Introduction

 "Misophonia" is a condition where individuals have a strong negative reaction to particular sounds from everyday life. The context in which these sounds occur often affects the reaction. Misophonia can cause physical and emotional discomfort, such as anxiety, in response to these sounds.¹

The term, "selective sound sensitivity syndrome" has also been used to describe misophonia, and it's sometimes called "sound-rage" by the general public. Both misophonia and the disorder hyperacusis fall under the broader category of decreased sound tolerance. The causes of misophonia are unknown, but a few risk factors may be responsible. 

What is misophonia?

People with misophonia are usually sensitive to specific everyday noises, often noticed from childhood. Common triggers include sounds made by others, like gum chewing, pen clicking, or foot tapping. 

However, other noises like trains, planes, or animal sounds can also be triggers. Additionally, certain movements, such as someone pointing a finger or swinging their legs, can also provoke strong negative reactions and feelings of distress, disgust, irritability, and anger.^{1, 2}

Diagnosis

In 2013, Schröder and colleagues studied 42 people with misophonia and created the Amsterdam Misophonia Scale, a tool to measure the type and severity of misophonia symptoms. This scale, inspired by a tool for obsessive-compulsive disorder, includes six criteria for diagnosing misophonia:³

• A specific sound made by a human (like eating or breathing noises) triggers an intense reaction starting with irritation or disgust that quickly turns into anger
• This anger leads to a strong sense of losing control, sometimes resulting in aggressive outbursts
• The person knows their reaction is excessive or unreasonable
• They try to avoid situations where the sound occurs or endure it with intense discomfort.
• These reactions cause significant distress or interfere with daily life, making it hard to work, socialize, or complete tasks
• These reactions aren't better explained by another condition, like obsessive-compulsive disorder or posttraumatic stress disorder

According to psychiatric criteria, people with misophonia in this study also had other conditions, such as mood disorders (7.1%), Tourette syndrome (4.8%), ADHD (4.8%), and others. About half of the participants had obsessive-compulsive personality disorder. Schröder and his team suggested that misophonia could be considered part of the obsessive-compulsive spectrum, a group of disorders with similar obsessive and compulsive features.⁴

This view contrasts with the original definition by Jastreboff and colleagues, who noted that misophonia can involve a wider range of reactions and triggers. For example, some triggers might come from animals or machines, not just humans, and not everyone reacts with anger. 

Some people may only feel irritation or annoyance and can control their reactions, avoiding aggressive outbursts. Additionally, some individuals feel their reactions are justified because they believe others are being inconsiderate. 

Misophonia reactions can range from mild discomfort to significant distress but don't always involve anger. Other findings suggest that in some cases, misophonia might be linked to neuropsychiatric disorders, although sensory over-responsivity can occur on its own. 

Interestingly, misophonia and sensory sensitivity have also been observed in children with obsessive-compulsive disorder.^{3, 4}

Causes 

Misophonia might develop as a reflexive reaction, like a Pavlovian response. Just as Pavlov's dogs learned to salivate at the sound of a bell because they associated it with food, people with misophonia might develop an automatic physical or emotional response to certain sounds.

Misophonia often involves a two-step process:

1. First, hearing a trigger sound causes an immediate physical reaction, like a muscle reflex. Often, this is a fight-or-flight response
2. Then, this physical reaction leads to an intense emotional response

So, the person first hears the sound, which causes a muscle reaction. This reaction then triggers strong emotions.

The misophonic response to sounds gets stronger over time because the physical reflex (like muscle tightening) becomes more intense with each exposure to the trigger sound. The stronger the muscle reaction, the stronger the emotional response becomes.

New trigger sounds can develop if they occur together with already established trigger sounds and the physical reflex they cause. So, if a new sound is often heard along with a known trigger sound, it can also become a trigger over time.⁵

Misophonia has been hypothesised to be a one- or two-step process:⁵

• Misophonia is a one-step process: the trigger stimulus directly elicits extreme emotions and fight-or-flight responses
• Misophonia is a two-step process: initially, hearing a trigger sound causes an immediate physical reaction, such as muscle tension. This physical response then leads to intense emotional reactions, including fight-or-flight responses like anxiety or anger

At first glance, misophonia seems like a uniform disorder, with everyone reacting similarly to a common set of sounds and experiencing similar emotional responses. These emotions are described as involuntary and unavoidable, making it seem like the sounds directly cause the emotional reaction. People with misophonia often try to stay calm when exposed to these triggers but find it impossible. 

Studies, like one by Edelstein and colleagues in 2013, have shown that these sounds cause physical changes, such as increased skin conductance and muscle tension, supporting reports of heightened arousal following misophonic triggers.²

The proposed mechanisms of misophonia disorder involve abnormal connections between the auditory and emotional (limbic) systems of the brain. Some individuals with misophonia experience pain linked to an overactive autonomic nervous system, which can activate a specific muscle within the ear (tensor tympani). This leads to symptoms like ear fullness and pulsation known as tensor tympani syndrome. This syndrome is commonly seen in those affected and often resolves with treatment for misophonia.

In essence, misophonia involves heightened autonomic and limbic system reactions due to abnormal connections among auditory, limbic, and autonomic systems in response to specific patterns of sound. It's important to note that misophonic triggers are not inherently unique sounds, as any sound can potentially become distressing to a misophonic individual.⁶

Causes (etiology)

Currently, the cause of misophonia is unknown. As in most disorders, there are many risk factors and potential causes. These include the following:⁷

Genetics 

There is evidence suggesting a genetic predisposition to misophonia, where individuals with a family history of the condition are more likely to develop it themselves. Genetic studies are ongoing to identify specific genes that may contribute to misophonia susceptibility.

Early life experiences 

Negative or traumatic experiences related to specific sounds during childhood or early adolescence can contribute to the development of misophonia. These experiences may create strong associations between certain sounds and negative emotions or physiological reactions.

Neurobiological factors

Misophonia involves aberrant connections between the auditory processing centres of the brain and the limbic system, which is responsible for emotional responses. Neuroimaging studies have shown abnormal neural responses in individuals with misophonia when exposed to trigger sounds.

Psychological factors

Individuals with anxiety disorders, obsessive-compulsive disorder (OCD), or high levels of neuroticism may be more susceptible to developing misophonia. These conditions are often characterised by heightened emotional reactivity and sensitivity to stimuli.

Co-occurring conditions

Misophonia frequently co-occurs with other neurodevelopmental disorders such as autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD). These conditions may share common neurobiological pathways that contribute to sensory processing abnormalities.

Environmental factors

Prolonged exposure to specific sounds or environments where trigger sounds are prevalent may exacerbate misophonic responses over time. This can lead to heightened sensitivity and more severe reactions to trigger sounds.

Personality traits

Certain personality traits, such as perfectionism or sensitivity to stimuli, may predispose individuals to misophonia. These traits can influence how individuals perceive and react to trigger sounds.

Risk factors

• Those assigned female at birth (AFAB) are often found to be more affected than those assigned male at birth (AMAB), although the reasons for this disparity are not fully understood
• Mental disorders, particularly anxiety disorders, OCD, and mood disorders, are commonly linked with misophonia
• Other potential risk factors include genetic predisposition, early life experiences such as trauma or negative associations with specific sounds, and certain personality traits like neuroticism or high sensitivity
• Environmental factors, such as prolonged exposure to trigger sounds, may also contribute to the development and severity of misophonia symptoms 

It's important to note that while these factors have been identified in studies, more research, especially longitudinal studies, is needed to establish definitive causal relationships and to better understand how these factors interact to influence misophonia.⁸

Understanding these risk factors helps in identifying individuals who may be at higher risk for developing misophonia and informs approaches for management and treatment. Early recognition and intervention can potentially mitigate the impact of misophonia on individuals' quality of life.

Research challenges in misophonia

Research on misophonia, both in understanding its mechanisms and finding effective treatments, faces significant hurdles due to the lack of a standardised definition. This lack of consensus makes it difficult to identify, diagnose, and assess the severity of misophonia. 

Different researchers use varying definitions and criteria to identify misophonia, leading to fragmented data that cannot be easily compared or combined. This inconsistency means that studies often examine different sub-groups of individuals, making meaningful comparisons impossible. 

A major issue in misophonia research is the failure to exclude individuals with hyperacusis from studies. Although the reactions to bothersome sounds in both conditions can be similar, treatments effective for hyperacusis do not work for misophonia, suggesting different underlying mechanisms. 

Hyperacusis is frequently found in those with misophonia but symptoms of misophonia should not be attributed to any co-occurring disorders. Therefore, for accurate research and treatment, it is crucial to separate patients with hyperacusis from those with only misophonia.⁹

Ignoring this distinction can lead to significant errors, as seen in past tinnitus research where hyperacusis was mistakenly included, leading to incorrect conclusions and the eventual retraction of study results.

Summary

Misophonia is a condition where specific sounds trigger intense negative emotions, such as anxiety or anger, significantly disrupting daily life. It is closely linked with anxiety disorders, OCD, and mood disorders, suggesting shared neurobiological pathways. 

Genetic predisposition, early life experiences, and neurobiological abnormalities in auditory-limbic connections play crucial roles in its development. Individuals with misophonia often exhibit heightened emotional reactivity and sensory sensitivity, exacerbated by personality traits like neuroticism. 

Research challenges include the absence of a universally accepted definition and the common inclusion of individuals with hyperacusis in studies, despite differing treatment responses. This oversight has paralleled issues seen in tinnitus research, leading to erroneous conclusions. 

Addressing these complexities through rigorous, longitudinal studies is essential to elucidate causal mechanisms and optimise tailored treatments. Understanding misophonia's multifaceted etiology and risk factors is pivotal for developing effective interventions and improving the quality of life for affected individuals.