Washington State College of Vet Med ? Pet Health Topics
Less commonly recognized signs that may be seen in a small number of dogs with hypothyroidism include dilation of the esophagus (megaesophagus) causing
regurgitation, and abnormal function of nerves or muscles leading to weakness or abnormal ability to walk.
World Small Animal Veterinary Association Congress 2001 Vancover, BC
Canine Idiopathic Megaesophagus: Pathogenesis, Diagnosis, and Therapy
By Robert Washabau, submitted by W. Jean Dodds, DVM
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.
NEUROMUSCULAR CASE OF THE MONTH
Resolution of megaesophagus in an adult female mixed breed dog after treatment of concomitant hypothyroidism
Contributed by Dr. Edouard Huber
Clinique v?t?rinaire Les Grilli?res
Montcherand, CH, 1354 (Switzerland)
An adult mix breed female hunting dog of 40kg was presented with a two month history of regurgitation and vomiting bile. Previous treatments included domperidon,
cimetidine, and clindamycin which did not result in clinical improvement.
The dog was obese, weak, and lethargic with generalized muscular atrophy. The hair coat was lusterless and a slight dry seborrhea and diffuse ventral alopecia
were noted. Vital signs were normal. Cranial and spinal nerve evaluations were considered normal. The dog regurgitated during the examination. Regurgitation
secondary to megaesophagus and possibly hypothyroidism were suspected.
|Leukocytes||27.2||(6-12 X 109/l)|
|Canine TSH||0.75||(0-0.32 ng/ml)|
|Free T4||<1.93||(9-33 ?mol/l)|
Based on the results of laboratory testing and the presence of esophageal dilatation on thoracic radiographs, a diagnosis of generalized acquired megaesophagus
possibly associated with hypothyroidism was made. A diagnosis of acquired myasthenia gravis was thought to be unlikely since the acetylcholine receptor
antibody titer (AChR antibody) was negative.
Treatment and Clinical Outcome
L-thyroxin was initiated at 20?g/kg twice daily and enrofloxacin at 5mg/kg for10 days in order to prevent a
possible pulmonary infection secondary to aspiration. Mestinon therapy, initiated prior to receiving results of the AChR antibody testing, was stopped
following receipt of the negative titer. One month later, the dog was alert, active, and regurgitation had ceased. The dog had lost 3kg in body
weight. Thoracic radiographs showed the megaesophagus had markedly diminished (about 60%). The thyroxin blood level was too high at 5.3?g/100ml
so the dosage of L-thyroxin was decreased to 20?g/kg daily. Over the following month the thyroxin level became normal (2.4?g/100ml). Eleven months
later, the vitality and the general health status was still very good and the dog had a normal weight of 32kg. The hair coat was bright and full.
Repeat thoracic radiographs showed complete resolution of the megaesophagus (Figure 2. ). The thyroxin level was within normal range. Two years
later, the dog is still receiving L-thyroxin supplementation and is doing very well without reoccurrence of megaesophagus.
In a retrospective study of 29 dogs with neurologic signs and hypothyro?dism (1), 4 dogs suffered from a concomitant megaesophagus. At long term (16
months), 1 dog showed clinical and radiological resolution, 2 dogs improved with occasional episodes of regurgitation, and 1 dog was euthanitized
because of worsening of clinical signs. Hypothyro?dism could initiate a metabolic myopathy or neuropathy associated with a megaesophagus. On the
other hand, hypothyro?dism and megaesophagus could be the simultaneous expression of two distinct immune mediated disorders: thyro?ditis and an
immune mediated neuropathy. In a prospective study of risk factors leading to megaesophagus (2), a causal relationship between megaesophagus and
hypothyroidism could not be demonstrated. In this case, the complete resolution of the megaesophagus and other clinical signs of hypothyroidism
following therapy with L-thyroxin is highly suggestive of but does not prove a causal relationship between megaesophagus and hypothyroidism.
- Jaggy A., Oliver J.E., Ferguson D.C., Mahaffey E.A., Glaus T. Neurological manifestations of hypothyroidism: a retrospective study of 29 dogs.
J. Vet. Int. Med. 1994, 8:328-336
- Gaynor A.R., Shofer E.S., Washabau R.J. Risk factors for acquired megaesophagus in dogs. J. Am. Vet. Med. Assoc. 1997, 211 : 1406-1412