The Cough:Its relation to allergies and ACID REFLUX
Author: Thomas C. Beller, MD
Cough is the number one complaint in doctor’s offices in the United States and presents a significant burden to the medical field along with significant discomfort and embarrassment to patients. Some causes of cough can be attributed to infections and others to chronic conditions such as asthma or chronic obstructive pulmonary disease. However, in many cases, a neuroinflammatory response contributes to cough. The term neuroinflammation is a relatively new term in medicine. It means nerves are contributing to an immune response in an effort to fend off infections. This neuroinflammatory response can be triggered by stomach acid, allergies or both.
The relationship between acid reflux and cough is already well established, but the details regarding cause and effect have been debated. Initial theories suggested acid might be traveling up the esophagus and then down the trachea into the airway directly causing irritation and inflammation. New theories suggest that nerves are involved, implying there is nervous communication between the stomach and the airway. This communication involves the release of neuromediators that cause irritation, something that is being called neuroinflammation.
To understand how allergies may be playing a role in stimulating acid reflux and in amplifying this neuroinflammatory process, we have to explore what exactly it means to have an allergy. This will require some understanding of how our body is designed to deal with certain infections, specifically parasites.
For the purposes of this article, parasites can be defined as multi-cellular organisms that cause infections, as opposed to smaller infections like bacteria and viruses. Parasites are highly susceptible to heat and chemicals, and in many modern societies, we have essentially eradicated parasites by chlorinating the water and cooking our food. There is a common theory that suggests this eradication of parasites is what is leading to the epidemic of increasing allergies. In fact countries with the cleanest water supply have the greatest burden of allergic disease. The theory suggests that taking parasites out of our exposure patterns creates immunologic confusion. In essence, the immune system is looking for parasites and eventually finds other organic substances that have some structural similarity to the old parasites. Once our body makes antibodies to these other substances, such as pollen, mold spores, pet dander, etc., we have the immunologic mistake called allergy. In short, allergy is an anti-parasitic mistake.
There is a well-established correlation between allergies, allergic asthma, and acid reflux. However, establishing a definitive cause-effect relationship has been difficult. It becomes easier to see a theoretical cause-effect relationship once you realize that the main purpose of stomach acid is to kill parasites. This notion is supported by the fact that the acid pump in the stomach is activated by a histamine receptor (histamine can be thought of as a parasitic weapon). This receptor is called the H2 receptor, and is blocked by drugs like Zantac or Pepcid. If parasites activate stomach acid, this suggests that allergens may also activate stomach acid. More important, parasites and allergens may also have direct effects on nerves that send neuroinflammatory signals to the airway. This means that patients with reflux-induced cough may not have excess acid, but rather that the nerves have become hypersensitive to normal amounts of stomach acid. This means that patients with “reflux induced cough” may not have true “reflux” at all. Rather than the stomach being the source of the problem, the nerves may be where the problem lies. This fits with the finding that most patients with reflux-induced cough do not have indigestion.
Most patients with reflux-induced cough do not have indigestion. The initial evidence suggesting that nerves are participating in reflux-induced cough came from a group of researchers who noticed that many patients with chronic cough complained of a sense of irritation in the throat. The researchers assumed that a type of neuropathy (nerve-mediated irritation) was involved. They tried using a drug called gabapentin, which slows nerve conduction, in order to reduce or eradicate cough in these patients. The results were dramatic with complete resolution of cough in more than two-thirds of the patients. These researchers were not aware that they were actually treating cough related to acid reflux, but more recent data has revealed that gabapentin and high doses of antacids (proton pump inhibitors such as Prilosec, Nexium, etc.) both work to reduce cough in these patients.
A new paradigm is developing. Allergies may be activating nerves that detect acid, thereby stimulating a neuroinflammatory signal that induces cough. Thus, allergies are stimulating cough as an expelling response to protect the body from parasitic invasion. Of course, parasites are not involved. Rather, allergens are false parasites, and cough is an appropriate response if we understand this immunologic mistake.
To better understand cough is to better understand one of the most common ailments afflicting us. This can only lead to better treatments and better outcomes. Our paradigm suggests many patients may not know their cough is related to activation of this reflux/neuroinflammatory pathway because they don’t have indigestion. It also suggests that patients with reflux-induced cough may benefit from a visit to an allergist.