Fragile X Syndrome (FXS) is the most common genetic cause of intellectual disability. People with Fragile X Syndrome commonly experience increased sensitivity to sensory stimuli as well as various behavioural and physical traits associated with the condition. Therefore, understanding how these sensory difficulties may be caused, and how they present, is crucial for effectively supporting those with FSXt.

In this article, we will introduce FSX before describing types of sensory processing difficulties and how they may present across the senses of sight, hearing, and touch. Importantly, we will also explore up-to-date research and look at examples of FXS-associated sensory behaviour.

Introduction to fragile X syndrome

Causes and prevalence

Fragile X Syndrome (FXS) is a genetic condition. It occurs more commonly in people who are assigned male at birth (AMAB) than people assigned female at birth (AFAB), with 1 in 7,000 males/people AMAB and 1 in 11,000 females/people AFAB diagnosed with FXS. In addition to being less likely to be diagnosed with FXS, women and people AFAB with FXS tend to experience milder symptoms of the disease.

The gene responsible for FXS is called FMR1. In people with FXS, the FMR1 gene is abnormal, resulting in a deficiency of the FMRP protein. This protein is important in brain development and many other nervous system functions. People with FXS have lowered levels of FMR1, which results in the onset of various traits associated with the condition. FXS is named accordingly because the causative FMR1 gene is found on the human X chromosome.

Traits

Fragile X syndrome is recognised as the main cause of inherited intellectual disability. In addition to this, FXS also causes a range of behavioural, physical, and intellectual characteristics that start in children and continue into adulthood. The most common features of FXS are listed below:

Behavioural

• Anxious or hyperactive behaviours: experiencing strong feelings of anxiety which can manifest in worrying, fearfulness, or avoidance. People with FXS may also struggle to remain still and are very frequently fidgeting or restless.
• Attention-deficit hyperactivity disorder (ADHD): people with ADHD often experience decreased ability to maintain attention for long periods. They can also act impulsively, often without considering the effects of their actions, and experience a range of other traits. Similar to as described above, hyperactivity manifests through fidgeting and very frequent movement in individuals with FXS
• Seizures: caused by irregular electrical activity in the brain, seizures result in temporary disruptions in an individual’s consciousness or movement. It is estimated that 15% of males/people with AMAB and 5% of females/people with AFAB with FXS experience seizures as part of the condition
• Sensory processing difficulties: people with FSX tend to experience hypersensitivity across their senses, most commonly with sight, sound, and touch. This can lead to stress or anxiety in their daily lives, impacting their own emotional well-being to their social interactions with others

Physical features

• Long, narrow face shape
• Large jaw, ears and forehead
• Flat feet
• Enlarged testicles in males

Intellectual

• Autism spectrum disorder (ASD), alternatively known as autism spectrum condition (ASC), or autism: ASD/ASC/autism manifests in the lack of awareness to social cues, repetitive interests and behaviours, and sensory processing difficulties. It can range from mild to severe in individuals with FXS.
• Learning disabilities: Fragile X Syndrome can result in being slower, or experiencing more difficulties, learning new concepts or skills.
• Developmental delay: children with FXS are often diagnosed upon being late in reaching key developmental milestones compared to other children, such as sitting, walking or talking.

Diagnosis and treatment

FXS is diagnosed through genetic testing where detection of an irregular FMR1 gene indicates FXS. This genetic testing is performed using a blood sample.

While there is no cure for FXS, accurate can inform therapeutic management and individualised support. Therapy may be offered for specific challenges for people with FXS, such as speech or mobility therapy. Similarly, medication can be beneficial for individuals who experience behavioural support needs such as anxiety and ADHD.

Educational management can also be tailored to suit the learning abilities of children with FXS. As they reach adulthood, individuals with FXS can also access genetic counselling to inform family planning and learn about the inheritance of the disease.

Sensory difficulties in people with FXS 

Introduction

A common aspect of FXS is sensory processing difficulty where people have different experiences of sensory stimuli compared to most people. 

The five human senses that can be affected are sight, smell, touch, taste, and hearing. Most commonly, an increased sensitivity to their senses results in hyperarousal which is a heightened state of responsiveness to the world around them. This underlies other traits associated with FXS such as anxiety, avoidance of social situations or unknown environments, and poor eye contact. Children with FXS who are also diagnosed with autism are more likely to experience sensory processing difficulties.¹

Sensory discrimination vs sensory modulation

The sensory sensitivity described in FXS can be categorised into two main groups: sensory discrimination and sensory modulation.²

Sensory discrimination is when information gathered from the senses is unable to be correctly processed into an appropriate action or response. For example, an individual with FXS might find it difficult to move or respond effectively to seeing a particular object move towards them.

Sensory modulation is the way information from the senses elicits an emotion or behaviour within an individual. For example, while people without FXS would automatically become more attentive after a loud noise, people with FXS might not experience heightened attention or awareness. This could also be the opposite, where a typically non-stressful sensory experience can cause great discomfort or anxiety for an individual with FXS.

Individuals with FXS are likely to experience both sensory discrimination and sensory modulation across their sight, smell, touch, taste, and hearing. In the following section, we will discuss how they both manifest in FXS. We will provide examples seen in people with FXS and describe their altered experience of sensory stimuli.

Auditory

People with FXS can have increased auditory sensitivity to sounds and speech. Studies have shown that different parts of the brain are activated in response to sound in people with FXS compared to the general population.³ It was also reported that individuals with FXS are less able to adapt or learn from repeated sounds than other people.⁴ Overall, their difference in auditory sensing could be associated with delayed language learning observed in FXS.⁵

Difficulties in auditory discrimination in an individual with FXS can be observed through:²

• Poor volume control when speaking: People with FXS may have trouble modulating their speech volume in relation to their surroundings
• Trouble distinguishing similar sounds or recognising repeated sounds
• Difficulty following spoken directions

Difficulties in auditory modulation in an individual with FXS can be observed through.²

• Distressed or avoidant behaviour when experiencing loud sounds or noisy environments

Visual

Responses to visual stimuli are also altered in people with FXS. It has been found that the function of both the eye and the brain is delayed, which may explain the difference in their sense of sight compared to others.^6,7

Visual-motor skills tend to be impaired in individuals with FXS, where the ability to translate visuals into an appropriate motor response is decreased.⁸ Other behaviours linked to the processing of visual information have also been documented, including the ability to identify textured objects and recognise objects after repeated sightings.⁹

Difficulties in visual discrimination in FXS can be observed through:²

• Poor understanding of spatial relationships or navigation. This can affect someone with FXS’s ability to orientate themselves in their surroundings, and can also reduce their daily fine motor skills
• Poor ability in reading or detecting sequences among visual stimuli
• Poor ability to identify facial features and expressions which can lead to difficulties responding to social cues

Difficulties in visual modulation in FXS can be observed through:²

• Low tolerance to bright lighting
• Decreased attention span when viewing written text
• Difficulty maintaining eye contact when interacting with others

Tactile

Lastly, increased sensitivity to touch is commonly seen in individuals with FXS.¹⁰ This not only affects an individual's ability to process tactile information through touching or holding objects with their hands but also impacts their oral-motor skills.

Difficulties in tactile discrimination in FXS can be observed through:²

• Poor oral awareness. This includes overstuffing their mouth while eating
• Inability to tolerate certain objects or textures in their mouth. This mostly affects their food sensitivities but can also lead to difficulties in brushing their teeth.

Difficulties in tactile discrimination in FXS can be observed through:²

• Distress over certain clothing textures on their skin
• Distress over being touched or physical contact with others
• Lack of awareness of wet surfaces. This is commonly seen in young children through an unawareness of soiled diapers

Assessment and management of sensory issues

Parents of children with FXS take on the largest responsibility in their care and management. However, dedicated healthcare professionals are also important for supporting FXS individuals and their sensory processing difficulties.

Sensory difficulties are currently assessed through questionnaires such as The Sensory Profile and The Sensory Processing Measure. There are additional tests tailored to neuromotor function in FXS such as the Movement Assessment Battery for Children.²  

Occupational therapists are largely responsible for managing care for individuals with FXS. They can help diagnose sensory processing difficulties and equip people with FXS and their parents/carers with coping skills to better manage them. 

Sometimes, occupational therapists may refer someone with FXS to more specialised healthcare professionals for more tailored care. This may include speech and language therapists, physical therapists (or physiotherapists), and psychologists.

For individuals who struggle with sensory discrimination, skill developmental plans may be put in place to improve daily skills such as writing or doing household chores. In people with FSX who experience sensory modulation, it is common for occupational therapists to create structured intervention plans to train more effective responses to sensory stimuli over time.

Summary

In conclusion, sensory processing difficulty is a feature of  Fragile X Syndrome that affects many other aspects of the condition. Therefore understanding its causes, and how it affects the individual is crucial to choosing therapeutic interventions and strategies that improve the quality of life for people with FXS.