Canine Idiopathic Megaesophagus: Pathogenesis, Diagnosis, and Therapy
Robert Washabau United States
Idiopathic megaesophagus is the most common cause of regurgitation in the dog. Aside from dysautonomia, megaesophagus is a rare finding in the domestic cat. The disorder is characterized by moderate to severe esophageal dilation and ineffective esophageal peristalsis. Several forms of the syndrome have been described: congenital idiopathic, acquired idiopathic, and acquired secondary megaesophagus.
Congenital idiopathic megaesophagus is a generalized dilation and hypomotility of the esophagus causing regurgitation and failure to thrive in puppies shortly after weaning. An increased breed incidence has been reported in the Irish setter, Great Dane, German Shepherd, Labrador Retriever, Chinese Shar-Pei, and Newfoundland breeds, but inheritability has been demonstrated only in the Miniature Schnauzer and Fox Terrier breeds. The pathogenesis of the congenital form is incompletely understood, although recent studies point to a defect in the vagal afferent innervation to the esophagus (Tan and Diamant, 1987; Holland et al., 1993; Holland et al., 1994). Congenital idiopathic megaesophagus has also been reported in several cats (Hoenig et al., 1990; Pearson et al., 1974), although megaesophagus may have been secondary to pyloric dysfunction in one group of cats (Pearson et al., 1974).
Acquired secondary megaesophagus may develop in association with a number of other conditions. Myasthenia gravis accounts for at least 25% of the secondary cases (Shelton et al., 1990). In some cases of myasthenia gravis, regurgitation and weight loss may be the only presenting signs of the disease, whereas in most other cases of acquired secondary mega-esophagus, regurgitation is but one of many clinical signs.
Most cases of adult-onset megaesophagus have no known etiology and are referred to as acquired idiopathic megaesophagus. The syndrome occurs spontaneously in adult dogs between seven to 15 years of age without sex or breed predilection. The disorder has been compared (erroneously) to esophageal achalasia in humans. Achalasia is a failure of relaxation of the lower esophageal sphincter and ineffective peristalsis of the esophageal body. A similar disorder has never been rigorously documented in the dog. Several important differences between idiopathic megaesophagus in the dog and achalasia in humans have been observed (Diamant et al., 1973). More recent studies have instead suggested a defect in the afferent neural response to esophageal distension (Washabau, 1992). The responses of the upper and lower esophageal sphincters to swallowing appear to be intact, but esophageal distension does not initiate peristaltic contractions in affected animals. The exact site of this abnormality in the afferent neural response has not yet been determined.
Regurgitation is the most frequent clinical sign associated with megaesophagus. The frequency of regurgitation may vary from as little as one episode every few days to many episodes per day. Regurgitation associated with megaesophagus occurs several minutes to several hours after feeding, whereas the regurgitation associated with oropharyngeal or cricopharyngeal disorders usually occurs immediately postprandially. As with many other esophageal disorders, affected animals suffer from malnutrition and aspiration pneumonia. Physical examination often reveals excessive salivation, mild to moderate cachexia, coughing, and pulmonary crackles or wheezes.
Routine hematology, serum biochemistry, and urinalysis should be performed in all cases to investigate possible secondary causes of megaesophagus (e.g., hypothyroidism, hypoadrenocorticism). Thereafter, survey radiographs will diagnose most cases of megaesophagus. A contrast study should always be performed to confirm the diagnosis, evaluate motility, and exclude foreign bodies or obstruction as the cause of the megaesophagus. Endoscopy may be performed but often accomplishes little more than to substantiate the diagnosis. Esophagitis is occasionally discovered during endoscopic evaluation.
If acquired secondary megaesophagus is suspected, additional diagnostic tests should be considered, for example: serology for nicotinic acetylcholine receptor antibody, thyroid function test (e.g., TSH assay, TSH stimulation), ACTH stimulation, serology for antinuclear antibody, serum creatine phosphokinase activity, electromyography and nerve conduction velocity, and muscle and biopsy. The additional workup will be dependent upon the individual case presentation.
The major differential diagnoses are those seen with acquired secondary megaesophagus, e.g., myasthenia gravis, esophagitis and dysmotility, Addison=s disease, polymyositis, etc.
Animals with secondary acquired megaesophagus should be appropriately diagnosed and treated. For example, dogs affected with myasthenia gravis should be treated with pyridostigmine (1.0-3.0 mg/kg q12h PO) and/or corticosteroids (prednisone 1.0–2.0 mg/kg q12h PO or SC); dogs affected with hypothyroidism should be treated with levothyroxine (0.22 mcg/kg q12h PO); and dogs affected with polymyositis should be treated with prednisone (1.0-2.0 mg/kg q12h PO). If secondary disease can be excluded, therapy for the congenital or acquired idiopathic megaesophagus patient should be directed at nutritional management and treatment of aspiration pneumonia.
Affected animals should be fed a high-calorie diet, in small frequent feedings, from an elevated or upright position to take advantage of gravity drainage through a non-peristaltic esophagus. Dietary consistency should be formulated to produce the fewest clinical signs. Some animals handle liquid diets quite well, while others do better with solids. Animals that cannot maintain adequate nutritional balance with oral intake should be fed by temporary or permanent tube gastrostomy. Gastrostomy tubes can be placed surgically or percutaneously with endoscopic guidance.
Pulmonary infections should be identified by culture and sensitivity, and an appropriate antibiotic selected for the offending organism(s). This may be accomplished by transtracheal wash or by bronchoalveolar lavage at the time of endoscopy.
Medical therapies have been advocated for stimulating esophageal peristalsis (e.g., metoclopramide or cisapride) or diminishing lower esophageal sphincter tone (e.g. anti-cholinergics or calcium channel antagonists) in affected animals. Metoclopramide and cisapride are smooth muscle prokinetic agents that will not likely have much of an effect on the striated muscle of the canine esophageal body (Washabau and Hall, 1995). Therefore, they cannot be recommended in the therapy of this disorder, especially in dogs. Calcium channel antagonists have potent hypotensive effects on vascular smooth muscle, but very little effect on canine lower esophageal sphincter smooth muscle (Washabau, 1993). Anti-cholinergic usage would likely be associated with too many side effects to be clinically useful. Unfortunately, at this time, there do not appear to be any clinically useful drugs for improving esophageal peristalsis in canine acquired idiopathic megaesophagus.
Historically, cardiomyotomy (Heller's myotomy) was recommended as a therapeutic measure in the belief that mega-esophagus was an achalasic disorder. Since the lower esophageal sphincter is normotensive and relaxes appropriately with swallowing in affected dogs (Washabau, 1992), cardiomyotomy cannot be recommended for the treatment of the disorder. Indeed, many animals treated with myotomy have had poorer outcomes than untreated animals.
Animals with congenital idiopathic megaesophagus have a fair prognosis. With adequate attention to caloric needs and prevention of aspiration pneumonia, many animals will develop improved esophageal motility over several months. The pet owner must be committed to potentially months of physical therapy.
The morbidity and mortality of acquired idiopathic megaesophagus remain unacceptably high. Many animals eventually succumb to the effects of chronic malnutrition and repeated episodes of aspiration pneumonia. A poor prognosis must be given in such cases. Animals with acquired secondary megaesophagus have a more favorable prognosis if the underlying disease can be promptly identified and successfully managed. Refractory cases result from chronic esophageal distension, myenteric nerve degeneration, and muscle atrophy.
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