An investigation of visual hallucinosis and visual sensory status in dementia

Visual hallucinosis occurs frequently amongst patients with dementia and there is some evidence of an association between hallucinosis and impaired visual function. This study examined the association between objectively rated visual acuity, ambient illumination levels and hallucinosis in patients with dementia.

Thirty dementia patients with visual hallucinosis plus 30 non-hallucinating dementia patients were recruited from referrals to community mental health teams in Leicestershire. Data were collected by means of a carer interview (including the Neuro-Psychiatric Inventory (NPI) and Clinical Dementia Rating Scale (CDR)), patient assessment (including Mini-Mental State Examination (MMSE) and measures of corrected visual acuity) and photometric assessment of the room most frequently occupied by the subject.

Hallucinators and non-hallucinators did not differ significantly in respect of age, cognitive status or global dementia severity. Fifty per cent of hallucinators had a Snellen visual acuity of 6/24 or worse compared with 27% of non-hallucinators. Comparison of LogMAR transformed values for distance (Snellen chart) and near-vision (Sussex test-types) revealed significantly worse visual acuity for hallucinators on both measures (Mann-Whitney p < 0.05 and p < 0.01, respectively). Ambient illumination in the centre of the room under conditions of natural plus artificial lighting was significantly lower for hallucinating subjects than for non-hallucinators (median values 200 versus 345 Lux respectively; Mann-Whitney p < 0.05).

These results are consistent with previous findings suggesting an association between visual hallucinations and poor visual acuity in patients with dementia, and introduce environmental illumination as another possible aetiologically relevant factor. Interventions aimed at improving visual function in this patient group now require evaluation.