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Sensory Modulation Disorder
Sensory Modulation Disorder (SMD) is characterised by a difficulty to appropriately regulate the type of and intensity of responses to sensory inputs from the environment. The responses are usually inconsistent with the demands of the situation and those affected usually have difficulties attaining a suitable range of attentional, emotional and physical responses to sensory stimuli.

SMD is classified under sensory processing disorder which comprises other sensory dysfunctions such as sensory-based motor disorder and sensory discrimination disorder. The proposed classification system by Miller and colleagues is a reconceptualisation of Ayres original model of sensory integration theory. The main features of SMD consists of: sensory overresponsivity, sensory underresponsivity and sensory seeking/craving behaviour. This could affect one or more of the following sensory systems: visual, auditory, olfactory, tactile, gustatory, proprioception and/or vestibular.

Sensory Overresponsivity
Individuals with sensory overresponsivity or hypersensitivity are more reactive to sensory stimulation than most people. The sensory inputs or sensations received are usually felt more easily, more intensely and for longer periods. There is greater likelihood of overresponsiveness when in new or unexpected situations. Moreover, situations that could potentially evoke a reaction may accumulate over a period time, which would precipitate an extreme reaction or response that is disproportionate to the situation. Behaviours associated with overresponsivitiy could be seen as more active, impulsive or aggressive. However, responses could also be more passive or withdrawn in order to avoid the overwhelming sensations. Such behaviours may result in sociol-emotional problems and affect daily functioning, academic skills and socialisation skills. This may be especially evident for children of school entry age as there is less control over the type of inputs from the environment when compared to being at home, as well as the increased learning demands in a school environment.

Sensory Underresponsivity
Sensory underresponsivity or hyposensitivity refers to a lack of response or awareness of sensory input from the environment. This reduced awareness may appear as a lack of motivation to engage in activities or socialise, resulting in a seemingly withdrawn and inattentive behaviour. Due to the lower level of sensations received, individuals with sensory underresponsivity will require sensory inputs of higher intensity in order to be engaged in activities or interactions.

Sensory Seeking / Craving
Individuals with sensory seeking/craving behaviour intensely desire particular sensations and will actively engage in actions like constant spinning, touching or making loud noises to provide these sensations to their bodies. Such actions may sometimes be inappropriate or unsafe and oftentimes interpreted as attention-seeking behaviour. Sensory seeking behaviour could also be confused with sensory over-and underresponsivity. For instance, seeking behaviours like rocking the body could be a way to regulate the overwhelming sensory input from the environment (i.e. hypersensitivity) and constant touching of objects could be a means to intensify sensory input as a result of hyposensitivity.

Signs and Symptoms
Individuals with SMD may exhibit a wide range of behaviours such as irritability, poor socialisation or adaptation and carelessness. Moreover, children with SMD are reported to have lower participation in daily activities as compared to typically developing children. This includes experiencing lower levels of performance and enjoyment of daily activities due to the sensory symptoms experienced. The following are some signs and symptoms usually exhibited in SMD based on the different sensory systems.

Visual
Hypersensitivity
 * bothered by bright lights and colours
 * difficulty seeing and concentrating under direct sunlight
 * easily distracted by other visual stimuli from the environment (e.g. movements, plants, animals)

Hyposensitivity
 * difficulty focusing on the movement of an object
 * difficulty estimating space between self and objects (i.e. constantly bumping into things)

Auditory
Hypersensitivity
 * distracted by sounds not usually noticed by others (e.g. refrigerator humming, clock ticking)
 * startle easily by loud unexpected sounds

Hyposensitivity
 * seems to be oblivious to certain sounds
 * difficulty responding to name being called
 * likes extremely loud noises

Olfactory
Hypersensitivity
 * bothered by certain perfumes or fragrances
 * negatively reacts to smells not usually noticed by others

Hyposensitivity
 * fails to notice pungent/unpleasant smells
 * unable to differentiate pleasant and unpleasant odours

Taste
Hypersensitivity
 * picky eating habits
 * difficulty eating food with particular textures
 * only eats hot or cold food

Hyposensitivity / seeking
 * craves salty, spicy, crunchy or chewy food types
 * prefers food with intense flavours

Tactile
Hypersensitivity (tactile defensiveness)
 * Fearful or anxious of unexpected touch
 * Sensitive to tags on clothes
 * Distressed about touching particular textures (e.g. sand, grass)

Hyposensitivity / seeking
 * Unaware of being touched or bumped into
 * Constantly touching objects, touches others or things too often or too hard

Proprioception
(i.e. position of joints and muscles)

Seeking
 * teeth grinding
 * likes jumping, wrestling and crashing activities
 * prefers tight clothing

Vestibular
(i.e. movement and relation to gravity)

Hypersensitivity Hyposensitivity / seeking
 * resistance to moving playground equipments like swings and slides
 * startles if someone moves them
 * loses balance easily
 * unaware of falling sensation
 * engaged in rocking or rhythmic movements
 * likes sudden or quick movements

Diagnosis
The topic of sensory processing or sensory integration difficulties have been around in the area of practice in occupational therapy. More recently, sensory dysfunctions including SMD have been acknowledged outside the area of occupational therapy in the following diagnostic classification references for mental health:
 * The Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early childhood, Revised (i.e. DC:0-3R)
 * The Diagnostic Manual for the Infancy and Eearly Childhood of the Interdisciplinary Council on Developmental and Learning Disorders (i.e.ICDL)
 * Psychodynamic Diagnostic Manual

Although SMD does not appear in the Diagnostic and Statistical Manual of Mental Disorders (5th edition; DSM-5) as a mental disorder, recent changes to the diagnostic criteria of autism spectrum disorder have included sensory overresponsivity, sensory underresponsivity and sensory seeking behaviour as core features of rigid and repetitive patterns of behaviours, activities or interest.

Coping Strategies / Treatment
Occupational therapy using the sensory integration approach could help to alleviate some of the difficulties experienced by individuals with SMD. Some ways to cope with sensory hypersensitivity is to slowly introduce any sensory stimuli such as noise, lights or physical touch. For instance, using dimmer lighting for individuals with light hypersensitivity and having the television or radio at low volume before increasing the sound for those with auditory hypersensitivity. Children with sensory hyposensitivity or sensory seeking behaviours, could benefit from physical activities and movement experiences like jumping and swinging which provide proprioceptive and vestibular input.

Prevalence
Sensory hypo and/or hypersensitivity is estimated to occur in 5% of typically developing children and approximately 40-80% of children with developmental disabilities like autism and ADHD. However, rates in typically developing children may be underestimated as sensory overresponsivity is found to occur in 16.5% of elementary school aged children. This is particularly evident in the auditory and tactile subtype. Most of the studies available are on the paediatric population, and although it does exists in adults, prevalence rates are yet to be studied and reported.

Comorbidity
In general SMD is more likely to co-occur with other developmental disorders. In autism, sensory hyper- and/or hyposensitivity is frequently associated with repetitive movement and insistence on sameness of behaviour. These behaviours could be a means to compensate for the lack of sensory input experienced from the environment (i.e. hyposensitivity) or to understand and manage the overload of sensory stimulation received (i.e. hypersensitivity). Sensory seeking behaviours in autism may also be expressed as stereotypies like rocking of the body and hand flapping at an attempt to self-regulate. These sensory related behaviours may also affect communication and socialisation for individuals with autism. For example, if visual and auditory information are not processed sufficiently, the development of verbal communication may be disrupted. In ADHD, sensory seeking behaviours like inattention, impulsive actions and restlessness usually co-occur. Also in schizophrenia, the visual and auditory systems are particularly affected. This is proposed to be a factor in the development of schizophrenic symptoms and the accompanying cognitive deficits.