Phantosmia
Encyclopedia
Phantosmia, or olfactory hallucinations, involves smelling odors that are not derived from any physical stimulus. These phantom odors can range from rotting flesh to a spring meadow, though most cases report unpleasant aromas. In some cases phantosmia has even led to the afflicted individual to believe that both the odor and source in fact exist, a condition known as olfactory delusions.
. During a temporal lobe seizure the victim rarely faints, but usually blacks out and cannot remember anything that happened during the seizure. Several people who have had these seizures did, however, recollect having phantosmia just prior to blacking out. Epilepsy
is a disease characterized by seizures. In the case of phantosmia, if smelling and something else become so strongly linked, the action of “something else” occurring can induce activation of the olfactory bulb even though there was no stimulus for the bulb present. This is an example of plasticity gone awry. Those with lesions on the temporal lobe, often brought about by a stroke but also from trauma to the head, also experience these olfactory hallucinations.
Other leading causes of phantosmia include neurological disorders such as schizophrenia
and Alzheimer’s disease. Both of these disorders have well documented cases of hallucinations, most commonly visual and auditory. Both also, however, have instances of phantosmia too, although not as frequently. In both cases, incidences of olfactory delusions are more common, especially in Alzheimer’s, where it is exceedingly difficult to convince the patient that these are in fact hallucinations and not real. Specifically in Alzheimer’s disease, atrophy in the temporal lobe has been known to occur. As evidenced in trauma and seizures, phantosmia is strongly associated with this area; leading to its appearance in some Alzheimer’s patients. Parkinson’s disease patients can also experience phantosmia, as well as parosmia
, however their appearance is more uncommon than the muscle tremors the patients experience.
Phantosmia can also be attributed to overactive olfactory receptor neurons (ORNs) or the loss of inhibitory neurons brought on by a sinus infection or some other type of head cold. These overactive ORNs could be activated by some other input through a fluke in wiring where they would normally remain inactive or inhibited, so a signal is sent to the olfactory bulb when no stimulus is present. The inhibitory neurons are also a part of the pathway just described. If extraneous information that is normally stopped at these impasses is allowed to continue along the pathway, the brain will interpret a signal that has no intrinsic value. This will provide a sensation without a proper stimulus.
In one study, a long-term (average 5.4 years after surgery) follow-up on 8 patients that underwent transnasal excision of olfactory epithelium revealed that 7 of them had their phantosmia permanently resolved. Before the surgery all of the patients had tests to show there were no psychological or pathological evidence for the hallucinations.
Causes
There are a few causes for phantosmia, but one of the most common and well-documented involves brain injury or seizures in the temporal lobeTemporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
. During a temporal lobe seizure the victim rarely faints, but usually blacks out and cannot remember anything that happened during the seizure. Several people who have had these seizures did, however, recollect having phantosmia just prior to blacking out. Epilepsy
Epilepsy
Epilepsy is a common chronic neurological disorder characterized by seizures. These seizures are transient signs and/or symptoms of abnormal, excessive or hypersynchronous neuronal activity in the brain.About 50 million people worldwide have epilepsy, and nearly two out of every three new cases...
is a disease characterized by seizures. In the case of phantosmia, if smelling and something else become so strongly linked, the action of “something else” occurring can induce activation of the olfactory bulb even though there was no stimulus for the bulb present. This is an example of plasticity gone awry. Those with lesions on the temporal lobe, often brought about by a stroke but also from trauma to the head, also experience these olfactory hallucinations.
Other leading causes of phantosmia include neurological disorders such as schizophrenia
Schizophrenia
Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...
and Alzheimer’s disease. Both of these disorders have well documented cases of hallucinations, most commonly visual and auditory. Both also, however, have instances of phantosmia too, although not as frequently. In both cases, incidences of olfactory delusions are more common, especially in Alzheimer’s, where it is exceedingly difficult to convince the patient that these are in fact hallucinations and not real. Specifically in Alzheimer’s disease, atrophy in the temporal lobe has been known to occur. As evidenced in trauma and seizures, phantosmia is strongly associated with this area; leading to its appearance in some Alzheimer’s patients. Parkinson’s disease patients can also experience phantosmia, as well as parosmia
Parosmia
Parosmia, also known as troposmia or cacosmia, is an olfactory dysfunction that is characterized by the inability of the brain to properly identify an odor’s “natural” smell....
, however their appearance is more uncommon than the muscle tremors the patients experience.
Phantosmia can also be attributed to overactive olfactory receptor neurons (ORNs) or the loss of inhibitory neurons brought on by a sinus infection or some other type of head cold. These overactive ORNs could be activated by some other input through a fluke in wiring where they would normally remain inactive or inhibited, so a signal is sent to the olfactory bulb when no stimulus is present. The inhibitory neurons are also a part of the pathway just described. If extraneous information that is normally stopped at these impasses is allowed to continue along the pathway, the brain will interpret a signal that has no intrinsic value. This will provide a sensation without a proper stimulus.
Treatment
At the onset of these hallucinations, the odor will last only for a few minutes and then dissipate. It may start with incidents occurring once a month, but then progresses to weekly and daily episodes.In one study, a long-term (average 5.4 years after surgery) follow-up on 8 patients that underwent transnasal excision of olfactory epithelium revealed that 7 of them had their phantosmia permanently resolved. Before the surgery all of the patients had tests to show there were no psychological or pathological evidence for the hallucinations.
External links
- "A Pungent Life: The Smells in My Head" by Jane G. Andrews in The New York TimesThe New York TimesThe New York Times is an American daily newspaper founded and continuously published in New York City since 1851. The New York Times has won 106 Pulitzer Prizes, the most of any news organization...