Megaesophagus
Megaesophagus is a dilation of the body of the esophagus associated with poor to no esophageal peristalsis. Megaesophagus has been described as an idiopathic congenital, idiopathic acquired, or secondary acquired disorder.52-66 Congenital disease has been reported in both the dog and the cat and an inherited basis has been proposed.52,53,55 The most common underlying disease process that results in a secondary megaesophagus is myasthenia gravis.63,64,67,68 Other underlying conditions reported include polyneuropathies, lead toxicity, polymyopathies, thymoma (causing a paraneoplastic form of myasthenia gravis), hypoadrenocorticism, dysautonomia, and possibly hypothyroidism, and others.57-62,65,66 Adult onset, idiopathic megaesophagus was termed esophageal achalasia in the past, as the pathophysiology was thought to be related to failure of a relaxation of the lower esophageal sphincter.
This has since been disproven, but as of yet the exact etiology and pathogenesis of primary megaesophagus is still unknown. It has been shown that some dogs have a normal lower and upper esophageal sphincter response to swallowing, but an absent or decreased response of the esophageal sphincters to intraluminal stimuli.69 This study, along with others, suggest that the underlying defect in dogs with idiopathic megaesophagus may lie in the vagal afferent innervation to the esophagus.70The most common clinical sign, regardless of the cause of the megaesophagus, is regurgitation. Patients with megaesophagus generally maintain their desire to eat, unless they develop aspiration pneumonia. Regurgitation may occur immediately or several hours after eating. Ptyalism and weight loss are common. Physical examination is often unremarkable unless an underlying condition is present (e.g., systemic weakness may
be seen with myasthenia gravis).
The results of a minimum diagnostic database are often normal. Thoracic radiographs usually show a diffusely dilated esophagus filled with air, food, and /or fluid (Figure 3.11). In questionable cases, contrast videofluoroscopy will help determine the extent of esophageal motility and whether any obstructive lesions are present. A definitive diagnosis involves ruling out other known causes of megaesophagus with appropriate testing, including serum acetylcholine receptor antibody titers, ACTH stimulation testing, lead concentrations, or others.68 Endoscopy may help rule out an obstructive disease, but is not generally necessary.If an underlying disease is present, it should be treated appropriately. For example, animals with myasthenia gravis often respond well to immunosuppression and anticholinesterase drugs.64,71 Other patients, especially those with a focal form, may have spontaneous regression of signs with supportive care.67 Unfortunately, to date, there is no definitive treatment for congenital or acquired idiopathic forms of megaesophagus, and these patients should be treated with supportive care. Aspiration pneumonia should be treated as it arises. Dietary support should be provided in form of a high quality, high calorie diet either via frequent, small upright feedings or through a gastrostomy tube. Some animals do better with a gruel consistency of the food while others regurgitate less with canned food formed into a meatball shape. Smooth muscle prokinetic agents (i.e., metoclopramide or cisapride) will not stimulate contraction of the canine esophagus as the esophagus is composed entirely of skeletal muscle.72 These drugs may be helpful in the cat since the distal esophagus consists of smooth muscle, but clinical or experimental efficacy studies are lacking. Surgical treatment of megaesophagus (gastroesophageal myotomy) has been associated with poor results.10
The prognosis for congenital or acquired idiopathic megaesophagus is guarded. Some young dogs have been shown to recover esophageal function as they mature.53 Although adult onset megaesophagus carries a poor prognosis, it has been reported to be transient in rare cases.73
3.3.9