Hallucinations are perceptions unrelated to active stimuli. They may originate anywhere along the visual pathway but occur most commonly within the globe or cortex. Hallucinations may be formed (eg, real objects such as animals, flowers, cars, or people) or unformed (eg, light, spots, dots, or geometric patterns).
Unlike illusions, hallucinations have no optical causes. Vitreous detachment with persistent vitreoretinal adhesions may produce photopsias (flashing lights) or vertical white flashes (so-called lightning streaks of Moore). Such hallucinations are often apparent in a dark environment and may be induced by saccades. Retinal detachment may produce persistent photopsias and floaters.
Photopsias resulting from outer retinal diseases are often continuous. They may be simple white lights or form geometric webs that take on colors, including silver and gold. Photopsias often herald the onset of autoimmune retinopathy, including cancer-associated retinopathy, a paraneoplastic process. In addition, photopsia may accompany a variety of retinal, retinal pigment epithelium, and choroidal abnormalities (eg, multiple evanescent white dot syndrome, acute zonal occult outer retinopathy, or birdshot chorioretinopathy; see BCSC Section 12, Retina and Vitreous, for more on these conditions).
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Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.