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Cacosmia
Last reviewed: 12.07.2025
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A disorder of the olfactory system, which manifests itself in the fact that ordinary odors seem unpleasant and even disgusting to a person, is defined as parosmia, troposmia or cacosmia (literally from Greek - bad smell).
In the ICD-10 section on perception-related symptoms and signs, this condition has the code R43.1. [ 1 ]
Epidemiology
The clinical statistics of cacosmia are unknown, but olfactory problems affect approximately 1-2% of people, and the disorder is more common in men. To date, the lack of specific olfactory tests to assess parosmia and poor self-esteem do not allow a realistic assessment of its frequency. In clinical practice, the patient quite often reports olfactory distortion, and the literature data confirm the prevalence of parosmia in the following conditions: head trauma (29-55%), post-upper respiratory tract infection (35-51%), sinus diseases (17-28%), toxins/drugs (17-28%) [ 2 ] and temporal lobe epilepsy (0.6-16%). [ 3 ] If, on the one hand, the frequency of parosmia is underestimated, on the other hand, the assessment of the relationship of parosmia with neurological pathology is even more significant.
Causes cacosmia
The main causes of olfactory disorders in the form of false perception of odors as unpleasant are rooted in the dysfunction of the olfactory analyzer, consisting of receptors of olfactory neurons (in the mucous membrane of the nasal cavity), the first cranial olfactory nerve, olfactory bulbs (bulbus olfactorius) - structures on the ventral part of the frontal lobes of the brain, where the initial synaptic processing of information about odor occurs, as well as the olfactory cortex - the cortex of the temporal lobe of the brain with the cortical part of the amygdala nuclei. [ 4 ]
But diseases of the nasal sinuses (chronic sinusitis) and upper respiratory tract infections with alteration of olfactory receptors, as clinical practice shows, more often lead to either a deterioration in the sense of smell - hyposmia, or to its temporary loss (anosmia).
While the development of cacosmia is observed in traumatic brain injuries [ 5 ], [ 6 ] (with chronic post-traumatic encephalopathy) and neurodegenerative diseases, which include:
- Alzheimer's disease;
- Parkinson's disease and other related limbic alpha-synucleinopathies (dementia with Lewy bodies, multiple system atrophy); [ 7 ], [ 8 ]
- Pick's disease (with atrophy of the temporal and frontal lobes of the brain);
- Huntington's chorea;
- multiple sclerosis; [ 9 ], [ 10 ]
- Parosmia is an early symptom of acute SARS-CoV-2 infection.[ 11 ]
Attacks of cacosmia occur in patients with temporal lobe epilepsy and alcoholic psychosis. [ 12 ]
Risk factors
Experts believe that risk factors leading to distorted perception of smells include pathologies affecting the olfactory nerve, olfactory bulb and olfactory tract of the brain:
- meningitis or encephalitis (including sclerosing);
- brain damage by fungi in the form of cerebral aspergillosis;
- congenital aplasia of the olfactory bulb;
- Kallmann syndrome;
- benign and malignant brain tumors.
Cacosmia may occur when cerebral structures are exposed to toxic substances, cytostatic drugs for cancer chemotherapy and radiation therapy, as well as some types of psychotic (delusional) disorders, such as schizophrenia. However, most often, patients with this disease experience phantosmia (olfactory hallucination) - a sensation of an unpleasant smell in its complete absence. [ 13 ]
Pathogenesis
Odors entering the nostrils in the form of microparticles are absorbed by the nasal mucosa, and information about them, sent to the brain along the neural circuit in the form of signals, is processed by the limbic system of the brain (including the olfactory cortex, the mediodorsal nucleus of the thalamus and the hippocampus) through sensory transduction.
Most likely, the pathogenesis of this olfactory disorder in upper respiratory tract infections is due to damage or a decrease in the number of functioning olfactory sensory neurons. And the mechanism of development of cacosmia in brain injuries and neurodegenerative diseases is associated with damage to the olfactory bulbs, tracts, subfrontal region, hippocampus, temporal and inferior frontal lobes.
In Parkinson's disease and dementia with Lewy bodies (abnormal accumulations of the protein alpha-synuclein in the cytoplasm of nerve cells), neurons, nerve fibers, or glial cells in the midbrain (substantia nigra) or cerebral cortex are affected.
Patients with Alzheimer's disease experience shrinkage of the olfactory bulbs and atrophy of the olfactory cortex.
In Pick's disease and corticobasal degeneration, damage to neurons in the brain occurs due to the deposition of agglomerates of hyperphosphorylated cytoskeletal tau protein in them. [ 14 ]
Symptoms cacosmia
Cacosmia (parosmia) is a symptom that manifests itself in the fact that ordinary odors - neutral or pleasant - are perceived as unpleasant.
Complications and consequences
Possible complications and consequences of this olfactory disorder may include headaches, nausea, loss of appetite and, of course, a decrease in the quality of life due to the loss of pleasure from food and various natural aromas.
Diagnostics cacosmia
Diagnosis includes a complete medical history and examination of the nasopharynx.
For information on how olfactory testing is performed, see:
Instrumental diagnostics are used: rhinoscopy, examination of cranial nerves and visualization of brain structures – CT or MRI.
Differential diagnosis
Differential diagnosis is carried out with olfactory hallucinations - phantosmia, hyposmia, parosmia.
Hyposmia is a partial loss of smell, while anosmia is a complete inability to perceive odors. Parosmia is a distorted perception of smell in the presence of an irritating odor. Phantosmia is an olfactory hallucination that occurs in the absence of odors. Both olfactory distortions are usually described as unpleasant (rotten, sewage, or burning). [ 16 ]
Who to contact?
Treatment cacosmia
Even treatment of upper respiratory tract infections with inflammation of the paranasal sinuses, which led to a false perception of odors, does not guarantee the restoration of normal olfactory function.
For Parkinson's disease, antiparkinsonian drugs based on L-dopa are used, for schizophrenia and other mental disorders - neuroleptic drugs, but none of the drugs will restore the ability to properly sense smells.
Patients who are particularly affected by cacosmia may have their olfactory bulbs removed to relieve the symptom, but then the sense of smell will disappear completely. [ 17 ]
Prevention
In cases of post-traumatic encephalopathy and neurodegenerative diseases, it is impossible to prevent any olfactory disorders.
Forecast
The prognosis for cacosmia depends on the cause: after sinusitis is cured, normal sense of smell may return; in other cases, there is no chance of its recovery.