Megaoesophagus
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Description
Megaoesophagus describes a dilated, hypomotile oesophagus that is not able to effectively transport food from the the pharnyx to the stomach. The condition may be congenital or acquired and, although it occurs in most of the major domestic species, it is most common in dogs and cats. The following sections describe the causes of megaoesophagus.
Congenital Causes of Megaoesophagus
Congenital Achalasia
Congenital megaoesophagus is sometimes called 'congenital achalasia' but this implies a defect of the cardiac sphincter of the stomach (as in man) whereas the canine condition affects the whole oesophagus. This disease is inherited in a number of breeds of dog, including the fox terrier (in an autosomal recessive manner), miniature Schnauzer (dominantly with 60% penetrance), great Dane, Irish setter, Chinese Shar-pei, Newfoundland and German shepherd. It is also an inherited disorder of the Siamese cat where it may occur with concurrent pyloric stenosis.
The disease is thought to result from a delay in the maturation of either upper motor neurones in the central swallowing centre in the brainstem or of the afferent sensory arm of the peristaltic reflex. The resultant hypomotility of the oesophagus leads to a functional obstruction and boluses of food do not reach the stomach and are eventually regurgitated. The condition is seen most commonly at 6-7 months during a period of rapid growth. Affected animals eat, regurgitate shortly afterwards and may then attempt to eat again. As with any disease that results in chronic regurgitation, affected animals lose weight due to malnutrition and may also develop aspiration pneumonia.
Diagnosis, treatment, prognosis.
Vascular Ring Anomalies
Vascular rings are congenital abnormalities of the aortic arch system which interfere with oesophageal function. Many vascular anomalies (such as an aberrant right subclavian artery) are clinically silent and up to 20% of dogs and cats possess such an anomaly. Clinically significant anomalies are found most commonly in large breed dogs, particularly Irish setters and German Shepherd dogs.
The most common clinically significant anomaly is a dextra-aorta or persistent right aortic arch, in which the aorta is formed by the fourth right branchial arch rather than the fourth left arch. In the normal animal, the aorta, pulmonary artery and ductus arteriosus (which connects the former vessels) all lie of the left side of the oesophagus but, when the aorta develops on the right, the ductus arteriosus must pass across the oesophagus to reach the aorta. The oesophagus is then trapped within a ring formed by the aorta (on the right), the main pulmonary artery (on the left), the ductus arteriosus (or ligamentum arteriosum) dorsally and the heart base ventrally. The compression and stricture produced by the vascular ring prevents the passage of solid food beyond the heart base and food accumulates cranial to the obstruction. This results in dilation (megaoesophagus) which is usually confined to the cranial thoracic region but, in chronic cases, may extend along its whole length.
The condition usually becomes apparent at weaning when animals start to eat solid food that cannot pass through the constricted region. Affected animals tend to regurgitate undigested solid food but, if they are presented for treatment before severe dilation occurs, surgical correction of the ring (if possible) should result in a good prognosis.
Myasthenia Gravis
The classical focal form of myasthenia gravis affects the extraocular muscles, the cranial oesophagus and the muscles innervated by cranial nerves V, VII and IX. 80% of animals diagnosed with myasthenia gravis have megaoesophagus at presentation. Myasthenia gravis may itself be either congenital or acquired.
Acquired Megaoesophagus
Megaoesophagus may occur in any animal in which there is a disruption of the brain centres, reflex arcs or muscles responsible for normal peristalsis and tone along the oesophagus. Potential causes include:
- Brain Centres
- Viral encephalitides including those syndromes caused by canine distemper virus.
- Ingestion of toxins, including lead, mercury, botulinum toxin and absorption of tetanus toxin
- Reflex arcs
- Peripheral neuropathies, including labrador retriever myopathy and giant axonal neuropathy of German Shepherd dogs
- Feline dysautonomia, also know as Key-Gaskell Syndrome.
- Muscle
- Myositis and polymyositis/polyneuritis
- Muscle weakness as occurs with severe hypokalaemia and hypoadrenocorticism (Addison's disease).
The result is the presence of a hypotonic oesophagus which is distended by accumulated food material. The thoracic oesophagus from the thoracic inlet to the diaphragm is usually affected affected animals regurgitate repeatedly, lose weight and may develop aspiration pneumonia.
Diagnosis
Diagnostic Tests
Hematological and biochemical analysis of blood samples may be useful to diagnose several underlying causes for acquired megaoesophagus, such as myositis with an elevated creatine kinase (CK) concentration.
Diagnostic Imaging
Most cases of megaoesophagus can be diagnosed on plain radiographs of the chest. The following features may be observed:
- A large radiolucent structure dorsal to the trachea on lateral radiographs, but it should be noted that it is normal for the oesophagus to contain air if the animal is under general anaesthesia.
- The tissue between the trachea and oesophagus may be compressed producing a radio-opaque tracheal stripe sign along the dorsal surface of the trachea on a lateral radiograph. The trachea may also appear to deviate ventrally. On a dorso-ventral radiograph of the chest, the trachea may be pushed to the right by the enlarged oesophagus.
- Chronic regurgitation increases the risk of developing aspiration pneumonia. If it is present, it produces an alveolar pattern in the cranial and ventral lung lobes.
- With vascular ring anomalies, the oesophagus is usually only dilated cranial to the base of the heart.
The presence of megaoesophagus can be confirmed by feeding barium contrast medium, either as a liquid or mixed with meat (a 'barium meal'). The advantage of the barium meal is that it distends the oesophagus (to show the extent of its dilation) and it provides better definition of the point of constriction in animals with a vascular ring anomaly.
Endoscopy allows the operator to visualise the dilated oesophagus directly but this is rarely required to reach a diagnosis.