Megaoesophagus

The term megaoesophagus is used to describe oesophageal dilatation. There are various different causes of such dilatation, including factors such as hypomotility, loss of normal peristalsis and obstruction._^1,2,3 Megaoesophagus may be congenital or acquired and this latter type can be distinguished into primary (idiopathic) or secondary forms._⁴

ETYMOLOGY

From the Greek mega (large) and issofagos (oesophagus).

PATHOGENESIS    

Megaoesophagus (congenital and acquired) can be caused by changes in the visco-elastic capacity of the oesophagus, by a lack of vagal response in the intraluminal distension of the oesophagus or by dysfunction of the afferent vagal fibres of the deglutination reflex._^2,5,6,7,8

Congenital megaoesophagus has a hereditary basis in the Wire Fox Terrier and the Miniature Schnauzer. Congenital megaoesophagus also occurs frequently in the German Shepherd dog, Great Dane, Labrador, Newfoundland, Shar-Pei and Irish Setter: although a genetic defect has been strongly suspected in these breeds, one has never been demonstrated. Congenital megaoesophagus can affect both purebred and mongrel dogs. The clinical signs may appear from 3 months to 1 year of age. Although congenital megaoesophagus is very rare in cats, the breed most predisposed to this condition seems to be the Siamese.

Acquiredmegaoesophagus is divided into idiopathic and secondary forms. Acquired megaoesophagus has been found in all breeds although some seem to be more predisposed (German Shepherd dog, Great Dane, Irish Setter). 

There are various causes of secondary megaoesophagus, some of which are rare. Myasthenia gravis, the underlying cause in almost 25% of cases,_^9,10 is an immune-mediated disease in which the production of antibodies against nicotinic receptors for acetylcholine (Ach) results in interruption of neuromuscular transmission And, therefore, muscle weakness. Myasthenia gravis is distinguished into congenital (rarer) and acquired forms. In its turn the acquired form is divided into two types: generalised and local. Most dogs with the generalised form have muscle weakness and megaoesophagus. In the focal form the muscle weakness predominantly affects the oesophagus, pharynx and facial muscles.

The diagnosis of myasthenia gravis is made on the basis of measurements of antibodies against the Ach receptors in the blood. It is important to appreciate that the concentration of anti-Ach antibodies is much lower in the local form than in the generalised one and, indeed, the examination may give negative results in animals with generalised acquired myasthenia gravis._¹¹ Furthermore, in the case of congenital myasthenia gravis, there is a deficiency of receptors for Ach and so antibodies are often not detectable in the blood in animals with this condition._¹

Other disorders that can cause secondary megaoesophagus include endocrine diseases such as hypoadrenocorticism_¹² and hypothyroidism,_^13,14 dysautonomia,_^15,16 toxic substances,_^17,18 vascular defects,_¹⁹ inflammation of the oesophagus and neoplasms such as thymoma_^20-22 (Table 1).

Idiopathic megaoesophagus is rare and often fatal. It occurs spontaneously in animals from 5 to 12 years of age which present with a dilated, hypomotile oesophagus. The aetiopathogenesis is unknown but appears to be related to a lack of opening of both the upper and lower oesophageal sphincters during oesophageal distension as a result of a defect in afferent sensory innervation or in the function of the oesophageal muscles._^7,23

CAUSES

Table 1. Causes of megaoesophagus

CLINICAL SIGNS  

The most common clinical sign is regurgitation. Pet owners often confuse regurgitation with vomiting, but it is crucial to distinguish between the two in order to make the correct diagnosis and, consequently, to be able to give the right treatment. Regurgitation is characterized by passive evacuation of fluids, saliva and undigested food from the oesophagus, while vomiting is accompanied by nausea and abdominal contractions.

Halitosis, ptyalism, regurgitation and weight loss are clinical signs of the megaoesophagus itself. Cough and dyspnoea are frequent complications caused by aspiration pneumonia secondary to the accidental inhalation of oesophageal matter into the lungs.

In the case of secondary acquired megaoesophagus other clinical signs may be present, depending on the underlying pathology, and include generalised weakness, muscle tenderness, problems with locomotion and gastrointestinal signs.

DIAGNOSIS

The diagnosis of megaoesophagus is made by imaging. X-rays of the neck and chest can show the dilated oesophagus containing air, fluids or ingesta (Fig. 1). Often, however, a radio-opaque barium meal is needed to be able to exclude stenosis, obstructions or foreign bodies or, when the oesophagus appears normal, to evaluate possible hypomotility in the absence of oesophageal dilatation. Fluoroscopy is an excellent technique for evaluating oesophageal motility, but when it is not possible, radiography with a contrast agent (oesophagograms) during and immediately after ingestion of a radio-opaque meal can be used (Figs. 2 and 3). Signs of aspiration pneumonia include alveolar and interstitial infiltrates (Fig. 4); the right, middle lobe is the area of the lung most frequently involved._²⁴

Oesophageal motility is better evaluated by fluoroscopy, scintigraphy andmanometry, but unfortunately these techniques are available in only very few veterinary referral centres. 

Oesophagoscopy (Fig. 5) is rarely necessary and should only be used in suspected cases of oesophagitis, stenosis, obstruction, foreign bodies or neoplasm.Once the presence of megaoesophagus has been confirmed, it must then be determined whether the condition is primary or secondary.  Subjects with primary megaoesophagus often present with a very dilated oesophagus with no peristaltic movements;  in cases of secondary megaoesophagus, there is less dilatation and some residual motility. A recent study^₂₅ showed a small, but statistically significant increase in the diameter of the oesophagus in non-myasthenic animals compared to myasthenic ones, but this finding is of limited importance in veterinary practice. Further examinations that may be useful are full blood count, blood biochemistry profile (including electrolytes and creatinine kinase), urinalysis and, depending on the clinical signs, assays of anti-acetylcholine antibodies, an ACTH stimulation test, a thyroid screen and other tests if indicated (e.g. electromyography, muscle biopsies).

TREATMENT

It is important to treat any underlying pathologies and severe complications, such as aspiration pneumonia and dyspnoea, which could further worsen the patient’s condition.

In animals with secondary megaoesophagus the causative disorder must be identified and treated  (e.g. myasthenia gravis with pyridostigmine or neostigmine; hypoadrenocorticism with corticosteroids and mineralocorticoids). Treatment can lead to an improvement in the clinical signs (even achieving a complete remission of the megaoesophagus) but over a variable time. It is extremely important to ensure that these patients have an adequate nutritional supply and that their vomiting and regurgitation is treated to avoid further problems.

Since there is a high risk of recurrence when drug treatment is suspended, the animal must be monitored closely and the owner encouraged to report any changes.

In the case of idiopathic megaoesophagus, the treatment is symptomatic and centred mainly on feeding techniques to minimise regurgitation. The animal needs small, frequent meals with a high calorie content to supply all the nutrition necessary. Animals with idiopathic megaoesophagus derive enormous benefit from being in a vertical position during a meal and remaining upright for several minutes after the meal. This can be achieved by putting the food somewhere high (e.g. on a table) or keeping the animal vertical (Figs 6, 7and 8). Regurgitation is decreased considerably in this way.

The type of diet chosen depends heavily on the patient’s clinical presentation and capacity to swallow. A liquid diet usually passes more easily through the oesophagus under the effect of gravity.

If the patient does not tolerate feeding by mouth, possible options are introduction of a gastric tube, percutaneous endoscopic gastrostomy (PEG) or various surgical techniques. Gastric tubes can be maintained for quite a long time (we have left a PEG tube in situ for several months in a patient with myasthenia gravis, without complications).  Oesophageal tubes are contraindicated since they can worsen the regurgitation and increase the risk of aspiration pneumonia.

Drugs to reduce the production of gastric juices (e.g. ranitidine, omeoprazole) can be used, particularly in the case that oesophagitis is suspected to be the principal cause of the megaoesophagus. 

The use of prokinetic drugs (metoclopramide, cisapride, etc.) is not advised, not only because of problems related to their action (metoclopramide) or limited availability (cisapride), but also because of a lack of documented evidence of their efficacy. Metoclopramide acts at the level of serotoninergic receptors in smooth muscles, but the canine oesophagus is formed entirely of striated muscle. Cisapride is not commercially available in various countries and it probably slows the transit of food through the oesophagus._⁴

PROGNOSIS

The prognosis of animals with congenital megaoesophagus is poor, with the possibility of recovering oesophageal function varying between 20 and 40%._⁴ Some animals can improve with growth, particularly if the oesophagus has not irreversibly lost its elasticity.

In some cases of acquired secondary megaoesophagus, oesophageal function can be restored if the underlying cause (e.g. Addison’s disease, oesophagitis) is treated effectively, whereas in other cases (e.g. myasthenia gravis) this does not always occur.  

The prognosis of idiopathic megaoesophagus depends heavily on how early the diagnosis is made and on the response to treatment. 

The commonest reasons for treatment failure are aspiration pneumonia, persistent regurgitation, which causes debilitation of the patient, and the lack of food intake which leads to cachexia and death (usually by euthanasia because of the patient’s poor quality of life).

References

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