Oropharyngeal swallowing impairment can be the result of oral, palatal, pharyngeal or pharyngoesophageal pathology. In humans, oropharyngeal swallowing impairment is particularly common amongst geriatric patients (70), secondary to aging or associated neurologic conditions. Elderly human patients have reduced lingual propulsion and delayed swallow response (70). They also have a smaller UES diameter, which could be due to reduced UES compliance or webbing, weak pharyngeal drive, and decreased hyolaryngeal traction (71, 72). In addition to aging, several neurologic conditions such as stroke (73), Alzheimer's disease (74), Parkinson's disease (75), neuromuscular diseases, and dementia (76) can cause pharyngeal weakness, discoordination, and/or UES dysfunction in humans. Similarly, pharyngeal weakness in dogs mainly occurs secondary to neuromuscular disorders [myasthenia gravis (77), muscular dystrophy (78), polymyositis (79), and polyneuropathies (80)]. Pharyngeal weakness typically occurs in middle-aged to older dogs, and can cause delayed propulsion of the bolus to the UES, with subsequent asynchrony between pharyngeal contraction and relaxation of the UES (81).

Esophageal swallowing impairment in canines is mainly caused by gastroesophageal disease with consequent esophagitis, structural lesions, or motility disorders. In both dogs and people, gastroesophageal disease due to GER ± hiatal herniation is the most common etiology (42–45). Brachycephalic (short-muzzled) breeds (French bulldogs, English bulldogs, pugs, and Boston terriers, boxers, shih-tzus) are frequently affected (Figures 3A–C) (82). Due to their nasofacial conformation and unique respiratory anatomy, they often have an upper airway tract obstruction, brachycephalic obstructive airway syndrome (BOAS), which increases negative intrathoracic pressure and causes subsequent hiatal herniation and GER (42, 50, 83). GER in dogs also frequently occurs during anesthesia secondary to reduction of LES tone (84, 85), and can lead to reflux esophagitis, esophageal dysmotility, and esophageal strictures (86) identical to peptic strictures in humans (Figures 4A,B,D) (32, 82). Other structural lesions such as tumors or vascular ring anomalies (87) occur in both species although dogs are at greater risk for esophageal foreign bodies (88) (Figure 4C) (82). Motility disorders in canine patients are frequently found in association with megaesophagus, which can be a congenital or acquired disease in dogs (Figure 4E) (82). Approximately 50% of dogs with megaesophagus have an acquired and idiopathic form, but there are many secondary causes of megaesophagus associated with polyneuropathies, polymyopathies, junctionopathies (myasthenia gravis, botulism, tick paralysis, tetanus, organophosphate poisoning), myopathies (inflammatory myopathies, dermatomyositis), and polyneuropathies (polyradiculoneuritis, dysautonomia) (89) in dogs. In humans, a sigmoid megaesophagus can develop as an end result of late-stage achalasia (90) (Figure 4F). Similarly, megaesophagus secondary to LES achalasia has been described in dogs and at one academic institution's teaching hospital, comprised 60% (14/23) of the megaesophagus cases seen over a 2-year span (40, 41). Esophageal motility disorders in the absence of megaesophagus such as juvenile esophageal dysmotility (51) or dysmotility secondary to GER have been identified in dogs (3, 42) but esophageal motility disorders in dogs are poorly characterized compared to those in humans in light of the limited application of HRM in animals (91).

The primary objectives of swallowing fluoroscopy are to localize the swallowing impairment and diagnose its etiology. Swallowing fluoroscopy can also detect tracheal aspiration or laryngeal penetration and guide the management of at-risk patients by modifying the diet consistency or specifically for human patients, teaching compensatory maneuvers.

In both dogs and people, structural abnormalities including strictures, vascular ring anomalies, foreign bodies, esophageal malignancy, and hiatal hernias or pharyngeal weakness, cricopharyngeus muscle achalasia, delayed opening of the upper esophageal sphincter, esophageal achalasia, esophageal dysmotility, and GERD can be diagnosed with swallowing fluoroscopy (3). Fluoroscopy can also confirm aerodigestive disorders in dogs and people with a respiratory history or signs (153, 154). However, there are several other esophageal disorders more commonly identified in humans. Examples include esophageal webs (thin, eccentric squamous epithelium membranes typically found in the proximal esophagus) or rings (thin extensions of tissue causing narrowing typically in the distal esophagus). Tissue in webs and rings is <3 mm in width compared to >3 mm in strictures (158). Other examples include spastic esophageal motility disorders such as distal esophageal spasm (112) or hypercontractile esophagus (113), viral and fungal infectious esophagitis (159), and numerous other causes of oropharyngeal (160) and esophageal swallowing impairment [diabetes mellitus (161), Alzheimer's disease (162), Parkinson's disease (163), Huntington's chorea (164), multiple sclerosis (165), and scleroderma] (16).

Swallowing fluoroscopy informs diet recommendations and safer feeding practices that can enhance swallow function and reduce aspiration risk in dogs and humans (3, 166). Duration of time spent upright in a Bailey chair to facilitate gravity-assisted feeding and treatment modifications can be advised for dogs with megaesophagus (41, 166). Follow-up fluoroscopy can also assess treatment responses or outcomes in dogs or humans that undergo hiatal hernia surgery or achalasia interventions (43, 167, 168).

During the fluoroscopic study, patients consume liquid and food boluses of various consistencies mixed with radio-opaque contrast material. In dogs, liquid contrast followed by soft canned food and kibble mixed with barium or iohexol are given, which is analogous to barium liquid, barium pudding, and barium biscuits used in humans. As patients swallow, images are captured as digitized recordings by a fluoroscopy unit at a preferred rate of 30 frames per second. Each frame is then analyzed frame-by-frame to assess swallow kinematics (2, 3).

Although the basic protocol is the same in humans, adjunctive techniques are used in people to minimize aspiration, expose impairments, and formulate therapeutic recommendations (57, 169–172). Firstly, patients are evaluated in both the lateral and anterior-posterior views to maximize visualization and diagnostic yield. Secondly, the patient is asked to perform multiple swallow tasks with varying bolus volumes (5 mL amounts, clinician-directed sips, self-directed swallows), viscosities (thin liquid, thick liquid nectar, thicker liquid honey), food textures (pudding and shortbread cookie), and delivery mechanisms (cup and spoon) to improve diagnostic yield (169, 170). Also, different temperatures, carbonation, and flavors are sometimes altered to change the sensory experience (173–176). Each distinct swallow type provides unique and complementary information for a holistic assessment of the patient. The shortbread cookie is the ideal method to assess oral clearance whereas large volume thin liquid boluses tend to reveal abnormalities in oropharyngeal dynamics (170). Higher viscosity barium or the addition of thickening gum-based agents may reduce the risk of aspiration and penetration. However, several studies show thickening agents can lead to increased post-swallow pharyngeal residues (177–180). Sour liquids can improve pharyngeal delays in swallows (174). The protocol can be further tailored to the particular patient. Larger, thicker boluses can be avoided if pharyngeal clearance of thin 5 mL liquid boluses already appears poor (169). Infants can be bottle-fed with different temperature milk-formulations at varying paces (175, 181). To evaluate the esophagus in more detail, double contrast examination can be performed where the human patient ingests gas producing effervescent tablets followed by barium. This technique fills the esophagus with both air and contrast to delineate mucosal irregularities (182, 183). A barium tablet (e.g., E-Z-DISK contains 700 mg of barium sulfate and is 13 mm in diameter) may also be used to better elucidate a stricture (184, 185).

Compensatory treatment strategies can then be trialed during the study (186). Patients may be asked to wait 3 s before swallowing (187, 188) or to hold their breath while swallowing (supraglottic swallow) to improve coordination and protect the airway from aspiration, respectively (186, 189). Patients may also be instructed to engage in a more forceful swallow (190) or adjust their head or neck postures to facilitate a stronger pharyngeal contraction (191). Specific lingual exercises may be tested during the videofluoroscopy to rehabilitate patients with stroke or traumatic brain injury (192, 193). Provocative maneuvers such as the water siphon test (patient asked to drink water while rolling into a right posterior oblique position) (194), cough stimulation, or Valsalva maneuvers can increase the detection of GER (195).

Although swallowing fluoroscopy is the gold standard diagnostic procedure to evaluate swallowing impairment in dogs, there are many challenges to conducting the study in this species. Variables such as body position (196), bolus size, bolus type (197), use of physical restraint (198), sedation (199), and equipment can impact the study (172). Positioning of the dog in lateral recumbency is associated with delayed cervical esophageal transit and fewer primary esophageal contractions compared to a standing or seated position (196) (Figure 7A). Seated positions increase the hydrostatic pressure against the lower esophageal sphincter (41) and upright feeding protocols accelerate bolus transit time due to gravity (200) (Figures 7B,C). Solid boluses increase primary peristalsis, but delay pharyngeal contraction and slow esophageal transit compared to liquid boluses (197). Larger boluses shorten the time to UES opening compared to medium sized boluses (197). However, it is virtually impossible to have a dog swallow a consistent bolus volume despite best efforts to facilitate this practice. Even if specified volumes or amounts are administered, dogs may intentionally fragment the bolus into several swallows, particularly if doing so minimizes their signs of swallowing impairment. Large and giant-breed dogs are more likely to swallow the entire solid bolus rapidly without chewing, whereas toy-breed dogs are more likely to chew and fragment the bolus before swallowing resulting in marked variation in bolus size.

Patient factors such as the dog's size, temperament, severity of disease, and willingness to eat or drink can also affect study quality and accuracy. Non-compliant or larger breed dogs are difficult to physically restrain, which introduces motion artifact and exposes personnel to radiation. Polycarbonate kennel devices and Bailey chairs can restrain the dog and limit radiation exposure to personnel. These restraint devices can also facilitate free-feeding protocols during which the dog voluntarily consumes liquid and solid boluses (41, 166, 198). However, sedation could still be required to calm anxious dogs to facilitate the study, which can potentially alter esophageal motility (201) and sphincter tone (199). Furthermore, stressed, anorexic, or severely ill dogs may refuse to voluntarily prehend the bolus, requiring force-feeding practices that increase the risk of aspiration pneumonia or pre-empt a complete evaluation. Additionally, the compensatory treatment strategies used in humans require complex verbal directions that cannot be relayed to dogs. Dynamic disorders that occur intermittently such as GER or sliding hiatal hernia might also be missed given the limited duration of the swallow fluoroscopy study.

Due to these enumerable factors, swallowing fluoroscopy procedures are challenging to perform in dogs and difficult to standardize. Study protocols vary between patients and veterinary institutions (Table 2) (3, 41, 43, 51, 81, 153, 166, 168, 196–198, 202, 203). A standardization initiative amongst veterinary institutions and practices would improve reproducibility, but even with a uniform methodology, variability exists amongst healthy dogs and interpreting radiologists (172, 198). Optimally, objective swallow metrics such as inter-swallow interval, time to UES opening, time to maximum pharyngeal contraction, pharyngeal constriction ratio (204), and esophageal transit time would be used (198). However, further research of these parameters is needed to establish normative data and prove their diagnostic validity in dogs. In humans, there are a multitude of established swallow metrics in healthy individuals with normative data (205, 206). The effects of age (207), gender (208), bolus volume (209, 210), viscosity (173, 177, 178, 180, 209–211), carbonation, and palatability (176, 212) on these parameters are much better understood in humans. The swallowing reflex and UES opening delays with age (207) and larger bolus volumes (209, 210), higher viscosity (178), and carbonation decrease the risk of penetration and aspiration in some populations (176, 212). There are also many standardized study protocols and training systems such as the Modified Barium Swallow Impairment Profile (MBSImp) protocol (169) to minimize interoperator and interrater variability. However, even for humans, a comprehensive, evidence-based set of practice guidelines that is used globally is lacking (213).

Swallowing fluoroscopy methodology should be more standardized in veterinary practice. This will help establish quantitative normative data in dogs. Separate reference ranges for swallow metrics should also be established for specific age groups from juvenile to mature adult to geriatric dogs. This would optimize evaluation of oropharyngeal and esophageal function in animals of different ages. Although standardization is essential, study methodology should also be tailored to specific patients and clinical scenarios. In patients with cricopharyngeus muscle dysfunction, smaller volumes or thicker consistencies can minimize aspiration and improve swallow safety although larger volume, thinner consistencies can reveal abnormalities and improve diagnostic yield (178, 180, 210, 211). For dogs with esophageal achalasia, timed barium esophagrams can be performed in which barium retention is assessed at specified time intervals following ingestion (214, 215). For patients with suspected hiatal herniation or GERD, provocative maneuvers akin to those used in humans could be performed to improve detection (83). The stomach can be maximally distended with food or air to stimulate TLESR and subsequent GER. Patients can be rotated into different body positions to elicit abnormalities. Lateral recumbency will apply more pressure to the fundus to encourage hiatal herniation (114) whereas a seated position will increase hydrostatic pressure against a hypertonic LES in patients with esophageal achalasia (41). Thus, both standardization and individualization are important to improve the diagnostic utility of swallowing fluoroscopy in veterinary medicine.

The clinical applications of swallowing fluoroscopy extend beyond diagnosis. It can be utilized to assess outcomes of medical (sildenafil, botulinum toxin, pneumatic dilation) (167, 216) or surgical management (myotomy with fundoplication) (167) of esophageal achalasia, prokinetic and proton pump inhibition or surgical treatment of GER or hiatal herniation (43, 168), and laser or surgical myectomy for cricopharyngeus muscle achalasia (39).

Fiberoptic endoscopic evaluation of swallowing (FEES) is a procedure commonly used in humans to evaluate oropharyngeal swallow function. FEES eliminates the use of specialized fluoroscopy equipment, contrast agents, and radiation required for swallowing fluoroscopy (217). Systematic reviews have shown an advantage of FEES over swallowing fluoroscopy to detect aspiration, penetration, and laryngopharyngeal residue in humans (218, 219). Guided observation of swallowing in the esophagus (GOOSE) can also be performed afterwards to identify a structural abnormality or evidence of delayed esophageal transit (220).

The nasal passage is routinely anesthetized with 4–5 drops of topical anesthetic in dogs, and the tip of a fiberoptic endoscope (2.9 mm outer diameter in dogs) is lubricated before being passed transnasally in an awake patient. The endoscope is advanced until the scope is positioned between the soft palate and tip of the epiglottis, facilitating observation of the base of the tongue, vallecula, larynx, and both pyriform sinuses. The patient is then offered boluses of liquid and food stained with food coloring. During each swallow, laryngeal and pharyngeal anatomy is directly observed to evaluate the integrity of pharyngeal function and document evidence of penetration or aspiration (217, 221).

Disadvantages of the procedure are that the endoscope can interfere with deglutition and pharyngeal contraction can cause transient image white-out. Patients may also experience excessive gagging, coughing, or anxiety with scope placement. FEES was successfully performed without sedation in 6 healthy dogs; however, the procedure warrants further assessment in dogs with swallowing impairment. Additionally, passage of the 2.9 mm diameter scope may not be feasible in smaller dogs or brachycephalic breeds with narrow nares (221).

HRM is utilized to diagnose esophageal motility disorders in humans after obstructing lesions have been ruled out. Motility disorders can be classified into disorders of EGJ outflow including esophageal achalasia (type I, II, or III) and EGJ outflow obstruction or disorders of peristalsis such as absent contractility, distal esophageal spasm, hypercontractile esophagus, and ineffective esophageal motility (91, 226). The key metrics analyzed with HRM are integrated relaxation pressure (IRP), distal contractile integral (DCI), and distal latency (Figure 9A) (222) to assess LES relaxation, strength of esophageal peristalsis, and latency of deglutitive inhibition, respectively, which help characterize the type of major or minor motility disorder. For example, an elevated IRP denotes an esophageal outflow obstruction or esophageal achalasia (Figure 9B) (222). An increased DCI is supportive of a hypercontractile disorder such as jackhammer esophagus (hypercontractile esophagus). A short distal latency indicates premature contractions as seen in distal esophageal spasm (91). Interpreting IRP, DCI, and distal latency in combination aids in subtyping esophageal achalasia as type I, II, or III (Figure 9C) (222). Type I (classic) achalasia is marked by the absence of peristalsis in 100% of swallows, an elevated median integrated relaxation pressure (IRP > 15 mm Hg) and distal contractile integral (DCI) <100 mm Hg/s/cm; type II by panesophageal pressurization in ≥20% of swallows (most common subtype) and elevated median IRP (>15 mm Hg), and type III as elevated median IRP (>15 mm Hg) and premature, contractions in ≥20% of swallows with DCI > 450 mm Hg/s/cm (227).

An accurate diagnosis and classification optimizes treatment and informs prognosis. Esophagogastric junction (EGJ) outflow obstruction may resolve spontaneously or with discontinuation of opioid medications (58). Spastic disorders including distal esophageal spasm and type III (spastic) achalasia benefit from peroral endoscopic myectomy (POEM) surgery while type I or II achalasia have better outcomes with interventions focused on the LES (pneumatic dilation or Heller's myotomy of the LES) (228–231). HRM can also diagnose hiatal hernias (232) or evaluate esophageal motility before (233) and after anti-reflux fundoplication surgery (234, 235).

In contrast to human medicine, HRM is still a relatively novel procedure in veterinary medicine and has been utilized predominantly in healthy dogs. However, the procedure has been successfully conducted in awake dogs (53, 236, 237) and results showed vigorous peristaltic contractions with solid compared to liquid boluses and a significant increase in LES pressure induced by cisapride compared to metoclopramide (53, 236). Thus, with further study, HRM could improve the characterization and treatment of esophageal motility disorders in dogs.

HRM can be conducted in both humans and dogs with a similar protocol except for a series of provocative maneuvers. The procedure begins with a temperature and pressure calibration. The calibrated manometric catheter is then placed transnasally in the awake patient. Humans should be positioned supine (12–30 degree angle) and dogs restrained in sternal recumbency or seated. The nasal passage is topically anesthetized with lidocaine jelly (238) ± a combination nasal spray of 1% tetracaine and 0.05% oxymetazoline (239) and the catheter is lubricated to facilitate passage of the flexible catheter into the nasopharynx and down the esophagus (Figures 8B,C) (53). Two to 3 mL of water can be administered orally to trigger peristalsis and advance the catheter past the LES. The distal 3–4 sensors are positioned intragastrically and the UES and LES should be visible on the image display as two bands of higher pressure above and below (Figures 5A,B) (53). Once the catheter position is set and the patient has acclimated to the catheter, baseline recording can begin. After baseline data is obtained, 5 mL boluses of water at room temperature are given and a swallow is recorded. This process is repeated until 10 consistent, intact swallows are obtained (227). In larger sized dogs and tall human patients with an esophageal length >30-cm, the manometry catheter may not span the entire esophagus, and the catheter is placed distally to span the distal esophagus and LES for acquisition of topographic data before being pulled proximally to repeat the procedure at the proximal esophagus, UES, and pharynx (53).

After the standard 10 water swallows, “provocative maneuvers” (240) are attempted in human patients to further test esophageal function. A multiple rapid swallow test (241) is performed by asking the person to swallow 5 times in quick successions. This helps reveal impairments in deglutitive inhibition or weak peristaltic reserve to diagnose achalasia (242) or ineffective esophageal motility (243), respectively. The individual is then moved to an upright position where at least 5 liquid swallows and another rapid swallow test are performed. The upright posture better replicates normal eating and improves detection of motility disorders (200) and hiatal herniation (244). If findings are equivocal, solid foods may be administered (245, 246) or post-prandial monitoring (247) is performed to improve diagnostic yield.

HRM has great potential in veterinary medicine, but there are many challenges to its implementation in dogs. Even with appropriate physical restraint of the patient, placement and retention of the manometric catheter is variably successful in the awake dog due to operator experience, patient non-compliance, and nasopharyngeal and esophageal anatomy. Dogs can be challenging to restrain during passage of the catheter transnasally, and are extremely sensitive to catheter insertion, particularly brachycephalic breeds with their shorter muzzle, upper airway obstruction, and respiratory distress. The catheter can be forcefully sneezed out due to nasal irritation, causing transient epistaxis and potential damage to the fragile pressure sensors along the catheter. In patients with megaesophagus or a sigmoid esophagus from esophageal achalasia, the catheter can coil and loop back on itself in the distended esophagus or meet physical resistance at the LES. Even if the catheter is placed successfully, it can be challenging to maintain in place, particularly while the dog is swallowing liquid or food boluses. Sedation can calm the dog to facilitate placement and retention, but sedatives or tranquilizers such as butorphanol and acepromazine can affect manometric parameters (237), and chronic opiate administration is known to affect esophageal function in humans (248).

Additionally, the provocative maneuvers applied in humans can be imitated in dogs, but are more challenging to replicate. For example, dogs can be kept in an upright position in a Bailey chair if they are amenable, but movement of their heads is uncontrolled and dogs with osteoarthrosis may be uncomfortable sitting on their haunches. Furthermore, this position does not emulate the physiologic feeding position of dogs as quadrupeds. Pre-determined bolus weights or volumes can be syringe-fed at specific intervals to perform a multiple rapid swallow test, but dogs may partially swallow the bolus at irregular intervals or retain material from multiple boluses in their mouths before swallowing at unpredictable times.

Furthermore, as with swallowing fluoroscopy, HRM methodology and data interpretation has not been standardized in dogs, whereas in humans there is a standardized protocol and consensus (the Chicago Classification v4.0) to diagnose and categorize esophageal disorders (91). In addition to the challenges of performing and interpreting HRM in dogs, the cost of manometry probes, hardware modules, and software costs is excessive for most veterinary practices and pet owners, and the procedures are not covered by veterinary insurance. The procedure is thus restricted to a few veterinary academic institutions in which it is predominantly utilized as a research tool.

Further evaluation of HRM in dogs with swallowing impairment is needed to develop a system analogous to the Chicago Classification. However, the many challenges encountered with HRM could impede its widespread application in veterinary practice. Meanwhile, the role of HRM in human medicine is expanding. Emerging applications in humans include the implementation of three-dimensional (3-D) HRM, in which there is a 9-cm segment lined with 12 pressure sensing loci and 8 radially dispersed pressure sensors at each locus, to evaluate pressure distributions at the pharynx, UES, and EGJ (249) or combination HRM-impedance technology to evaluate post-prandial TLESRs (250) or belching disorders (247). HRM-impedance technology is also being trialed in humans to assess bolus transit and post-residue swallows in patients with oropharyngeal (251) and esophageal dysmotility (252, 253).

Once metrics and normal reference ranges are established, HRM should be performed in dogs with swallowing abnormalities to appreciate the spectrum of esophageal dysfunction. Performing HRM in clinical patients could confirm esophageal motility disorders similar to those found in humans including ineffective esophageal motility, hypercontractile esophagus, distal esophageal spasm, and esophageal achalasia. HRM could also help differentiate the causes of oropharyngeal dysfunction or assist in the diagnosis of hiatal herniation and GER in dogs. Improved detection of these disorders will invariably improve treatment and patient outcomes.

pH-metry is used in humans to diagnose GERD, classify phenotype as erosive or non-erosive reflux disease (NERD), and direct treatment. Human patients symptomatic for GER are initially treated with a proton pump inhibitor (PPI), but up to 33% of patients do not respond to a 2-week PPI course (261, 262). For PPI-refractory patients, upper endoscopy and ambulatory pH monitoring are recommended to evaluate for confirmatory evidence of GER. pH monitoring can also be used in conjunction with methods that assess gastric motility to assess whether delayed gastric emptying is exacerbating GER (20, 263).

In contrast, ambulatory reflux monitoring is understudied and underutilized in veterinary medicine. pH monitoring in dogs has mainly been performed under anesthesia with catheter-based techniques (29, 164–185). Numerous factors such as age (84, 264), sex (265, 266), breed (267), body size (268), type of surgery (84, 269), length of pre-operative fasting (85, 264, 270), body position (84, 269), anesthetic agents (morphine, acepromazine, inhalant gases) (271–275), and use of maropitant (276, 277), acid suppressant (omeprazole and esomeprazole) and prokinetic medications (metoclopramide and cisapride) (54, 276, 278–281) have been evaluated in association with peri-anesthetic GER. However, results have been variable and often conflicting possibly due to differing anesthetic protocols, definitions of reflux, and methods of pH measurement (Table 3) (45, 54, 84, 85, 264–283). For example, in some studies, increasing age and prolonged pre-anesthetic fasting were identified as risk factors for GER (84, 270), but were found to be protective in others (264). Changes in body position were associated with acid reflux in one study (269), but had minimal effect on GER in other studies (84, 275). Medications such as metoclopramide and omeprazole (280, 281) were initially found to reduce reflux under anesthesia, but subsequent publications refuted this claim (276). pH/impedance technology has now shown that PPIs only raise esophageal pH enough to mitigate acid reflux (54, 279), but cisapride significantly decreases both acid and non-acid reflux (54).

The multichannel intraluminal impedance/pH catheter (MII-pH) is a 2.13 mm (6.4Fr) diameter catheter made of polyurethane. It has 6–8 impedance sensors, spaced 2-cm apart, and 1–2 pH sensors. After calibration in the appropriate pH buffer solutions, the catheter is placed transnasally or transorally (for assessment of GER in anesthetized canine patients) into the esophagus such that the proximal pH sensor is 5-cm above the LES or 6-cm proximal to the EGJ (Figures 10A,B). The catheter is affixed to the patient's face or around the dog's muzzle and kept in place for the duration of the study. Catheter-free ambulatory pH monitoring can be performed with a wireless Bravo pH capsule. Although impedance data cannot be obtained, pH data can be collected for up to 96 h, which minimizes variance and increases diagnostic sensitivity (284). The pH capsule is deployed with endoscopic guidance and tethered to the esophageal mucosa 6-cm proximal to the EGJ with a suction and locking pin mechanism (Figure 10C). The capsule measures pH every 6 s and transmits data to a receiver every 12 s using radio telemetry. Within 5–7 days, the capsule naturally detaches from the esophagus and passes through the intestinal tract. Data from the receiver can then be uploaded to a computer software program for analysis (285, 286). The primary outcome measure assessed is distal esophageal acid exposure time (AET). Acid exposure time >6% denotes pathologic GER, and <4% is considered physiologic in humans. Symptom association indices are also evaluated to determine the correlation of reflux events and patient reported symptoms of heartburn and regurgitation (255).

pH testing should be utilized more frequently in veterinary medicine to evaluate GER in non-anesthetized canine patients. Unfortunately, transnasal placement in an awake dog and retention of the probe for 24 h is challenging. However, ambulatory pH monitoring utilizing the wireless pH capsule (Bravo) is catheter-free and enables continuous pH data collection in the awake patient for up to 96 h. Results can show daily fluctuations in pH in association with the patient's symptoms, meal or water intake, body position, and activity (284, 287). As previously mentioned, the major limitation of wireless pH capsule monitoring is that it lacks impedance technology and therefore cannot diagnose non-acid reflux events or belching/rumination (repetitive regurgitation of undigested food due to a learned behavior) disorders. Additionally, minor technical complications such as breaks in data transmission, early dislodgement, or capsule retention can occur. However, non-trivial complications including esophageal wall trauma or capsule aspiration are rare (288). Another drawback to placing pH capsules in dogs is that the procedure requires general anesthesia and optimal positioning for placement is reliant on human studies. In humans, the capsule is consistently placed 6-cm proximal to the EGJ, but dogs have variable esophageal and sphincter lengths (114). Thus, a standard 6-cm distance may not be applicable to all dogs and could contribute to variation in pH readings. Another significant limitation to using pH monitoring in dogs is that normative data and metrics for analysis have not been established. In a study that used nuclear scintigraphy to evaluate reflux in healthy dogs, reflux events occurred on average twice every 5 min, however, pH monitoring was not performed to discern whether these were acid reflux events (289). A study that performed ambulatory pH monitoring in 7 healthy dogs documented a median of 10 acid reflux events (range 1–65) over a median study duration of 45 h (45).

In contrast to canines, there are conclusive criteria to diagnose GERD in humans. These have been summarized and published in the Lyon Consensus of 2018. They include both endoscopic criteria and metrics such as number of reflux episodes per 24 h, AET, SI (symptom index), and SAP (symptom association probability) (255). These metrics can even be assimilated into a DeMeester composite score (290) which directly correlates with endoscopic findings of erosive esophagitis. Endoscopic findings are even graded with established classification schemes such as the Los Angeles Classification system (291).

For example, if advanced grade erosive esophagitis (Los Angeles classification grade C and D) (Figure 4D), long-segment Barrett's mucosa, or peptic strictures are seen on endoscopy or distal esophageal AET > 6% on ambulatory pH or pH-impedance monitoring in a human patient, GERD is confirmed (255). However, <50% of human patients have endoscopic evidence of esophagitis (292, 293). These patients may have non-erosive reflux disease (NERD) or functional esophageal disorders.

If endoscopic findings are normal, but AET is >6%, NERD is diagnosed. If endoscopy and AET are both normal, further testing with pH-impedance is warranted to document non-acid reflux. Additionally, symptom-reflux association metrics SI (294) and SAP (295) should be analyzed. SI is the percentage of symptom events preceded by reflux episodes and SAP is the probability that symptoms and reflux events are associated. Patients with SI > 50% and SAP > 95% are predictive of better responses to medical and anti-reflux surgical therapy (296, 297). pH monitoring can also be repeated after anti-reflux surgery to assess surgical response. Symptom-reflux association is also used to diagnose functional esophageal disorders of altered nociception. Human patients with normal endoscopy and AET, but high symptom-reflux association may have reflux hypersensitivity. Alternatively, if symptom-reflux correlation is poor, functional heartburn is possible and cognitive behavioral therapy may be necessary (255).

The Lyon consensus also recognizes the heterogeneity of GERD in humans. For example, although an AET <4% is considered normal and > 6% defines pathologic reflux, patients may be diagnosed with borderline or inconclusive GERD based on low-grade esophagitis on endoscopy, equivocal AET between 4 and 6%, or normal AET but positive symptom-reflux associations.

In such cases, further testing with impedance and HRM may be warranted to interrogate for non-acid reflux, evaluate esophageal mucosal permeability, assess LES tone, and screen for poor esophageal contractility and delayed acid clearance (255). Novel impedance metrics including baseline impedance (298, 299) and post-reflux swallow-induced peristaltic wave (PSPW) index (300) can be used to assess esophageal mucosal integrity and peristalsis following a reflux episode, respectively. Although normative values for these impedance metrics are not yet available in humans, low baseline impedance is indicative of alterations in intercellular space and tight junctions secondary to reflux, and abnormal PSPW reflects diminished peristalsis and prolonged acid clearance (298, 301).

Additional pH/impedance studies should be performed in awake, healthy dogs to establish normative reference ranges for key metrics. This will provide a contextual basis to perform studies in clinical canine patients that help differentiate physiologic reflux, pathologic reflux, and functional/hypersensitivity conditions. A relevant patient population to study would be brachycephalic dogs given the high prevalence of GER and hiatal herniation (42, 50). Dogs with signs of aerodigestive disease would also be a pertinent patient group. The wireless Bravo pH capsule could help evaluate whether cough or nasal symptoms were correlated to reflux events. Once a classification system is developed, pH monitoring can be used in dogs, as it is in humans, to diagnose GERD phenotype, direct therapy, and assess response to medical or surgical treatments.

Electrodiagnostic testing, including electromyography, nerve conduction velocity testing, and repetitive nerve stimulation, can confirm neuromuscular causes of swallowing impairment (315–317). Results can also guide selection of muscle and nerve biopsy sites (318). In humans, electromyography can also be used as a screening tool for dysphagia (319) and as biofeedback to guide swallowing rehabilitation (320). In dogs, electrodiagnostics have elucidated esophageal physiology (321) and explored pathology in patients with laryngeal paralysis (322) and megaesophagus (323–325). This has led to the hypotheses that vagal afferent dysfunction and secondary alterations in biomechanical properties explain the pathogenesis of idiopathic megaesophagus in dogs (323–326). Electromyography has also played an integral role in facilitating the diagnosis of inflammatory myopathies, including polymyositis, an immune-mediated disorder well-documented in Boxers and Newfoundlands (134).

Electromyography is performed by inserting electrodes into skeletal muscles of interest such as pharyngeal, laryngeal, esophageal, thoracic, and pelvic limb musculature. Abnormal spontaneous electrical activity, characterized by scattered fibrillation potentials, positive sharp waves, and complex repetitive discharges, is consistent with a myopathy or neuropathy. Motor and sensory nerve conduction velocity testing and repetitive nerve stimulation measure electrical activity in muscles following nerve stimulation (316). Muscle biopsies can then be obtained to analyze affected myofiber types and identify features of inflammation, atrophy, or necrosis. Immunofluorescence staining on muscle biopsies can also characterize antibodies, major histocompatibility complexes, or T-lymphocytes to suggest polymyositis or identify protein deficiencies associated with muscular dystrophy (134). Nerve biopsies can be analyzed for inflammatory infiltrate, axonal degeneration, axonal dystrophies, demyelination, and nerve regeneration (318).

In humans, conventional electromyography electrodes can be placed intramuscularly in awake patients. Surface electromyography with adhesive electrodes (319) or skin patches can also be performed (320) in awake patients and can characterize swallowing disorders (327). In dogs, general anesthesia is required, which makes the procedure more costly and labor-intensive. Furthermore, procedural technique must be adhered to strictly to limit confounding variables. Electrodes must be grounded properly and inserted at various depths and locations in the muscles. Motion artifact must be minimized and the temperature of the muscle should be held constant because varying temperatures can affect readings (316). These requirements and specifications make these procedures complex and challenging to perform in veterinary practice.

EndoFLIP has numerous indications in humans, but the primary use is to evaluate distensibility of the EGJ for pathology. EndoFLIP can complement or further support HRM and esophagography, particularly for patients in whom results are mismatched or equivocal (333, 334). An abnormally low EGJ-DI is expected in human patients with EGJ outflow obstruction or achalasia as compared to an elevated EGJ-DI in patients with GERD (331, 335). However, FLIP can also be used intra-operatively or post-operatively in achalasia and GERD patients to tailor surgical procedures. Real-time intra-operative data can guide surgeons in adjusting the extent of the myotomy or tightness of the fundoplication wrap (Figure 11B). Post-operative DI can then be compared to pre-op and intra-operative data to assess surgical outcome and predict clinical outcome (218–221). FLIP can also be used to measure stricture dimensions and guide balloon dilation in patients (336). This is particularly helpful for strictures in the pharyngoesophageal region because this area can be difficult to visualize endoscopically (337). In human patients with eosinophilic esophagitis, FLIP can monitor disease activity and track fibrotic remodeling that can occur with chronic inflammation (338). A less common indication of FLIP is to diagnose hiatal herniation by identifying a double-sphincter image caused by the separation of the LES from the crural diaphragm with the hiatal hernia in between (339). EndoFLIP could theoretically be used for the same indications in dogs; however, only EndoFLIP 1.0 has been evaluated in dogs to date in brachycephalic breeds following hiatal hernia surgery (Figure 11C) (43, 168).

The EndoFLIP balloon catheter is placed transorally under sedation or light anesthesia. The catheter is positioned with a couple sensors in the stomach and the mechanical pump fills the balloon to a volume of 20 mL with an electrolyte solution of known conductance. At this point, the EGJ should be visualized as a narrowing of the hourglass shape on the image display. After 15–30 s, additional 10 mL aliquots of volume are instilled with wait periods of 30–60 s between until the balloon is distended to the recommended volume (70 mL). The volumetric distension should trigger secondary peristalsis for the contractile pattern to be observed with EndoFLIP 2.0. EGJ-DI can also be assessed once the volume is at least 60 mL and pressure is at least 15 mmHg. After all measurements are obtained, the balloon is deflated and the catheter is removed (329).

Although EndoFLIP is an attractive technological modality, there are a few barriers to entry in veterinary medicine. Firstly, the cost of EndoFLIP 2.0 equipment costs ~$70,000, excluding the cost of the single-use catheters ($350 each) (329). In the study of brachycephalics undergoing open hiatal hernia surgery, significant changes to EGJ geometry and DI were not found. Furthermore, the characteristic double-sphincter view indicative of a hiatal hernia was not observed in any of the dogs pre-operatively (43). Brachycephalic breeds most commonly have type I hiatal herniation, which is intermittent and dynamic (43, 168). Thus, measuring EGJ-DI at static, set volumes and time-points may not be adequate to detect sliding herniation. The mechanics of the EGJ are also incredibly complex, involving the hiatus, phrenoesophageal ligament, crural diaphragm, angle of His, and the LES (36). By obtaining measurements intraluminally, EndoFLIP may not be capturing all the relevant exatraluminal components and pathophysiology. It is also unclear whether distension of the EndoFLIP 2.0 balloon will stimulate the same contractile patterns in dogs because the musculature of the distal esophagus is different from that in humans (35). Another potential drawback to EndoFLIP is the patient must be anesthetized for the procedure, which alters LES pressure profiles and puts patients at heightened risk for aspiration pneumonia and GER. Additionally, variables including body position (244), obesity (340), and brachycephalic conformation (341) can alter the anatomy and mechanics of the EGJ junction. Finally, a logistical hurdle may be providing adequate training to personnel to ensure that EndoFLIP is conducted consistently and interpreted properly.

If the safety and feasibility of EndoFLIP is proven in dogs, the technology may be useful to characterize EGJ outflow obstruction, esophageal achalasia, or GERD disorders in dogs. It is also possible that EndoFLIP could document hiatal herniation in brachycephalic breeds with a higher intra-balloon pressure, a different anesthetic protocol, or the newer EndoFLIP 2.0 module. EndoFLIP could also be used in the operating room to assess GEJ distensibility and predict patient outcomes following fundoplication. EndoFLIP could potentially be used to evaluate dogs with cricopharyngeal disease, but first requires further study of the UES in humans. Furthermore, a study in healthy dogs showed greater variability in UES measurements and significant effects of body position (55). Other potential confounding variables such as patient size and body condition may also significantly affect measurements.