Peduncular hallucinosis

Peduncular hallucinosis (PH), or Lhermitte's peduncular hallucinosis, is a rare neurological disorder, which causes vivid visual hallucinations that typically occur in dark environments, and last for several minutes. Unlike some other kinds of hallucinations, those that patients with PH experience are very realistic, and often involve people and environments that are familiar to the affected individuals. Because the content of the hallucinations is never exceptionally bizarre, patients can rarely distinguish between the hallucinations and reality.

In 1922, the French neurologist Jean Lhermitte documented the case of a patient who was experiencing visual hallucinations that were suggestive of localized damage to the midbrain and pons. After other similar case studies were published, this syndrome was labeled "peduncular hallucinosis."

The accumulation of additional cases by Lhermitte and by others influenced academic medical debate about hallucinations and about behavioral neurology.

Lhermitte provided a full account of his work in this area in his book "Les hallucinations: clinique et physiopathologie," which was published in Paris in 1951 by Doin publishing.

Contemporary researchers, with access to new technologies in medical brain imaging, have confirmed the brain localization of these unusual hallucinations.

History
The first documented case of peduncular hallucinosis was by French neurologist and neuropsychiatrist Jean Lhermitte which described a 72-year-old woman’s visual hallucinations of colorfully dressed people and children which occurred at dusk. The hallucinations occurred during normal conscious state and the patient’s neurological signs were associated with those characteristic of an infarct to the midbrain and pons. Von Bogaert, Lhermitte’s colleague, named this type of hallucination “peduncular,” in reference to the cerebral peduncles, as well as to the midbrain and its surroundings. In 1925, Von Bogaert was the first to describe the pathophysiology of peduncular hallucinosis through an autopsy of a patient. His autopsy revealed the infarction of many areas of the brain including the inferolateral red nucleus, superior colliculus, periaqueductal gray, third nerve nucleus, superior cerebellar peduncle, substantia nigra, and pulvinar. Later in 1932, Lhermitte, Levy, and Trelles discovered an association between peduncular hallucinosis and “pigmentary degeneration of the periaqueductal gray and the degeneration of the occulomotor nucleus.” Posterior thalamic lesions were also found to be linked to peduncular hallucinosis by De Morsier. More recently, magnetic resonance imaging (MRI) has been used to localize lesions in the brain characteristic of peduncular hallucinosis. In 1987, the first case of peduncular hallucinosis was reported in which MRI was used to locate a midbrain lesion.

Etiology
Peduncular hallucinosis is attributed to a range of various pathologies such as vascular and infectious midbrain, pontine and thalamic lesions, local subarachnoid hemorrhage, compression by tumors, basilar migraine, basilar vascular hypoplasia, and following regional surgical or angiographic interventions. These pathologies are mainly near the base of the brain and the hallucinations have gone away in patients that had their pathology corrected such as the removal of a tumor. The most commonly reported hallucinations are animals, people or children, scary or deformed faces and heads, landscapes, and people walking in a line.

Symptoms
They are normally colorful, vivid images and occur during wakefulness, and predominately at night. Lilliputian hallucinations (also called Alice in Wonderland syndrome), hallucinations in which people or animals appear smaller than they would be in real life, are common in cases of peduncular hallucinosis. Most patients exhibit abnormal sleep patterns characterized by insomnia and daytime drowsiness. Peduncular hallucinosis has been described as a “release phenomenon” due to damage to the ascending reticular activating system, which is supported by the sleep disturbance characteristic of this syndrome. In most cases, people are aware that the hallucinations are not real. However, some people experience agitation and delusion and mistake their hallucinations for reality.

Lesions
The lesions that disturb brainstem reticular formation or thalamic targets seem to be the source behind peduncular hallucinosis. For example, lesions affecting the dorsal raphe system can lead to hallucinations by preventing ascending inhibition to the dorsal lateral geniculate nucleus (LGN). This inhibition may hyper-excite the LGN, inducing visual hallucinations. Lesions of the retina and areas proximal to it can remove or alter the inputs coming in from the external environment to the visual system. Peduncular hallucinosis therefore might emanate from disturbances in distal portions of the visual system. Lesions in the frontal and temporal lobes can also lead to complex visual hallucinations because the lobes connect to the visual system via the lateral geniculate nucleus and medial pulvinar. Additionally, visual processing and salience can be disrupted by thalamic lesions which affect important structures such as the pulvinar.

The effect lesions on the brainstem have on the ascending reticular activating system (ARAS) has also been hypothesized. It was proposed that since the ARAS plays a role in consciousness and waking, the lesions of the brainstem common to peduncular hallucinosis may “disrupt ARAS impulses from the brainstem reticular formation” and consequently lead to the sleep disturbances characteristic of peduncular hallucinosis. The use of drugs such as Olanzapine may help treat sleep disturbances as it has been found to “improve sleep continuity, sleep quality, and [to] increase slow wave sleep.”

Correlation between other diseases
People diagnosed with Parkinson's disease, narcolepsy-cataplexy syndrome, delirium tremens, Lewy Body Dementia, and temporal lobe epilepsy are more prone to complex visual hallucinations such as peduncular hallucinosis. Peduncular hallucinosis is more common in patients with a long duration of Parkinson's disease and also with a long treatment history, depression, and cognitive impairment. Paranoid delusions are common in these patients even though the hallucinations can occur during clear sensorium.

Differences from other visual hallucinations
Other visual hallucinations tend to stem from psychological disorders. Whereas a person with a psychological disorder thinks their hallucinations are real, people with peduncular hallucinosis normally know that the visual hallucinations they see are not real. Peduncular hallucinations are independent of seizures unlike some other visual hallucinations. The hallucinations also have the quality of being vivid and colorful and also tend to have some sort of animation.

Case Studies
Peduncular hallucinosis is a rare occurrence so what is known comes from case studies.

Peduncular hallucinosis occurring after surgery
In one study, a 68-year-old woman was diagnosed with peduncular hallucinosis after microvascular decompression for trigeminal neuralgia. Her trigeminal neuralgia went away but the procedure led to hallucinations characteristic of peduncular hallucinosis. She was completely conscious at the time of her hallucinations of furniture, stars, and a ranch. The study concluded that peduncular hallucinosis does not require brainstem injury and that a cerebellar lesion could have been the causation.

In another study, a 50-year-old man also underwent microvascular decompression for trigeminal neuralgia. Two days after surgery he developed peduncular hallucinosis which disappeared after two days. It was concluded that the peduncular hallucinosis “was probably related to petrosal vein obliteration and was followed by venous congestion and posterior fossa edema.” Also, to prevent peduncular hallucinosis and other complications due to venous congestion, these types of surgeries should protect the petrosal vein when possible.

An 11-year-old girl in one incident developed peduncular hallucinosis, which included disturbed sleep, after an endoscopic third ventriculostomy was performed on her. A CT scan after the operation revealed bilateral subdural hygroma. The hygroma was drained which improved her consciousness and the peduncular hallucinosis eventually subsided after two weeks.

A 10-year-old girl in another case had a hypothalamic astrocytoma removed which caused a postoperative clot leading to diencephalon and midbrain compression. After placement of an external ventricular drainage device, the girl became more oriented but developed peduncular hallucinosis which consisted of imaginary friends, birds, and an angel. A mild tranquilizer was used to help control the hallucinations which subsided by the end of the week.

Peduncular hallucinosis unrelated to surgery
A 69-year-old woman with a recent history of ataxia and intracranial hypertension experienced vivid hallucinations combined with mental confusion, agitation, and sleep disturbances. An MRI of the brain located aright tentorial meningioma which was removed and diagnosed as fibroblastic meningioma. Post-operatively the patient was given the drugs dexamethasone and carbamazepine and there were no more occurrences of hallucinations. In a similar case, a 44-year-old woman admitted to the hospital displayed ataxia, dysphonia, and papilledema. Following an MRI, a meningioma which had been compressing the midbrain was removed. Agitation and visual hallucinations began the day after surgery and a CT scan revealed moderate brain stem edema. Dexamethasone and carbamazepine were administered to this patient as well, resulting in the disappearance of the agitation and hallucinations within three days. These case studies indicated that besides the lesions commonly associated with peduncular hallucinosis, meningiomas and brain stem compression can also be causes.

A 59-year-old man with a history of hypertension—who had not been taking his antihypertensive medication for three days— experienced visual hallucinations of a waterfall with full awareness of the unreality of his visions. The patient was given aspirin and antihypertensives which caused the hallucinations to disappear as the blood pressure was controlled. An MRI taken two days later found bilateral paramedian pontomesencephalic lesions of ischemic origin. The doctors concluded that the high blood pressure caused the lesions, which consequently led to the symptoms characteristic of peduncular hallucinosis.

In one case, a 76-year-old woman with Parkinson's disease was admitted to the hospital due to acute dyspnea. Her visual hallucinations had been occurring for a year and were non-distressing and without bizarre delusions or auditory hallucinations. She understood that her hallucinations were not real. She displayed decreased short-term recall and attention span. The patient continued having the hallucinations despite discontinuation of the dopamine antagonist she was taking. An MRI of the brain found a hyper-intense focus adjacent to the right thalamus. The patient was given quetiapine which decreased the amount of hallucinations.

A 53-year-old man diagnosed with peduncular hallucinosis reported visual hallucinations of human figures, birds, flowers, and cartoon bunnies. He understood that the hallucinations were not real. His cognitive functioning was normal, except for ataxia and papilledema. A brain CT scan revealed lesions in the cerebral hemispheres and posterior cranial fossa due to a cerebellar metastatic tumor which consequently led to compression of the midbrain. He was treated with mannitol and dexamethasone which led to the disappearance of hallucinations after 4 days.

An 8-year-old girl was admitted to the hospital due to headache, blurred vision, and difficulty walking for a period of 6 months. She had colorful visual hallucinations of snakes, frogs, and birds. She exhibited daytime somnolence and restlessness at night. A CT scan of the brain found a cerebellar pilocytic astrocytoma which was causing posterior midbrain compression. Upon removal of the tumor, the hallucinations subsided.

Treatment
Treatment of any kind of complex visual hallucination requires an understanding of the different pathologies in order to correctly diagnose and treat. Obviously if a person is taking a pro-hallucinogenic medication, then the first step is to stop taking it. Sometimes improvement will occur spontaneously and pharmacotherapy is not necessary. While there is not a lot of evidence of effective pharmacological treatment, antipsychotics and anticonvulsants have been used in some cases to control hallucinations. Since peduncular hallucinosis occurs due to an excess of serotonin, modern antipsychotics are used to block both dopamine and serotonin receptors, preventing the overstimulation of the lateral geniculate nucleus. The drug generically called carbamazepine increases GABA which prevents the LGN from firing, thereby increasing the inhibition of the LGN. Regular antipsychotics as well as antidepressants can also be helpful in reducing or eliminating peduncular hallucinosis.

More invasive treatments include corrective surgery such as cataract surgery, laser photocoagulation of the retina, and use of optical correcting devices. Tumor removal can also help to relieve compression in the brain, which consequently can decrease or eliminate peduncular hallucinosis. Some hallucinations may be due to underlying cardiovascular disease so in these cases the appropriate treatment includes control of hypertension and diabetes. As described, the type of treatment varies widely depending on the causation behind the complex visual hallucinations.