Delusional disorder is an illness characterized by at least 1 month of delusions but no other psychotic symptoms according to the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5).  Delusions are false beliefs based on incorrect inference about external reality that persist despite the evidence to the contrary and these beliefs are not ordinarily accepted by other members of the person's culture or subculture. Delusions can be characterized as persecutory (i.e., belief one is going to be harmed by an individual, organization or group), referential (i.e., belief gestures, comments, or environmental cues are directed at oneself), grandiose (i.e., belief that the individual has exceptional abilities, wealth, or fame), erotomanic (i.e., ani ndividual’s false belief that another individual is in love with them), nihilistic (i.e., conviction that a major catastrophe will occur), or somatic (i.e., beliefs focused on bodily function or sensation).
Nonbizarre delusions are about situations that could occur in real life, such as being followed, being loved, having an infection, and being deceived by one's spouse. Bizarre delusions are clearly implausible. Delusions that express a loss of control over mind or body are generally considered to be bizarre and include belief that one’s thoughts have been removed by an outside force, that alien thoughts have been put into one’s mind, or that one’s body or actions are being acted on or manipulated by an outside force. 
Making a distinction between a delusion and an overvalued idea is important, the latter representing an unreasonable belief that is not firmly held.  Additionally, personal beliefs should be evaluated with great respect to complexity of cultural and religious differences; some cultures have widely accepted beliefs that may be considered delusional in other cultures.
Unfortunately, patients with delusional disorder do not have good insight into their pathological experiences. Interestingly, despite significant delusions, many other psychosocial abilities remain intact, as if the delusions are circumscribed. Indeed, this is one of the key differences between delusional disorder and other primary psychotic disorders. However, the individual may rarely seek psychiatric help, remain isolated, and often present to internists, surgeons, dermatologists, policemen, and lawyers rather than psychiatrists.
Mrs. K is a 39-year-old woman who was brought to the inpatient psychiatric unit by police after being arrested for trespassing on Mr. L’s property. Upon arrival, Mrs. K was adamant about being released, stating that she was simply entering her husband’s home, adamantly declaring that Mr. L was her husband. She elaborated a story about how much the two of them loved each other, when they got married, and how she was currently pregnant with his child. In actuality, Mr. L used to be Mrs. K’s boss, and had fired her because of her inappropriate romantic advances several years prior. Mrs. K was married to another man in Florida, with whom she denied any relationship, stating that she was kidnapped for 4 years, and after escaping, had come to California to be with her husband, Mr. L. Mrs. K was diagnosed with delusional disorder, erotomanic type, and was started on risperidone.
Theo Manschreck  outlined 3 steps in the initial evaluation of patients who present with delusions.
First, establish whether pathology is present. This represents a clinical judgment that is sometimes difficult to make. Some comments that appear delusional may be true. In contrast, some reports that initially seem believable may later be identified as delusions as the symptoms worsen, the delusions become less encapsulated (i.e., begin to extend to more people or situations), and more information comes to light. The clinical judgment that delusions are present should be made after taking into account the degree of plausibility, systemization, and the possible presence of culturally sanctioned beliefs that are different from one's own beliefs. Making the distinction between a true observation, a firm belief, an overvalued idea, and a delusion is sometimes a challenging task. Often, the extremeness and inappropriateness of the patient's behaviors, rather than the simple truth or falsity of the belief, indicate its delusional nature. [19, 4]
The second step is determining the presence or absence of important characteristics and symptoms often associated with delusions, such as confusion, agitation, perceptual disturbances, physical symptoms, and prominent mood abnormalities.  Studies have shown that the most common symptoms reported were self-reference (40%), irritability (30%), depressive mood (20%), and aggressiveness (15%). 
The third step is to present a systematic differential diagnosis. A thorough history, mental status examination, and laboratory/radiologic evaluation should be performed to rule out other medical and psychiatric conditions that are commonly present with delusions. CNS illness is high on the differential diagnosis of any psychotic disorder, especially so in the onset of delusional disorder in patients older than the typical onset of schizophrenia. Delusional disorder should be seen as a diagnosis of exclusion. 
Diagnostic criteria (DSM-5)
The specific DSM-5 criteria for delusional disorder are as follows: 
Presence of one or more delusions with a duration of one month or longer.
The criteria for schizophrenia has never been met. Note: Hallucinations, if present are not prominent and are related to the delusional theme (e.g., the sensation of being infected with insects is associated with the delusions of infestation).
Apart from the impact of the delusion(s) or its ramifications, functioning is not markedly impaired, and behavior is not obviously bizarre or odd.
If manic or major depressive episodes have occurred, these have been brief relative to the duration of the delusional periods.
The disturbance is not better explained by another mental disorder such as obsessive-compulsive disorder, and is not attributable to the physiological effects of a substance or medication or another medical condition.
Subtypes are defined as erotomanic, grandiose, jealous, persecutory, somatic, mixed, and unspecified. The diagnosis is further specified “with bizarre content” when delusions are clearly implausible, not understandable, and not derived from ordinary life experiences.
The following duration specifiers are used only after 1-year duration of the disorder:
First episode, currently in acute episode
First episode, currently in partial remission
First episode, currently in full remission
Multiple episodes, currently in acute episode
Multiple episodes, currently in partial remission
Multiple episodes, currently in full remission
United states statistics
The prevalence of delusional disorder in the United States is estimated in the DSM-5 to be around 0.02%  , which is considerably lower than the prevalence of schizophrenia (1%) and mood disorders (5%).  Our current understanding of delusional disorder, however, is limited by scarce scientific data that mostly consist of individual case descriptions or small uncontrolled case studies, which are therefore difficult or impossible to duplicate. 
A British study reported that of 227 patients presenting to mental health centers with a first episode of psychosis during the 3-year study period, 7% were diagnosed with persistent delusional disorder, as compared to 11% with schizophrenia and 19% with psychotic depression. 
Sexual differences in incidence
The female-to-male ratio has been reported to vary from 1.18  -3:1  . Men are more likely than women to develop paranoid delusions; women are more likely than men to develop delusions of erotomania.  Associated factors include being married, being employed, recent immigration, low socioeconomic status, celibacy among men, and widowhood among women. [9, 2]
Age-related differences in incidence
The mean age of onset is 40 years and ranges from 18-90 years.  A Spanish study conducted by de Portugal et al (2008) looked at medical records of 370 people diagnosed with delusional disorder and found that the mean age in this population was 55 years, with 56.5% of the patients being female.
The etiology of delusional disorder is unknown, and several difficulties exist in conducting research in this area:
Patients currently diagnosed with delusional disorder may represent a heterogeneous group of patients with delusions as the predominant symptom.
Patients often do not present for treatment, and thus they do not commonly make themselves available for research studies.
However, strong indications exist that delusional disorder is a distinct condition, different from schizophrenia or mood disorder. Naturalistic studies indicated that delusional disorder has a relatively stable course.
The definition of this condition has changed over time and continues to be a work in progress.
The relationship to the more severe psychosis is yet unclear. According to the DSM-5, on average, global function is generally better than that observed in schizophrenia. Although the diagnosis is generally stable, a proportion of individuals go on to develop schizophrenia. Delusional disorder has a significant familial relationship with both schizophrenia and schizotypal personality disorder. Although it can occur in younger age groups, the condition is more prevalent in older individuals. 
Biological factors may play some role in the development of delusional disorder, as delusions are associated with a wide range of nonpsychiatric medical conditions. Among patients with neurologic disorders (primarily dementia, head injury, and seizures) problems with the basal ganglia and temporal lobe are most commonly associated with delusions. [7, 12] However, a case report of a somatic delusion involving reduplication of body parts implicated the temporal and parietal lobes, showing hypoperfusion of both regions. 
Campana et al  used eye tracking movement tests to understand the relationship between frontal field functions and clinical symptoms of delusional disorder. They found that compared with normal participants, patients with delusional disorder showed abnormalities of voluntary saccadic eye movements and smooth pursuit eye movements, a dysfunction similar to that seen in patients with schizophrenia.
Hyperdopaminergic states have been implicated in the development of delusions. Recently, Morimoto et al  reported that 13 patients with delusional disorder were reported to have increased levels of plasma homovanillic acid (HVA) (a dopamine metabolite) compared with control subjects. Patients responded well to treatment with low-dose haloperidol (average 2.7 mg/d) and showed decreased posttreatment plasma level of HVA, which correlated with the improvement of their symptoms.
The same authors reported an increased prevalence of a polymorphism at the D2 receptor gene at amino acid 311 (cysteine-for-serine substitution) among individuals with delusional disorder in their sample, particularly those with persecutory delusions. Individuals that had more TCAT repeats within the first intron of the tyrosine hydroxylase gene had higher levels of HVA, although it is unclear if they corrected for multiple statistical comparisons. 
The fields of cognitive and experimental psychology suggest that persons with delusions selectively attend to available information, which appears to overlap with hypochondriacal patient populations.  They make conclusions based on insufficient information, attribute negative events to external personal causes, and have difficulty in envisaging others’ intentions and motivations. 
Conway et al  reported that patients with delusional disorder made probability decisions based on fewer data compared with normal controls. Despite using fewer data, they were as certain as controls regarding the accuracy of their decisions.
Two neuropsychological models proposed for schizophrenia may also have some validity in delusional disorder. A cognitive bias model (CBM) proposes that paranoia is a defense against thoughts that threaten the idealized self, to protect a fragile self-esteem. Positive events are attributed to the self whereas negative events are ascribed to the external environment. In contrast, the cognitive deficit model (CDM) focuses on cognitive impairments and distortions of threat evaluating mechanisms as the cause for delusion formation. 
The status examination (including cognitive examination) is usually normal with exception of the presence of abnormal delusional beliefs.
In general, patients are well groomed and well-dressed without evidence of gross impairment. Speech, psychomotor activity, and eye contact may be affected by the emotional state associated with delusions, but are otherwise normal.
Mood and affect are consistent with delusional content; for example, patients with persecutory delusions may be suspicious and anxious. Mild dysphoria may be present without regard of type of delusions.
Tactile and olfactory hallucinations may be present and may be prominent if they are related to the delusional theme (eg, the sensation of being infested by insects, the perception of body odor).  Systemic or CNS causes of tactile and olfactory hallucinations, such as substance intoxication and withdrawal, and temporal lobe epilepsy, should be ruled out. Auditory or visual hallucinations are characteristic of more severe psychotic disorders (eg, schizophrenia) and should lead away from a diagnosis of delusional disorder.
Memory and cognition are intact. Level of consciousness is unimpaired.
Patients usually have little insight and impaired judgment regarding their pathology. Police, family members, coworkers, and physicians other than psychiatrists are usually the first to suspect the problem and seek psychiatric consultation. Seeking corroborative information, when permitted by the patient, is often crucial. Recall that it is permissible to seek collateral history but that collateral history should not be withheld from the patient.
Assessment of homicidal or suicidal ideation is extremely important in evaluating patients with delusional disorder. The presence of homicidal or suicidal thoughts related to delusions should be actively screened for and the risk of carrying out violent plans should be carefully assessed. Reid (2005) pointed out that some types of this illness—erotomanic, jealous, and persecutory—are associated with higher risk for violence than others.  History of previous violent acts as well as history of how aggressive feelings were managed in the past may help to assess the risk. Access to weapons should be explored.
The central theme of delusions is that another person, usually of higher status, is in love with the patient. The object of delusion is generally perceived to belong to a higher social class, being married, or otherwise unattainable. [3, 5]
Patients may attempt to contact the object of the delusion by making phone calls, sending letters and gifts, making visits, and even stalking. Some cases lead to assaultive behaviors as a result of attempts to pursue the object of delusional love or attempting to "rescue" her/him from some imagined danger. 
Patients believe that they possess some great and unrecognized talent, have made some important discovery, have a special relationship with a prominent person, or have special religious insight. 
Grandiose delusions in the absence of mania are relatively uncommon, and the distinction of this subtype of disorder is debatable. Many patients with paranoid type show some degree of grandiosity in their delusions. 
Grandiosity in narcissistic personality disorder is by definition nonpsychotic and not directly related to an elevated mood state, as in bipolar disorders. Narcissistic patients will concurrently show a lack of empathy, exploitive behavior, and a sense of entitlement in addition to grandiosity.
The main theme of the delusions is that her or his spouse or lover is unfaithful. Some degree of infidelity may occur; however, patients with delusional jealousy support their accusation with delusional interpretation of "evidence" (eg, disarrayed clothing, spots on the sheets). [1, 4]
Patients may attempt to confront their spouses and intervene in imagined infidelity. Jealousy may evoke anger and empower the jealous individual with a sense of righteousness to justify their acts of aggression. Both the intimate partner and the (perceived) lover may be the targets of aggression and violence. This disorder can sometimes lead to acts of violence, including suicide and homicide. 
Patients with this type believe that they are being persecuted and harmed.  In contrast to persecutory delusions of schizophrenia, the delusions are systematized, coherent, and defended with clear logic. No deterioration in social functioning and personality is observed. 
Patients are often involved in formal litigation against their perceived persecutors. Munro  refers to an article by Freckelton who identifies the following characteristics of deluded litigants: determination to succeed against all odds, tendency to identify the barriers as conspiracies, endless drive to right a wrong, quarrelsome behaviors, and "saturating the field" with multiple complaints and suspiciousness. 
Patients often experience some degree of emotional distress such as irritability, anger, and resentment.  In extreme situations, they may resort to violence against those who they believe are hurting them. 
The distinction between normality, overvalued ideas, and delusions is difficult to make in some of the cases. 
The core belief of this type of disorder is delusions around bodily functions and sensations. The most common are the belief that one is infested with insects or parasites, emitting a foul odor, parts of the body are not functioning, the belief that their body or parts of the body are misshapen or ugly, and the reduplication of body parts. [1, 13]
Patients are totally convinced in physical nature of this disorder, which is contrary to patients with hypochondriasis who may admit that their fear of having a medical illness is groundless. 
Patients are usually first seen by dermatologists, cosmetic surgeons, urologists, gastroenterologists, and other medical specialists. 
Sensory experiences associated with this illness (eg, sensation of parasites crawling under the skin) are viewed as components of systemized delusions.  This must be distinguished from bizarre somatic delusions occasionally seen in schizophrenia (eg, a delusion that a colony of lobsters is living in the patient’s stomach).
Delusional themes fall outside the specific categories or cannot be clearly determined. 
Misidentification syndromes such as Capgras syndrome (characterized by a belief that a familiar person has been replaced by an identical impostor) or Fregoli syndrome (a belief that a familiar person is disguised as someone else) fall into this category. Misidentification syndromes are rare and frequently are associated with other psychiatric conditions (eg, schizophrenia) or organic illnesses (eg, dementia, epilepsy). 
Another unusual syndrome is Cotard syndrome, in which patients believe that they have lost all their possessions, status, and strength as well as their entire being, including their organs.  Described first in the 19th century, it is a rare condition, which is usually considered a precursor to a schizophrenic or depressive episode. 
|Neurodegenerative disorders||Alzheimer disease, Pick disease, Huntington disease, Parkinson Disease, basal ganglia calcification (Fahr disease), multiple sclerosis, metachromatic leukodystrophy|
|Other CNS disorders||Brain tumors, especially temporal lobe and deep hemispheric tumors; epilepsy, especially complex partial seizure disorder; head trauma (subdural hematoma); anoxic brain injury; fat embolism|
|Vascular disease||Atherosclerotic vascular disease, especially when associated with diffuse, temporoparietal, or subcortical lesions; hypertensive encephalopathy; subarachnoid hemorrhage, temporal arteritis|
|Infectious disease||Human immunodeficiency virus/acquired immune deficiency syndrome (AIDS), opportunistic infections in AIDS, encephalitis lethargica, Creutzfeldt-Jakob disease, syphilis, malaria, acute viral encephalitis|
|Metabolic disorder||Hypercalcemia, hyponatremia, hypoglycemia, uremia, hepatic encephalopathy, porphyria|
|Endocrinopathies||Addison disease, Cushing syndrome, hyperthyroidism or hypothyroidism, panhypopituitarism|
|Vitamin deficiencies||Vitamin B-12 deficiency, folate deficiency, thiamine deficiency, niacin deficiency|
|Medications||Adrenocorticotropic hormones, anabolic steroids, corticosteroids, cimetidine, antibiotics (eg, cephalosporins, penicillin), disulfiram, anticholinergic agents|
|Substances||Amphetamines, cocaine, alcohol, cannabis, hallucinogens|
|Toxins||Mercury, arsenic, manganese, thallium|
Delusional symptoms are preferentially associated with disorders involving the limbic system and basal ganglia. 
Fifty percent of patients with Huntington disease and individuals with idiopathic basal ganglia calcifications developed delusions at some point of their illness. 
Head trauma has been associated with development of delusions. Koponen et al  found patients with traumatic brain injury were diagnosed with delusional disorder in 5% of the cases during a 30-year follow-up (3 out of 60 assessed patients).
Table 2. Related Psychiatric Disorders and Differentiating Features (Open Table in a new window)
|Delirium||Fluctuating level of consciousness, altered sleep/wake cycle, hallucinations and impaired cognition are features of delirium that are absent in delusional disorder.|
|Dementia||Delusions (usually persecutory) are common in Alzheimer and other types of dementia (the prevalence ranges from 15-50%) and may present first, before subclinical cognitive deficits become apparent. Neuropsychological testing may be warranted to detect cognitive impairments. Additionally, elderly patients with delusional disorder were found to have an incidence of dementia that was twice as high as in the general population over a 10-year follow-up period. |
|Substance-related disorders (intoxication, withdrawal, substance-induced psychotic disorder with delusion)||Amphetamines and cocaine are the most commonly described substances to be associated with delusions, typically of persecutory type. Other illicit drugs (especially hallucinogens, anabolic steroids) and alcohol have been related to the development of delusions. (For example, alcohol withdrawal is a common condition, which may present with tactile or somatic delusions). Prescribed substances (especially steroids, dopamine agonists), OTC medications (especially sympathomimetics), and herbal products may also be associated with delusions. Careful substance and medication use history with specific attention to temporal relationship between substance use and onset/persistence of delusional symptoms may aid in differential diagnosis.|
|Mood disorders with delusional symptoms (manic or depressive type)||Mood symptoms are common in persons with delusional disorder and often represent a proportionate emotional response to perceived delusional experiences. However, given that mood disorders are common in the general population, they may present as comorbid conditions, often predating delusional disorder. Mood symptoms of mood disorders contrary to mood symptoms of delusional disorder are prominent and meet criteria for a full mood episode (depressive, manic, or mixed). Delusions associated with mood disorders usually develop after the onset of mood symptoms and progress secondary to mood abnormalities. Mood symptoms of delusional disorder are generally mild and delusions usually exist in the absence of mood abnormalities.|
|Schizophrenia||Delusions of schizophrenia are bizarre in nature, and thematically-associated hallucinations are common. Additionally, disorganized thought process, speech, or behaviors is present. Negative symptoms and deterioration in function are prominent. Cognitive deficits are common.|
|Hypochondriasis||Patients with hypochondriasis are usually able to doubt (at least for a short while) their convictions of having illness when presented with reassuring data. Most of them have a long history of illness preoccupation, and their fears are usually not limited to a single symptom or organ system.|
|Body dysmorphic disorder (BDD)||Many patients with BDD hold their beliefs with conviction that reaches level of delusions, leading to a significant overlap between these conditions.|
|Obsessive-compulsive disorder (OCD)||Patients with OCD show a varying degree of insight into their obsessions and compulsions. If reality testing is lost and conviction in their beliefs reaches the level of delusions, both disorders may be present.|
|Paranoid personality disorder||Differentiation between extreme characterological suspiciousness and frank delusions may be difficult. History of pervasive distrust beginning by early adulthood is suggestive of personality disorder, while the delusional disorder most commonly presents as an acute illness of middle life. Additionally, patients with paranoid personality disorder frequently appear to be unemotional and lack warmth in their relationships.|
|Shared psychotic disorder||Symptoms emerge in the context of a close relationship with another person with delusional beliefs and diminish with separation from that other person.|
Treatment & Management
Delusional disorder is challenging to treat for various reasons, including patients' frequent denial that they have any problem, especially of a psychological nature, difficulties in developing a therapeutic alliance, and social/interpersonal conflicts.
Treatment principles include the following:
Establish a therapeutic alliance and negotiate acceptable symptomatic treatment goals. Start where "the patient is at," and offer empathy, concern, and interest in the experiences of the individual.
With the appropriate permission from the patient, include the patient's family in decision-making and educate them.
Consider the impact of culture for treatment planning.
Avoid direct confrontation of the delusional symptoms to enhance the possibility of treatment compliance and response.
Use medication judiciously to target core symptoms and associated problems (eg, anger).
Use outpatient treatment unless there is potential for harm or violence.
Tailor treatment strategies to the individual needs of the patient and focus on maintaining social function and improving quality of life.
Recognize and treat coexisting psychiatric disorders.
Inpatient hospitalization should be considered if a patient’s delusions cause him or her to be a threat to self, others, or if he or she is deemed to be gravely disabled.
The evidence for the psychopharmacological treatment of delusional disorder would commonly be considered "grade C" (case series) or "grade D" (single case studies) evidence in many evidence-based medicine hierarchies. This is in contrast to randomized, blinded studies (grade A) or nonrandomized or nonblinded, but still systematically conducted, studies (grade B).
Antipsychotics have been used since the 1970s when the first report was published on the use of pimozide for the treatment of monosymptomatic hypochondriacal psychosis (now classified as a delusional disorder, somatic type by DSM-5). Of approximately 1000 treated cases of delusional disorder from 1965-1985, a subanalysis of 257 best-described cases revealed that delusional disorder has a relatively good prognosis when adequately treated — 52.6% of the patients recovered, 28.2% achieved partial recovery, and 19.2% did not improve. Treatment response was positive regardless of the specific delusional content. The data concluded that pimozide (68.5% recovery rate and 22.4% partial recovery rate) may be better than other typical antipsychotics (22.6% recovery and 45.3% partial recovery). 
Data since that time still consists mostly of case reports. The most recent review of treatment for delusional disorder included 224 case reports published since 1995, though only 134 case reports were well described.  The following is the summary of their findings:
In general, delusional disorders were reported to be fairly responsive to treatment (50% of the published patients reported symptom-free recovery and 90% of patients showed at least some improvement).
Combination treatment was common. Polypharmacy was common, most often including a combination of antipsychotic and antidepressant medication. In addition, patients commonly received more than one antipsychotic over the course of their illness, and medication treatments were also complemented by other interventions, such as cognitive-behavioral therapy or even (in a single case) electroconvulsive therapy (ECT).
In contrast to previous findings, no significant difference was observed between treatment with pimozide and other antipsychotics. Indeed, no difference was observed between typical and atypical antipsychotic agents.
Somatic delusions appeared potentially more responsive to antipsychotic therapy than other types of delusions (regardless of whether this treatment was pimozide or other antipsychotics). However, this apparent difference may mostly result from the generally poor response rates for delusional disorder with persecutory delusions (50% improvement rates, with no reports of complete recovery).
No other predictors of a positive outcome have been studied or clearly elucidated (eg, age, gender, symptom severity, positive family history, or premorbid function).
A systematic review of the literature shows that olanzapine and risperidone are the most common atypical antipsychotics used. [30, 31] Four reports (5 cases) of individuals with delusions presumably refractory to previous antipsychotic treatment reported that clozapine was associated with an improved quality of life and a decrease in symptoms associated with the delusion, although the central delusional theme often persisted. In contrast, as indicated above, some cases of delusional disorder appear refractory even to clozapine treatment. 
Reviews of treatment of delusional disorder have not systematically addressed the question of what particular dose of antipsychotics is needed to achieve remission of symptoms. However, a study of 11 patients with delusional disorder appeared to be adequately treated on fairly low doses of antipsychotic (4.7 mg of haloperidol). 
Antidepressants have been successfully used for the treatment of delusional disorder, although primarily of the somatic type. The data consist of case reports showing improvement with selective serotonin reuptake inhibitor (SSRI)  and clomipramine treatments [33, 34] . Several case reports documented successful treatment with SSRI for culture-bound syndromes (conditions that would be diagnosed as somatic type of delusional disorder in Western cultures). 
A single case report of successful ECT use for somatic delusions exists. 
In summary, a reasonable pharmacological treatment approach for the patient with delusional disorder is a standard trial of an antipsychotic or, for somatic delusions, an SSRI at starting doses commonly used to treat psychotic or mood disorders.
For most patients with delusional disorder, some form of supportive therapy is helpful. The goals of supportive therapy include facilitating treatment adherence and providing education about the illness and its treatment. Educational and social interventions can include social skills training (eg, not discussing delusional beliefs in social settings) and minimizing risk factors that may increase symptoms, including sensory impairment, isolation, stress, and precipitants of violence. Providing realistic guidance and assistance in dealing with problems stemming from the delusional system may be very helpful. 
Cognitive therapeutic approaches may be useful for some patients and this is best studied in persecutory type. The therapist helps the patient to identify maladaptive thoughts by means of Socratic questioning and behavioral experiments and then replaces them with alternative, more adaptive beliefs and attributions. Discussion of the unrealistic nature of delusional beliefs should be done gently and only after rapport with the patient has been established. [38, 37, 39]
A recent study evaluated the effectiveness of cognitive-behavioral therapy (CBT) versus attention placebo control (APC) as a means to treat delusions in delusional disorder. Using the Maudsley Assessment of Delusions Schedule (MADS), the study found that both APC and CBT improved belief and mood parameters associated with delusions. However, CBT produced more of an impact when compared to APC on strength of conviction, affect relating to belief, and positive actions of beliefs, suggesting CBT as a successful means of treating delusional disorder. 
According to Liberman  , another technique that may be applicable to a wider population of persons with delusional disorder is behavioral principles and social skills training to provide the individual with effective means of "feeling in control" and less subject to viewing others' efforts to harm him/her as allowing "them" to be controlling. Social skills training focuses on promoting interpersonal competence, confidence (with successful use of more competent social skills) and comfort in interacting with those who the individual feels are judging and having harmful intent toward him/her. Taking control and initiative can dissipate the feeling of loss of control that feeds into and reinforces the delusions.
The literature also states that insight-oriented therapy may be indicated, rarely  or contraindicated for delusional disorder  . However, reports exist of successful treatment.  Goals in insight-oriented therapy include development of the therapeutic alliance; containment of projected feelings of hatred, badness, and impotence; measured interpretation; and, ultimately, development of a sense of creative doubt in the internal perception of the world through empathy with the patient's defensive position.