(6) They are much more common in the elderly, and (7) they generally occur in the setting of visual loss, which has been gradual (most commonly, cataract formation).336 The Charles Bonnet syndrome is benign and usually self-limited; however, some patients may continue to hallucinate for years with little response to anticonvulsants or other medications.

Although this condition is classically seen in elderly patients, an increasing number of reports indicate that it may also occur in children with bilateral anterior visual pathway disease.269,353,422 Because children are often reluctant to report visual hallucinations, this condition is probably frequently overlooked. Medical treatment is rarely effective, but children with this disorder benefit from simple reassurance.

The formed and unformed visual hallucinations that occur in the hemianopic field of adults with occipitovascular disease may be of the irritative or release variety.60 Children with congenital hemianopic defects do not complain of similar hallucinations, probably because they have never experienced vision in the affected hemifield and therefore do not have the visual association area connections that become deafferented in adults.

Lilliputian Hallucinations

Lilliputian hallucinations refers to the perception of very small, perfectly formed figures, usually active and mobile, gaily colored, and pleasant to look upon.351 Despite their unique character, they seem to be a nonspecific symptom, because they have been reported in various forms of intoxication, visual deprivation, acute infection, epilepsy, and CNS tumor and infarction. Lilliputian hallucinations have been reported in children with scarlet fever and measles.351

Palinopsia

Palinopsia is a rare symptom in which there is visual perseveration beyond the physiological afterimage.69,175,227 It is experienced as a persistence or reappearance of portions of a recently viewed scene. Some cases consist of freeze-frame or stroboscopic images of a moving stimulus.175 Palinopsia is usually accompanied by other visual hallucinations or a hemianopia. When a visual field defect is present, symptoms usually involve the hemianopic field.274 It is rare for palinopsia to occur as an isolated visual phenomenon. Palinopsia has been noted predominantly with vascular or neoplastic lesions of the posterior portions of the cerebral hemispheres, most of which have been right-sided.29 Less commonly, it has been reported in association with seizures, hallucinogenic drug use, antidepressant therapy (Trazadone), encephalopathy, and migraine.175 Palinopsia occasionally responds to anticonvulsant therapy.43 Although a number of elaborate theories

have been advanced to explain the existence of palinopsia (enhancement of the normal physiological afterimage, release hallucination, sensory seizure, involuntary visual memory), its neuropharmacological basis remains unclear.

Peduncular Hallucinosis

Peduncular hallucinosis is a rare phenomenon in which vascular disease of the cerebral peduncles or associated midbrain structures is associated with moving, intensely colorful visual imagery that changes in a kaleidoscopic fashion, is nonthreatening, and is often pleasurable to the patient.123,419 The hallucinations may consist of geometric patterns and designs or as more elaborate pictures, such as landscapes, country and mountain scenes, flowers, birds, animals, or human beings.287 Although formed visual hallucinations do not generally have strong localizing value, peduncular hallucinosis is usually associated with other neuro-ophthalmo- logic signs of midbrain dysfunction, allowing clinical localization of the lesion.123 Autopsy studies and neuroimaging have confirmed lesions intrinsic to or compressing the

mid-brain.50,91,104,123,283,395

Peduncular hallucinosis is believed to be a special form of release hallucination caused by diminished activity in the reticular activating system and other ascending brainstem pathways, leading to abnormal activity in the temporal lobes. Sleep disturbances often coexist, and it has been suggested that peduncular hallucinosis may be due to a dissociation of the sleep mechanism, causing dream activity to be released while consciousness remains normal or nearly so.287

Hypnagogic Hallucinations

Hypnagogic hallucinations are fragments of rapid eye movement (REM) sleep that occur during entry into sleep. They may be visual or auditory. The visual ones consist of vivid scenes, objects, animals, or people that may be frightening to the child or elementary hallucinations, such as flashes or patterns.149,150 Hypnagogic visual hallucinations may occur in normal children,419 but the child should be evaluated for narcolepsy if there is also a history of sleep attacks, cataplexy, or sleep paralysis.149,150

Posterior Reversible Encephalopathy Syndrome

Posterior reversible encephalopathy syndrome (PRES) is becoming an increasingly recognized etiology of transient visual symptoms and seizures of occipital onset in children. Characteristic symptoms include visual disturbances, altered mentation, seizures, headache, and vomiting, transient cortical

blindness, and complex visual hallucinations during the recovery period.430 T2-weighted MR imaging demonstrates focal, symmetrical areas of increased signal intensity involving both gray and white matter, with no significant mass effect or cortical effacement. Vision normalizes and the associated MR abnormalities resolve following successful lowering of blood pressure, suggesting that the MR abnormalities are caused by extravasation of fluid and protein across the blood-brain barrier rather than infarction. These signal abnormalities reflect the presence of vasogenic edema that predominantly affects the posterior cerebral hemispheres. Most reported pediatric cases have occurred in the presence of known hypertension, renal disease, or use of immunosuppressive agents, when the suspicion for this diagnosis may be high.

When symptoms of PRES occur de novo in an otherwise well child, diagnosis may be more difficult. The diagnosis of PRES should be considered in any child presenting with new-onset seizures, encephalopathy, and visual symptoms.168 Visual symptoms include blurred vision and hallucinatory activity related to occipital seizures.430 The pathophysiology of PRES involves vasogenic rather than cytotoxic edema. Cerebral autoregulation maintains a relatively constant cerebral blood flow over a range of mean arterial blood pressure, thus protecting the brain from acute changes in blood pressure. However, at high mean arterial pressures, autoregulation fails, leading to arteriolar vasodilation and endothelial dysfunction; this, in turn, results in disruption of the blood-brain barrier and capillary leakage. The white matter is predominantly affected, because as it is less tightly packed than the cortex. The posterior brain appears to be preferentially affected because of diminished sympathetic innervation.

Children with acute onset of hypertension are probably more at risk than those with chronic hypertension, because they have not yet developed adaptive vascular changes. PRES can also be seen with use of immunosuppressive drugs. The pathophysiology in this setting is less clear, but may involve damage to the vascular endothelium, resulting in vasospasm, reduced tissue perfusion, activation of the coagulation cascade, and extravasation of fluid. Although abnormalities may be seen on CT scanning, fluid-attenu- ated inversion recovery (FLAIR) MR sequences are much better at showing the extent of the hyperintense lesions and the degree of cortical involvement. The calcarine and paramedian part of the occipital lobe are classically spared, helping to distinguish PRES from bilateral posterior cerebral artery infarction.

Neurodegenerative Disease

As in adults with Alzheimer’s disease, transient visual disturbances may occur as early symptoms in a variety of neu-

rodegenerative diseases that eventually involve the optic nerves or higher cortical centers. Examples include episodic visual loss in the early stages of ornithine decarboxylase deficiency (a hyperammonemia syndrome),364 transient homonymous hemianopia in subacute sclerosing panencephalitis,194 and formed visual hallucinations in juvenile ceroid lipofuscinosis.215,367,440

Multiple Sclerosis

Transient visual disturbances may occur in children with multiple sclerosis. Recognized causes of transient visual disturbances include mild or subclinical episodes of optic neuritis, Uhthoff symptom, phosphenes induced by ocular motion, and the Pulfrich phenomenon. (The Pulfrich effect is a well-known visual illusion in which a pendulum swinging in a frontal plane in front of a subject is perceived as moving in an oval trajectory, with the plane of the oval being parallel to the floor. It is noted most commonly in patients who have recovered from optic neuritis in one eye.)420

Schizophrenia

Schizophrenic hallucinations are most often auditory in nature, but they may also be visual.419 The visual hallucinations are usually of frightening objects, such as skeletons or ghosts, or may represent a recently deceased relative or friend.114 Schizophrenic hallucinations are not influenced by eye closure or opening, as opposed to drug-induced visual disturbances that tend to exacerbate with the eyes closed.419 Visual hallucinations have also been described in children with reactive psychosis, depressive syndromes, and organic brain syndromes114 and are reported most frequently in psychoses of late childhood.96 Differential diagnosis from temporal lobe epilepsy is sometimes difficult due to overlap in symptomatology.

Children with less severe psychiatric disorders, such as emotional and behavior problems, also experience hallucinations in the form of fantasies and “pretend companions” that may possibly aid them in coping with their situational disturbances.96 Such children do not appear to be at increased risk for psychosis, depressive illness, organic brain damage, or other psychiatric disorders.115

Hallucinogenic Drug Use

While LSD, mescaline, and psilocybin ingestion can all produce visual hallucinations, the hallucinatory phenomena associated with LSD have been studied most extensively. Ingestion of LSD can produce several organic mental disorders, causing hallucinations. The first is an acute dose-related