Visual Hallucinations and Illusions

Visual hallucinations and illusions comprise some of the most vivid and sometimes bizarre symptoms in neuro-ophthalmology. Hallucinations are defined as perceptions that occur in the absence of a corresponding external sensory stimulus. Visual hallucinations can be classified as unformed/simple (e.g., dots, flashes, zig-zags) or formed/complex (actual objects or people). In contrast, illusions are misinterpretations of a true sensory stimulus. Visual hallucinations and illusions are generally positive phenomena, in contrast to visual loss, which is a negative phenomenon. The causes of visual hallucinations and illusions can be grouped into several major categories: migraine, release phenomena (in the setting of impaired vision), entoptic (ocular) phenomena, alcohol and drug-related, seizures, neurogenerative disease, central nervous system lesions, psychiatric disease, and narcolepsy. This chapter details the various categories, but first the history, examination, and diagnostic and therapeutic considerations in patients with hallucinations and illusions are reviewed.