Clinical Geriatrics

Hallucination is defined as “false sensory perception not associated with real external stimuli.” Hallucinations in patients with PD are reported as visual hallucinations in 30%,1 auditory hallucinations in 10%,2 and, rarely, tactile hallucinations.3

Visual hallucinations experienced by patients with PD are complex. They occur when the patients are awake and alert,2,4 as opposed to hypnogogic hallucinations. Hallucinations can occur at any time of the day or night, but are more common at nighttime. Initially, patients may be aware of the nonreality of these experiences but gradually become paranoid and delusional.5 The most common are realistic images of animals, familiar or strange people, or children. These hallucinations are different from more common hallucinatory syndromes seen in patients with schizophrenia or in those who take illicit drugs. They are not accompanied by flashing lights, elaborate shifting patterns, distortion of time and space, or abstract repeating units.6,7 Most hallucinations are pleasant and not threatening. Occasionally, visual images are incomplete and resemble hallucinations in the visually impaired patient (Charles Bonnet’s syndrome).8 Fenelon et al9 classified the hallucinations in patients with PD as minor hallucinations and formed visual hallucination. Minor hallucinations are subclassified into three types: presence hallucinations (64%), which are described as a perception such as “someone is behind me”; passage hallucinations (33%) such as brief vision of an object passing sideways; and illusions such as transformation of an object into an animal.

Risk Factors

Risk factors that have been reported include dementia, duration of therapy with dopaminergic medications, duration of disease, age of patients,10,11 anticholinergic medications, and sleep disorders.12-15 The mnemonic “SADDAD” can be used to recall these risk factors for hallucinations in PD:

• Sleep disorders
• Age of patients
• Dementia
• Duration of therapy
• Anticholinergic medications
• Duration of disease

Severe cognitive impairment or dementia is a major and independent predictive factor for visual hallucinations.4,9,16 In one study, the prevalence of visual hallucinations was reported as 70% in patients with PD who have dementia.9 It is not clear whether cognitive impairment and hallucinations occur independently as the natural progression of disease or if they have causative links. Studies using positron emission tomography have shown marked occipital hypometabolism in patients with PD who have dementia.17 This suggests that degenerative process in the visual cortex can be a cause for visual hallucinations.

Several medications have been implicated in causing hallucinations. Common drugs that are implicated include anticholinergics such as benztropine, dopaminergics such as ropinirole,18 and amantadine. Because both anticholinergics (antimuscarinic) and dopaminergics may induce hallucinations, disturbances of dopaminergic-cholinergic balance have been hypothesized as the pathophysiology.5 No dose-response relation between dopaminergic drugs and hallucinations has been reported in recent prospective studies. Goetz et al19 showed that there is no simple relationship between visual hallucinations and high plasma levels of levodopa or changes in plasma level.

Increased age has been associated with the presence of hallucinations, although this factor can be confounded by duration of illness, which is described as a major risk factor by Fenelon et al.9 It was proposed hypothetically that most hallucinations occur in the “off” state, but Sanchez-Ramos et al4 showed that the majority of their patients (60%) were “on” (able to move) while experiencing their hallucinations.