Researchers have uncovered a link between patients with increased sensitivity to scents and chemicals, a condition known as sensory hyperreactivity ( SHR ).

This study, published in the journal Environmental Health Perspectives ( EHP ), confirms that some chemically intolerant people with a history of coughing and upper- and lower-airway symptoms can connect their symptoms to a clear and measurable neurochemical alteration in airway mucosa.

SHR-related symptoms such as throat irritation, eye irritation, excess phlegm production, and coughing have often been mistakenly attributed to allergies or asthma.
The authors stress that although the symptoms may be similar, there is a distinct need to “emphasize the discrepancy between SHR and allergies/asthma, though these groups have similar airway symptoms and are often confused.”

The primary difference between patients with allergies/asthma and those with SHR is the absence of mucosal inflammation among the latter. Among allergy/asthma sufferers, this inflammation results in high levels of nerve growth factor ( NGF ), a protein that is essential for the survival and development of sensory neurons.

The authors hypothesized that a neurochemical alteration lay at the root of SHR. Although SHR patients do not experience mucosal inflammation ( a source of high NGF levels ), they have been shown to have an increased cough sensitivity to inhaled capsaicin.
Capsaicin is known to stimulate sensory nerve endings and induce coughing when inhaled. The researchers therefore theorized that the NGF reaction in SHR patients “may be derived from hyperreactive nerve endings.”

The researchers studied 13 nonsmoking patients who had been referred to an asthma and allergy clinic due to symptoms suggestive of those conditions, but for whom allergy tests were negative and asthma medications ineffective. The subjects were given increasing levels of nebulized capsaicin. After this provocation, the subjects’ coughs were counted and levels of NGF in nasal discharge measured. Results were then compared to a control group of 14 participants without a history of allergies/asthma or respiratory difficulties.

Within the study, all subjects exhibited a dose-dependent response to inhaled capsaicin. However, the SHR group coughed significantly more than the control group. The SHR group also exhibited other symptoms from the capsaicin inhalation including throat irritation, heavy breathing, eye irritation, excess phlegm production, and runny nose, while the control group registered few or no additional symptoms. The authors observed a significant correlation between the number of coughs, change in NGF levels, and symptom scores for throat irritation, phlegm production, and runny nose.

According to the article, “patients with SHR do have enhanced cough sensitivity to inhaled capsaicin, which correlates to a small but significant increase in NGF in nasal lavage fluid after capsaicin provocation. This indicates a neurochemical imbalance of the respiratory system in patients with SHR.”
In other words, the neurochemical imbalance causes SHR sufferers to react abnormally to chemicals and scents, even in “concentrations normally regarded as harmless,” the study says.

Source: Environmental Health Perspectives ( NIEHS ), 2005


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