Peduncular hallucinosis (PH) represents complex, realistic visual hallucinations secondary to lesions of the midbrain and/or the thalamus.
Hallucinations are defined by the American Psychiatric Association as “perception-like experience with the clarity and impact of true perception, but without the external stimulation of the relevant sensory organ”. In contrast, illusions are misinterpretations of actual external stimuli.
Peduncular hallucinosis was first described by Lhermitte in 1922. A 72-year-old woman presented with stroke localized to the cerebellar peduncle and pons. In addition to neurological deficits, she developed sleep disturbances and new onset of hallucinations. She initially described her hallucinations as vivid, strange-looking animals (also known as zoopsia) walking around the room, with increase in frequency around dusk. Her hallucinations changed overtime, transitioning to humans in strange clothing. However, she had clear insight during the whole process that these images were not real. In addition, the hallucinations did not cause any emotional distress nor did she experience associated auditory hallucinations.
The term “l’hallucinose pédonculaire”, translated to “peduncular hallucinosis”, was coined by Van Bogaert in 1927. Currently, this term does not only refer to lesions in the peduncle but also includes those in the midbrain and thalamus.
Peduncular hallucinosis are typically complex visual with realistic, dynamic scenes, and often include familiar people or places. Benke’s cohort of five patients experienced recurring hallucinations, with subjects performing stereotypic behaviors every couple of weeks that lasted several minutes. Patients have difficulty distinguishing their hallucinations from reality, though they often have insight into the hallucinations. These hallucinations are not fanatical and are so realistic that most will start interacting, either verbally or physically, with the people or environment of their hallucinations.
Peduncular hallucinosis can occur at any time of the day but is more frequent around nighttime or in dimly lit areas. In between hallucinations, patients have intact memory and are able to consistently describe their hallucinations accurately, despite poor cognitive function in other aspects of their life. The initial episode usually occurs acutely after an ischemic or hemorrhagic event of the midbrain, thalamus or pons, with posterior circulation infarction as the most common cause.
Frequent concomitant clinical findings include associated brainstem symptoms and signs such as ocular motor impairment, sleep and arousal disturbances, cerebellar disturbances (e.g., ataxia, dysarthria, and dysmetria). As opposed to the release hallucinations of Charles Bonnet syndrome, peduncular hallucinosis patients have intact vision and visual fields. After their neurological event, patient can have impaired short-term memory recall, behavioral and cognitive deficiencies seen on mini mental status exam. However, there have been case reports of peduncular hallucinosis without the classic symptoms of ocular movement abnormalities or lesion localization to the brainstem. Hallucinations were the only presenting symptom and neuroimaging confirmed diagnosis. Peduncular hallucinosis tend to improve over several months and can cease more rapidly with resolution of the primary lesion. They can occur in conjunction with other neurocognitive disorders such as Parkinson disease and sleep-wake cycle disorders. However, there is not enough evidence to support or deny any association for increase in development of dementia or mortality. As these hallucinations are so realistic, patients tend to question their reality and faulty awareness can cause distress, affecting other aspects of their life.
Ischemia of the posterior circulation is the most common cause of PH, though there are reports of hemorrhage, brain masses, demyelination, vasospasm, encephalitis, and venous congestion. The exact pathophysiology of PH is not known. Lhermitte thought PH was caused by lesions of the protuberantial calotte bulb, requiring damage to cranial nerves III, IV, and VI, and dysregulation of the brainstem, causing subcortical areas responsible for dreaming to become active while awake.
Imbalance in the brainstem between excitatory cholinergic (pontine tegmentum), inhibitory serotonergic (dorsal raphe nuclei), and other neurotransmitters disrupt the thalamic thresholds of sensory input. Loss of inhibitory serotonergic input leads to overexcitation of the dorsal lateral geniculate nucleus (LGN) of the thalamus, resulting in visual, auditory, and/or tactile hallucinations. Conversely, disruption to the thalamus, which is involved in higher order visual processing can lead to PH. The dorsal raphe nucleus is also responsible for the sleep-wake cycle, REM and non-REM sleep, which could explain why patients with lesions can experience nighttime wakefulness and hypersomnolence during the day. Another theory behind PH includes overactivation of the reticular activating system and patients entering REM sleep while their consciousness is intact.
The diagnosis of peduncular hallucinosis are based on clinical history including an initial CNS insult, sleep pattern changes and typical vivid hallucinations. Such a diagnosis shall not overlap with any DSM V diagnosis.
Lesions implicated in peduncular hallucinosis include:
- Basal ganglia: striatum and globus pallidus
- Pineal meningioma
Peduncular hallucinosis is a diagnosis of exclusion which necessitates ruling out other causes of visual hallucinations:
- Charles Bonnet syndrome (CBS): Onset of visual hallucinations occur in the setting of progressive blindness such as macular degeneration. Hallucinations are complex, visual, and non-threatening, occurring in poorly-lit settings. They tend to disappear with eye closure in CBS, in comparison to PH which can worsen. Patients preserve intact insight.
- Lewy body dementia: Hallucinations are complex, visual and non-threatening with patients maintaining intact insight of its artificial nature. Patients have extrapyramidal symptoms such as akathisia (irrepressible restlessness), dystonia (involuntary muscle contraction), Parkinson-like symptoms (e.g. rigidity), and tremor. They also have chronic, progressive neurocognitive decline and impaired spatial visuospatial performance.
- Hypnogogic/hypnopompic hallucinations: Hallucinations while falling asleep (hypnogogic) and during waking (hypnopompic) is seen in patients with narcolepsy. Narcolepsy is characterized by daytime somnolence, sleep attacks and cataplexy (hypotonia induced by emotions). Patients enter rapid eye movement (REM) sleep inappropriately and have visual, auditory, or tactile hallucinations while still awake with intact consciousness.
- Psychiatric disorders: e.g. schizophrenia
- Drug use: particularly LSD
- Delirium, seizures, migraines
- Brain tumors
Peduncular hallucinosis is a diagnosis of exclusion. Clinicians must complete a thorough workup to rule out other possible causes of visual hallucinations such as psychiatric disorders, drug use, delirium, etc. Magnetic resonance imaging (MRI) can be useful in identifying the brain stem lesion and help determine the underlying etiology. If there is a treatable etiology, hallucinations can resolve with rectification; for example, resection of the mass can be curative in patients with juvenile pilocytic astrocytoma. Visual hallucinations can sometimes resolve spontaneously over the course of weeks to months.
There has been anecdotal success of atypical antipsychotics usage, including olanzapine, risperidone, and quetiapine, to help alleviate the emotional turmoil and cause remission of hallucinations. It is thought that atypical antipsychotics block dopamine and modulate serotonin activity, correcting disinhibition of the LGN. However, it is important to note that atypical antipsychotics carry a black box warning of increased mortality in elderly patients with dementia-associated psychosis. Similarly, antiepileptics such as clonazepam or carbamazepine have been reported helpful in several cases of peduncular hallucinosis.
- American Psychiatric Association. Glossary of Technical Terms. In: Diagnostic and Statistical Manual of Mental Disorders. American Psychiatric Association; 2013. doi:10.1176/appi.books.9780890425596.glossaryoftechnicalterms
- Lhermitte J. Syndrome de la calotte du pedoncule cerebral. Les troubles psycho-sensoriels dans les lesions du mesocephale. Rev Neurol. 1922;38:1359-1365. https://ci.nii.ac.jp/naid/10012678114.
- Drouin E, Péréon Y. Peduncular hallucinosis according to Jean Lhermitte. Rev Neurol (Paris). 2019;175(6):377-379. doi:10.1016/j.neurol.2018.11.005
- Kosty JA, Mejia-Munne J, Dossani R, Savardekar A, Guthikonda B. Jacques Jean Lhermitte and the syndrome of peduncular hallucinosis. Neurosurg Focus. 2019;47(3):E9. doi:10.3171/2019.6.FOCUS19342
- Bujarski KA, Sperling MR. Hallucinations. In: Encyclopedia of Human Behavior: Second Edition. Elsevier Inc.; 2012:283-289. doi:10.1016/B978-0-12-375000-6.00187-7
- Benke T. Peduncular hallucinosis: A syndrome of impaired reality monitoring. J Neurol. 2006;253(12):1561-1571. doi:10.1007/s00415-0060-0254-4
- Galetta KM, Prasad S. Historical Trends in the Diagnosis of Peduncular Hallucinosis. J Neuro-Ophthalmology. 2018;38(4):438-441. doi:10.1097/WNO.0000000000000599
- Couse M, Wojtanowicz T, Comeau S, Bota R. Peduncular hallucinosis associated with a pontine cavernoma. Ment Illn. 2018;10(1). doi:10.4081/mi.2018.7586
- De Raykeer RP, Hoertel N, Manetti A, Rene M, Blumenstock Y, Limosin F. A case of chronic peduncular hallucinosis in a 90-year-old woman successfully treated with olanzapine. J Clin Psychopharmacol. 2016;36(3):285-286. doi:10.1097/JCP.0000000000000497
- Lucci B, Marcon GM. Peduncular hallucinosis: A case report. In: Rivista Di Neurobiologia. Vol 36. Ochsner Clinic, L.L.C. and Alton Ochsner Medical Foundation; 1990:575-578.
- Miyazawa T, Fukui S, Otani N, et al. Peduncular hallucinosis due to a pineal meningioma. Case report. J Neurosurg. 2001;95(3):500-502. doi:10.3171/jns.2001.95.3.0500
- Talih FR. A Probable case of peduncular hallucinosis secondary to a cerebral peduncular lesion successfully treated with an atypical antipsychotic. Innov Clin Neurosci. 2013;10(5-6):28-31.
- Vetrugno R, Vella A, Mascalchi M, et al. Peduncular hallucinosis: A polysomnographic and spect study of a patient and efficacy of serotonergic therapy. Sleep Med. 2009;10(10):1158-1160. doi:10.1016/j.sleep.2009.05.005
- Maiuri F, Iaconetta G, Sardo L, Buonamassa S. Peduncular hallucinations associated with large posterior fossa meningiomas. Clin Neurol Neurosurg. 2002;104(1):41-43. doi:10.1016/S0303-8467(01)00184-6