Chapter 6: The Patient With Illusions, Hallucinations, and Disorders of Higher Cortical Function
Patients are often reluctant to report “seeing things” due to concerns that it may reflect mental illness; however, many visual hallucinations or illusions are not a harbinger of psychiatric disease but are instead indicators of brain, ocular, or optic nerve pathology. The ophthalmologist should therefore make sure to ask patients about these symptoms. Illusions are false perceptions of visual stimuli. For example, a person looking at stationary high-contrast borders may perceive an illusion of movement. A hallucination is the subjective perception of an object or event when no sensory stimulus is present. Illusions should disappear with eye closure; hallucinations often do not.
When assessing a patient who reports illusions or hallucinations, the first step is to ascertain the patient’s mental status and the function of the patient’s afferent visual system. Patients with dementia or altered sensorium (eg, delirium, hypnosis) are prone to hallucinations. Reduced visual acuity or visual field loss often leads to positive visual phenomena. The ophthalmologist should determine the corrected distance visual acuity, assess color vision, and test for perimetry. By localizing the disorder anatomically, the ophthalmologist is able to establish a likely pathophysiology, or at least a differential diagnosis.
Celesia GG. The mystery of photopsias, visual hallucinations, and distortions. Suppl Clin Neurophysiol. 2006;59:97–103.
Winterkorn JM, Fraser CE, Nirenberg MJ. Ophthalmic features of neurodegenerative diseases. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2010, module 1.
The Patient With Visual Illusions and Distortions
The alterations of perception that make up the spectrum of illusions can arise at various points in the visual system. Nonvisual causes of illusions and hallucinations are listed in Table 6-1.
Fraunfelder FT, Fraunfelder FW, Chambers WA. Drug-Induced Ocular Side Effects. 7th ed. Philadelphia: Saunders/Elsevier; 2014.
Table 6-1 Nonvisual Causes of Illusions and Hallucinations
Many illusions have an ocular basis and may be classified as optical or retinal (due to alterations in photochemical transduction).
Irregularities in the eye’s refracting elements can produce visual distortion and resulting illusions such as ghost images (monocular diplopia), movement, or multiple images. Any alteration in the tear film (eg, dry eye syndrome) or in the corneal surface (eg, keratoconus, corneal edema, or scarring) can distort vision. Peripheral iridectomy may result in dysphotopsia and shadowing of images. Early cataract may produce visual distortion rather than decreased visual acuity, especially in eyes with early oil-droplet nuclear sclerotic changes or posterior subcapsular changes. The crystalline lens acts as a filter, changing the transmitted spectrum of light, and thereby producing altered perception of shape and color. Cataract extraction may also produce changes in brightness and color perception. After intraocular lens implantation, some patients experience temporal field visual disturbances such as an arc of light or a dark area, termed positive or negative dysphotopsia, respectively; these disturbances are typically transient and resolve spontaneously.
Any disruption of normal foveal architecture (eg, epiretinal membrane, macular edema, macular hole, or subretinal disease) can cause visual distortion. Changes in the position of the retinal photoreceptors can cause various alterations in vision.
Metamorphopsia (perceptual distortion) is characteristic of macular disorders. Patients with these disorders usually report that linear objects appear curved or discontinuous. In patients with intraretinal edema, the retinal elements are often pushed apart, causing micropsia (perceived image shrinkage). Macropsia (perceived image enlargement) is much rarer and can occur if the photoreceptors are pushed together.
The retina may be the source of changes in color perception (dyschromatopsia) associated with drug effects (eg, digoxin-induced yellowing of vision, sildenafil-induced blue tinge). Other changes in color perception may be related to vitamin A deficiency and choroidal or retinal ischemia, which can also lead to persistent afterimages or dazzle (brightness that temporarily confuses vision).
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.