The experience of persistently smelling an unpleasant odor, such as dog feces, when no source is present can be deeply unsettling and disruptive to daily life. This sensory distortion is a medical phenomenon known as an olfactory hallucination, or phantosmia. The brain perceives a smell that does not exist in the environment, which signals that something in the complex olfactory pathway is malfunctioning.
Understanding Phantosmia: The Phantom Smell
Phantosmia is the medical term for perceiving an odor in the absence of an external stimulus, essentially a smell hallucination. The olfactory system begins when odor molecules interact with specialized receptor neurons high in the nasal cavity’s olfactory epithelium. These neurons then transmit electrical signals along the olfactory nerve to the brain’s olfactory bulb and cortex, where the scent is identified. The phantom smell is often described as unpleasant or foul, a specific type of phantosmia known as cacosmia. Common descriptions include burnt toast, metallic, rotten, or sewage-like odors. This occurs because misfiring or damaged nerve cells tend to generate a signal that the brain interprets as a universally noxious or warning scent.
Peripheral Causes: Issues Within the Nasal Cavity
The most common causes of phantom smells are rooted in the peripheral olfactory system, meaning the nose, sinuses, and surrounding structures. In these cases, the signal is spontaneously generated at the receptor level before it reaches the brain. Chronic sinusitis, a long-term inflammation of the sinuses, is a frequent culprit because persistent swelling can irritate or damage the delicate olfactory neuroepithelium. This inflammation leads to aberrant neural activity, causing receptors to fire irregularly without an actual odorant present.
Similarly, nasal polyps or tumors within the nasal passages can physically obstruct airflow and cause localized pressure or irritation on the olfactory nerve endings, often leading to a phantom scent localized to the affected nostril. Upper respiratory tract infections, including those caused by viruses like COVID-19, are also well-documented causes of phantosmia. The infection can temporarily damage or permanently injure the olfactory receptor neurons, leading to sensorineural dysfunction. As these neurons attempt to regenerate, they may fire spontaneously or incorrectly, sending erroneous signals to the brain. This peripheral damage is often a transient issue, though it can sometimes persist for months or even years after the initial illness resolves.
Central Causes: Signals Originating in the Brain
When a phantom smell is not related to a nasal or sinus issue, the cause often lies in the central nervous system, where the signal originates in the brain’s processing centers. Temporal lobe seizures, particularly those affecting the uncus or piriform cortex, can generate a brief, foul-smelling olfactory aura as an initial sign of a seizure. This is known as ictal phantosmia and results from hyperexcitability in the brain regions responsible for interpreting smell.
Brain lesions or tumors, especially those near the temporal lobe or frontal lobe, can also physically press upon or irritate the olfactory cortex, causing the spontaneous perception of odors. The presence of a mass can directly interfere with the neural integration that processes smell. Head trauma, such as a severe concussion, can likewise damage the olfactory bulb or the central processing areas, causing long-term misfires in the signal pathway. Migraines represent another central cause where phantosmia can occur as part of the sensory aura preceding or accompanying the headache. The perception of a phantom smell in this context is thought to be related to the wave of altered electrical activity that sweeps across the brain during a migraine event.
Identifying When Medical Attention is Necessary
While phantosmia is frequently benign and often resolves on its own, certain symptoms necessitate a prompt medical evaluation. It is advisable to consult a physician, likely an Ear, Nose, and Throat (ENT) specialist or a neurologist, if the phantom smell is persistent and worsening over several weeks. A strong indicator that the issue is localized to the nasal cavity is if the smell is only perceived in one nostril.
Specific red flags point toward a potentially more serious central cause, requiring immediate attention. These include the phantom smell being accompanied by new onset seizures, severe headaches, or visual disturbances. Other concerning neurological symptoms are memory loss, dizziness, or changes in balance. Medical evaluation through imaging, such as a CT or MRI scan, may be necessary to rule out structural issues like tumors or lesions impacting the olfactory pathways.