Prevalence and classification of hallucinations in multiple sensory modalities in schizophrenia spectrum disorders

Auditory hallucinations are experienced by 60–80% of all patients diagnosed with a schizophrenia spectrum disorder. However, in this patient group, the prevalence of hallucinations in multiple sensory modalities, i.e. multimodal hallucinations (MMHs), is unknown. To assess the prevalence of MMHs in patients diagnosed with a schizophrenia spectrum disorder, data were analyzed from 750 patients who participated in the Dutch Genetic Risk and Outcome of Psychosis (GROUP) study. We drew on the section of the CASH (Comprehensive Assessment of Symptoms and History) that probes into the lifetime presence of auditory, visual, somatic/tactile, and olfactory hallucinations. A lifetime prevalence of 80% was found in this group for hallucinations in any of these modalities. Within the whole group, 27% of the participants reported unimodal hallucinations and 53% MMHs. There were no significant differences in prevalence rate for Dutch versus migrant participants from Morocco, Turkey, Surinam or the (former) Dutch Antilles. We conclude that MMHs, rather than auditory hallucinations, are the most frequent perceptual symptom of patients diagnosed with a schizophrenia spectrum disorder. Our data also suggest that hallucinations experienced in a single sensory modality (notably auditory ones) stochastically increase the risk for more sensory modalities to join in. We recommend that future studies take into account all 14 sensory modalities in which hallucinations can be experienced. For this we provide a classification of MMHs that allows characterization of their serial versus simultaneous occurrence and their congruent versus incongruent nature.