CHAPTER 6
The Patient With Illusions, Hallucinations, and Disorders of Higher Cortical Function
Patients are often reluctant to complain of “seeing things” lest they be dispatched to a psychiatrist. However, many visual hallucinations or illusions are not a harbinger of psychiatric disease but of ocular, optic nerve, or brain pathology. The clinician should ask about these symptoms because patients may be reluctant to volunteer such information. Illusions are false perceptions of visual information that is present in the external environment. For example, a person looking at stationary high-contrast borders may perceive the 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.
The first step in assessing patients with such visual disorders is to ascertain their mental status and the status of their afferent visual pathways. 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. Determination of best-corrected visual acuity, assessment of color perception, and perimetry testing should be performed. The primary goal is to localize the disorder anatomically, which helps 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.
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.
Table 6-1
Ocular Origin
Many illusions have an ocular basis. Such illusions may be classified as those due to optical causes and those due to alterations in photochemical transduction in the retina.
Optical causes
Irregularities of the eye’s refracting elements produce visual distortion and resulting illusions such as ghost images, movement, or multiple images. Any alteration in the tear film (eg, early tear breakup time, dry eye syndrome, or reduced blink rate) or in the corneal surface (eg, keratoconus or scarring) can distort vision. Early cataract may produce visual irregularities and distortion rather than decreased visual acuity alone, especially with early oil-droplet nuclear sclerotic or posterior subcapsular changes. The lens also acts as a filter, changing the spectrum of the light transmitted; thus, spectral changes may be seen as alterations of shape and color. Conversely, cataract extraction produces changes in brightness and color perception. After the implantation of an intraocular lens, some patients experience temporal field visual disturbances (eg, arc of light, dark or dim area). Also known as positive or negative dysphotopsias, such visual disturbances are typically transient and resolve spontaneously.
Retinal causes
Any disruption of normal foveal architecture, such as an epiretinal membrane, macular edema, a 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 such macular disorders; the patient usually reports that linear objects appear curved or discontinuous. 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 citrate–induced blue tinge). Other changes in color perception may be related to hypovitaminosis A and choroidal or retinal ischemia (eg, giant cell arteritis), which can also lead to persistent afterimages.
Optic Nerve Origin
After an episode of demyelinating optic neuritis, a relative conduction delay within the affected optic