With such a wide range of potential aetiologies and endless possibilities for clinical overlap (glaucoma in a patient with Parkinson’s disease, cognitive impairment in a patient with Macular degeneration), how should a patient with visual hallucinations be managed? Clearly the answer will depend on the clinical context; however, the general principles are summarised in Figure 2. Whatever the patient’s phenomenology, it is important to review their medication to minimise anti-cholinergic load and to consider whether the hallucinations/illusions may have been precipitated by concurrent infection (often a UTI in the elderly). The question of whether to investigate depends largely on the match between their
hallucinations and clinical context: if the symptoms are wrong, look for another cause. Hallucinations of a familiar dog in a patient with Parkinson’s disease would not warrant further investigation but hallucinations of grid patterns confined to one hemifield might prompt imaging of the visual cortex. Similarly, prolonged hallucinations of whispering figures in a patient with Macular disease might warrant referral to an old age psychiatry service whereas a brief hallucination of an Edwardian tea party would not. Two investigations deserve special mention. Sleep studies may have a place in the investigation of brainstem/cholinergic hallucinations as these have been found to relate to REM sleep behaviour disorder and daytime REM intrusions. Visual evoked potentials may help characterise a visual pathway lesion but there are, as yet,
no known VEP markers for susceptibility to hallucinations