Primary neurologic causes
Visual hallucinations in neurodegenerative diseases
Patients with neurodegenerative diseases such as Parkinson disease (PD), dementia with Lewy bodies (DLB), or Creutzfeldt-Jakob disease (CJD) commonly experience hallucinations as a feature of their condition. However, the true cause of these hallucinations often cannot be directly attributed to any specific pathophysiology because these patients often have multiple coexisting risk factors, such as advanced age, major depressive disorder, use of neuroactive medications, and co-occurring somatic illness. Though the prevalence of visual hallucinations varies widely between studies, with 15% to 40% reported in patients with PD, the prevalence roughly doubles in patients with PD-associated dementia (30% to 60%), and is reported by 60% to 90% of those with DLB.16-18 Hallucinations are generally thought to be less common in Alzheimer disease; such patients most commonly experience visual hallucinations, although the reported prevalence ranges widely (4% to 59%).19,20 Notably, similarly to hallucinations experienced in patients with delirium, and in contrast to those with psychosis, visual hallucinations are more common than auditory hallucinations in neurodegenerative diseases.20 Hallucinations are not common in individuals with CJD but are a key defining feature of the Heidenhain variant of CJD, which makes up approximately 5% of cases.21
Content, perceived physical properties, and reaction. Similar to the visual hallucinations experienced by patients with psychosis or delirium, those experienced in patients with PD, DLB, or CJD are often complex, most commonly of people, followed by animals and objects. The presence of “passage hallucinations”—in which a person or animal is seen in a patient’s peripheral vision, but passes out of their visual field before the entity can be directly visualized—is common.20 Those with PD also commonly have visual hallucinations in which the form of an object appears distorted (dysmorphopsia) or the color of an object appears distorted (metachromatopsia), though these would better be classified as illusions because a real object is being perceived with distortion.22
Hallucinations are more common in the evening and at night. “Presence hallucinations” are a common type of hallucination that cannot be directly related to a specific sensory modality such as vision, though they are commonly described by patients with PD as a seen or perceived image (usually a person) that is not directly in the individual’s visual field.17 These presence hallucinations are often described as being behind the patient or in a visualized scene of what was about to happen. Before developing the dementia and myoclonus also seen in sporadic CJD, patients with the Heidenhain variant of CJD describe illusions such as metachromatopsia, dysmorphia, and micropsia that eventually develop into frank visual hallucinations, which have been poorly reported in medical literature.22,23 There are no generalizable trends in the temporal nature of visual hallucinations in patients with neurodegenerative diseases. In most cases of visual hallucinations in patients with PD and dementia, insight relating to the perception varies widely based on the patient’s cognitive status. Subsequently, patients’ reactions to the hallucinations also vary widely.
Visual hallucinations in epileptic seizures
Occipital lobe epilepsies represent 1% to 4.6% of all epilepsies; however, these represent 20% to 30% of benign childhood partial epilepsies.24,25 These are commonly associated with various types of visual hallucinations depending upon the location of the seizure onset within the occipital lobe. These are referred to as visual auras.26 Visual auras are classified into simple visual hallucinations, complex visual hallucinations, visual illusions, and ictal amaurosis (hemifield blindness or complete blindness).
Content, perceived physical properties, and reaction. Simple visual hallucinations are often described as brief, stereotypical flashing lights of various shapes and colors. These images may flicker, change shape, or take on a geometric or irregular pattern. Appearances can be repetitive and stereotyped, are often reported as moving horizontally from the periphery to the center of the visual field, and can spread to the entire visual field. Most often, these hallucinations occur for 5 to 30 seconds, and have no discernible provoking factors. Complex visual hallucinations consist of formed images of animals, people, or elaborate scenes. These are believed to reflect activation of a larger area of cortex in the temporo-parieto-occipital region, which is the visual association cortex. Very rarely, occipital lobe seizures can manifest with ictal amaurosis.24
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