Diagnosis and management of megaesophagus in dogs (Proceedings)

Anatomy and physiology

The canine esophagus is a complex structure comprised of two layers of oblique skeletal muscle traversing the thorax from the upper esophageal sphincter in the pharynx to the lower esophageal sphincter entering the stomach. Cranially, at the pharyngoesophageal junction, the muscles of the upper esophageal sphincter remain closed at all times except during a swallow, which helps to prevent aspiration of gastrointestinal ingesta. The gastroesophageal junction is not a true sphincter but remains closed due to gastric compression on the esophagus, the muscular sling of the right crus of the diaphragm, the gastric rugal folds, and the oblique angle of the junction. At this junction, an inner layer of smooth muscle comprising the junction merges with the stomach wall.

The esophagus is innervated by the vagus nerve, and afferent vagal receptors are stimulated by the presence of food and liquid in the pharynx and upper esophagus, causing the dog to swallow and the esophagus to contract. Coordinated contractions of the esophagus assist movement of food and water bolused from the mouth to the stomach.

Megaesophagus definition

Megaesophagus can be defined as loss of tone and motility of the esophagus, often resulting in diffuse dilation and clinical signs of regurgitation. It can be either congenital or acquired.

Congenital megaesophagus

The pathophysiology of congenital megaesophagus remains unclear, but defects in the vagal afferent innervation of the esophagus or abnormalities of the esophageal musculature are suspected. Congenital megaesophagus often presents in puppies as they start to wean and is typically evident by 3 months of age. Dogs with milder disease may not present until 1 year old. Typical clinical signs include regurgitation and failure to thrive. Breeds with increased prevalence include wire-haired fox terriers (inherited, autosomal-recessive), miniature schnauzers (inherited, autosomal-dominant or autosomal-recessive with partial penetrance), Great Danes, German shepherds, Labrador retrievers, Newfoundlands, Chinese shar-peis, and Irish setters.

Acquired megaesophagus

Etiology

There are numerous potential causes of megaesophagus in the adult dog. Myasthenia gravis (MG), caused by autoantibody production against nicotinic acetylcholine receptors at neuromuscular junctions, is common and responsible for up to 30% of cases. In endemic areas, such as Kansas and Missouri, dysautonomia should also be considered as an underlying cause for megaesophagus, with 61% of dogs with dysautonomia documented to have radiographic evidence of megaesophagus. One study found that peripheral neuropathies, laryngeal paralysis, severe esophagitis (caused by gastroesophageal reflux, hiatal hernia, Spirocerca lupi infection, or ulceration from neoplasia or foreign object), and chronic or recurrent gastric dilatation with or without volvulus were also associated with increased risk of developing megaesophagus. A dilated esophagus can also occur cranial to a mass, stricture, vascular ring anomaly, or obstructing foreign object. Although hypothyroidism has been cited as a potential cause of megaesophagus, there are no data directly associating the two conditions; however hypothyroidism may occur concurrently with MG. Megaesophagus has also been reported with hypoadrenocorticism, suspected to be due to impaired muscle carbohydrate metabolism and depletion of glycogen stores in muscle. Less frequently reported diseases associated with megaesophagus include CNS disorders (distemper, trauma, neoplasia), other polyneuropathies (polyneuritis, polyradiculoneuritis, lead toxicity, and bilateral vagal nerve damage), neuromuscular junction disorders (botulism, tetanus, anticholinesterase toxicity), muscular disorders (polymyositis, glycogen storage disease, trypanosomiasis), and miscellaneous causes such as systemic lupus erythematous, gastric dilatation and volvulus, thymoma, and pyloric stenosis. These underlying diseases should be considered and screened for in affected patients so that specific therapy can be provided whenever possible; however, the majority of cases of adult-onset canine megaesophagus are ultimately deemed idiopathic.

Signalment

Dogs that develop acquired megaesophagus are typically middle aged to older, but this can affect young dogs as well; dogs with MG have a bimodal age distribution (<5yrs and >7yrs), and dogs with dysautonomia have a median age of 18 months. Female dogs may be at greater risk for development of MS, but no sex predilection exists for megaesophagus in general. Any breed can be affected, but certain underlying conditions have breed predilections. Breeds at increased risk for MG include: German shepherds, golden retrievers, Labrador retrievers, Akitas, Newfoundlands, and Great Danes. Breeds at increased risk for dysautonomia include: German shorthaired pointers, Labrador retrievers, and German shepherd dogs have been reported most often affected by dysautonomia; however this may reflect the rural population of dogs in this area rather than a true predilection for disease in these breeds. Idiopathic megaesophagus occurs more often in large breed dogs.

Clinical signs

Regurgitation is the classic clinical sign observed with megaesophagus. Distinguishing regurgitation from vomiting is important, as these clinical signs lead to separate differential diagnoses and diagnostic pathways. Regurgitation is typically characterized as a passive motion producing food or liquid, compared with the more active abdominal heaving that occurs with vomiting. Bile should only be produced with vomiting. With megaesophagus, regurgitation may happen right after a meal or hours later, and regurgitated food may take a tubular form. Dogs with megaesophagus may also show signs of ptyalism, halitosis, and vomiting. If aspiration pneumonia has developed, lethargy, dyspnea, cough, and nasal discharge may occur. Dogs with MG may have either the focal (megaesophagus) form or the generalized form characterized by neuromuscular weakness. Additional clinical signs related to dysautonomia include vomiting, diarrhea, anorexia, lethargy, weight loss, dysuria, dyspnea, and nasal discharge. Dogs with laryngeal paralysis may have a voice change and inspiratory stridor.

Physical examination

A complete physical examination is important, to help identify underlying or concurrent disorders. On physical examination, dogs with megaesophagus may be bright and alert or lethargic, especially if aspiration pneumonia has developed. Some patients have substantial weight loss. Occasionally a swelling of the ventral neck can be palpated representing food in the esophagus. Dyspnea may be present, and crackles in the lungs may be ausculted. Abdominal palpation is typically normal. Dogs with MG may show generalized weakness, and a complete neuromuscular exam should be performed. With dysautonomia, dogs may have decreased anal tone, absent pupillary light reflex, elevated nictitating membranes, dehydration, distended (easily expressible) urinary bladders, xerostomia, and mydriasis.

Diagnostic tests

Survey thoracic radiographs are usually diagnostic for generalized megaesophagus and will show a dilated esophagus containing food, fluid, or air; taking the opposite lateral radiographic view may help confirm the diagnosis if there is question based on a single lateral and ventrodorsal view. Thoracic radiographs should also be evaluated for presence of aspiration pneumonia. In cases with focal or partial megaesophagus due to an obstruction (foreign body, neoplasia) where survey films may not be diagnostic, a contrast esophagram with barium can be helpful; however, aspiration of barium is a potential complication of this procedure.

In adult animals, after megaesophagus has been identified, a search for an underlying cause should begin. A minimum database, with CBC, biochemical profile and electrolytes, and urinalysis is recommended. Further testing can be decided based on initial results. An inflammatory leukogram would be supportive of pneumonia. The CBC differential (lack of stress leukogram) and electrolytes (hyponatremia and hyperkalemia) can be assessed for evidence of hypoadrenocorticism. Elevated creatine kinase would support a myositis. Proteinuria would support systemic lupus erythematous; an antinuclear antibody would also support lupus. Testing for the presence of autoantibodies to acetylcholine receptors in serum should be considered in all patients with acquired megaesophagus; this can be run by the Comparative Neuromuscular Laboratory at the University of California, San Diego (http://medicine.ucsd.edu/vet_neuromuscular), and is a sensitive and specific test. Muscle biopsies can be performed in rare cases suspected to have MG but who are seronegative. Electromyogram and nerve conduction velocity testing can be helpful in select cases. If there is evidence for hypoadrenocorticism (electrolyte abnormalities or lack of a stress leukogram) an ACTH stimulation test should be performed. For dysautonomia, definitive diagnosis is made by histopathology of autonomic ganglia, usually at necropsy; however a variety of tests can be performed to support an antemortem diagnosis. Most dogs with dysautonomia will show rapid constriction of the pupils when pilocarpine ophthalmic solution 1% (diluted to 0.01-0.1%) is administered, whereas a normal dog may constrict only after 45-60 minutes; this is due to denervation hypersensitivity and postganglionic neuron loss. Dogs with dysautonomia may also have decreased tear production by Schirmer tear test, and relative bradycardia with lack of response to an atropine response test (0.04mg/kg atropine SQ, monitor HR q 15 minutes for 1 hr). Esophagoscopy can also be performed to rule out obstructive, inflammatory, and neoplastic disease; although this is not performed in all cases, it should be considered prior to deeming a case idiopathic. If esophagoscopy is performed, care should be taken during anesthesia to minimize risk of aspiration pneumonia.

Therapeutic strategies

Treatment should be addressed towards an underlying cause of megaesophagus whenever possible, in addition to supportive care with the therapy goals of minimizing regurgitation, providing ample nutrition, and resolving and/or preventing aspiration pneumonia.

MG is treated with pyridostigmine bromide (1-3mg/kg PO q8-12 hours), and antibody concentrations should be checked every 4-6 weeks; if antibody titers return to normal, treatment can be discontinued. Clinicians should monitor for both relapses and progression from focal to generalized MG. In cases not responding to pyridostigmine, immunosuppressive therapy (prednisone, azathioprine) can be instituted; however these should not be administered until aspiration pneumonia has resolved. Dogs with MG should also be spayed/neutered to prevent breeding, and because heat cycles and pregnancy can exacerbate clinical signs of MG. Risk assessment should also be used when considering vaccines for any dogs with MG, as immune stimulation may worsen their clinical signs. Other identified diseases should also be treated when identified (hypoadrenocorticism, systemic lupus erythematous, etc.). No specific therapy exists for treatment of dysautonomia; cases are treated supportively based on organ involvement, including pilocarpine for photophobia and tear production, bethanechol for detrusor function, and promotility agents for the gastrointestinal tract.

Nutritional management is critical for dogs with megaesophagus to ensure adequate caloric intake and minimize regurgitation. If dogs are fed by mouth, they should feed in an upright position, and remain in that position for up to 20 minutes after a meal to allow gravity to assist the movement of food and liquid into their stomachs. Individual dogs may do better with kibble, canned, or liquid diet because of altered swallowing or motility related to their disease. Some dogs may require a high caloric diet in order to receive an adequate daily intake, while others can consume maintenance diets. Most dogs with megaesophagus will have the least regurgitation if fed smaller amounts more frequently throughout the day. In dogs with severe esophagitis or whose regurgitation is frequent, a percutaneous gastrostomy tube can be placed to bypass the esophagus for feeding. There are multiple internet support groups available that provide moral support, suggestions, and instructional videos for owners of dogs with megaesophagus.

Prokinetic agents (cisapride, metoclopramide) are used by some clinicians to assist with esophageal motility, but their actions are for smooth muscle peristalsis which would be more effective in cats than dogs; metoclopramide can also be used for its ability to increase the tone of the lower esophageal sphincter. Bethanechol (5-15mg/dog PO q 8 hours) may be an effective promotility agent to promote esophageal peristalsis in dogs and improve lower esophageal sphincter tone.

Aspiration pneumonia is common in dogs with megaesophagus and is often the cause of death. Whenever possible, aerobic culture of airway fluid should be performed, collected either by trans- or endo-tracheal wash or bronchoalveolar lavage. Aerobic bacteria and Mycoplasma sp. are commonly identified; Mycoplasma PCR may be preferred to culture, as these organisms can be difficult to culture. Antimicrobial therapy should be based on culture results. Coupage and nebulization may improve clearance of bacteria and mucus from the lower airways. Follow-up radiographs are important to guide duration of therapy; treatment is recommended for at least 1 week beyond resolution of clinical and radiographic evidence of pneumonia.

Prognosis

Success depends on early diagnosis, appropriate feeding techniques, and recognition and treatment of aspiration pneumonia. Although overall prognosis for resolution of congenital megaesophagus in puppies is only 20-40%, some puppies will grow out of the condition, especially miniature schnauzers who typically return to normal by 6-12 months of age. Some puppies have mild megaesophagus, adapt well to elevated feedings and live a fairly normal life; owners must be committed and educated about monitoring for signs of aspiration pneumonia. For dogs with dysautonomia or idiopathic megaesophagus, the prognosis is poor. Some clinicians believe a subpopulation of idiopathic dogs have seronegative MG and advocate treating idiopathic patients as MG patients. For those dogs with documented MG, clinical remission may occur anywhere from 1 month to 1 year, and most dogs regain esophageal tone. Dogs with megaesophagus secondary to other diseases may also regain tone and improve if the primary disease is addressed.

Selected references

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Detweiler DA, Biller DS, Hoskinson JJ, et al. Radiographic findings of canine dysautonomia in twenty-four dogs. Vet Radiol Ultrasound 2001;42:108-112.

Gaynor AR, Shofer FS, Washabau RJ. Risk factors for acquired megaesophagus in dogs. J Am Vet Med Assoc 1997;211:1406-1412.

Harkin KR, Andrews GA, Nietfeld JC. Dysautonomia in dogs: 65 cases (1993-2000). J Am Vet Med Assoc 2002;220:633-639.

Bartges JW, Nielson DL. Reversible megaesophagus associated with atypical primary hypoadrenocorticism in a dog. J Am Vet Med Assoc 1992;201:889.

Johnson BM, DeNovo RC, Mears EA. Canine Megaesophagus. In Current Veterinary Therapy XIV, eds Bonagura JD, Twedt DC.Saunders Elsevier, St. Louis, MO, 2009.

Shelton GD. Myasthenia gravis and disorders of neuromuscular transmission. Vet Clinics of North America 2002;32:189-206.

Shelton GD, Schule A, Kass PH. Risk factors for acquired myasthenia gravis in dogs:1154 cases (1991-1995). J Am Vet Med Assoc 1997;211:1428-1431.