**2.1 Sensory processing difficulties in autism spectrum disorder**

Difficulty in processing sensory information and, in particular, extreme sensitivity or insensitivity to sensory input from the environment is common in autism [18], with prevalence estimates of 90–95% [19–21]. Such difficulties may exacerbate social communication deficits and increase the frequency of restrictive and repetitive behaviour, and may occur because of impaired regulation of central nervous system arousal [22]. Hyper-reactivity, reflecting the autonomic nervous system 'fight or flight' response, may result in behaviours such as aggression, hypervigilance or withdrawal (owing to poor tolerance of noise, touch, smell or movement), or additional 'safe space' needs. Hyporeactivity, in contrast, is characterised by reduced awareness of sensory stimuli within the environment [23]. Impaired sensory processing may also result in poor motor control, affecting participation in daily life.

Sensory modulation difficulties (i.e. difficulty recognising and/or integrating sensory information) in children with autism probably pose substantial burden to children and families, limiting participation in leisure activities [24, 25], and are linked to problems with activities of daily living, such as eating, sleeping, dressing, toileting and personal hygiene . Such difficulties represent a long-term challenge for health services in terms of treating potential consequences, such as behaviours that challenge and mental health disorders. Awareness and management of sensory difficulties in mainstream educational settings is also likely to affect peer relationships and educational outcomes. The potential pathway of effect is unconfirmed (i.e. the mechanism(s) by which sensory difficulties affect key outcomes [26]), but it is plausible that reducing sensory processing difficulties (SPDs) could lead to improvements across behavioural, social and educational domains.
