The experience of smelling odors like incense, smoke, or chemicals when no source is present is medically known as phantosmia, or an olfactory hallucination. This phenomenon involves perceiving a smell that does not exist in the surrounding environment. Phantosmia is a relatively common olfactory disorder, though it can be unsettling. The perceived odors vary widely but often involve unpleasant, foul, or burning scents, such as spoiled food, smoke, or a metallic smell.
Understanding Phantosmia The Phantom Smell
The perception of a phantom smell occurs due to a misfiring within the olfactory pathway, which begins in the nasal cavity and ends in the brain. Specialized sensory receptors in the nose, called the olfactory neuroepithelium, detect odor molecules and convert that information into electrical signals. These signals travel along the olfactory nerve to the olfactory bulb and then to the olfactory cortex in the brain, where the smell is identified.
In phantosmia, the brain generates the odor perception without the initial stimulus reaching the nasal receptors. This can result from a peripheral cause, where sensory neurons in the nose malfunction and send erroneous signals. Alternatively, the disruption may occur centrally within the brain’s processing centers, where neural circuits become incorrectly stimulated. The resulting phantom odor is created internally by the brain, often defaulting to a strong or pungent scent.
Underlying Causes of Olfactory Hallucinations
Phantosmia triggers are categorized based on whether the problem originates in the nose (peripheral causes) or within the brain (central causes). Peripheral causes are the most common and often involve inflammation or damage to the nasal cavity lining. Conditions such as chronic sinusitis or nasal polyps can irritate or obstruct the olfactory neuroepithelium.
Upper respiratory infections, including colds, influenza, and COVID-19, are frequent triggers because they temporarily damage olfactory sensory neurons. Impaired neurons may send distorted signals to the brain, leading to the phantom odor perception. Head trauma, even minor concussions affecting the nasal passages or the olfactory nerve area, can also cause peripheral phantosmia.
Central causes involve the brain’s interpretation of smell and are often associated with neurological events. Migraine headaches can feature phantosmia as part of the aura, where a phantom smell precedes or accompanies the head pain. Certain types of epilepsy, particularly temporal lobe epilepsy, can cause brief, recurrent olfactory hallucinations, often known as an olfactory aura.
Other central causes include brain tumors, strokes affecting olfactory processing regions, and neurodegenerative conditions like Parkinson’s disease. In these cases, the integrity of the neural network responsible for smell is compromised, causing it to spontaneously activate and generate the perception of an odor. A smoky or incense-like scent is frequently reported because the brain tends to hallucinate strong, noxious odors, such as burnt rubber, cigarette smoke, or chemical smells.
Red Flags and Indicators for Medical Consultation
While many cases of phantosmia are temporary, a medical evaluation is necessary if certain indicators suggest a serious underlying condition. Consultation is warranted if the phantom smell has a sudden onset, persists for more than a few weeks, or significantly interferes with daily life. Details like the odor’s intensity, whether it is perceived in one or both nostrils, and the frequency of episodes help guide the diagnosis.
Neurological Red Flags
A medical evaluation is urgent if the phantom smell is accompanied by neurological symptoms, suggesting a central cause. These red flags include:
- Unexplained dizziness
- Changes in vision
- Muscle jerks
- Loss of consciousness
The diagnostic process begins with a thorough physical examination of the head and neck, and a detailed review of the patient’s medical history, including recent infections, head injuries, or current medications.
The initial consultation involves specific questions about the phantom smell, including episode duration and odor description. This information helps differentiate phantosmia from other smell disorders, such as parosmia, where a real odor is perceived incorrectly. The patient’s history of headaches, seizures, or recent viral illness provides clues about whether the cause is peripheral or central.
Diagnostic Steps and Treatment Approaches
To determine the precise cause of phantosmia, a physician may recommend advanced diagnostic procedures following the initial exam. A nasal endoscopy, using a flexible tube with a camera, allows the doctor to visually check for peripheral issues like chronic inflammation, polyps, or structural abnormalities. Imaging tests are employed if a central cause is suspected.
Imaging and Testing
A computed tomography (CT) scan or magnetic resonance imaging (MRI) of the head can identify issues within the brain, such as tumors, past strokes, or abnormal tissue disrupting the olfactory pathways. If seizures are possible, an electroencephalogram (EEG) measures the brain’s electrical activity. These tools pinpoint the origin of the aberrant signal generation, whether in the nasal lining or the olfactory cortex.
Treatment for phantosmia depends on the underlying cause identified. If the cause is a peripheral issue like chronic sinusitis or nasal polyps, treatment focuses on resolving inflammation, often using nasal steroid sprays or surgery to remove the polyps. For cases linked to neurological conditions, management involves adjusting medications, such as anti-seizure drugs or migraine prophylactic therapies, to stabilize neural activity. Symptom management for peripheral causes may also include nasal saline rinses.