[Visual hallucinations and illusions, symptoms frequently misdiagnosed by the practitioner]

Visual hallucinations or illusions are not a rare symptom. However, they are often unrecognized. Unawareness of the meaning of these symptoms often mislead both the patient and his physician.

To define and describe the types of visual illusions and hallucinations which can be commonly encountered in neuro-ophthalmological practice.

Overview article.

Hallucinations are a perception not based on sensory input, whereas illusions are a misinterpretation of a correct sensory input. Both phenomenon can be due to medication or drug, or to an altered mental status. Visual hallucinations can be formed (objects, people) or unformed (light, geometric figures). They can be generated either by a lesion on the antechiasmatic pathway, by a seizure phenomenon, by a migrainous phenomenon, or by a release phenomenon secondary to visual differentiation. Investigations will be directed towards a retinopathy, an optic neuropathy, a chiasmal or retrochiasmal lesion, or a bilateral antechiasmal lesion (Charles Bonnet syndrome). Visual illusions include meta-morphopsias, micro- macropsias, polyopia, palinopsia (visual perseveration), achromatopsia, Pulfrich phenomenon, or subjective vertical deviation. Illusions can be due to lesions of the retina, the optic nerve, the visual cortex (primary or associative), or the graviceptive pathways.

As most patients do not spontaneously mention their symptoms, history taking is essential. The first step is to rule out medication or an altered mental status as the possible cause of these symptoms. Then, careful visual function examination should provide a good insight in the location of the lesion.