Delusions of Grandeur
Delusions of grandeur is characterized by a deep conviction that one possesses superior qualities that far exceeds the average person, such as a belief that they are a genius, wealthy, famous, or omnipotent. They will delude themselves into believing such things when in reality the opposite is usually the case.
They may be poor, but think they are wealth, such as the case with many narcissists. They may be relatively unknown, yet believe they are famous. They may also believe they are the son of god or some other nonsensical delusion. People who suffer from delusions of grandeur often have diagnosable mental disorders such as schizophrenia, bipolar disorder, or delusional disorder. 
Grandiose delusions are just one of many symptoms of delusional disorder. There are several “types” of delusions that someone may suffer from. Some of the more common ones are of course grandiosity, persecutory, somatic, jealousy, and erotomanic delusions. Persecutory delusional disorder is characterized by a belief that they are being persecuted or followed by someone. Somatic delusions involve irrational beliefs of having a physical illness. Erotomanic delusions are characterized by having a loving relationship with someone famous or of high status whom they have most likely never met before. 
Causes of Delusions of Grandeur
The exact cause of someone developing these types of delusions is not known. However, one’s environment and genetics may play crucial roles in the development of delusions of grandeur. Some other possible causes may be neurotransmitter concentrations in the brain, receptor density, brain anatomy, drugs, brain injuries, encephalitis, or hemispheric activation.
According to a study done in 2006, it was shown that there was genetic associations between delusional disorder and paranoid schizophrenia. In this study, researchers looked at the gene human leukocyte antigen (HLA). They found that HLA is significantly associated with delusional disorder, as well as with paranoid schizophrenia.  According to another study, it was shown that the left hemisphere of the brain appears to be more of a primary cause of delusional disorder than right hemisphere impairment. 
Symptoms of Delusions of Grandeur
Though delusions of grandeur may seem somewhat analogous to symptoms of narcissistic personality disorder, the symptoms of delusions of grandeur are much more hyperbolic and irrational to say the least. Delusional disorder is characterized by having either bizarre or non-bizarre delusions that have occurred for at least one month.  Below, you will see some of the more common symptoms of delusions of grandeur.
Feelings of superiority
Delusion of being wealthy
Delusion of being omnipotent or devine
Delusion of being famous
Delusions of Grandeur Diagnostic Criteria
To get diagnosed with having delusions of grandeur you will need to get a psychiatric evaluation by a psychiatrist or an accredited therapist. You can expect to be asked questions about your family history, as well as the symptoms you’re currently experiencing. Diagnosing delusional disorder is actually quite rare. So, upon getting a psychiatric evaluation, the healthcare professional should also take into consideration of the possibility that another major disorder such as bipolar, schizophrenia, or a brain injury may be causing their delusions of grandeur.
Treatments for Delusions of Grandeur
Treatments for delusions of grandeur will depend on which mental disorder they have been diagnosed with. Someone with delusions of grandeur may be diagnosed with delusional disorder, schizophrenia, or another mental illness. It is also pertinent to note that treatment for delusions of grandeur is often quite rare due to the fact that many individuals who suffer from this delusion believe that they are divine or superior to other people. So, it is unlikely that they would admit or realize that they have a problem. The same case can be made for those suffering from narcissistic personality disorder as well.
1) “Delusions Of Grandeur: Causes, Symptoms, Treatment.” Mental Health Daily. https://mentalhealthdaily.com/2015/05/12/delusions-of-grandeur-causes-symptoms-treatment/
2) “Delusional Disorder.” Harvard Medical School. https://www.health.harvard.edu/diseases-and-conditions/delusional-disorder
3) “Genetic associations between delusional disorder and paranoid schizophrenia: A novel etiologic approach.” NCBI. https://www.ncbi.nlm.nih.gov/pubmed/16786814
4) “A general neuropsychological model of delusion.” NCBI. https://www.ncbi.nlm.nih.gov/pubmed/20198522
5) “Delusional Disorder Symptoms.” Psych Central. https://psychcentral.com/disorders/delusional-disorder-symptoms/