Delusional disorder (DSM-IV 297.1 and ICD-10 F22)(1.,2) is a psychotic illness with some superficial resemblances to schizophrenia from which, however, it is quite distinct. It presents with a stable and well-defined delusional system which is typically 'encapsulated' from a personality which retains many normal aspects, unlike the situation in schizophrenia where there is widespread personality disorganization in addition to the psychotic features. Nevertheless, although many normal aspects of the personality are preserved, the individual's way of life becomes progressively overwhelmed by the intensity and intrusiveness of the delusional beliefs. Hallucinations may be present but are not usually prominent. This is a chronic disorder, probably lifelong in most instances, which retains an unjustified reputation for being untreatable. Because of the nature of their delusions, many patients are unwilling to accept that they have a mental disorder or that they require psychiatric treatment but, if they can be persuaded to co-operate and accept appropriate medication, the condition can be shown to respond to treatment in a remarkably high proportion of cases.

Delusional disorder is the name now applied to the illness previously known as 'paranoia', and the terms are virtually synonymous. Paranoia and its related disorders were regarded as an important group of psychiatric illnesses until the early part of the twentieth century. Then, because of changing diagnostic and classificatory approaches, especially the tendency in some quarters to overdiagnose schizophrenia, the diagnosis of paranoia and a companion illness, paraphrenia, practically stopped and they all but disappeared from standard classificatory systems. In 1987, paranoia was again officially recognized by DSM-IIIR (3) but was renamed delusional (paranoid) disorder—since simplified to delusional disorder. It is currently the only officially acknowledged member of the old group of paranoid illnesses appearing in DSM-IV and ICD-10.

Although the diagnosis of paranoia almost ceased for many years, the illness and its sufferers did not disappear. When the phenomena of the disorder came to attention, one of two things tended to happen. Either the patient was labelled as schizophrenic or else a specific feature of the delusional symptomatology was seized upon and spurious syndromes were described. Therefore, as will be noted in the brief historical review below, we have a multiplicity of apparently disparate diagnoses such as de Clérambault's syndrome (delusional erotomania), the Othello syndrome (delusional jealousy), querulant paranoia (a form of persecutory delusional disorder), monosymptomatic hypochondriacal psychosis (delusional disorder with somatic preoccupations), and many others. The result has been an extraordinarily scattered literature with cases recorded in a variety of medical and non-medical sources, but very few in psychiatric publications until recently. It is only since the publication of DSM-IIIR that a serious attempt has begun to resolve a profoundly confusing situation and once again to diagnose paranoia/delusional disorder on the basis of its own intrinsic features. Later in this section, some of the problems still bedevilling nomenclature will be discussed.

Jaspers, in discussing paranoia, said: 'Why are the paranoics as defined by Kraepelin so rare, yet when they do occur they are so typical?' This is one of the outstanding paradoxes concerning delusional disorder. There are striking similarities from case to case and the illness has features which clearly distinguish it from other psychoses, yet even now diagnostic practices often lead to its being confused with illnesses such as schizophrenia.

Many psychiatric illnesses are associated with persistent delusions, but DSM-IV and ICD-10 provide criteria to differentiate delusional disorder as an illness in its own right and these are now widely accepted. This section adopts this official approach but with two caveats. The first is that the descriptions are bald and not very helpful to the clinician who has not actually seen cases of the disorder. The second is that the category of delusional disorder (persistent delusional disorders in ICD-10) may well be over-restrictive at present. However, it should be noted that some well-respected authorities (4,5) take a somewhat different approach, regarding 'delusional disorders' as all psychiatric illnesses with delusions and then subcategorizing according to the underlying syndrome, which might be severe mood disorder, schizophrenia, delusional disorder (as in DSM-IV and ICD-10), etc. Therefore the reader must be aware of each author's particular criteria for the diagnosis.

Emil Kraepelin (1856-1926) clearly described paranoia and he included it in a continuum of illnesses with delusional features, which also subsumed paraphrenia and paranoid schizophrenia. This so-called paranoid spectrum will be described later. Paranoid schizophrenia continues to be a widely used diagnosis, but nowadays it officially belongs to the group of schizophrenias rather than with the delusional disorders. Paraphrenia is not officially acknowledged in DSM-IV or ICD-10, but cases fitting its traditional description are not uncommonly seen in practice. A short account of its putative features is provided later in this chapter and an argument advanced for its reacceptance as a discrete disorder within a paranoid spectrum. A somewhat contentious diagnosis—late-onset paraphrenia—has relevance to the contention that paraphrenia does exist, but as a concept it is not widely used outside the United Kingdom; its features will be considered when describing paraphrenia itself.

At present, 'delusional disorder' is both an illness category and essentially the only syndrome contained within that category. In recent years, another diagnosis—delusional misidentification syndrome (DMIS)—has come into increasing prominence. This group of disorders was first specifically reported in 1923 by Capgras and Reboul-Lachaux(6) who described the phenomenon of delusional conviction that someone in the patient's environment has been replaced by an almost exact double. For a long time the 'Capgras syndrome' led a rather marginal existence in the literature, sustained mainly by occasional anecdotal descriptions and spurious psychodynamic explanations, but in the past decade there have been considerably more case reports, descriptions have become more objective, and clinical subtypes have been distinguished. Most importantly, sound psychological and neuropathological work has been carried out and there has been an increasing ability to demonstrate scientifically the presence of significant cerebral pathologies in a high proportion of sufferers.

DMIS is not currently recognized by DSM-IV and ICD-10, but it has a number of clinical features similar to those of delusional disorder and there is no doubt that it warrants official acknowledgement and, it is suggested, inclusion in an expanded category of delusional disorders.

Finally, there is an important phenomenon which is found in association with all illnesses with delusions, but is especially prominent in delusional disorder. This is named 'shared psychotic disorder' in DSM-IV and 'induced delusional disorder' in ICD-10, but is often still referred to by its long-established name folie á deux. Here, the primary patient has a bona fide delusional disorder and a secondary patient comes to accept the abnormal beliefs as true. The secondary patient is usually a highly impressionable individual living in prolonged close contact with the other; he or she is not truly deluded, but retains the beliefs tenaciously as long as the intimate relationship is maintained. A less common variety is when two individuals each have genuine delusional disorders and, through close proximity, come to share identical abnormal beliefs. Folie á deux is not uncommon and, as will be explained later, there are very practical reasons why the clinician should be aware of its possible presence and the ways in which it may influence management of the case.

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