Cacosmia

Disorder of the functions of the olfactory system, which manifests itself in the fact that ordinary smells seem unpleasant and even disgusting to a person, is defined as parosmia, troposmia or cacosmia (literally from Greek - a bad smell).

In the ICD-10 section on perception-related symptoms and signs, the condition is coded R43.1. [1]

Epidemiology

The clinical statistics of cacosmia are unknown, but problems with the sense of smell are present in about 1-2% of people, moreover, more often its disorder is noted 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 often reports a distorted sense of smell, and the literature data confirm the prevalence of parosmia in the following conditions: head trauma (29–55%), post-infection of the upper respiratory tract (35–51%), diseases of the sinuses (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 between parosmia and neurological pathology is even more significant.

Causes of the cacosmia

The main causes of impaired  smell  in the form of a false perception of odors as unpleasant lie in the disorder of the functions 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 the brain, where the initial synaptic processing of information about the smell occurs, as well as the olfactory cortex - the cortex of the temporal lobe of the brain with the cortical part of the amygdala. [4]

But diseases of the sinuses (chronic sinusitis) and infections of the upper respiratory tract with alteration of olfactory receptors, as clinical practice shows, often lead either to a deterioration of the sense of smell -  hyposmia , or to its temporary loss (anosmia).

While development cacosmia seen with 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 (Lewy body dementia, multiple systemic atrophy); [7], [8]
• Pick's disease (with atrophy of the cortical and frontal lobes of the brain);
• chorea of Huntington;
• multiple sclerosis; [9], [10]
• Parosmia is an early symptom of acute SARS-CoV-2 infection. [11]

Cacosmia attacks occur in patients with  temporal lobe epilepsy  and alcoholic psychosis. [12]

Risk factors

Experts consider pathologies that affect the olfactory nerve, olfactory bulb and the olfactory tract of the brain as risk factors leading to a distorted perception of odors:

• meningitis or encephalitis (including sclerosing);
• brain damage by fungi in the form of cerebral aspergillosis;
• congenital aplasia of the olfactory bulb;
• Kalman's syndrome;
• benign and malignant brain tumors.

Cacosmia can occur when the cerebral structures are exposed to toxic substances, cytostatic drugs for cancer chemotherapy and radiation therapy, as well as in some types of psychotic (delusional) disorders, for example, schizophrenia. However, most often in patients with this disease, phantosmia (olfactory hallucination) is observed - a feeling of an unpleasant odor in its complete absence. [13]

Pathogenesis

Odors penetrating into 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, through sensory transduction is processed by the limbic system of the brain (including the olfactory cortex, mediodorsal nucleus of the thalamus and hippocampus).

Most likely, the pathogenesis of this olfactory disorder in upper respiratory infection is due to damage or a decrease in the number of functioning olfactory sensory neurons. And the mechanism of development of kakosmia in brain injuries and neurodegenerative diseases is associated with damage to the olfactory bulbs, tracts, subfrontal region, hippocampus, temporal and lower frontal lobes.

Parkinson's disease and Lewy body dementia (abnormal accumulations of the protein alpha-synuclein in the cytoplasm of nerve cells) affect neurons, nerve fibers, or glial cells in the midbrain (substantia nigra) or cerebral cortex.

In patients with Alzheimer's disease, there is a decrease in the olfactory bulbs and atrophy of the olfactory cortex.

And in Pick's disease and corticobasal degeneration, damage to brain neurons occurs due to the deposition of agglomerates of hyperphosphorylated cytoskeletal tau protein in them. [14]

Symptoms of the 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 disorder of smell can be headaches, nausea, impaired appetite and, of course, a decrease in the quality of life due to the loss of pleasure from food and a variety of natural aromas.

Diagnostics of the cacosmia

Diagnosis includes a complete history and examination of the nasopharynx. 

For how the olfactory study is done, see:

• Smell organ examination
• Olfactometry [15]

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, and anosmia is a complete inability to perceive odors. Parosmia is a distorted perception of odor in the presence of an irritating odor. Phanthosmia is an olfactory hallucination that occurs in the absence of odors. Both olfactory distortions are usually described as unpleasant (rotten, sewage, or burning odor). [16]

Treatment of the cacosmia

Even the treatment of upper respiratory tract infections with sinus inflammation, which has led to 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 - antipsychotics, but none of the drugs will restore the ability to correctly smell.

For patients who are particularly affected by cacosmia, the olfactory bulbs can be removed to relieve this symptom, but then the sensation of odors will completely disappear. [17]

Prevention

In cases of post-traumatic encephalopathies and neurodegenerative diseases, it is impossible to prevent any olfactory disorders.

Forecast

Its prognosis also depends on the cause of kakosmia: after the cure of sinusitis, the normal sense of smell may return; in other cases, there is no chance of recovery.