an objective with a subjective evaluation, as confirmed by the case of pain sensitivity, classically considered reduced in ASD subjects. In this regard, the study by Yasuda et al. offers very interesting findings (Table 2).17

As shown in Table 2, over the last years several parent report questionnaires concerning sensory reactivity have been used in ASD individuals, some not specific for autism (e.g., Short Sensory Profile – SSP), others specific for autism (e.g., Sensory Experiences Questionnaire, Version 3.0 – SEQ-3.0). These questionnaires (only sporadically associated with observational tools)12,16,20 can provide valuable findings; however, as they are based on data from an external perspective, they cannot describe the first-person sensorial experience of patients. It would be very important to understand what they really perceive, but unfortunately only some are able to describe it, due to the frequently severe impairment of language and cognitive skills. ASD patients who are able to describe the sensory experiences can provide us with information that is also at least partially applicable to individuals who cannot report such experiences. This aspect was addressed in the studies by Chamak et al.18 and Elwin et al.19 (Table 2).

There are different ways by which sensory abnormalities may negatively impact the life of ASD individuals and their families,9–16,18–20 including an impairment of: social communication and activities; adaptive behavior (e.g., “problem behaviors”: see Discussion); range of interests (restricted, repetitive); everyday routines (e.g., avoiding behaviors due to sensorial distress); and cognition, the last one hypothetically due to the reduced number of signals that ASD individuals can utilize to understand and interact with the environment, as recently suggested by Haigh.21 In addition, eating behavior can be affected by sensory abnormalities, leading to a food selectivity that can in turn cause inadequate nutrition22, as well as sleep, due particularly to a hyperarousal mechanism.23

Several hypotheses have been formulated to explain the sensory abnormalities in ASD patients, including that of “enhanced perceptual functioning,” perhaps due to the increased functioning of the brain regions involved in primary perceptual functions,24 and the “intense world hypothesis of autism,” where the core brain pathology is increased reactivity and plasticity of local neuronal circuits, leading to increased perception, attention, and memory, making the world aversively intense for the child with ASD.25 But during recent years, there has been an increasing interest in the interpretation models suggesting an impairment not only of unisensory modalities (e.g., auditory alone) but also of multisensory integration, probably related to a brain connectivity impairment, consisting in particular of poor long range connectivity.26 According to this perspective, children and other individuals with ASD have a reduced ability to integrate sensory information across different modalities (auditory, visual, etc.), which would contribute to core autism symptoms, such as social communication impairments.11,14,27,28 In addition, environmental factors may influence these symptoms, as recently suggested by Kirby et al., who studied the contexts surrounding the sensory and repetitive behaviors of 32 ASD children through the behavioral coding of naturalistic video recordings at home. They found that hyper-responsive behaviors were associated with daily living activities and stimuli initiated by the family, while sensory seeking behaviors were associated with free play activities and stimuli initiated by the child. However, the conclusions of this intriguing study should be taken with caution due to the relatively narrow size of the sample considered.29

In the context of sensory interventions, sensory integration therapies (child-centered) and sensory-based interventions (adult-directed) are distinguished. The former are clinic-based interventions using play activities and sensory-enhanced interactions in order to improve the adaptive responses to sensory experiences. Through gross motor activities activating the vestibular and somatosensory systems, these interventions aim to enhance the capacity to integrate sensory information, leading the child to adopt more organized and adaptive behaviors, including improved joint attention, social skills, motor planning, and perceptual skills. In this context, the therapist chooses a “just-right challenge” (i.e., an activity that is just a little above what the child is now able to do without difficulty) from the child's emerging skills and supports his/her adaptive responses to the challenge. Conversely, sensory-based interventions are classroom-based and use single-sensory strategies (for example therapy balls or weighted vests) in order to influence the state of arousal, most often, with the aim of reducing a high arousal state that can be clinically manifested as restlessness, hyperactivity, and self-stimulating behaviors.30

Which of these two types of intervention is the most effective? The answer comes from Case-Smith et al.,30 who performed a systematic review concerning sensory interventions in children with ASD and sensory processing abnormalities (2000–2012), including five studies about the effects of sensory integration therapies and 14 studies about the effects of sensory-based interventions. Concerning sensory integration therapies, two randomized controlled trials showed positive effects on Goal Attainment Scaling,31,32 decreased mannerisms,31 and improved self-care and social function,32 while other studies showed positive effects on decreasing behaviors related to sensory abnormalities. On the contrary, the studies about sensory-based interventions showed only a few positive effects, suggesting a lack of efficacy.30

Sensory experiences in ASD individuals are reported as a distress/anxiety as well as a source of fascination/interest.33 On one hand, distress/anxiety can lead to avoidance behaviors against disturbing stimuli or may even trigger reactions with intense agitation and hetero- or self-directed aggression (see the so-called “problem behaviors”). On the other hand, the absorbent stimuli sources can lead to restricted and repetitive behaviors, from which it is very hard to deflect these individuals’ attention. For one reason or another, the impact of sensory abnormalities of children with ASD on their daily lives is considerable and probably underestimated because of their communication difficulties, thus it should always be specifically investigated.

The following question arises spontaneously: should sensory abnormalities be considered core autism features or only comorbidity characteristics? Based on the data available today, this question remains open. In fact, sensory symptoms are common and often invalidating in ASD children, but are not specific for autism, being a feature, as was mentioned above, frequently described also in subjects with intellectual disability without autism. However, the atypical sensory reactivity of these subjects may be the key to understand many of their abnormal behaviors and, therefore, it is a relevant aspect to be taken into account in the daily management of these individuals in all contexts in which they live. For example, according to Leekam et al., the distress due to specific sensory inputs can arouse problem behaviors in low-functioning ASD individuals who are unable to report their discomfort. Understanding what specific sensory inputs cause discomfort in a given individual is the prerequisite for reorganizing the environment in which he/she lives and his/her daily routine in order to reduce as much as possible such discomfort; in this perspective, a desensitization program may be useful. In some cases the use of sensory rooms, where individuals have pleasant sensory experiences of various kinds, can be very useful.11

In conclusion, a formal evaluation of sensory function should be always performed within the neurobehavioral assessment of ASD children and should be repeated periodically during the follow-up, in order to meet the needs of these individuals, to prevent as far as possible the appearance of problem behaviors, and ultimately to alleviate the difficulties of their families. Moreover, further research should be performed, according to Lane et al.,14 in order to better understand the possible neurobiological (genetic, neurophysiological, and neuro-imaging) pathways of sensory abnormalities in ASD patients.

The authors declare no conflicts of interest.