Feline megaoesophagus

This condition refers to oesophageal dilatation with absence of effective oesophageal peristalsis. Megaoesophagus has been termed oesophageal achalasia, oesophageal dilatation, oesophageal hypomotility, and oesophageal neuromuscular disease. Both congenital idiopathic (CIM) and acquired forms of megaoesophagus occur. Congenital megaoesophagus has been reported in Great Danes, German Shepherds, Irish Setters, Newfoundlands, Shar Peis, and Greyhounds. The condition occurs as an inherited disease in Wire-Haired Fox Terriers (autosomal recessive) and Miniature Schnauzers (autosomal dominant or 60% penetrance autosomal recessive). A suspected hereditary form has been reported in Bouvier des Flandres dogs (see Bouvier des Flandres myopathy). Idiopathic megaoesophagus is also reported in cats, with a predisposition noted in Siamese and Siamese-related breeds. The congenital form is usually apparent in animals around the time of weaning. Less commonly, adult-onset idiopathic megaoesophagus may be detected. Readers should refer to other texts for more information on megaoesophagus associated with obstructive oesophageal disease such as neoplasia, granulomas, vascular rings, strictures, periesophageal masses, and foreign bodies.


Barium meal showing large white-filled oesophagus

Acquired megaoesophagus may occur in dogs or cats at any age, although in one study, older (mean = 8 years), heavier dogs were at risk, including German Shepherds, Golden Retrievers and Irish Setters. In many cases, the cause is unknown; however, the condition has been observed in association with certain systemic neuromuscular disorders such as myasthenia gravis, botulism, hypoadrenocorticism (associated with glucocorticoid deficiency with or without concurrent mineralocorticoid deficiency), polymyositis, dermatomyositis, myotonic myopathy, Nemaline myopathy, polyradiculoneuritis, distemper, giant axonal neuropathy (German Shepherds), tick paralysis, lead toxicosis, thallium toxicosis, canine and feline muscular dystrophies and dystrophy-like conditions, laryngeal paralysis-polyneuropathy complex, dysautonomia, glycogen storage disorders, feline Mannosidosis, sensory ganglioradiculitis, and spinal muscular atrophy. In acquired myasthenia gravis in dogs, megaoesophagus may be the only clinical sign. It has also been reported sporadically in canine pituitary dwarfs, dogs with tetanus, and Labrador Retriever puppies with familial reflex myoclonus. In one report, megaoesophagus was noted in English Springer Spaniels with a polysystemic disorder comprising dyserythropoiesis, polymyopathy, and cardiac disease. Megaoesophagus may also occur with bilateral vagal nerve damage due to surgery, trauma, or neoplasia as well as with various brainstem lesions - neoplasia, distemper encephalitis, granulomatous meningitis / meningoencephalomyelitis, trauma, and infarction. It has also been observed in dogs secondary to tiger snake envenomation.

It has been stated that the relationship between hypothyroidism and megaoesophagus has yet to be established. In one report, megaoesophagus was found in 5 dogs with hypothyroidism and myasthenia gravis.

Causes

The pathogenesis remains elusive. Megaoesophagus may result from lesions involving the oesophageal muscle, or afferent/efferent pathways controlling oesophageal motility. Afferent pathways include oesophageal sensory receptors, afferent fibres in the vagus nerve and its branches (e.g., cranial laryngeal nerve), and the solitary tract/nucleus complex. The efferent limb of the reflex comprises the vagus nerves (including special visceral efferent axons from the nucleus ambiguous and general visceral efferent fibres from the parasympathetic vagal nucleus), neuromuscular junction, and oesophageal muscle (primarily skeletal, with less involvement of smooth muscle). Electrolytic lesions of the nucleus ambiguous in dogs and of the parasympathetic nucleus of the vagus in cats produce oesophageal dysfunction similar to the clinical syndrome. A reduction of the normal number of neuronal cell bodies in the nucleus ambiguous has been recorded in clinically affected dogs, but not in affected cats. These neuroanatomical differences between dogs and cats with megaoesophagus probably relate to differences in proportion of striated and smooth oesophageal muscle between the two species. In one study in 12 week old Chinese Shar Peis with CIM, no histological lesions were found in the nucleus ambiguous or parasympathetic nucleus of the vagus, or in ganglion cells of the myenteric plexus. CIM appears to be associated with loss of peristaltic function in the oesophagus due to developmental immaturity of innervation and/or musculature . Based upon studies of upper and lower oesophageal responses to intraesophageal balloon distension, CIM was considered to be at least partly due to a faulty afferent component of the reflex neural pathway that controls swallowing. Seemingly consistent with this finding, more recent studies on dogs with CIM showed no electrophysiological evidence for segmental demyelination or axonal degeneration in cervical vagal motor fibres innervating striated muscle of the thoracic oesophagus portion and no EMG abnormalities indicative of oesophageal muscle denervation or primary myopathy. Additionally, demonstration of a normal Hering-Breuer lung inflation reflex is consistent with an organ specific, selective vagal afferent dysfunction in dogs with CIM. Note that few cases of megaoesophagus appear to be related to disturbances of gastroesophageal sphincter function. In one study, resting caudal oesophageal sphincter pressure was similar in clinically normal dogs and in dogs with congenital or acquired idiopathic megaoesophagus.

Clinical signs

Clinically, megaoesophagus is characterized by postprandial regurgitation of undigested food, with radiographic evidence of megaoesophagus, usually to the level of the diaphragm. Abnormal oesophageal motility may be demonstrated by contrast radiography/fluoroscopy. In some dogs, respiratory signs such as cough, dyspnea and/or abnormal secretions may be the only signs observed.

Treatment

Recommended management includes elevated feeding and/or gastrostomy tube feeding of high caloric diets. Surgical treatment remains controversial. Pharmacological management, using drugs that relax the gastroesophageal sphincter or increase strength of oesophageal contractions, has been disappointing, although sildenafil, a phosphodiesterase-5 inhibitor, is reported to have profound effects on oesophageal motility in cats by modifying propagation and amplitude of oesophageal contractions. Nifedipine, a calcium channel blocker, resulted in temporary clinical improvement (2 months) in an adult German Shepherd with megaoesophagus.

Prognosis

Prognosis of congenital megaoesophagus in young animals is guarded. Some animals appear normal by the time they mature, based on radiographic, manometric, and clinical examination, while others show no clinical improvement with time. Acquired, idiopathic megaoesophagus generally has a poor prognosis for recovery, although transient megaoesophagus followed by spontaneous recovery has been reported occasionally. The prognosis for secondary megaoesophagus varies with the underlying cause, for example, cats with megaoesophagus and dysautonomia have a poor prognosis, while clinical improvement has been noted in dogs following treatment of the primary disease process, e.g., myasthenia gravis, hypoadrenocorticism, hypothyroidism, botulism, tetanus, and lead poisoning. Cachexia becomes an important complication and death is a common consequence of inhalation pneumonia.