The hallucinations were not vague flickers or shapes at the edge of vision. A 56-year-old Japanese woman saw and heard the King of Hell. She developed delusions of jealousy, then delusions of possession, accompanied by impulsive and violent behavior. Her thinking declined.
The cause was not a psychiatric illness. It was a small blood vessel malformation in her right thalamus, discovered incidentally 18 years earlier and left alone because surgery had not been feasible at the time.
The report, in The Primary Care Companion for CNS Disorders, was written by Mizuki Kudo, Yuko Daa, and Hirofumi Hirakawa of the Department of Neuropsychiatry at Oita University Faculty of Medicine. Their diagnosis was peduncular hallucinosis with a thalamic cavernous angioma, a syndrome in which brain injury produces hallucinations elaborate enough to be mistaken for psychosis.
Hallucinations with Plot and Characters
Peduncular hallucinosis is unusual among neurological hallucination syndromes because of how detailed the content is. The case report describes it as a form of complex visual hallucination that includes vivid images of people or animals, reported in association with thalamic lesions.
That is a meaningful distinction. Damage to the visual pathways more often produces simple phenomena: flashes, geometric patterns, distortions. Complex hallucinations, populated with recognizable figures doing recognizable things, suggest something different is happening. The brain is not misreading noise. It is generating content.
The syndrome takes its name from the cerebral peduncles in the midbrain, though later reports established that thalamic lesions can produce it as well.
What the Scans Showed
The imaging findings are worth walking through, because they trace how a lesion that sat quietly for nearly two decades eventually rewrote her perception.
MRI showed a punctate dark lesion with surrounding brightness on one sequence, consistent with fluid buildup, extending from the right putamen into the thalamus. On another sequence, the lesion itself appeared dark, a signature of old bleeding. There was also brain shrinkage, particularly in the temporal lobes. MR angiography confirmed the vascular lesion in the right thalamus.
A SPECT scan measuring cerebral blood flow found reduced perfusion in both hemispheres, at 32.2 milliliters per 100 grams per minute on the right and 34.2 on the left, with marked underperfusion in the right temporal and right occipital lobes.
Her symptoms had built gradually. Somnolence appeared four years before admission. Left-sided weakness followed a year later. The hallucinations began at age 54, and the delusions of possession at 55.
The authors propose an anatomical explanation. The thalamus contains relay stations for sensory traffic, one for vision and one for hearing, each wired to its corresponding cortex through bundles called thalamic radiations. They suggest that damage to the pathway running toward the right occipital lobe accounts for the visual hallucinations, and damage to the fibers carrying auditory signals toward the right temporal lobe accounts for the auditory ones. That is the team's interpretation of a single case rather than an established mechanism, and they call for further study.
Common Lesions, Uncommon Consequences
Cavernous malformations, also called cavernomas or cavernous angiomas, are clusters of abnormal thin-walled capillaries lacking normal vessel structure and containing no functioning brain tissue.
They are more common than most people realize. Estimates from autopsy studies and imaging series put prevalence between 0.2 and 0.9 percent of the general population. A population study of 2,715 older adults scanned for nonclinical reasons found cavernous malformations in 0.46 percent, but only one participant, 0.037 percent of the group, developed symptoms from one during the study period.
That gap is the whole story. Most people with a cavernoma never know it, and between roughly half and 80 percent cause no symptoms at all. According to StatPearls, a reference hosted by the National Library of Medicine, these lesions may be silent or may cause seizures, headaches, focal neurological deficits, or symptomatic bleeding. Up to 20 percent run in families, and up to 20 percent sit in the brainstem. A prior bleed raises the risk of another.
Location determines everything about what a bleed does. A cavernoma in a quiet region can bleed without producing a single symptom. One in the thalamus, sitting at the junction where sensory information is sorted and forwarded, can do what happened here.
Distinguishing a Brain Lesion from a Psychiatric Illness
She was admitted to a psychiatry department. Hallucinations, delusions of jealousy and possession, and violent behavior are the presentation that gets someone there.
What separated this case was history and imaging. She had a known vascular lesion. She had developed somnolence and one-sided weakness in the years before the psychiatric symptoms appeared, which is not how primary psychotic disorders behave. Neurological signs preceding psychiatric ones is a pattern that points away from a primary psychiatric diagnosis.
Treatment reflected the hybrid nature of the problem. She was started on aripiprazole at 6 milligrams daily, which partially improved the hallucinations and delusions, and she was discharged. Partially is the operative word. Antipsychotic medication addressed the output, not the lesion generating it.
For readers, the useful thought is narrow rather than alarming. Cavernous malformations are common and almost always uneventful. But new hallucinations arriving alongside neurological changes such as weakness, unusual sleepiness, or a decline in thinking is a combination that warrants medical evaluation rather than a psychiatric assumption.
Key Questions Answered
What is peduncular hallucinosis?
A syndrome of complex visual hallucinations, often including detailed images of people or animals, linked to lesions in the midbrain or thalamus.
What caused it in this case?
A cavernous angioma in the right thalamus that had been known about for 18 years and had bled, producing fluid buildup and changes in nearby tissue.
How is it different from psychiatric hallucinations?
The content is often unusually elaborate, and it typically appears alongside neurological findings such as weakness or altered alertness.
How common are cavernous malformations?
Prevalence estimates run from roughly 0.2 to 0.9 percent of the population, but symptomatic cases are far rarer, in one study 0.037 percent.
Do they always need treatment?
No. Most are found incidentally and never cause problems. Management depends on location, symptoms, and whether the lesion has bled.
Did medication resolve her symptoms?
Aripiprazole partially improved the hallucinations and delusions. The report describes partial improvement, not resolution.
