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Sensory processing disorder or SPD is a neurological disorder causing difficulties with taking in, processing, and responding to sensory information about the environment and from within the own body (visual, auditory, tactile, olfaction, gustatory, vestibular, and proprioception).

Sensory integration is an unconscious process wherein the brain processes and organizes information provided by the senses. It allows us to pay attention to what is important, and ignore other stimuli. It is how we are able to respond to situations correctly, and forms the basis for learning and social behaviour ^[1].

For those identified as having SPD, sensory integration does not happen in a normal manner. The information is often registered, interpreted and processed differently by the brain. The result can be unusual ways of responding or behaving, or finding things harder to do. Difficulties may typically present as difficulties planning and organizing, problems with doing the activities of everyday life (self care, work and leisure activities), and for some with extreme sensitivity, sensory input may result in extreme avoidance of activities, agitation, distress, fear or confusion ^[2] .

Occupational therapist A. Jean Ayres was the first to propose a theory in which deficits in the processing and interpretation of sensation from the body and the environment could lead to sensorimotor and learning problems in children ^[3] . It was originally termed sensory integration dysfunction. More recently, the term SPD has become commonly used in an effort to distinguish between the diagnosis (SPD) and the theory.

SPD is currently controversial as a stand-alone diagnosis. At present, doctors recognize sensory problems as a common aspect of autism and attention deficit hyperactive disorder, but not as a distinct diagnosis. As such, SPD is not currently recognized in any standard medical manuals such as the ICD-10^[4] or the DSM-IV-TR ^[5] . Inclusion in the DSM-5 has been suggested, and is currently under review ^[6]. Inclusion would lead to an official recognition of SPD as a stand-alone disorder. SPD is included in Stanley Greenspan’s Diagnostic Manual for Infancy and Early Childhood and in the Regulation Disorders of Sensory Processing part of the The Zero to Three’s Diagnostic Classification ^[7] .

SPD is often associated with a range of neurological, psychiatric, behavioral and language disorders ^[8] . There is no known cure; however, there are many treatments available ^[9] .

Sensory processing disorders are linked with functional impairments. Five main impairments have been outlined ^[10] :

1. Impaired social skills and reduced participation in play;

2. Diminished frequency, length, or complexity of adaptive responses;

3. Reduced self-confidence or self-esteem or both;

4. Poor adaptive or daily life skills;

5. Diminished fine-, gross-, and sensory-motor skill development ^[10].

A new nosology was recently proposed to differentiate between different diagnostic subtypes ^[11] . Three main categories and subtypes of each were identified.

Type I: Sensory Modulation Disorder (SMD)

Type II: Sensory Discrimination Disorder (SDD)

Type III: Sensory Based Motor Disorder (SBMD)

Habituation occurs when the body begins to recognise a stimulus as familiar and stops reacting to it. Sensitisation occurs when the body recognises a stimulus as important or potentially dangerous, and has a heightened reaction to it. SMD occurs when habituation and sensitisation fail to work correctly. Those with SMD may show inappropriately increased or decreased reactions to stimuli ^[12] . They may become overly excited or completely inattentive. Three subgroups of SMD were identified:

Sensory Overresponsivity (SOR): Responding to a stimulus more quickly, more intensely, or for a longer duration than typical. This may occur in one or several sensory systems.

Sensory Underresponsivity (SUR): Disregarding or failing to respond to a stimulus. This is not due to a lack of motivation, as is often assumed, but is a result of being unaware of the possibilities for action. SUR is rarely detected until children are school-age. Proprioceptive and tactile systems are often affected, leading to clumsiness and inferior tactile discrimination. Thus, SUR is often co-morbid with developmental coordination disorder or sensory discrimination disorder.

Sensory Seeking (SS): Those who desire an unusual level of sensation. They seek out situations with more intense sensations, such as loud noises. They may fail to respect personal boundaries, or may engage in unwelcome behaviours such as constant movement, and thus can have social difficulties. SS can disrupt attention to a debilitating degree.

A disruption of one’s ability to identify or locate stimuli correctly, and to tell the difference between stimuli. Different systems may be affected. SDD in the tactile, proprioceptive, and vestibular systems may lead to awkward motor abilities ^[11]. If the visual or auditory systems are affected, a learning disability may result ^[11].

Poor postural or volitional movement. This has been divided into two subgroups ^[11].

Postural Disorder: This is characterised by difficulties stabilising the body in response to the situational demands. Symptoms include inappropriate muscle tension, hypotonic or hypertonic muscle tone, inadequate control of movement, or inadequate muscle contraction to achieve movement against resistance. It often occurs in combination with SDD and SMD, and may also occur with dyspraxia.

Dyspraxia: An inability to plan or perform precise actions. The appearance of awkwardness and poor coordination is common. People with dyspraxia may have difficulty identifying their location in space, and may bump into things or accidentally break things due to misjudgement. They may persist with familiar strategies even when they are unsuccessful, due to an inability to formulate novel ideas. Fine motor skills are also often affected.

Sensory processing disorders vary between individuals in their characteristics and intensity. Some people are so mildly afflicted, the disorder is barely noticeable, while others are so impaired they have trouble with daily functioning.

A scientific work group (SWG) was set up to investigate SPD, consisting of scientists funded by the National Institutes of Health (NIH) ^[13] . Pennington’s model of syndrome validation was used, which states that the greater the degree of divergence shown between the syndrome being investigated and existing disorders, the more likely that a discreet disorder exists ^[14] .

Two factors are considered relevant to the development of SPD: Prenatal environmental factors, and genetic and familial associations. Methodological and ethical considerations limit human studies, so several key studies have been carried out into the etiology of SPD in non-human primates. A 2007 study into SPD in non-human primates found that exposure to prenatal alcohol or stress resulted in sensory overresponsiveness (SOR) that was not seen in normal monkeys ^[15]. SOR primates also had significantly poorer muscle tone and worse righting reactions at birth. When removed from their mothers after six months, SOR primates showed significantly increased cortisol levels. At 32 months, the affected primates demonstrated inferior cognitive ability at a learning task compared to normally developing monkeys.

Prenatal stress was found to significantly increase withdrawal responses to stimuli compared to controls, meaning SPD primates reacted consistently to a higher degree to a stimuli than controls ^[16] . These results indicate that prenatal stressors can contribute to vulnerability to SPD ^[15][16]. It was hypothesised that this SOR was related to altered dopamine functioning in the striatum. The hypothesis was tested and revealed an association between increased striatal dopamine type 2 (D2) receptor binding and SOR .

Twin studies have found that auditory and tactile over-responsivity is significantly higher in identical than non-identical twins, indicating a genetic component to SPD-SOR ^[17] .

SPD has a basis in the nervous system. Children with sensory modulation disorder show greater and more frequent electrodermal responses to stimulation than control children ^[18]. They also habituate more slowly to repeated stimulation ^[18]

The parasympathetic nervous system is also implicated. Children with SPD have been found to have lower cardiac vagal tone, suggesting inefficient parasympathetic functioning compared to children developing normally ^[19]. Consistent with this finding, children with SPD also demonstrated lower heart period than typical children ^[19].

Sensory gating has recently been used to investigate the central nervous system’s role in SPD. Children display less gating than adults, with children with SPD demonstrating less auditory gating than normal children ^{[20] [21]}. When presented with tones of different frequency and intensity, children with SPD (specifically those with SOR and SUR) did not demonstrate an increased response to an increasing intense stimulus as normal children did. Sensory gating failed to improve with age for children with SPD ^[20]. These results indicate that SPD brains are less organised than typical, with a reduced ability to filter out irrelevant or repeated stimuli. Maturation of sensory gating proceeds in a different manner also.

The results of these studies indicate that children with SPD have different brain and nervous system mechanisms to normally developing children.

The treatment of sensory processing disorder is somewhat of a controversial issue^[22] and conceptually is criticised particularly in the areas of medicine, education, and neuropsychology^[23]. Some of these treatments (for example, sensorimotor handling) have a questionable rationale and no empirical evidence. Other treatments (for example, prism lenses, physical exercise, and auditory integration training) have had studies with small positive outcomes, but few conclusions can be made about them due to methodological problems with the studies^[24]. Although replicable treatments have been described and valid outcome measures are known, gaps exist in knowledge related to sensory integration disorder and therapy^[24]. Empirical support is limited, therefore systematic evaluation is needed if these interventions are used^[25].

The main form of sensory integration therapy is based on work by Dr. A Jene Ayres (1972, 1979 as cited in^[26] ). This is a type of occupational therapy that places a child in a room specifically designed to stimulate and challenge all of the senses. During the session, the therapist works closely with the child to provide a level of sensory stimulation that the child can cope with, and encourage movement within the room.

Sensory integration therapy is driven by four main principles^[24]:

• Just Right Challenge (the child must be able to successfully meet the challenges that are presented through playful activities)

• Adaptive Response (the child adapts his behaviour with new and useful strategies in response to the challenges presented)

• Active Engagement (the child will want to participate because the activities are fun)

• Child Directed (the child's preferences are used to initiate therapeutic experiences within the session).

Children with lower sensitivity (hyposensitivity) may be exposed to strong sensations such as stroking with a brush, vibrations or rubbing. Play may involve a range of materials to stimulate the senses such as play dough or finger painting. Children with heightened sensitivity (hypersensitivity) may be exposed to peaceful activities including quiet music and gentle rocking in a softly lit room. It has been reported that therapeutic listening (listening with the whole body) therapies have yielded positive results when combined with a sensory diet^[27]. Treats and rewards may be used to encourage children to tolerate activities they would normally avoid.

The aim of the sensory integration approach is exposure of various sensory stimuli based on the individual’s ability and functioning. It is then the hope that changes will occur within the brain, stimulating new neural pathways to allow sensory information to be processed and integrate more effectively^[28]. While occupational therapists using a sensory integration frame of reference work on increasing a child's ability to tolerate and integrate sensory input, other OTs may focus on environmental accommodations that parents and school staff can use to enhance the child's function at home, school, and in the community^[29]. These may include selecting soft, tag-free clothing, avoiding fluorescent lighting, and providing ear plugs for "emergency" use (such as for fire drills).

There is a growing evidence base that points to and supports the notion that adults also show signs of sensory processing difficulties. In the United Kingdom early research and improved clinical outcomes for clients assessed as having sensory processing difficulties is indicating that the therapy may be an appropriate treatment^[30] for a range of presentations seen in adult clients including for those with Autism and Asperger's Syndrome, as well as adults with dyspraxia and some mental health difficulties^[31] that therapists suggest may arise from the difficulties adults with sensory processing difficulties encounter trying to negotiate the challenges and demands of engaging in everyday life^[32].

Therapies in use to treat SPD are highly individualized with a range of different treatments used by therapists^[33]. As the treatment is so individualized to the specific difficulties that the client is exhibiting, it is difficult to find consistencies to compare, contrast and also replicate findings. Some research into sensory integration interventions concludes^[34] that the validity of outcomes in sensory integration therapy studies is under threat by weak fidelity of the therapies used. Due to inconsistent research results and methodological issues it has been seen by some as costly and ineffective ^[22]. A review of three controversial educational practices: Perceptual motor programs, sensory integration, and tinted lenses. Education and treatment of children, 32(2), 313-342.</ref>. This being said there are many studies that yield positive results and there is an emergence of randomized control trials that show positive results for the use sensory integration as a treatment when compared to no treatment or an alternative treatment^[35]. However, it seems further research and work is needed to replicate findings and develop consistency in approach and measures for sensory integration therapies to be accepted by other scientific disciplines^[24].

Difficulties of sensory processing have been observed in many disorders. Sensory processing disorder is commonly found to be co-morbid in individuals with autism spectrum disorder (ASD)^[36]. Studies have shown that when compared to community controls, participants with ASD were significantly different on a range of sections of their sensory profile - body position and movement; movement affecting activity level; sensory and visual input affecting emotional response and activity level^[37]. It is suggested that these difficulties in sensory processing could contribute to the behavioral and emotional problems seen in individuals with ASD. Literature does suggest that individuals with ASD display difficulties in processing their sensory world and this could affect social interactions and behavioral responses to situations. Although sensory processing difficulties are found in individuals with ASD regardless of behavioral problems, it has been found that there was higher rates of sensory processing dysfunction in children with autism who displayed behavioral or emotional problems in comparison to those that did not not have significant behavioral and emotional problems^[38].

Children with ASD were found to have significantly lower levels of baseline arousal than children with SPD or normally developing children ^[39]. Meanwhile, children with SPD had a greater reactivity to sensory stimuli than ASD and normal children ^[39].

Although abnormal responses to sensory stimuli are more common and prominent in those with ASD, there is a lack of evidence in the literature that reactions to sensory stimuli differentiate ASD from other developmental disorders^[40]. Some studies suggest that differences are greater for under responsivity (for example, walking into things) than for over-responsivity (for example distress from loud noises) or for seeking (for example rhythmic movements). However, individuals with ASD both seek and avoid sensory stimuli that would be viewed as normal^[41]. It is clear from the literature that sensory processing disorder is prevalent among those with ASD, however the type and severity seem to vary between individuals which effects their level of functioning in their sensory world. Also it has been noted that sensory processing difficulties are more common in children with ASD as opposed to adults with ASD^[42] and that adults with ASD are closer to community controls that children with ASD when examining sensory modulation^[37]. Further research is needed to examine whether this is due to the effect that treatments may be having on the individuals or whether the disorder itself lessens in strength and prevalence over time.

Some argue that sensory related disorders may be misdiagnosed as ADHD, however recent studies have shown that they can co-exist and are distinct from each other^[43]. Sensory modulation disorder (a subtype of sensory processing disorder) was examined and compared to ADHD in a study by Miller, Neilsen and Schoen (2011). Although symptoms overlap individuals with sensory modulation disorder were found to have a distinct profile separate from ADHD, differing in physiological reactivity to sensory stimuli and measures of attentional, emotional and sensory-related behaviors. Children with Sensory Modulation Disorder (a subtype of SPD) showed more difficulty in adaptation and sensory related problems as well as being more withdrawn and anxious when compared to individiuals with ADHD. Sensory problems were found in individuals with ADHD alone when compared to normal samples however their attentional difficulties were significantly greater than those with SMD. One area of thought that may shed some light on the similarities between ADHD and sensory processing disorder comes from research that describes a treatable inherited sensory over stimulation disorder called Hypokalemic sensory overstimulation^[44] that meets the diagnostic criteria for both ADHD and sensory integration dysfunction.

Difficulties of sensory processing have also been noted in individuals with Williams syndrome. The profile of sensory difficulties exhibited seems to be similar across this population. Research suggests they have difficulty with awareness of sensory events, screening out sounds and muscle movement, with 90 percent in the abnormal range and over half in the definite abnormal range^[45].

Individuals with Intellectual Developmental Disability (IDD) also exhibit a range of sensory processing problems across all levels of the disorder. However, studies have found that those in the mild and severe range may be more vulnerable^[46]. Self-stimulating behaviors that many individuals with IDD exhibit may be linked to their seeking out sensory stimulation or self-injurious behaviors, such as head banging, may be linked to a higher sensory threshold.

Impaired multi-sensory processing has also been observed in individuals with schizophrenia. It has been reported that when individuals with schizophrenia take part in experiments using variants of the McGurk effect(which relies on integration of visual and auditory stimuli) they are less susceptible to the illusions than normal participants. It is possible that there is a deficit in integrating multiple sensory inputs. As it is suggested that individuals with schizophrenia have dysfunctions in the posterior superior temporal cortex (where the dorsal stream is located) - it is suggested that this is likely to be the reason for the sensory processing difficulties^[47].

It is clear that difficulties in sensory processing exist in a number of disorders as outlined above, making it difficult for both practitioners and researchers to tease out where sensory processing disorder symptoms stand alone or are symptomatic of another co-morbid disorder. This being said, symptoms still need to be treated and there are various therapies and interventions that are in practice today to address the difficulties experienced with sensory processing disorder.

SPD is currently a very controversial diagnosis. Its current exclusion from diagnostic manuals such as the ICD and the DSM mean that it is not officially recognised as a discreet disorder. Many professionals consider it to be only a symptom of other disorders, such as those outlined above. While it is true that sensory integration problems are a symptom of several disorders, it is currently being argued that SPD is a standalone disorder, sufficiently different to be recognised as an independent disorder. Reasons for this movement include eventual reimbursement of medical costs from insurance companies, and funding for research into the area. Without the disorder being officially recognised, both of those are difficult goals to achieve.

Sensory processing is extremely important to make sense of our worlds. Disorders of sensory processing can effect the functioning of the person and impact greatly on their lives and the lives of those around them. Both the diagnosis and treatment of sensory processing disorder is still under scrutiny and harbours much debate from multiple discipines. However, early diagnosis and appropriate intervention is paramount for those that the disorder effects. Scientific research and replication of findings are important in working towards this as is open communication across discilpines and multidiscilpinary approaches.