NPD has a high rate of comorbidity with other mental disorders. Individuals with NPD are prone to bouts of depression, often meeting criteria for co-occurring depressive disorders. In addition, NPD is associated with bipolar disorder, anorexia, and substance use disorders, especially cocaine. As far as other personality disorders, NPD may be associated with histrionic, borderline, antisocial, and paranoid personality disorders.

The World Health Organization's (WHO) "International Statistical Classification of Diseases and Related Health Problems", 10th Edition (ICD-10) lists narcissistic personality disorder under "Other specific personality disorders". It is a requirement of ICD-10 that a diagnosis of any specific personality disorder also satisfies a set of general personality disorder criteria.

The PCL-R, the PCL:SV, and the PCL:YV are highly regarded and widely used in criminal justice settings, particularly in North America. They may be used for risk assessment and for assessing treatment potential and be used as part of the decisions regarding bail, sentence, which prison to use, parole, and regarding whether a youth should be tried as a juvenile or as an adult. There have been several criticisms against its use in legal settings. They include the general criticisms against the PCL-R, the availability of other risk assessment tools which may have advantages, and the excessive pessimism surrounding the prognosis and treatment possibilities of those who are diagnosed with psychopathy.

The interrater reliability of the PCL-R can be high when used carefully in research but tend to be poor in applied settings. In particular Factor 1 items are somewhat subjective. In sexually violent predator cases the PCL-R scores given by prosecution experts were consistently higher than those given by defense experts in one study. The scoring may also be influenced by other differences between raters. In one study it was estimated that of the PCL-R variance, about 45% was due to true offender differences, 20% was due to which side the rater testified for, and 30% was due to other rater differences.

To aid a criminal investigation, certain interrogation approaches may be used to exploit and leverage the personality traits of suspects thought to have psychopathy and make them more likely to divulge information.

The prognosis for psychopathy in forensic and clinical settings is quite poor, with some studies reporting that treatment may worsen the antisocial aspects of psychopathy as measured by recidivism rates, though it is noted that one of the frequently cited studies finding increased criminal recidivism after treatment, a 2011 retrospective study of a treatment program in the 1960s, had several serious methodological problems and likely would not be approved of today. However, some relatively rigorous quasi-experimental studies using more modern treatment methods have found improvements regarding reducing future violent and other criminal behavior, regardless of PCL-R scores, although none were randomized controlled trials. Various other studies have found improvements in risk factors for crime such as substance abuse. No study has of yet in a 2013 review examined if the personality traits that form the core character disturbances of psychopathy could be changed by such treatments.

Malignant narcissism is a psychological syndrome comprising an extreme mix of narcissism, antisocial behavior, aggression, and sadism. Often grandiose, and always ready to raise hostility levels, the malignant narcissist undermines organizations in which they are involved, and dehumanizes the people with whom they associate.

Malignant narcissism is a hypothetical, experimental diagnostic category. Narcissistic personality disorder is found in the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-IV-TR), while malignant narcissism is not. As a hypothetical syndrome, malignant narcissism could include aspects of narcissistic personality disorder as well as paranoia. The importance of malignant narcissism and of projection as a defense mechanism has been confirmed in paranoia, as well as "the patient's vulnerability to malignant narcissistic regression".

Sadistic personality disorder is a personality disorder involving which appeared in an appendix of the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-III-R). The later versions of the DSM (DSM-IV, DSM-IV-TR and DSM-5) do not include it.

The words "sadism" and "sadist" are derived from Marquis de Sade.

Sadistic personality disorder has been found to occur frequently in unison with other personality disorders. Studies have also found that sadistic personality disorder is the personality disorder with the highest level of comorbidity to other types of psychopathological disorders. In contrast, sadism has also been found in patients who do not display any or other forms of psychopathic disorders. One personality disorder that is often found to occur alongside sadistic personality disorder is conduct disorder, not an adult disorder but one of childhood and adolescence. Studies have found other types of illnesses, such as alcoholism, to have a high rate of comorbidity with sadistic personality disorder.

Researchers have had some level of difficulty distinguishing sadistic personality disorder from other forms of personality disorders due to its high level of comorbidity with other disorders.

The social psychologist Erich Fromm first coined the term "malignant narcissism" in 1964, describing it as a "severe mental sickness" representing "the quintessence of evil". He characterized the condition as "the most severe pathology and the root of the most vicious destructiveness and inhumanity". Edith Weigert (1967) saw malignant narcissism as a "regressive escape from frustration by distortion and denial of reality", while Herbert Rosenfeld (1971) described it as "a disturbing form of narcissistic personality where grandiosity is built around aggression and the destructive aspects of the self become idealized".

Developing their ideas further, the psychoanalyst Otto Kernberg pointed out that the antisocial personality was fundamentally narcissistic and without morality. Malignant narcissism includes a sadistic element creating, in essence, a sadistic psychopath. In his article, "malignant narcissism" and psychopathy are employed interchangeably. Kernberg first proposed malignant narcissism as a psychiatric diagnosis in 1984, but so far it has not been accepted in any of the medical manuals, such as the ICD-10 or the DSM-5.

Kernberg described malignant narcissism as a syndrome characterized by a narcissistic personality disorder (NPD), antisocial features, paranoid traits, and egosyntonic aggression. Other symptoms may include an absence of conscience, a psychological need for power, and a sense of importance (grandiosity). Pollock wrote: "The malignant narcissist is presented as pathologically grandiose, lacking in conscience and behavioral regulation with characteristic demonstrations of joyful cruelty and sadism".

Kernberg believed that malignant narcissism should be considered part of a spectrum of pathological narcissism, which he saw as ranging from Hervey M. Cleckley's antisocial character (today's psychopath or antisocial personality) at the high end of severity, through malignant narcissism, and then to narcissistic personality disorder at the low end. The malignant narcissist thus represents a less extreme form of pathological narcissism than psychopathy. Malignant narcissism can be distinguished from psychopathy, according to Kernberg, because of the malignant narcissist's capacity to internalize "both aggressive and idealized superego precursors, leading to the idealization of the aggressive, sadistic features of the pathological grandiose self of these patients". According to Kernberg, the psychopath's paranoid stance against external influences makes him or her unwilling to internalize even the values of the "aggressor", while malignant narcissists "have the capacity to admire powerful people, and can depend on sadistic and powerful but reliable parental images". Malignant narcissists, in contrast to psychopaths, are also said to be capable of developing "some identification with other powerful idealized figures as part of a cohesive 'gang'...which permits at least some loyalty and good object relations to be internalized". "Some of them may present rationalized antisocial behavior – for example, as leaders of sadistic gangs or terrorist groups...with the capacity for loyalty to their own comrades".

Grandiosity refers to an unrealistic sense of superiority—a sustained view of oneself as better than others that causes the narcissist to view others with disdain or as inferior—as well as to a sense of uniqueness: the belief that few others have anything in common with oneself and that one can only be understood by a few or very special people. It also occurs in reactive attachment disorder.

Grandiosity is chiefly associated with narcissistic personality disorder, but also commonly features in manic or hypomanic episodes of bipolar disorder.

Neurosis is a class of functional mental disorders involving chronic distress but neither delusions nor hallucinations. The term is no longer used by the professional psychiatric community in the United States, having been eliminated from the "Diagnostic and Statistical Manual of Mental Disorders" in 1980 with the publication of DSM III. It is still used in the .

Neurosis should not be mistaken for psychosis, which refers to a loss of touch with reality. Neither should it be mistaken for neuroticism, a fundamental personality trait proposed in the big Five personality traits theory.

There are many different neuroses: obsessive–compulsive disorder, obsessive–compulsive personality disorder, impulse control disorder, anxiety disorder, hysteria, and a great variety of phobias.

According to C. George Boeree, professor emeritus at Shippensburg University, the symptoms of neurosis may involve:

Neurosis may be defined simply as a "poor ability to adapt to one's environment, an inability to change one's life patterns, and the inability to develop a richer, more complex, more satisfying personality."

The diagnostic criteria for Dorian Gray syndrome are:

- Signs of dysmorphophobia

- Arrested development (inability to mature)

- Using at least two different medical-lifestyle products and services:

- Hair-growth restoration (e.g. finasteride)

- Antiadiposita to lose weight (e.g. orlistat)

- Anti-impotence drugs (e.g. sildenafil)

- Anti-depressant drugs (e.g. fluoxetine)

- Cosmetic dermatology (e.g. laser resurfacing)

- Cosmetic surgery (e.g. a face-lift, liposuction)

The Dorian Gray syndrome arises from the concurring and overlapping clinical concepts of the narcissistic personality, dysmorphophobia, and paraphilia. Psychodynamically, the man afflicted with DGS presents an interplay among his narcissistic tendencies ("timeless beauty"), his arrested development (inability to psychologically mature), and his use of "medical lifestyle" products and services — hair restoration, drugs (for impotence, weight-loss, and mood modification), laser dermatology, and plastic surgery — in order to remain young.

Although the DGS patient displays diagnostic features of said mental disorders, the syndrome describes a common, underlying psychodynamics of mental illness, which is characterized by narcissistic defences against time-dependent maturation, expressed by actively seeking the timeless beauty of youth. The article "Das Dorian Gray syndrom" (2005) reported that approximately 3.0 per cent of the population of Germany present features of the Dorian Gray syndrome.

Pathological grandiosity has been associated with one of the two subtypes of Narcissistic Personality Disorder. (Gabbard, 1989) Characteristics of the narcissist-grandiose subtype (as opposed to the narcissist-vulnerable subtype) include:

- Being labeled the “oblivious narcissists”

- Observed lack of insight into the impact they have on others

- More likely to regulate self-esteem through overt self-enhancement

- Denial of weaknesses

- Inflated demands of entitlement

- Consistent anger in unmet expectations

- Devaluation of people that threaten self-esteem

- Diminished awareness of the dissonance between their expectations and reality, along with the impact this has on relationships

- Overt presentation of grandiose fantasies

- Conflict within the environment is generally experienced as external to these individuals and not a measure of their own unrealistic expectations

The differences between grandiose and vulnerable narcissist subtypes have been studied (Dickinson & Pincus, 2003):

The grandiosity section of the Diagnostic Interview for Narcissism (DIN) (Second edition) is as follows:

1. The person exaggerates talents, capacity and achievements in an unrealistic way.

2. The person believes in his/her invulnerability or does not recognize his/her limitations.

3. The person has grandiose fantasies.

4. The person believes that he/she does not need other people.

5. The person overexamines and downgrades other people, projects, statements, or dreams in an unrealistic manner.

Multiple complex developmental disorder is likely to be caused by a number of different various genetic factors. Each individual with MCDD is unique from one another and displays different symptoms. Various neuropsychological disorders can also be found in family members of people with MCDD.

Counterdependency is the state of refusal of attachment, the denial of personal need and dependency, and may extend to the omnipotence and refusal of dialogue found in destructive narcissism, for example.

The current diagnostic criteria for MCDD are a matter of debate due to it not being in the DSM-IV or ICD-10. Various websites contain various diagnostic criteria. At least three of the following categories should be present. Co-occurring clusters of symptoms must also not be better explained by being symptoms of another disorder such as experiencing mood swings due to autism, cognitive difficulties due to schizophrenia, and so on. The exact diagnostic criteria for MCDD remain unclear but may be a useful diagnosis for people who do not fall into any specific category. It could also be argued that MCDD is a vague and unhelpful term for these patients.

The roots of counterdependency can be found in the age-appropriate negativism of two-year-olds and teens, where it serves the temporary purpose of distancing one from the parental figure[s]. As Selma Fraiberg put it, the two-year-old "says 'no' with splendid authority to almost any question addressed to him...as if he establishes his independence, his separateness from his mother, by being opposite". Where the mother has difficulty accepting the child's need for active distancing, the child may remain stuck in the counterdependent phase of development because of developmental trauma.

In similar fashion, the teenager needs to be able to establish the fact of their separate mind to their parents, even if only through a sustained state of cold rejection; and again unresolved adolescent issues can lead to a mechanical counterdependence and unruly assertiveness in later life.

Katoptronophilia is a paraphilia for mirrors (the Greek word for "mirror" is "katoptron"). It may include activities such as having sex in front of mirrors, masturbating in front of mirrors, enacting other paraphilias in front of a mirror, having an orgy in front of a mirror, or enacting stripping fetishism in front of mirrors.

Enacting katoptronophiliac fantasies may involve constructing environments for erotic activity in which one is completely surrounded by mirrors, sometimes including even on the ceiling. A person who is a "katoptronophiliac" may put mirrors all over their house so they can have sex in any room in the house.

Many pornographic films show porn stars having sex in front of mirrors.

Many people enjoy having sex in front of mirrors and have mirrors in their bedrooms in which they can watch themselves have sex. They sometimes engage in this activity for their personal enjoyment. On a deeper level this could relate to the person's need to reflect and critique themselves, and also being on a mental state of narcissism. The person often is solely absorbed in themselves and likes to watch their actions so as to admire.

Regardless of location, all rhabdoid tumours are highly aggressive, have a poor prognosis, and tend to occur in children less than two years of age.

Although often described as benign, a teratoma does have malignant potential. In a UK study of 351 infants and children diagnosed with "benign" teratoma reported 227 with MT, 124 with IT. Five years after surgery, event-free survival was 92.2% and 85.9%, respectively, and overall survival was 99% and 95.1%. A similar study in Italy reported on 183 infants and children diagnosed with teratoma. At 10 years after surgery, event free and overall survival were 90.4% and 98%, respectively.

Depending on which tissue(s) it contains, a teratoma may secrete a variety of chemicals with systemic effects. Some teratomas secrete the "pregnancy hormone" human chorionic gonadotropin (βhCG), which can be used in clinical practice to monitor the successful treatment or relapse in patients with a known HCG-secreting teratoma. This hormone is not recommended as a diagnostic marker, because most teratomas do not secrete it. Some teratomas secrete thyroxine, in some cases to such a degree that it can lead to clinical hyperthyroidism in the patient. Of special concern is the secretion of alpha-fetoprotein (AFP); under some circumstances AFP can be used as a diagnostic marker specific for the presence of yolk sac cells within the teratoma. These cells can develop into a frankly malignant tumor known as yolk sac tumor or endodermal sinus tumor.

Adequate follow-up requires close observation, involving repeated physical examination, scanning (ultrasound, MRI, or CT), and measurement of AFP and/or βhCG.

The diagnosis of a mediastinal germ cell tumor should be considered in all young males with a mediastinal mass. In addition to physical examination and routine laboratory studies, initial evaluation should include CT of the chest and abdomen, and determination of serum levels of HCG and alpha-fetoprotein.

Extraspinal ependymoma, usually considered to be a glioma (a type of non-germ cell tumor), may be an unusual form of mature teratoma.

Diet and lifestyle are believed to play a large role in whether colorectal polyps form. Studies show there to be a protective link between consumption of cooked green vegetables, brown rice, legumes, and dried fruit and decreased incidence of colorectal polyps.

The histologic diagnosis of malignant rhabdoid tumour depends on identification of characteristic rhabdoid cells—large cells with eccentrically located nuclei and abundant, eosinophilic cytoplasm. However, the histology can be heterogeneous and the diagnosis of MRT can often be difficult. Misclassifications can occur.

In MRTs, the INI1 gene (SMARCB1)on chromosome 22q functions as a classic tumour suppressor gene. Inactivation of INI1 can occur via deletion, mutation, or acquired UPD.

In a recent study, SNP array karyotyping identified deletions or LOH of 22q in 49/51 rhabdoid tumours. Of these, 14 were copy neutral LOH (or acquired UPD), which is detectable by SNP array karyotyping, but not by FISH, cytogenetics, or arrayCGH. MLPA detected a single exon homozygous deletion in one sample that was below the resolution of the SNP array. SNP array karyotyping can be used to distinguish, for example, a medulloblastoma with an isochromosome 17q from a primary rhabdoid tumour with loss of 22q11.2. When indicated, molecular analysis of INI1 using MLPA and direct sequencing may then be employed. Once the tumour-associated changes are found, an analysis of germline DNA from the patient and the parents can be done to rule out an inherited or de novo germline mutation or deletion of INI1, so that appropriate recurrence risk assessments can be made.