EXTRA ESOPHAGEAL REFLUX AND SYMPTOMS OF THE EAR, NOSE, THROAT (cont'd)

How Reflux Affects the Airway

The larynx, or voice box, acts as a two-way valve to prevent aspiration of food and liquids into the lungs during swallowing. During breathing, the larynx is open, allowing air to move in and out of the lungs via the trachea, but during swallowing, the larynx closes off the airway.

Even so, the proximity of the airway to the esophageal entrance makes the potential for reflux to enter the airway very high in certain situations. When a person is lying flat, refluxing gastric contents can readily flow into the esophagus to the level of the larynx and throat. Babies are especially prone to EER because their lower esophageal sphincter doesn’t reach maturity until 18 months and therefore doesn’t always function fully to keep gastric contents in the stomach.

Acid and the digestive enzyme pepsin are responsible for the damaging effects of reflux. Stomach tissue is most protected against their damaging effects, but even so, erosions and ulcers can develop. While not nearly as resistant to the effects of acid and pepsin as the stomach tissue, the esophagus is adapted to handle some acid exposure that occurs due to intermittent physiological reflux. Peristalsis, which is rhythmic, wavelike movement of the esophagus, helps push reflux back into the stomach (Figure 2A). The esophagus secretes mucus that forms a protective barrier against the corrosive effects of the reflux. The esophagus is also lined by a specialized layer of cells called stratified squamous epithelium that secretes bicarbonate ions, which can neutralize the acid (Figure 2B and C). Even these protective mechanisms in place, acid may still penetrate the epithelial layers and irritate nerve endings, which results in the sensation of “heartburn.”

Unlike the stomach and esophagus, the respiratory tract is extremely sensitive to the damaging effects of gastric fluids. The respiratory tract is lined by a cell layer called pseudo-stratified, ciliated columnar epithelium, which is very different in structure from the epithelium in the esophagus (Figure 3). Respiratory epithelia possess tiny hairs called cilia that perform a protective function. The sweeping action of the cilia keeps the airway clear by removing mucus from the respiratory tract. The cilia also help eliminate infectious and allergenic materials from the body because any bacteria, viruses, dust, pollen or mold that float onto the cilia also are swept out of the airway.

When repeatedly exposed to reflux, the respiratory columnar epithelium sometimes will change into stratified squamous epithelium like that found in the esophagus (Figure 3). Although more resistant to the damaging effects of reflux, squamous cells do not possess cilia. By causing the loss of cilia, EER compromises an essential cleansing process in the respiratory tract.

Sometimes when explaining to patients the difference between the ability of the esophagus and the airway to tolerate reflux, we use the analogy of the esophagus being a rubber hose and the airway being tissue paper. While not entirely accurate, it does paint a picture of the relative sensitivities of the two.

Although direct irritation of the respiratory tract by acid and pepsin can bring about the signs and symptoms of airway disease, reflux can also have an indirect, or referred, effect on the airway even if it never leaves the esophagus. This indirect effect comes about because the esophagus and airway are connected to a common central nerve called the vagus. When acid refluxing into the lower esophagus interacts with special structures called receptors, nerve impulses can arc backwards along the vagus to cause spastic closure of the larynx (laryngospasm). This reflexive response of the airway to reflux is yet another way in which the body protects itself from aspiration.