The gastrointestinal tract (or GI tract) of the dog is a living tube that begins at the mouth, travels via the esophagus to the stomach and, after a myriad of twists and turns in the small and large intestines, ends its journey at the anus. The GI tract is responsible for processing and absorbing all the food and liquid that the body takes in. The teeth crush and tear the food into smaller pieces and saliva mixes with it in the mouth to moisten it and to begin the breakdown of complex carbohydrate components. After it is swallowed, the food (now called ingesta) passes through the esophagus to the stomach, where it is mixed with a strong acid solution which begins to break down meat proteins. After extensive mixing in the stomach, the ingesta, which is now mostly liquid in consistency, passes out of the stomach into the first portion of the small intestine, called the duodenum. Very shortly after the liquid ingesta enters the duodenum, digestive enzymes from the pancreas and bile from the liver are added in order to complete the digestion of proteins, carbohydrates, and fats. After digestion is completed, the journey through the rest of the small intestine (i.e., the ileum and jejunum) is devoted to absorbing the nutrients that were processed earlier. After the necessary nutrients have been absorbed, the small intestinal contents now pass into the large intestine, or colon, where the excess liquid is absorbed. By the time the colon contents reach the rectum, which is the last part of this tube, the feces (or stool) are firm enough to be passed easily to the outside of the body, through an opening called the anus.

Problems with this process can occur at any point along the way, causing many possible signs, such as salivation, regurgitation, vomiting, diarrhea, or constipation. We will address some of these problems in the various topics included in this series, beginning with a serious swallowing disorder known as Megaesophagus.

The swallowing process in general
Food enters the pharynx (or throat) and is pushed into the esophagus by movements of the tongue. The esophagus, which is the muscular tube connecting the pharynx to the stomach, is normally narrow and flat unless ingested food is moving through it. The relaxation and opening of the esophagus is coordinated with the action of the tongue as it pushes the ingested food from the pharynx into the esophagus during the act of swallowing. Although the subsequent esophageal contractions are not strong ones, they nevertheless require active participation on the part of the esophagus. The muscular walls contract in response to stretching, causing a gentle, propulsive "wave" of contractions from the pharynx toward the stomach. The gastroesophageal sphincter, which keeps the entrance to the stomach closed, is stimulated to open by the presence of ingesta (ingested food) at its opening.

The esophagus, on its way to the stomach, travels down the neck (usually on the left) and enters the thoracic (chest) cavity at the beginning of the sternum ("breast-bone"). It then travels through the entire length of the thoracic cavity, passing above the trachea ("wind-pipe"), heart, and vena cava, which is the large vein returning all of the blood to the heart from the hindquarters. Just prior to entering the stomach, it passes through an opening in the diaphragm, which is the sheet of powerful muscle fibers that assist in breathing and separate the thoracic cavity from the abdominal cavity.

What is Megaesophagus?
With megaesophagus, the esophagus is abnormally stretched far beyond its normal collapsed state. In this condition, it is unable to move food actively toward the stomach and is unable to empty itself of the ingesta (ingested food and liquid) that have already entered it. This situation can be caused by one of several different abnormalities. In order to treat, or at least manage, megaesophagus adequately, it is very important to determine the underlying cause.

Possible causes:

 
• Abnormal gastro-esophageal (G-E) sphincter, such that ingesta are moved normally into the esophagus from the pharynx, but cannot exit the esophagus properly into the stomach. The end result is a gradual overfilling of the esophagus, causing excessive stretching. With time, just like the latex in a long-inflated balloon, the esophagus simply stays stretched out, even if its contents are removed. Abnormal G-E sphincter control may result from a congenital defect; scar tissue formation following some form of physical damage; a hiatal hernia; abnormal muscle and/or nerve function to that location; or other rare structural or functional abnormalities.


• "Idiopathic" --The term means that "medical science" has not come up with an adequate explanation! Nonetheless, idiopathic megaesophagus occurs when some abnormal neuromuscular (i.e., involving both nerve and muscle interaction) function of the esophagus results in abnormal transport of ingesta from the pharynx to the stomach. In dogs and cats, this disorder is believed to be hereditary, with Great Danes being the most commonly affected breed of dog, followed, in prevalence, by German shepherds and Irish setters. In cats, Siameses are the most prone.


• Acquired-- With acquired megaesophagus, the esophagus is normal at birth but becomes abnormal after birth. Several precipitating causes are possible.

• In puppies, often a cause for spontaneous acquired megaesophagus can never be found. Sometimes, though, it may be due to an acquired generalized muscle disorder, such as myasthenia gravis, botulism, lead poisoning, an infection caused by Canine Distemper or Toxoplasmosis, or one of several other rare neuromuscular diseases.


• In adolescent or adult dogs, in addition to the disorders mentioned above, causes may include a hormone disorder known as hypoadrenocorticism (or Addison's disease, in which the adrenal glands are underactive); polymyositis (a destructive, inflammatory muscle disorder); and Systemic Lupus Erythematosus (a severe autoimmune disorder).

• Congenital Vascular Ring Anomalies: These are congenital (i.e., from birth--does NOT imply heritability) malformations of the primary blood vessels entering and leaving the heart. The consequence of one of these congenital defects is a physical obstruction of the normal flow of ingesta through the esophagus. The most common defect is a Persistent Right Aortic Arch (PRAA). In this defect, the aorta (which is the main artery leaving the heart and carrying blood toward the body) develops on the right side, instead of on the left, which is the normal configuration. The resulting arrangement of the blood vessels causes the esophagus to be encircled by the aorta as it exits the heart, causing a physical constriction of the esophagus. Vascular ring anomalies occur more often in young dogs than in young cats. In dogs, there is a breed predisposition, with Irish setters, Boston terriers, and German shepherds being seen most often. In cats, no breed predilection has been identified.

An X-ray procedure known as a "barium swallow" can confirm a diagnosis of megaesophagus and can often identify the location of the defect causing it. In this procedure, a special, barium-containing dye outlines the dilated esophagus. With vascular ring anomalies like PRAA (Persistent Right Aortic Arch), a constriction is seen over the base of the heart, with the dilatated, barium-filled portion of the esophagus seen only in front of that location. With an esophageal stricture caused by scar tissue or from a congenital lack of muscle fibers in a particular location, the dilatated area is seen in front of the level of the specific defect. With a generalized, neuromuscular defect in the esophagus, or with a constriction at the level of the gastroesophageal (G-E) sphincter, the dilatation occurs over the entire length of the esophagus. To narrow down the possible causes for a general dilatation, "fluoroscopy" may be required. With fluoroscopy, the X-ray procedure is an active process which displays the actual movements of the esophagus and its related structures in "real time." Using this procedure, a physical constriction may be distinguished from a functional abnormality.

Treatment:
Whether or not megaesophagus can actually be corrected depends upon what caused the dilatation to occur in the first place. Removal of a foreign object lodged in the esphagus, correction of a hiatal hernia, or surgical repositioning or ligation of an abnormal heart vessel (or vessel remnant) are all potential "cures" for the underlying cause. If the stretching of esophagus has not persisted for too long, and the underlying cause can be removed, then a dog may be able to live a completely normal life.

However, most situations involve management, rather than cure. The difficulties that megaesophagus imposes upon the body are serious and sometimes life-threatening. Because nutrients cannot easily reach the stomach for processing and digestion, malnutrition can become a significant problem. Many cases can be improved dramatically by simply changing the angle of the dog's body during and after eating, and by feeding a diet that is soft and easily transported. Even young puppies can be taught to eat with their front paws on a step-stool and to stay in that position for 20 minutes following feeding. For those that have difficulty with this, creative companies and individuals have fashioned upper-body slings to maintain the recommended position for an adequate length of time. This feeding regimen helps many dogs maintain their caloric and fluid intake. A potentially more severe complication of megaesophagus is aspiration pneumonia. Even with impeccable management, this is a serious risk. The pooling of liquids from saliva or water drinking, along with unavoidable small quantities of food and a host of nasty bacteria, cause a constant situation where fluid is "sloshing" around in the esophagus, ready to flow forward whenever the dog lowers his head. If the dog breathes in at the wrong time, it is extremely easy to aspirate this fluid into the lungs, causing pneumonia. Almost ALL cases of megaesophagus get pneumonia at some time or another, sometimes many times. Treatment for the pneumonia is broad-spectrum antibiotic therapy, sometimes for prolonged periods. Other than early euthanasia because of management difficulties, intractable aspiration pneumonia is the most common reason for euthanasia in megaesophagus cases. Nevertheless, there are many dogs who have had megaesophagus for years and have led very happy, healthy lives.