From the Guidelines
Olfactory hallucinations, also known as phantosmia, are most commonly caused by neurological disorders, psychiatric conditions, or degenerative diseases, and the most recent evidence suggests that temporal lobe dysfunction, degenerative or psychiatric disease, and trauma are among the leading causes 1.
Causes of Olfactory Hallucinations
The causes of olfactory hallucinations can be grouped into several categories, including:
- Neurological disorders: epilepsy, particularly temporal lobe seizures, can trigger smell hallucinations as electrical activity disrupts the brain regions responsible for processing olfactory information 1
- Psychiatric conditions: schizophrenia, depression, and bipolar disorder can manifest with sensory hallucinations, including smell 1
- Degenerative diseases: neurodegenerative diseases like Parkinson's and Alzheimer's often include olfactory dysfunction as an early symptom 1
- Trauma: head trauma or brain injuries affecting the olfactory bulb or temporal lobe may lead to smell distortions 1
- Infections: sinus infections, nasal polyps, or upper respiratory infections can irritate olfactory neurons, causing phantom smells 1
- Medications: certain medications like antibiotics, antidepressants, and antipsychotics may have phantosmia as a side effect 1
- Toxins: exposure to toxins, including certain chemicals or drugs, can damage the olfactory system 1
- Tumors: tumors pressing on parts of the brain involved in smell processing can also trigger these hallucinations 1
Diagnosis and Treatment
Diagnosis of olfactory hallucinations depends on identifying and addressing the underlying cause, which may require consultation with a neurologist, psychiatrist, or ENT specialist 1. Treatment options vary depending on the underlying cause, but may include medications, therapy, or lifestyle changes.
Key Findings
- The sense of smell is mediated by the olfactory nerve (CN I), and abnormalities thereof can be grouped into clinical categories, including quantitative disturbances, qualitative disturbances, and discrimination disturbances 1
- Olfactory dysfunction is associated with neurodegenerative, cognitive, and mood disorders, including Alzheimer disease, Parkinson disease, and depression 1
- The ACR Appropriateness Criteria recommend that most patients with olfactory symptoms do not require imaging, unless history or physical examination warrants it 1
From the Research
Causes of Olfactory Hallucinations
- Olfactory hallucinations can be caused by various factors, including neurological disorders such as temporal lobe epilepsy (TLE) 2 and schizophrenia 3, 4
- In TLE, olfactory hallucinations may be related to dysfunction of the temporo-limbic neural substrates involved in olfactory perception 2
- In schizophrenia, olfactory hallucinations may be associated with tactile hallucinations and may suggest a novel cause of olfactory hallucinations 3
- Other factors that may contribute to olfactory hallucinations include substance use, severe life events, and mood disorders such as anxiety and depression 4
- A common pathogenic mechanism, such as neuronal hyperexcitability and channelopathies, may underlie the comorbidity of olfactory hallucinations with other conditions, including migraine and epilepsy 5
Neurological Basis of Olfactory Hallucinations
- The hippocampus is considered a key region in the brain involved in olfactory hallucinations, particularly in TLE and schizophrenia 6
- Abnormal network connectivity between the dentate gyrus and the Cornus Ammonis area 3 subregions of the hippocampus may contribute to the development of olfactory hallucinations 6
- Ectopic mossy fiber projection in the hippocampus may be a commonality between TLE and schizophrenia, potentially contributing to olfactory hallucinations 6
Clinical Implications of Olfactory Hallucinations
- Olfactory hallucinations can have a significant impact on an individual's quality of life and may be associated with increased risk of developing psychopathology 4
- The presence of olfactory hallucinations may require a comprehensive assessment to determine the underlying cause and develop an effective treatment plan 4
- Antiepileptic drugs, such as topiramate, may be used to treat olfactory hallucinations, particularly in cases where they are comorbid with epilepsy or migraine 5