Aerodigestive disorders are caused by swallowing abnormalities, defects in airway protection or a combination of both, resulting in the initiation or worsening of respiratory disease. They may be mechanical or functional defects and can cause a spectrum of conditions ranging from coughing without any overt gastro-oesophageal (GE) signs to severe inhalation pneumonia.
Diagnosing a chronic cough without gastro-oesophageal signs
In coughing dogs, inhalation/aspiration should be suspected even in the absence of clinical GE signs. One study of 31 dogs presenting exclusively for a chronic cough found swallowing abnormalities fluoroscopically in over 80 percent, including in 9 out of 11 dogs with normal thoracic radiographs (Grobman et al., 2019).
In coughing dogs, inhalation/aspiration should be suspected even in the absence of clinical gastro-oesophageal signs
Radiographic evidence will be absent if only the upper respiratory tract is affected, ie in cases of laryngitis and/or tracheitis. Even if refluxed material reaches the lungs, it initially causes chemical pneumonitis, and radiographic changes may not be visible or may be delayed until there is a secondary infection and pneumonia.
Inhalation pneumonia may be visible, but changes may be overlooked because it typically affects the ventral middle lung fields overlying the soft tissue density of the cardiac silhouette in the lateral thoracic view.
The risk of aspiration/inhalation pneumonia in dogs with megaoesophagus (Figure 1) or after a tie-back procedure for laryngeal paralysis (LP) is well recognised in veterinary practice. Tie-back surgery prevents the normal closure of the laryngeal folds, but a more generalised progressive polyneuropathy, termed “geriatric onset laryngeal paralysis and polyneuropathy” (GOLPP), may concurrently affect the function of pharyngeal muscles involved in airway protective mechanisms.
Inhalation and sometimes nasal reflux are also risks in vomiting dogs, particularly if they are obtunded with an impaired vomiting reflex, as their airways are unprotected.
The rest of this article is restricted to aerodigestive disorders caused by oesophageal conditions that may be exacerbated by laryngeal dysfunction or other airway disorders.
Pathophysiology of aerodigestive diseases
A chronic cough without overt GE signs is thought to be caused by aerosolised refluxate containing gastric acid, digestive enzymes and bile acids. It may result in one or more of the following conditions: laryngitis, tracheitis, bronchitis, bronchiolitis or pneumonitis. This mechanism may also be involved in the progression of chronic airway disease and the development of pulmonary fibrosis.
The presence of bile acids in the saliva and bronchoalveolar lavage fluid has demonstrated extra-oesophageal reflux and aspiration in dogs with respiratory diseases (Kouki et al., 2023). Fluoroscopic studies confirm that asymptomatic GE reflux can occur, although subtle signs of reflux oesophagitis, such as lip-smacking, neck extension, dry swallowing, night restlessness and a “pain-face”, may be overlooked.
Subtle signs of reflux oesophagitis, such as lip-smacking, neck extension, dry swallowing, night restlessness and a “pain-face”, may be overlooked
GE reflux is normally prevented by a combination of positive intra-abdominal pressure and a “flap valve” that operates as the stomach expands in addition to the resting tone in the lower oesophageal sphincter (LOS). Thus, reflux can occur not only when there is transient relaxation of the LOS but also if a hiatal hernia is present. This allows the LOS to migrate cranially to the diaphragm and become subject to negative intrathoracic pressure, opening the LOS.
Increases in intra-abdominal pressure caused by obesity or pregnancy can also increase the risk of reflux.
Comorbidities and predispositions to aerodigestive diseases
Although aerodigestive disorders can occur in any dog, brachycephalic dogs are over-represented in the literature. In one study, 43 out of 51 brachycephalic dogs had evidence of GE reflux detected by oesophageal pH monitoring (Appelgrein et al., 2022). Several anatomical oesophageal abnormalities are recognised in brachycephalic dogs, including redundant oesophagus, idiopathic dysmotility and hiatal hernia.
Oesophageal hiatal size in brachycephalic dogs, measured by computed tomography, has been shown to be approximately twice the surface area compared to its surface area in weight-matched non-brachycephalic dogs (Conte et al., 2020). These anatomical defects certainly predispose brachycephalics to GE reflux. However, these abnormalities may be exacerbated by the concurrent presence of brachycephalic obstructive airway syndrome (BOAS).
BOAS comprises one or more of the following defects: stenotic nares, overlong soft palate, everted laryngeal saccules, laryngeal collapse and hypoplastic trachea. These can result in severe airway obstruction, and because of their dyspnoea, affected dogs must put in extra effort to inhale, resulting in increased negative intrathoracic pressure. This may provoke herniation of an anatomically abnormal hiatus; the stomach slides cranially as it is literally “sucked” into the thoracic cavity. A similar scenario occurs in dogs with LP.
Identification of aerodigestive disorders
Identification of a sliding hiatal hernia by plain thoracic radiographs is insensitive, as the herniation is intermittent. Barium swallows with fluoroscopy are much more sensitive, and using long-handled paddles to keep hands out of the X-ray beam while compressing the abdomen improves success (Reeve et al., 2017). However, the fluoroscopic equipment needed and the expertise to interpret the sometimes subtle, dynamic changes are mainly only found in referral centres.
In the author’s opinion, flexible oesophagoscopy is probably the more sensitive diagnostic technique for aerodigestive disorders, but there are no studies comparing the two methods yet.
Endoscopic identification of a sliding hiatal hernia can still be tricky; the stomach may not herniate while the patient is anaesthetised and intubated, as it is no longer dyspnoeic. Nevertheless, evidence of a hernia may be obvious with a fold or crease around the LOS and distal oesophagitis (Figure 2).
There are, however, a couple of manoeuvres that can be executed while performing endoscopy to make the herniation overt. Manual compression of the abdomen can cause herniation (Video 1). A more elegant technique is to occlude the endotracheal tube for three spontaneous breaths during oesophagoscopy (Broux et al., 2017). The increased negative intrathoracic pressure during this manoeuvre provokes herniation (Figure 3; Video 2). In humans, hiatal hernia can also be detected by a retroflexed view of the gastric cardia, but this has not yet been reported in dogs.
Treatment of aerodigestive disorders
Once aerodigestive disorders are recognised, medical treatment with proton pump inhibitors to block acid production and cisapride (more effective than metoclopramide) to increase the tone in the LOS may alleviate any respiratory signs.
In dogs affected by BOAS or LP, surgical correction may resolve the herniation (Poncet et al., 2006). However, dogs with BOAS may still need surgical correction of any hiatal hernia if the surgical treatment of the BOAS and medical therapy for the reflux fail.
Surgical correction (herniorrhaphy) involves a combination of oesophagopexy, phrenoplasty (apposition of the diaphragmatic crurae), fundic gastropexy and, sometimes, fundoplication (Figure 4).
