This symptom, which is known to doctors as an "olfactory hallucination," is potentially very worrisome. Olfactory hallucinations are occasionally a symptom of a brain tumor, so you need to bring this problem to the attention of a neurologist immediately. Olfactory hallucinations may also be a symptom of epilepsy; once again, you would be best served by consulting with a neurologist.

In general, a variety of things can go wrong with one’s sense of smell. Another name for what you describe is "phantosmia" (phantom smells). Some folks suffer from "parosmia," which is a distortion of the sense of smell. In other words, there really is an odor present, but the patient does not perceive it correctly. A fresh bowl of strawberries may smell like garbage. In one particular form of parosmia, cacosmia, the patient frequently smells fecal matter when this odor is not actually present. Absent or reduced sense of smell (anosmia and hyposmia) has already been addressed in this column.

What causes these odd disorders? Aside from the occasional patient with psychiatric problems, most disturbances of the sense of smell are probably due to an injury to the olfactory pathways. Smell is a chemical sense– when you smell something, your brain is perceiving specific airborne molecules. These molecules bind to receptors in the olfactory epithelium, which is the tissue that lines the back of the roof of the nasal cavity. Nerve impulses are generated, which travel first to the olfactory bulb, then to other areas deeper within the brain. Damage to any of these tissues may cause a distorted sense of smell.

What do I mean by "damage"? Infection (especially viral infection, such as the common cold, or flu) and trauma are perhaps the most common mechanisms of injury, but tumor is easily the most worrisome mechanism of injury. Trauma includes both blunt head trauma, such as might be suffered from a car accident, and surgical trauma. Exposure to toxic chemicals and ingestion of certain drugs can also injure one’s sense of smell, although these more commonly cause anosmia or hyposmia than phantosmia.

Because of the very real concern for possible disease within your brain, such as a brain tumor, you should first see a neurologist. You may also need to see an ear, nose and throat specialist, but I personally would start with a neurologist.