A Review of Multimodal Hallucinations

Categorization, Assessment, Theoretical Perspectives, and Clinical Recommendations

Disclosures

Schizophr Bull. 2021;47(1):237-248. 

In This Article

Prevalence

Overall, studies show that hallucinations in one modality incrementally increase the risk of hallucinations in one or more other modalities.[26,27] There also seems to be an inverse relationship between the number of modalities and proportion of people reporting them,[15,16] as well as specific patterns of frequencies of MMHs across disorders, which will be discussed in the following sections.

Schizophrenia and Bipolar Disorder

Since AHs were thought to be the cardinal symptom of psychotic disorders, other hallucinatory modalities were typically overlooked. Recent studies have, however, shown that, for schizophrenia, the weighted mean prevalence of VHs is around 27% (based on 29 studies)[6] compared to 79% for AHs.[28] Prevalence estimates for olfactory hallucinations vary from 6% to 26%, gustatory hallucination 1%–31% and somatic/tactile hallucinations 4%–19%.[15,28,29]

Evidence suggests that VHs in psychosis almost always (90% of cases) occur in combination with another hallucination modality (auditory, somatic, or other),[30,31] in contrast to AHs, which can occur independently of other modalities about half of the time.[32] The overall lifetime prevalence of any hallucinations for schizophrenia is approximately 80%, with MMHs being twice as common as unimodal ones (53% vs 27%).[16,33] Such higher prevalence of MMHs over unimodal hallucinations was found across studies of bipolar disorder patients as well,[3] suggesting continuity across psychotic illnesses. However, none of these studies specifically looked at simultaneous MMHs.

Both serial and simultaneous MMHs were investigated in a group of 22 individuals with schizophrenia and VHs by Dudley et al.[13] Ninety-six percent of patients experienced serial MMHs vs 86% experiencing simultaneous MMHs, indicating most had a combination of the 2. MMHs were again more common than unimodal ones (see Figure 2). By contrast, others report that hallucinations in simultaneous multiple modalities are rare,[34] suggesting that the relatively small sample in Dudley's study[13] might not necessarily represent the prevalence of simultaneous MMHs in psychosis more generally. Further replications are needed.

Figure 2.

Bar chart showing the prevalence (in n%) of hallucination modalities in combination with visual hallucinations across 4 different disorders. Adapted from data in Table 2 in Dudley.

Eye Disease and Neurodegenerative Disorders

Dudley et al[14] compared hallucinations across different disorders in participants with VHs using the North East Visual Hallucination Interview (NEVHI)[35] and found that the frequencies of unimodal vs multimodal experiences varied across neurodegenerative disorders (see Figure 2). The Lewy Bodies Dementia (LBD) group had the highest prevalence of MMHs, followed by those with Parkinson's disease (PD) and then by eye disease patients. Unimodal hallucinations remained the most prevalent for all groups, which is at odds with a study by Llorca.[30] In their larger sample of 200 PD patients, a combination of hallucinations in 2–3 modalities was more common than unimodal ones, perhaps suggesting that bigger samples are needed to properly detect the prevalence of MMHs in neurodegenerative disorders.

In the Dudley study,[14] despite the higher frequency of unimodal hallucinations reported by participants, MMHs were found to be more irritating, distressing, and frightening than their unimodal counterparts. Furthermore, patients' VHs in the context of MMHs were accompanied by a significantly stronger degree of conviction about their veracity, suggesting some important clinical implications that need further exploration.

Developmental Aspects

Hallucinations are frequently observed in children and adolescents in both clinical and nonclinical contexts.[36,37] A phenomenological analysis of the hallucinations reported in the National Institute of Mental Health (NIMH) childhood-onset schizophrenia cohort revealed that the number of sensory modalities involved may serve as an indicator of the neurodevelopmental weight of the disorder.[26] This idea of MMHs as a proxy of developmental vulnerability was also confirmed in a case series showing that the number of sensory modalities involved in early-onset hallucinations was related to the probability that a given child ever experienced prior traumatic events.[38]

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