Phantom Smells More Common Than Once Thought, Study Suggests
Olfactory hallucinations (OH), or phantom smells, were present in over 18% of people with Parkinson’s disease and were associated with the presence of other hallucination types, but not objective sense of smell, according to a recent study.
The findings overall suggest that olfactory hallucinations are a more prevalent Parkinson’s symptom than had been thought.
“More studies are needed for a better understanding, recognition, and possible treatment of these symptoms,” the researchers wrote.
The study, “Olfactory hallucinations as a non-motor sign of Parkinson’s disease – A cross sectional study amongst patients in a tertiary movement disorder center,” was published in Parkinsonism & Related Disorders.
Parkinson’s disease is often recognized by its motor symptoms, including tremors, slow movements, and muscle rigidity. But the neurodegenerative condition is also characterized by numerous nonmotor symptoms, including but not limited to a loss of smell, cognitive difficulties, mood changes, and gastrointestinal problems.
Hallucinations of some kind are thought to occur in about 30%–40% of Parkinson’s patients, with olfactory hallucinations, or the detection of phantom smells that are not really there, occurring in somewhere between 2.1%-11% of patients. However, the true frequency of olfactory hallucinations is difficult to determine, as this symptom is often missed in clinical evaluations.
A team of researchers at the University of Texas Health Science Center in Houston aimed to determine the prevalence of olfactory hallucinations in Parkinson’s patients seen at their movement disorder clinic between 2017 and 2018.
Patients completed a questionnaire about the presence and nature of olfactory hallucinations and their influence on quality of life. They also underwent a University of Pennsylvania Smell Identification Test (UPSIT) to evaluate objective smell loss, or hyposmia.
Overall, 137 people completed the UPSIT and the questionnaire. These participants had an average age of 66.3 years, and 63% were men.
Signs of olfactory hallucinations were reported in 25 patients (18.2%). The most commonly described hallucinations were smells of smoke or cigarette smoke, and 32% of people said their hallucinations were unpleasant. While most people said that their hallucinations were brief, two people reported hallucinations lasting over an hour.
Almost half of people with olfactory hallucinations (44%) reported that they believed their hallucinations were real.
Among those with olfactory hallucinations, 32% also had visual hallucinations, 24% had auditory hallucinations, and 12% had tactile hallucinations.
People with olfactory hallucinations had a significantly higher rate of other types of hallucinations than did those without olfactory hallucinations, in whom 25.9% of patients had experienced visual, auditory, or tactile hallucinations.
No significant differences were observed in the objective sense of smell between people with or without olfactory hallucinations. Specifically, 84% of people with olfactory hallucinations had smell loss, compared with nearly 70% of those without olfactory hallucinations.
A discrepancy was sometimes observed between the subjective level of hyposmia among patients with olfactory hallucinations and their objective hyposmia measurements, with eight out of 25 people in this group perceiving their sense of smell as good, very good, or acceptable when their objective smell loss was moderate to severe.
Overall, the findings show that olfactory hallucinations may be more prevalent among Parkinson’s patients than has previously been described.
“Our results underline the need for a routine interview to identify OH in [Parkinson’s] patients, as these are more frequent than previously thought, and can be unpleasant in up to a third of affected patients,” the researchers wrote, adding that a validated questionnaire specifically aimed at understanding olfactory hallucination symptoms should be developed.
The mechanisms underlying olfactory hallucinations in Parkinson’s need further investigation, the researchers noted. While the olfactory bulb, a brain region involved in smell detection, could be involved, the relationship between olfactory hallucinations and other hallucination types, but not actual smell loss, suggest that other, more central brain regions may be involved, the team hypothesized.