Visual hallucinations in migraine with aura
The estimated prevalence of migraine in the general population is 15% to 29%; 31% of those with migraine experience auras.33-35 Approximately 99% of those with migraine auras experience some type of associated visual phenomena.33,36 The pathophysiology of migraine is believed to be related to spreading cortical depression, in which a slowly propagating wave of neuroelectric depolarization travels over the cortex, followed by a depression of normal brain activity. Visual aura is thought to occur due to the resulting changes in cortical activity in the visual cortex; however, the exact electrophysiology of visual migraine aura is not entirely known.37,38 Though most patients with visual migraine aura experience simple visual hallucinations, complex hallucinations have been reported in the (very rare) cases of migraine coma and familial hemiplegic migraine.39
Content and associated signs/symptoms. The most common hallucinated entities reported by patients with migraine with aura are zigzag, flashing/sparkling, black and white curved figure(s) in the center of the visual field, commonly called a scintillating phosphene or scintillating scotoma.36 The perceived entity is often singular and gradually moves from the center to the periphery of the visual field. These visual hallucinations appear in front of all other objects in the visual field and do not interact with the environment or observer, or resemble or morph into any real-world objects, though they may change in contour, size, and color. The scintillating nature of the hallucination often resolves within minutes, usually leaving a scotoma, or area of vision loss, in the area, with resolution back to baseline vision within 1 hour. The straight, zigzag, and usually black-and-white nature of the scintillating phosphenes of migraine are in notable contrast to the colorful, often circular visual hallucinations experienced in patients with occipital lobe seizures.25
Visual hallucinations in peduncular hallucinosis
Peduncular hallucinosis is a syndrome of predominantly dreamlike visual hallucinations that occurs in the setting of lesions in the midbrain and/or thalamus.40 A recent review of the lesion etiology found that approximately 63% are caused by focal infarction and approximately 15% are caused by mass lesions; subarachnoid hemorrhage, intracerebral hemorrhage, and demyelination cause approximately 5% of cases each.40 Additionally, a review of the affected brainstem anatomy showed almost all lesions were found in the paramedian reticular formations of the midbrain and pons, with the vast majority of lesions affecting or adjacent to the oculomotor and raphe nuclei of the midbrain.39 Due to the commonly involved visual pathway, some researchers have suggested these hallucinations may be the result of a release phenomenon.39
Content and associated signs/symptoms. The visual hallucinations of peduncular hallucinosis usually start 1 to 5 days after the causal lesion forms, last several minutes to hours, and most stop after 1 to 3 weeks; however, cases of hallucinations lasting for years have been reported. These hallucinations have a diurnal pattern of usually appearing while the patient is resting in the evening and/or preparing for sleep. The characteristics of visual hallucinations vary widely from simple distortions in how real objects appear to colorful and vivid hallucinated events and people who can interact with the observer. The content of the visual hallucinations often changes in nature during the hallucination, or from one hallucination to the next. The hallucinated entities can be worldly or extraterrestrial. Once these patients fall asleep, they often have equally vivid and unusual dreams, with content similar to their visual hallucinations. Due to the anatomical involvement of the nigrostriatal pathway and oculomotor nuclei, co-occurring parkinsonism, ataxia, and oculomotor nerve palsy are common and can be a key clinical feature in establishing the diagnosis. Though patients with peduncular hallucinations commonly fear their hallucinations, they often eventually gain insight, which eases their anxiety.39
Other causes
Visual hallucinations in visual impairment
Visual hallucinations are a diagnostic requirement for Charles Bonnet syndrome, in which individuals with vision loss experience visual hallucinations in the corresponding field of vision loss.41 A lesion at any point in the visual pathway that produces visual loss can lead to Charles Bonnet syndrome; however, age-related macular degeneration is the most common cause.42 The hallucinations of Charles Bonnet syndrome are believed to be a release phenomenon, given the defective visual pathway and resultant dysfunction in visual processing. The prevalence of Charles Bonnet syndrome ranges widely by study. Larger studies report a prevalence of 11% to 27% in patients with age-related macular degeneration, depending on the severity of vision loss.43,44 Because there are many causes of Charles Bonnet syndrome, and because a recent study found that only 15% of patients with this syndrome told their eye care clinician and that 21% had not reported their hallucinatory symptoms to anyone, the true prevalence is unknown.42 Though the onset of visual hallucinations correlates with the onset of vision loss, there appears to be no association between the nature or complexity of the hallucinations and the severity or progression of the patient’s vision loss.45 Some studies have reported either the onset of or a higher frequency of visual hallucinations at a time of visual recovery (for example, treatment or exudative age-related macular degeneration), which suggests that hallucinations may be triggered by fluctuations in visual acuity.46,47 Additional risk factors for experiencing visual hallucinations in the setting of visual pathway deficit include a history of stroke, social isolation, poor cognitive function, poor lighting, and age ≥65.
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