These are visual images that the patient claims to see but that other observers do not. Visual hallucinations can be characterized as unformed (e.g., dots, flashes, zig-zags) or formed (actual objects or people). They may occur in patients with damage to the afferent visual pathways, sensory deprivation, migraine, seizures, brain stem lesions, drug toxicity or abuse, and psychiatric illnesses. y
Patients with visual loss due to a lesion anywhere within the afferent visual pathways may complain of hallucinations (positive visual phenomena) within the defective field.y This clinical scenario has been termed Charles Bonnet's syndrome. The complexity of the visual phenomenon is nonlocalizing. Patients may find these visual images pleasant or disturbing and may be embarrassed to volunteer their occurrence to their physician. Physiologically, they may represent visual phenomena released by lack of inhibitory input. y This mechanism may also explain visual hallucinations associated with severe sensory deprivation in prisoners of war.
Migraine visual aura may include positive phenomena such as stars, sparks, flashes, simple geometrical forms, enlarging scintillating scotomas, and fortification spectra.U Typically, auras develop over more than 4 minutes, last less than 1 hour, and precede or rarely accompany or follow the headache. y Uncommonly the positive phenomena may persist in patients with or without "migrainous infarction." y
Epileptic visual hallucinations of occipital lobe origin may be very similar to migraine aura, and clinically the two disorders may be difficult to distinguish. y Secondary generalization, loss of consciousness, and ictal occipital discharges on electroencephalogram (EEG) may support a diagnosis of epilepsy, whereas transient visual loss, headache, and family history are more suggestive of migraine. Epileptiform discharges from other parts of the brain (the temporal lobe, for instance) may also produce positive visual phenomena. Other clinical features, however (autonomic, for example), are expected. As with release hallucinations, the complexity of the visual phenomena generally does not help localize the seizure focus.
Peduncular hallucinations, consisting of vivid and life-like visual images of concrete objects, are rare sequelae of ventral midbrain injury. The hallucinations are frequently accompanied by sleep and cognitive disturbances. One clinicopathological study y suggested that their expression requires bilateral destruction of the medial substantia nigra pars reticulata.
Illicit drug use (cocaine, lysergic acid diethylamide [LSD], marijuana), medications (digoxin, anticholinergics, and dopaminergics), and parasympatholytic eye drops (atropine) may also be responsible for visual hallucinations. A psychotic psychiatric disorder is suggested when complex visual hallucinations are accompanied by occasional auditory hallucinations. y Normal people may experience both formed and unformed visual images upon wakening (hypnopompic) or upon going to sleep (hypnagogic). A number of degenerative neurological diseases (Parkinson's disease, progressive supranuclear palsy), in the setting of medication use (levodopa, dopamine agonists, monoamine oxidase [MAO] inhibitors, and anticholinergics) can make visual hallucinations a common clinical disorder. Other disorders such as diffuse Lewy body disease may have visual hallucinations as a primary symptom as well.
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