Mega Esophagus

At necropsy, we observed dilation of the distal esophagus . When we cut open the esophagus, we found it was impacted with food particles, which had caused a bulbous distension. The dilation was limited to the distal esophagus just proximal to the stomach. Histologically, the muscular wall of the esophagus in this region was thin, and there was compression and thinning of the mucosa. Pulmonary lesions consisted of multifocal alveolar neutrophilic and histiocytic infiltration associated with material which was refractile under polarized light. This is the typical microscopic appearance of aspiration pneumonia, and we interpreted the refractile material as unswallowed food particles. We made a diagnosis of idiopathic megaesophagus, with secondary malnutrition and aspiration pneumonia. Megaesophagus has been documented in humans1, dogs2, cats2,3, mice4, rats6,7, and a number of other animals2,5. Megaesophagus can be congenital or acquired. Acquired megaesophagus occurs gradually, secondary to neurologic disorders or acutely due to trauma8. In dogs, there is a breed predisposition (German Shepherd, Great Dane, and Wire Fox Terrier) for the development of juvenile onset megaesophagus2. The increased incidence in these breeds is speculated to involve delayed maturation of the nerve plexus innervating the esophagus and lower esophageal sphincter. These dogs often gradually acquire normal esophageal peristalsis and control of the lower esophageal sphincter over a period of a few weeks to two months2. There is also a predisposition in certain canine breeds to develop megaesophagus secondary to patent right aortic arch2. The role of genetic factors in other canine breeds and in other animals, including humans, is controversial. In humans, the majority of megaesophagus cases are secondary to achalasia. People diagnosed with achalasia-induced megaesophagus range in age from 25 to 60 years1. In achalasia-induced megaesophagus, there is an insufficient or uncoordinated relaxation of the lower esophageal sphincter and decreased peristalsis in the lower esophagus. The achalasia can be due either to muscular or neurologic abnormalities1. The neural component is considered to be more commonly involved1. Other less common causes of megaesophagus in people include systemic disease, anatomic anomalies, myasthenia gravis, stroke, amyloidosis, hypothyroidism, vagal afferent dysfunction, Trypanosoma cruzi infections, and foreign body obstruction of the esophagus1,5,8. In rodents, megaesophagus is rare6,7. However, two strains of rodents, the ICRC/HiCri mouse and BDE/HAN rat, spontaneously develop megaesophagus4,7. In ICRC/HiCri mice, the esophageal mucosa is hyperplastic with hyperkeratosis and parakeratosis. Chemical carcinogens can accelerate the occurrence of megaesophagus in this strain secondary to esophageal papilloma development4. In BDE/HAN rats, there is a 20% incidence of dilation in normal appearing animals of various ages. Severe dilation was lethal in 14 rats in one study7. Other strains of rats at the same facility (Wistar, Sprague-Dawley, and E-3) were negative for esophageal lesions. Dilation of the esophagus was most commonly found at the esophageal hiatus in the BDE/HAN rats. The obstruction and dilation in the Sprague-Dawley rat in this report was just proximal to the esophago-gastric junction. There are several models of megaesophagus in domestic animals. Megaesophagus due to vagal nerve neuropathy can be induced experimentally in animals by the oral administration of acrylamide in dogs9; the heavy metal lead in cats, dogs, or cattle3; or in any animal by the swallowing of a lump of food or foreign object that cannot pass the lower esophageal sphincter1,5. It can also be produced experimentally in dogs by vagotomy, or by an electrolytic lesion in the medulla9. Due to the anatomical differences between people and animals, some concern has been raised regarding the appropriateness of the experimental models for human megaesophagus. Canines and rodents have striated muscle throughout the length of the esophagus, while in humans the proximal esophagus has striated muscle which gradually changes to smooth muscle in the lower esophagus5,6. In addition, a reproducible animal model of achalasia-induced megaesophagus has not been produced. The prognosis for resolution of advanced megaesophagus is poor. Treatment of megaesophagus is often unrewarding. Some forms of megaesophagus respond to isosorbide dinitrate and calcium channel blockers treatment, physically by pneumatic dilatation, or by surgical myotomy of the lower esophageal sphincter. In most cases, if there is an underlying degenerative neural lesion or other physiologic abnormality in the esophagus, the treatment is primarily for temporary amelioration of symptoms1.

"Esophageal dilation in young animals may be the result of a vascular ring anomaly or due to an unknown cause. Idiopathic megaesophagus may also occur in adult dogs, and the esophagus may dilate secondary to systemic diseases such as myasthenia gravis, systemic lupus erythematosus, polymyositis, distemper, hypoadrenocorticism, heavy metal toxcity, hypothyroidism, CNS neoplasia, or trauma. Esophageal dilation has been produced experimentally by chronic administration of cholinesterase inhibitors; it may also occur anterior to an esophageal stricture, neoplasia, or extraesophageal compression.

The cardinal sin is regurgitation. A puppy with congenital megaesophagus characteristically begins to regurgitate when it begins to eat solid food. Initially, regurgitation occurs soon after swallowing; as the condition progresses, the esophagus enlarges and food in retained longer. Effected pups are generally unthrifty and smaller than their littermates. Pressure applied to the abdomen may cause ballooning of the esophagus at the thoracic inlet. aspiration pneumonia is a common complication, and the associated signs are fever, cough, and nasal discharge. Adults with megaesophagus regurgitate and, generally, have lost weight. Respiratory signs may predominate, with little or no history of regurgitation. Thoracic radiographs reveal air, fluid, or food in a dilated esophagus. The thoracic esophagus is usually uniformly dilated, and a large ventral deviation may present anterior to the heart. The cervical esophagus may also be dilated. Strictures, foreign bodies, or a vascular ring anomaly should be ruled out with an esophagogram or esophagoscopy.

In adult dogs, associated diseases (eg, myasthenia gravis) should be treated. Surgery is indicated for a vascular ring anomaly. Medical management is indicated for congenital or acquired idiopathic megaesophagus. The consistency of the diet that best prevents regurgitation varies from dog to dog; a soft gruel works well in many, but dry food works better for some. A feeding schedule of frequent small meals is preferred. Feeding from an elevated dish (which requires the dog to eat with forelimbs higher than hind) or holding the dog in an upright position for 10-15 min. after eating allows gravity to assist feed passage into the stomach. Surgery at the gastroesophageal junction generally does not improve the signs of idiopathic megaesophagus; the overall prognosis is poor, and many die from aspiration pneumonia."

Submitted by Anne Staudt