To the Editor: Peduncular hallucinosis was first described by Lhermitt as odd visual hallucinations stemming from lesions affecting the midbrain, pons, and thalamus.1 We report on an elderly female with Parkinson’s disease who possibly exhibits this rare form of perceptual disturbance.
Our patient is a 76-year-old Caucasian woman with a 3-year history of Parkinson’s disease, presenting with acute dyspnea which resulted in her hospitalization. She reported visual hallucinations of "mechanical bugs walking around the hospital" and "two people fighting" in the corner of her room. The visual hallucinations began 1 year prior to her hospitalization and were nondistressing. She had insight into the visual hallucinations. Our patient was treated with carbidopa--dopa until 4 months prior to admission. The patient’s mental status exam was notable for bradyphasia, psychomotor retardation, and limited range of affect. She had signs of a pill rolling tremor on her left hand. She was otherwise alert and oriented times 3 with no fluctuations in consciousness. She reports visual hallucinations in the absence of bizarre delusions or auditory hallucinations. Cognitive examination was notable for deficits in short-term recall (which improved with cues) and attention span.
An MRI scan of the brain revealed a hyper-intense focus adjacent to the right thalamus consistent with an old lacunar infarct. An EEG revealed no eliptiform discharges or significant slowing.
Our patient was treated with quetiapine for the visual hallucinations, with a noted decrease in the number of hallucinations for the remainder of the hospital stay.
Visual hallucinations often suggest a wide range of etiologies. Hallucinations and delusions occur in up to 40% of patients with Parkinson’s disease. Visual hallucinations are typically associated as a side effect of dopamine agonists, such as carbidopa--dopa, in about 20% of Parkinson’s disease patients; however, our patient’s visual hallucinations persisted despite discontinuation of carbidopa--dopa.2 The differential diagnosis for visual hallucinations includes neurodegenerative dementias, such as Parkinson’s dementia, postictal states, intoxications/delirium tremens, migraine headache with aura, and narcolepsy.3
Peduncular hallucinosis is a rare form of visual hallucination characterized by intense, vividly colored, nonstereotypical visual images of people, animals, and plants that are nonthreatening to the patient. The exact mechanism for peduncular hallucinosis is unknown. One theory is that when "normal" afferent input is decreased, for example by diminished visual acuity, spontaneous cerebral activity of the visual system is disinhibited, resulting in visual hallucinations. However, given the rarity of such cases, it may be that cerebral pathology may render some elderly patients vulnerable to this disinhibited phenomena associated with pontine lesions.4 However, according to Cubo et al.,5 visual hallucinations are more likely to occur with more severe overall Parkinson’s symptoms and longer duration of Parkinson’s disease. Our patient only had Parkinson’s disease for 3 years and was only mildly impaired by the illness. Thus, based on clinical and radiographic findings, peduncular hallucinosis was considered in the differential diagnosis of our patient’s visual hallucinations. In closing, the emergence of new-onset visual hallucinations in the elderly warrants an MRI of the brain and, although rare, peduncular hallucinosis should be considered in the differential diagnosis, especially with brain stem or thalamic infarcts.