A delusion is a fixed false belief based on an inaccurate interpretation of an external reality despite evidence to the contrary. The belief is not congruent with one’s culture or subculture, and almost everyone else agrees it is false. An individual with delusional disorder has a difficult time discerning, or distinguishing, between external reality and their own belief. Similar to schizophrenia, delusional disorder causes a major disconnect between an individual’s internal world (thoughts, feelings, and perceptions) and an individual’s external world (relationships, work, and hobbies). Unlike schizophrenia, individuals with delusional disorder do not suffer from disorganized thinking or moderate to severe hallucinations that impact the ability to function and interact with others.

One interesting aspect of delusional disorders is that overall functioning is more or less preserved as compared to other psychotic disorders such as schizophrenia. Individuals with delusional disorder often continue to function at home and at work and relate to others in a normal manner, and their functionality is not impacted and behavior is not obviously bizarre as long as interactions do not lead to discussions of topics related to the delusion. However, the preoccupation with their delusional ideas can disrupt their overall life. Aside from discussing the specific delusion, it would not be easy to tell that an individual has a diagnosis of delusional disorder. Impairment can be significant in one’s occupation, however, making it difficult for those with delusional disorder to hold a steady job if they are unable to focus on other issues than the delusional thought content. Additionally, there may be social isolation which can lead to additional psychological symptoms such as depression.

Diagnosis

The diagnosis of a delusional disorder occurs when a person has one or more delusional thoughts for one month or more, that has no explanation by another physiological, substance-induced, or medical condition or any other mental health condition. An individual’s cultural and religious beliefs merit consideration before coming to the diagnosis. Cultural beliefs may also impact the content of delusions. In some cultures, a specific delusion can be perceived as possible, and therefore is not considered to be an impairment on a person’s life, nor abnormal. A person also cannot have met Criterion A for schizophrenia at any point (two or more of hallucinations, delusions, disorganized speech, grossly disorganized behavior, or negative symptoms of schizophrenia).

There are two main classifications of delusions: bizarre and non-bizarre. Bizarre delusions include delusions involving a phenomenon that is impossible, not understandable, and unrelated to normal life. An example of a bizarre delusion would be an individual believing their organs have been removed and replaced overnight while they were asleep. A bizarre delusion such as this is not only impossible, but can be demonstrated as false by exhibiting no signs of invasion or physical marks. Even after their delusions are proven to be untrue, or impossible, individuals experiencing delusions will continue their belief with certainty and conviction. Non-bizarre delusions involve situations that are possible, such as being manipulated or harmed, that remain fixed false beliefs even without evidence.

Types of Delusions

Some of the most frequently encountered types of delusions are

• delusional jealousy. This delusion can be displayed by the belief that one’s romantic, or sexual partner, is unfaithful, accompanied by abnormal or socially unacceptable behavior that is related to these thoughts. Also known as Othello syndrome, this type is more common in males; it can sometimes correlate with suicidal or homicidal ideations, and hence safety is an important consideration in evaluation and management. It is important to note that if an individual has been in prior relationships where their significant other had been unfaithful, this belief would not merit a delusional disorder as there is a real reason for that concern, fear, and initial mistrust. In such a situation, initial skepticism or mistrust of a partner might be considered normal.
• erotomanic. A delusion that another person, more frequently someone of higher status, is in love with the individual. Also known as psychose passionelle. These patients are usually socially withdrawn, dependent, sexually inhibited with a poor level of social and/or occupational functioning. Paradoxical conduct is an important characteristic wherein all denials of affection are rationalized as affirmations. Males with this type of delusion tend to be more aggressive than their female counterparts.
• grandiose. A conviction that one has great talent; made a great discovery; or has power, knowledge or a relationship with someone famous or a deity. This is also known as megalomania, which is the delusion of increased self-importance. Grandiose delusions often have religious content.
• persecutory. The central theme is being conspired against, threatened, attacked, harassed, or obstructed in pursuit of long-term goals. This is one of the most common types of delusions and patients can be anxious, irritable, aggressive, or even assaultive at times, often believing they need to protect themselves.
  

  Figure 1. A body dysmorphic delusion may lead someone to believe that body parts are distorted, but more common types of somatic delusions have someone concerned about the way they smell.

• somatic. These delusions involve imaginary bodily functions and sensations. Also called monosymptomatic hypochondriacal psychosis, somatic delusions can severely impair reality. The patient is unarguably convinced of the severity of symptoms, even after seeing a medical professional or doctor who lets them know they are healthy and their concern is not a reality. The most common type of somatic delusions is that the person emits a foul odor or severe bad breath. Others include beliefs of infestation, for example with parasites, or body dysmorphic delusion (that parts of the body are distorted or misshapen). These patients also tend to have high levels of anxiety and nervousness.
• thought broadcasting. This is the delusion that one’s thoughts are projected and can be perceived by others.
• thought insertion. A delusion that an external source or entity has inserted thoughts into one’s mind and one’s thoughts are not their own.
• mixed. No single theme is prevalent, but rather a mix of two or more delusions.

Differential Diagnosis

Differential diagnosis involves ruling out other causes of delusions such as drug-induced conditions, dementia, head injury or cancer, infections, metabolic disorders, and endocrine disorders. Other psychiatric disorders must then be ruled out. In delusional disorder, mood symptoms tend to be brief or absent, and unlike schizophrenia, delusions are usually non-bizarre, and hallucinations are minimal or absent. Interviews are important tools to obtain information about the patient’s life situation and past history to help make a diagnosis. Clinicians generally review earlier medical records to gather a full history. Clinicians also try to interview the patient’s immediate family, as interviews can be helpful in determining the presence of delusions. The mental status examination (MSE) is used to assess the patient’s current mental condition. A psychological questionnaire sometimes used in the diagnosis of the delusional disorder is the Peters Delusion Inventory (PDI), which focuses on identifying and understanding delusional thinking. However, this questionnaire is more likely to be used in research than in clinical practice.

Etiology

Figure 2. The basal ganglia is located deep within the brain under the cerebral cortex.

Delusions can be caused in several ways, including from both substance use and medical conditions including traumatic brain injury and other forms of damage to brain tissue including strokes and dementia. Neurological conditions involving the limbic system and basal ganglia in those with intact cortical functioning can also cause delusions (Joseph & Siddiqui, 2020). The main functions of the limbic system involve processing emotion, memory, motivation, and learning. The basal ganglia is a structure that plays a role in emotional and cognitive functioning, but is mainly responsible for motor control and movement.

Hypersensitive persons and ego defense mechanisms like reaction formation, projection, and denial are some psychodynamic theories for delusional disorder. Social isolation, envy, distrust, suspicion, and low self-esteem are some of the factors, which when becoming intolerable, lead to a person seeking an explanation and thus form a delusion as a solution. Immigrants with language barriers, deaf and visually impaired persons, as well as the elderly, are special populations who are more vulnerable to delusions due to a greater sense of vulnerability.

Epidemiology

The lifetime risk of delusional disorder in the general population has been estimated to range from 0.05%-0.1%, based on data from various sources, including case registries, case series, and population-based samples. According to the DSM-5, the lifetime prevalence of delusional disorder is about 0.02%. The prevalence of delusional disorder is much rarer than other conditions like schizophrenia, bipolar disorder, and other mood disorders, possibly due to under-reporting, as those with delusional disorder may not seek mental health attention unless forced by family or friends. The mean age of onset is about 40 years, but the range is from 18 years to 90 years. The persecutory and jealous types of delusion are more common in males, while the erotomanic variety is more common in females (Joseph & Siddiqui, 2020).

Treatment

The treatment of delusional disorder is difficult considering the lack of insight. A good doctor-patient relationship is a key to treatment success. Treatment includes psychotherapy by establishing trust and building a therapeutic alliance.

A patient’s history of medication compliance is the best guide to select appropriate antipsychotic medication. An antipsychotic should be started for a trial period of six weeks after which there is an evaluation of the effectiveness of the medication. Start a low dose and gradually increase the dosage as needed. Another drug from another class can be tried after six weeks if no benefit is noted from the initial treatment.

Some delusional disorders may respond well to antipsychotic medications; often the best approach is a combined treatment that includes antipsychotics and psychotherapy.