Although the conventional teaching is that lesions affecting the anterior optic radiations (ie, temporal lobe) typically cause formed hallucinations while more posterior lesions (ie, parietal and occipital lobes) typically produce unformed hallucinations, there are many exceptions. In rare cases, lesions involving the mesencephalon may cause formed hallucinations (peduncular hallucinosis) that can be constant; they are usually associated with an inverted sleep-wake cycle. Associated symptoms may occur if adjacent midbrain structures are involved.
Interesting cortical phenomena may occur with disorders of both the dominant and, more frequently, the nondominant temporal, parietal, and occipital lobes. Palinopsia is visual perseveration after removal of the original stimulus (multiple afterimages) and can be divided into two categories: illusory and hallucinatory. Illusory palinopsia is triggered by contrast and/or motion; the visual afterimage or trails appear in the same location as the original stimulus. Migraine, hallucinogenic drugs such as lysergic acid diethylamide (LSD), and medications (eg, clomiphene, nefazodone, topiramate, trazodone, and zonisamide) can cause illusory palinopsia. In contrast, hallucinatory palinopsia is not affected by environmental conditions, and the perseverated image can occur in a different location than the original stimulus. When the afterimages are associated with homonymous hemianopia and appear in the blind hemifield, a posterior cortical lesion is usually present.
Gersztenkorn D, Lee AG. Palinopsia revamped: a systematic review of the literature. Surv Ophthalmol. 2015;60(1):1–35.
Yun SH, Lavin PJ, Schatz MP, Lesser RL. Topiramate-induced palinopsia: a case series and review of the literature. J Neuroophthalmol. 2015;35(2):148–151.
Temporal, parietal, and occipital lobe lesions
Temporal lobe lesions most often produce olfactory and gustatory hallucinations; however, they can also cause complex, formed hallucinations in either the ipsilateral or the contralateral visual field. Epilepsy is the most common cause of hallucinations from this region. An accompanying visual aura implies that the seizure began focally.
In the parietal lobe, hallucinations may be either formed or unformed, whereas occipital lobe disorders commonly cause unformed hallucinations. Patients with occipital lobe lesions may describe white or colored flashes of light, kaleidoscopic colors, moving discs, flickering, or a hexagonal array (eg, chicken-wire or honeycomb patterns). A complete whiteout of vision suggests bilateral occipital lobe ischemia.
Hallucinations within a homonymous hemianopia or quadrantanopia are generally complex; they may be static or move throughout the visual field.
Celesia GG. Positive spontaneous visual phenomena. In: Celesia GG, ed. Disorders of Visual Processing. Edinburgh: Elsevier; 2005:353–370. Handbook of Clinical Neurophysiology; vol 5.
Abnormal excitatory activity in the cerebral cortex, followed by a wave of depressed neuronal function (cortical spreading depression), is believed to cause the visual phenomena of migraine. In migraine with aura, the binocular hemifield visual phenomena can last 5–60 minutes and are always followed by a headache (see Chapter 12). In addition to the classic fortification spectra (teichopsia), false sensation of motion (scintillations), and false perception of colors, patients may also experience “Alice in Wonderland effect” (micropsia and/or macropsia), formed or unformed images, or visual distortion. Common patient descriptions of visual phenomena experienced during a migraine with aura include heat waves, cracked glass, kaleidoscopic vision, or fragmented vision. Patients may also experience typical aura without headache, previously termed acephalgic migraine.
Persistent positive visual phenomena
These visual phenomena are often described as snow, television static, dots (black-and-white or colored), colored pixelated images, or a rainlike pattern affecting the entire visual field. Such visual phenomena usually last for months to years and, while bothersome, rarely interfere with visual function. They are commonly, but not exclusively, noted in patients with a personal or family history of migraine. Prophylactic treatment with migraine medication is usually unsuccessful. Persistent positive visual phenomena are distinct from migraine aura and cannot be attributed to another disorder. Isolated visual snow (with no other neurological or visual symptoms) may be a normal phenomenon.
Schankin CJ, Maniyar FH, Digre KB, Goadsby PJ. ‘Visual snow’—a disorder distinct from persistent migraine aura. Brain. 2014;137(Pt 5):1419–1428.
Sleep-associated visual hallucinations
Hypnagogic hallucinations are vivid perceptual experiences that occur at sleep onset, whereas hypnopompic hallucinations are similar experiences that occur during awakening. Both phenomena are frequently associated with sleep disorders (eg, insomnia and daytime sleepiness) and may be suggestive of narcolepsy.
Charles Bonnet syndrome
Visually impaired patients with preserved cognitive status may experience visual hallucinations, a condition known as Charles Bonnet syndrome. Patients may have formed or unformed hallucinations that are either persistent or abruptly intermittent. Hallucinations may be elementary or highly organized and complex. Patients with this syndrome have a clear sensorium and realize the hallucinations are not real. If the cause of the vision loss is known and there is no homonymous visual field defect, neuroimaging is unnecessary. Common underlying conditions include age-related macular degeneration, glaucoma, diabetic retinopathy, and cerebral infarction. Medical treatment is often unsuccessful in suppressing the hallucinations.
Menon GJ, Rahman I, Menon SJ, Dutton GN. Complex visual hallucinations in the visually impaired: the Charles Bonnet syndrome. Surv Ophthalmol. 2003;48(1):58–72.
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.