Sensory Modulation Difficulties in Autism Spectrum Disorders: A brief primer
It is essential in understanding disruptive behaviour patterns in ASD to be aware of the important role of sensory modulation, and that assessment, diagnosis and treatment should be considered from a multi-disciplinary perspective.
Dr. J. M. Rao, M.B., B.S., D.P.M., M.R.C.Psych., F.R.C.P. (Canada)
Associate Professor, Department of Psychiatry, Developmental Disabilities Program, The University of Western Ontario, London, Ontario
This is a brief outline of sensory modulation difficulties in autism. It is essential in understanding disruptive behaviour patterns in ASD to be aware of the important role of sensory modulation, and that assessment, diagnosis and treatment should be considered from a multi-disciplinary perspective.
Aggression and self-injury are two of the most common reasons for referral to a specialized clinic for adults with autism and other developmental disabilities. The majority of these individuals do not have a mental health diagnosis.
It becomes clearer in the course of assessing these persons that many factors either individually or together contribute to the initiation and maintenance of such behaviours. We have identified nine different factors (Multi Factor Checklist) that may have a role in the development and establishment of these behaviours, namely, bio-medical, psychiatric, emotional, environmental, communication, sensory modulation, functional behaviour and executive function deficits (Rao, 2019. In preparation for publication). These factors may affect the behaviours to varying extents in different persons.
One of the factors that has a powerful influence on the behaviours of adults with autism is related to the sensory system. Sensory processing disorder is the term that is used to describe difficulties in processing sensory information and executing an action based on it. Neuroscientists use the term Multi-Sensory Integration to describe the convergence of two or more sensory modalities and the neural mechanism that is involved. The association between the sensory difficulties and ASD have been recognized for several decades. Unfortunately, much of the discussion is focused on whether sensory processing deficits form a core feature or an associated feature of autism. There is not much literature examining the association between self-injury/aggression and sensory difficulties in this population.
Miller et al describe three main types of sensory processing difficulties. One of these categories that we routinely encounter in our practice is Sensory Modulation Disorders.
The importance of the sensory modulation function in maintaining equilibrium and adaptation becomes obvious when one considers the fact that about 80 to 85% of the central and peripheral nervous system is involved in sensing, processing and responding to sensory information. Sensory modulation challenges are characterized by an inability to grade the degree, intensity, and nature of responses to sensory input. Sensory discrimination challenges represent problems in discerning the characteristics of sensory stimuli, leading to a lessened ability to interpret and differentiate temporal, spatial, categorical discriminant qualities of stimuli. In other words, there are problems in terms of the understanding of sensory information and in acting on them.
Affected individuals may under-respond to sensory input, over-respond to them or seek sensations to the detriment of their adaptation to their environment.
Those who under-respond to sensory stimuli in their environment or stimuli from within their own bodies seem withdrawn, self-absorbed and slow to respond. Children who are under-responders are often considered disinterested in the class-room, even defiant, as they do not appear to actively participate in the activities in the class-room. Many do not register pain at a ‘normal’ threshold, are unaware of minor injuries and irritants and become isolated.
Children who are over-responders often react too strongly to their environment, as they register sensory information at a low threshold. As a result, they are unable to tolerate noises, activity and stimulation in their class-rooms and either become ‘defensive’ (avoiding stimuli) or ‘offensive’ (agitated, aggressive). These children may also be considered as oppositional, disruptive, and may be subjected to punitive measures. In these children even mildly aversive sensory stimuli can provoke a greater neural response. One of the regions of the brain concerned with emotional processing, Insula, is over-reactive in these individuals. Insula, in conjunction with Amygdala is associated with anxiety. Amygdala determines the relevance of a sensory stimulus, whether it is a threat or not. In autism this seems to fail in its top-down regulation and as a result the individual’s brain cannot determine the saliency of a stimulus leading to emotional over-reactiveness and anxiety to ordinary stimuli.
The sensory seekers on the other hand constantly crave sensory stimulation. In their need to find stimulation, they constantly explore, hum, make noises, jump, bang on objects and can be quite disruptive.
When there are difficulties in interpreting, processing and responding to sensory input, the brain cannot generate an accurate model of the world and responds maladaptively. This may provoke anxiety, discomfort and distress. Given the poor meta-cognitive skills (problem solving) that these children have, it may lead to severe behavioural reactions such as self-injury and aggression.
Sensory processing difficulties are quite prevalent in Autism Spectrum Disorder. 95% of children with ASD may have some degree of sensory dysfunction (Tomcheck and Dunn, 2007). In adults with ASD, 94.4% may have sensory processing difficulty, establishing that these difficulties extend across the life-span (Crane et al, 2009). Given such high prevalence it is apparent that sensory difficulties invariably overlap many types of behaviours that one may consider in individuals with ASD.
Organization of the sensory system and its developmental origins.
It may be helpful to consider the various types of sensory systems individually and look at their developmental trajectory.
‘Touch’ is of special interest in ASD. The tactile sensation is not only involved in touching but is also necessary for discriminating between pleasant and painful stimuli, the textures of the objects touched and in identifying objects, and has a protective function. The ability to discriminate sensations and to localize sensations is necessary for us to construct a three dimensional experience of our environment. It is also involved in the perception of pressure and temperature. Most ASD children and adults are sensitive to light touch and avoid it. It usually begins to develop around 7 weeks of gestation and the touch sensors cover the entire body by 32 weeks. By the time the baby is born this sensation is finely developed. The consequences of disruption in early development is that the child is over-stimulated by even ‘ordinary’ sensory experiences, and may display a number of behaviours such as avoidance of tactile stimuli, extreme discomfort to touch, temperature variance, textural variations. Disruption of touch discrimination ability leads to the child’s inability to be comfortable, avoiding certain foods, inability to localize tactile stimuli. Many of these children develop tactile defensiveness which is an attempt to avoid a necessary activity such as the touch of the breast skin on the lips, leading to avoidance of feeding. Examples are that of children who have strong preferences for particular textures of clothing. Some may not tolerate tight bands around their waist or socks on their feet; some may not tolerate denims but prefer sweat-pants.
The system of taste starts developing before 30 days of fetal life. The newborn has a fully developed taste system and is able to discriminate between bitter, sour and sweet. If the threshold for the gustatory sensation is low, the system can be easily overwhelmed. As a result, infants develop difficulties coordinating suckling and swallowing. Many of these children may develop idiosyncratic eating patterns and may suffer nutritional deficiencies. One of our patients had refused to eat anything but french fries and cannot tolerate salt, sour and sweet. The severe, potentially life-threatening nutritional concerns lead to hospitalization, insertion of a G tube and a gradual habituation to other flavors.
The vestibular system is concerned with perception of balance and motion. The semi-circular canals in the inner ear are finely developed motion and position sensors of exquisite simplicity and sophistication. They convey information to the vestibular processing centers of the brain. They start developing by around 44 days of fetal life. By just over 20 weeks, they are fully developed. Vestibular senses are important to the fetus as it moves in the womb changing position. The system can become easily overstimulated and may result in distress, excessive motor activity, difficulty in settling and tolerating movement. Many adults and children with vestibular modulation difficulties rock their head side-to-side continuously. Some individuals with autism refuse to go on escalators. In these, there is a disconnection between the input through the visual system which indicates motion and the vestibular system that does not register motion as the individual is standing still on a step while the escalator moves. Some cannot maintain equilibrium when they go through a poorly lit place as the visual orienting information is cut off and the vestibular system cannot maintain its sense of position and motion. One of our clinic attendees has such issues, was treated for years with anxiolytic medications and behaviour therapy until it was realized that his difficulties were sensory based.
The structures of hearing are not fully developed till 24 weeks of gestation, though the fetus hears muffled sounds and the mother’s voice earlier, by about 16 weeks. By 4 weeks of fetal life the hearing system has its rudimentary anatomic beginnings. The function of hearing is necessary not only for identifying sounds but is essential for attention, orienting and vigilance. There are consequences if the fetus is unable to tolerate and habituate to various sounds. This may lead to problems such as auditory defensiveness, anxiety and avoidance of certain decibels of sounds.
Many individuals with ASD have abnormal or hyper acute hearing. They respond to some sounds with distress, such as loud sounds, crying sounds of babies, din of multiple sounds. They often cover their ears or prefer to wear headphones. Some adults with autism in our clinic population describe ordinary sounds as painful, “as though they are bouncing off a room with cathedral ceilings”. It may be difficult for some to separate sounds if they occur simultaneously, as in a social setting. Some in our clinic state that they cannot separate the main conversation from collateral conversations. They avoid crowds, malls, social gatherings. A class-room environment is not the ideal place for them as they cannot concentrate, become restless, and develop avoidant responses.
The anatomical development of the system of smell starts around 5 weeks of gestation and is done by 8 weeks in terms of basic apparatus. Babies develop the sense of smell around 11 to 16 weeks of gestation. They develop a predilection for smelling, initially the amniotic fluid, and subsequently the breast milk and their mother’s smell. Obviously, this is of survival value. The preference for identifying or seeking others through the smell sense alone fades as the other senses develop. However, many children and adults with ASD continue to use this modality to augment other senses such as vision in their relationship with people. Many with ASD prefer or avoid foods based on their odour. Preference for certain odours often disrupts their social functioning.
The visual system is probably one of the earliest to start developing. By about the 22nd day of fetal life, the process has begun. However, it continues to develop till about the 36th week of gestation. The visual system continues to develop even after birth to almost the 5th month of infancy. The visual system works with the vestibular system in providing information to the brain about the three-dimensional aspects of objects and space and enables the maintenance of equilibrium. Depth perception is often an issue with many of our patients, resulting in inability to walk on different surfaces, difficulty in climbing stairs or stepping on to a sidewalk. A patient of ours had refused to walk and would flop on the floor and scoot across to the extent that he would not leave the house. After many medication changes and interventions, he was assessed at our clinic and was found to have severe problems with depth vision. With focused rehabilitation, he was discharged home and is functioning well. Many individuals have double vision as a result of a failure in binocular vision. Recognition of these difficulties routinely is important.
Proprioception is the sense that helps us understand the position of our body in space. In other words, how our body is placed in space (sitting, standing), and how our limbs are placed relative to the body. There are many mechanoreceptors in our muscles, joints, tendons, and even skin. Not only do they have spatial functions, they also help in the control of tone in the muscles. They are essential for grading and co-ordination of movements. For example, a person with grading difficulties may not be able to grasp a plastic bottle or their care provider’s arm, applying the right amount of pressure. Typically, they slam doors shut, bang objects on the counter, grasp hands and arms too tightly causing bruises unintentionally.
Proprioception in conjunction with other sensory systems such as visual and vestibular helps develop a three dimensional location of objects in space. It is also important for control of movement and awareness of our body.
One manifestation of proprioceptive difficulties is that they cannot sit in a place without moving too much. They rock, bounce on the couch, change position, jump, move aimlessly. Proprioceptive modulation difficulties are common in our clinic population.
The brain is constantly updated with spatial information of the body and environment through several sensory modalities. It is possible that when such information inflow is slowed down, or if the brain does not receive enough data, there is difficulty in mapping the body or its spatial position. As a result, movement in joints and effort in muscles may be activated to increase the flow of proprioceptive information to the brain. This may result in constant motion, rocking, bouncing, fidgeting, jumping. Some are soothed by movement related activities such as car rides, swinging on a swing, swimming. A weighted blanket or vest may provide the anchor-sensation to soothe and calm. Many such children are misdiagnosed as ADHD and usually do not respond to stimulants. Even more children come to us with a school history of severe disciplinary interventions, labelled as having oppositional defiant or conduct disorders.
There is a relative lack of understanding of how proprioception develops, partly because of difficulties in accurately assessing proprioceptive function in children. Proprioception in typically developing children has never been fully mapped and no consistent assessment protocol has been established.
However, being aware of the symptoms of proprioceptive modulation difficulties is essential in developing a comprehensive care plan.
This is a brief, rudimentary outline of sensory modulation difficulties in autism. It is essential in our assessment and understanding of disruptive behaviour patterns in ASD that we are fully cognizant of the role that many factors such as sensory modulation play. It is also important that we approach assessment, diagnosis and treatment from a multi-disciplinary perspective. Sensory modulation difficulties cause significant distress to persons with ASD. Working with an Occupational Therapist with sensory expertise should be an essential requirement, particularly when this effort is coordinated with Speech and Language Pathologist, Behaviour Consultant and a medical specialist.
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Lane A E, et al: Sensory processing subtypes in autism: association with adaptive behavior. J Autism Dev Disord
Lane S J, et al: Examining the Neuroscience Evidence for Sensory driven Neuroplasticity: implications for sensory-based occupational therapy for children and adolescents. The American Journal of Occupational Therapy
Miller L J. et al: Perspectives on sensory processing disorder: a call for translational research. Frontiers in Integrative Neuroscience
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Rao, J M: Multi Factor Assessment Checklist: in preparation for submission.
This article was first published on this site in 2021.