In 1906 Alois Alzheimer described the case of Auguste Deter, who was admitted to the hospital where he was working, in 1901. Based on the neuropathological findings and the clinical picture, the disease was named Alzheimer’s disease in the 8th edition of Kraepelin’s authoritative textbook on psychiatry [1]. Prior to admission, Auguste Deter had over some time developed paranoid delusions in addition to cognitive symptoms. The first few days after admission, Alzheimer described symptoms such as emotional instability, paranoid delusions, screaming and possible hallucinations [2]. The case of Auguste Deter shows that right from the initial recognition of the condition, psychotic symptoms were recognised as symptoms occurring in the context of dementia or, more specifically, in Alzheimer’s disease. Strangely enough, Kraepelin did not mention these symptoms in his psychiatry textbook.

Psychosis in dementia (PiD) remains an elusive concept. Psychotic symptoms often seem to overlap with cognitive symptoms. If a person tells us that yesterday she had a visit from her mother, who has been deceased for many years, is she disoriented in time or is she psychotic? If a person looks out of the window and sees the house where she grew up, even though that place is 100 km away, is she disoriented to place or is she psychotic? If a person wants to fight with his mirror image, is he disoriented to person or is he psychotic?

Furthermore, psychotic symptoms in dementia often co-occur with other neuropsychiatric symptoms, such as agitation or anxiety, which makes it difficult to discern the specific psychotic symptoms.

Usually, we rely on verbal reports when we decide whether a person has hallucinations or delusions. However, with the progression of dementia, language impairments become more prominent, and the identification of psychotic symptoms has to be made from vocal signs or gestures, rather than language. This may cause both underestimation and overestimation of the extent of the psychotic symptoms.

In the late 1990s, the concept of behavioural and psychological symptoms of dementia (BPSD) was heavily promoted [3]. The aim of this effort was to highlight the importance of noncognitive symptoms in dementia, which are associated with a number of negative outcomes for the person with dementia, for their caregivers and for society generally. However, an unfortunate side effect of this initiative has been the tendency to view BPSD as a homogenous symptom group, lumping together aggression, disinhibition, aberrant motor behaviour and psychotic symptoms under terms like challenging behaviour, disruptive behaviour or simply agitation. This may have caused an erroneous view that psychotic symptoms are similar to other BPSD in terms of aetiology or treatment approaches. In a number of research studies, including clinical trials, the outcome has been a sum score of an assessment scale for BPSD (or neuropsychiatric symptoms (NPS)), such as the Neuropsychiatric Inventory (NPI) scale [4] or the Behavioural Pathology in Alzheimer’s Disease (BEHAVE-AD) scale [5], which makes interpretation of the results difficult.

The most common psychotic symptoms in dementia are delusions and hallucinations. The Schneiderian first-rank psychotic symptoms commonly encountered in persons with schizophrenia (e.g. thought withdrawal, thought insertion, thought broadcasting, hearing voices talking to each other or commenting on the patient’s action) are very rare in dementia. Delusions in dementia are typically non-bizarre and simple and paranoid in character, like ideas about infidelity, abandonment, poisoning or theft. The content of the delusional thoughts is usually conceivable, which underlines the need for a comprehensive assessment of the symptoms. Sometimes things really are stolen or the food might taste strange in the nursing home. Language disturbances, which are common, particularly in severe dementia, may make it difficult to interpret delusions. Other types of delusions that are rather common in dementia include misidentifications, beliefs about living in another person’s home, thinking that family members have been replaced by identical-looking imposters (Capgras’ syndrome) or duplicated or experiencing people on the television to be present in the room.

In dementia, visual hallucinations are more common than auditory hallucinations. This is opposite to what is normally the case in other psychotic disorders, such as schizophrenia or paranoid psychosis. In dementia with Lewy bodies (DLB), visual hallucinations are particularly prominent, and they are included among the core diagnostic features [10]. In the early stages of DLB, hallucinations are typically well formed and detailed, and the patient often shows some level of insight about the perceptions not representing reality. As the disease progresses, more complex hallucinations, both visual and auditory, may occur. Auditory hallucinations are rarely commenting or commanding voices, but rather sounds, individual words or phrases. Musical hallucinations have been described.

Other types of hallucinations, such as tactile hallucinations or olfactory hallucinations, are not common and may be an indication of the co-occurrence of delirium.

Other forms of psychotic symptoms, such as disorganised speech or motor activity, are rarely reported in the context of PiD. However, one should bear in mind that the scales (inventories) most commonly used to assess psychotic symptoms in dementia (NPI, BEHAVE-AD) focus entirely on hallucinations or delusions. A more in-depth investigation of other types of psychotic symptoms in dementia might be warranted. This may in particular apply to disorganised motor behaviour, which is one of the key clinical domains of psychosis in schizophrenia. Disorganised motor behaviour is highly prevalent in dementia but is often categorised as a type of agitation, distinct from psychosis.

With the increasing prevalence of dementia, PiD will soon become the second most prevalent psychotic disorder, second only to schizophrenia [15].

However, prevalence estimates for PiD vary widely. A comprehensive review reported prevalence rates for delusions from 9 to 63 % (median 36 %) and for hallucinations from 4 to 41 % (median 18 %) [16]. This probably reflects different sample characteristics and different cutoffs for defining the condition. Misidentification symptoms, which are quite common in dementia, are in some studies classified as delusions, but not in others. Psychotic symptoms are more frequent among inpatients (e.g. in nursing homes) than in outpatient settings (e.g. memory clinic samples). Population-based studies have reported prevalence rates of around 22 % for delusions and 13 % for hallucinations [17]. Five-year period prevalence rates were around 60 % for delusions and nearly 40 % for hallucinations [18]. Even higher prevalence rates are found among the oldest group of the elderly (85+) [19].

In general, prevalence rates based on clinical criteria seem to give lower figures than rates based on assessment scales.