Aspiration pneumonia is a common clinical diagnosis in dogs, and underlying disorders associated with this disorder include vomiting, laryngeal disease or surgery, megaesophagus or esophageal dysfunction, and decreased level of consciousness.1–8 Aspiration pneumonia results from inhalation of oropharyngeal or gastrointestinal contents into the respiratory tract, which triggers chemical, bacteriologic, and immunologic damage to the airways9–11 and can progress to acute respiratory distress syndrome.12 Early clinical recognition of aspiration pneumonia allows appropriate treatment of both the airway injury and the underlying disease process that lead to aspiration, thereby preventing additional damage to the lower portion of the respiratory tract through repeated aspiration.
Aspiration pneumonia is clinically identified through the development of respiratory tract signs and radiographically detectable infiltrates in a person or animal at risk. To the authors' knowledge, definitive diagnosis of aspiration pneumonia via evaluation of biochemical markers such as pepsin in bronchoalveolar lavage fluid has not been described in the veterinary medical literature; therefore, identification of underlying disorders associated with aspiration is essential to limit the development of aspiration pneumonia in hospitalized patients and to provide preventive advice for owners of dogs that have certain diseases.
Despite the common occurrence of this disease syndrome, little information is available on the relative incidence of underlying diseases and rate of survival in affected dogs. The purpose of the study reported here was to evaluate the number and types of underlying disorders detected in dogs with aspiration pneumonia and determine the survival rate among affected dogs.
In humans, the mortality rate associated with multilobar lung disease is higher than that associated with involvement of a single lung lobe13; thus, a secondary goal of the study was to establish the relationship between radiographic severity of disease and outcome. We hypothesized that dogs with multiple underlying disorders associated with aspiration pneumonia or radiographic evidence of more extensive pulmonary infiltrates would be hospitalized for a longer period and have a decreased survival rate, compared with dogs with a single causative disorder or less extensive pulmonary infiltrates. It was envisaged that results of the study would provide clinically useful prognostic information for dogs with aspiration pneumonia.
Materials and Methods
Case selection—Medical records of dogs dated January 1, 2004, to January 1, 2006, at the Veterinary Medical Teaching Hospital of the University of California, Davis, were searched to identify dogs with a clinical diagnosis of aspiration pneumonia. This time frame was chosen to generate a sufficient number of cases for evaluation from the period prior to transition from conventional film screen radiography to digital radiography. This was specifically done so as to eliminate any confounding variability caused by differing imaging techniques. Dogs were included in the study if aspiration pneumonia was listed in the medical record under the record field for the final clinical diagnoses and radiographic images illustrating pulmonary infiltrates were available for review. Cases with other forms of pneumonia were not evaluated.
Medical records review—Medical records were examined by 2 of the authors (DAK, LRJ), and clinical information was abstracted. At the discretion of the primary care clinician, diagnostic tests to identify the underlying causes of aspiration were performed as clinically indicated on the basis of the history and physical examination findings. Historical features, clinicopathologic findings, and results of diagnostic tests were reviewed by a board-certified internist (LRJ) to verify the diagnosis by use of criteria (variably documented in individual records) as follows: witnessed or suspected regurgitation or vomiting episodes followed by acute onset of respiratory difficulty, cough, or tachypnea; physical examination findings consistent with lower respiratory tract disease or radiographic detection of pulmonary infiltrates in a dog at risk for aspiration; and identification of systemic disorders or interventions potentially associated with aspiration, including anesthesia, esophageal dysfunction, laryngeal disease, gastrointestinal tract disease, and decreased consciousness in dogs with clinical or radiographic evidence of pneumonia. Medical records and radiographic reports were also scrutinized for comments that reflected clinical suspicion of an aspiration event and clinical or clinicopathologic evidence of hypoxemia. Records were also reviewed to identify all potential syndromes that could contribute to an aspiration event.
Assessments were performed to elucidate gastrointestinal and extragastrointestinal causes of vomiting; these included serum biochemical analyses and abdominal ultrasonography to assess intestinal function, abdominal radiography (with and without administration of contrast medium) to identify gastrointestinal structural abnormalities and motility disorders, and histologic examination of gastrointestinal biopsy samples to identify inflammatory or infectious disease. Additional evaluations included urinalysis, renal and hepatic ultrasonography, and assessment of canine pancreatic-like immunoreactivity. Disorders associated with regurgitation or dysphagia were investigated via cervical and thoracic radiography and videofluoroscopy (after administration of liquid or barium-soaked kibble to assess esophageal motility) where indicated. Underlying causes for megaesophagus were identified via serum anti-acetylcholine receptor antibody detection, assessment of blood lead concentration, ACTH stimulation test (detection of hypoadrenocorticism), and endoscopy. Dogs with focal (esophageal) myasthenia gravis were considered to have only esophageal disease, whereas those with esophageal disease and lower motor neuron signs were considered to have both esophageal and neurologic disease. Disorders of laryngeal structure or function were diagnosed via direct observation of the larynx while dogs were in a light plane of anesthesia. Dogs that had had arytenoid lateralization surgery performed > 2 months prior to evaluation for aspiration pneumonia at the hospital were placed in the same group as dogs with other laryngeal disease. Prolonged recumbency because of neurologic or neuromuscular disease and seizures or obtundation associated with neurologic disorders were recorded as potential causes of aspiration. Dogs that developed aspiration pneumonia in an immediate perioperative period were assigned decreased level of consciousness as an underlying cause for aspiration. All additional historical information and diagnostic test results were examined to detect any other disease process that could result in development of aspiration pneumonia. Dogs were assigned to categories with regard to the type of disease identified and were also classified by the number of different underlying etiologies.
Thoracic radiographs that were obtained soon after the aspiration event were reviewed by a board-certified radiologist (REP); however, the exact timing (in hours) of radiography in relation to a potential aspiration event could not be determined. Thoracic radiographs (2-, 3-, and 4-view sets) obtained for each dog were evaluated for the type and severity of pulmonary infiltrates. The radiologist was unaware of the history, clinical course of disease, or outcome for each case. A score was assigned on the basis of the severity (relative opacity) of interstitial and alveolar infiltrates. The scoring system applied for an interstitial pattern was as follows: 1 = mild, 2 = moderate, and 3 = severe. The scoring system applied for an alveolar infiltrate was as follows: 4 = mild, 5 = moderate, and 6 = severe. To generate an overall radiographic assessment of the extent of lung involvement, the score was multiplied by the number of apparently affected lung regions to provide a subjective radiographic severity score, with higher numbers reflecting more extensive pulmonary involvement (Figure 1). This methodology has not been established as a rigorous determination of disease severity for aspiration pneumonia, but it was considered clinically relevant for the purposes of the study. For the purposes of the study, lung regions were defined on the basis of the lobar branching pattern in dogs14; because the left cranial lung lobe in dogs is comprised of cranial and caudal segments (and each was considered independently), the location of infiltrates was described as left cranial-cranial segment, left cranial-caudal segment, or left caudal, right cranial, right middle, accessory, or right caudal lobe.
Dorsoventral radiographic views of the thorax of 2 dogs with aspiration pneumonia. For each dog, a score was assigned on the basis of the severity (relative opacity) of interstitial and alveolar infiltrates. An interstitial pattern was scored as 1 (mild), 2 (moderate), and 3 (severe) and an alveolar infiltrate was scored as 4 (mild), 5 (moderate), and 6 (severe). The score was multiplied by the number of apparently affected lung regions to provide a subjective radiographic severity score, with higher numbers reflecting more extensive pulmonary involvement. Because the left cranial lung lobe in dogs is comprised of cranial and caudal segments (and each was considered independently), the location of infiltrates is described as left cranial-cranial segment, left cranial-caudal segment, or left caudal, right cranial, right middle, accessory, or right caudal lobe. A—In this dog, a severe alveolar infiltrate (score 6) is identified in the right cranial lung lobe. The subjective radiographic severity score for this dog is 6. B—In this dog, a severe alveolar infiltrate (score 6) is identified in the left cranial-cranial segment, left cranial-caudal segment, and left caudal lobe (3 affected regions). The subjective radiographic severity score for this dog is 18.
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1748
Dorsoventral radiographic views of the thorax of 2 dogs with aspiration pneumonia. For each dog, a score was assigned on the basis of the severity (relative opacity) of interstitial and alveolar infiltrates. An interstitial pattern was scored as 1 (mild), 2 (moderate), and 3 (severe) and an alveolar infiltrate was scored as 4 (mild), 5 (moderate), and 6 (severe). The score was multiplied by the number of apparently affected lung regions to provide a subjective radiographic severity score, with higher numbers reflecting more extensive pulmonary involvement. Because the left cranial lung lobe in dogs is comprised of cranial and caudal segments (and each was considered independently), the location of infiltrates is described as left cranial-cranial segment, left cranial-caudal segment, or left caudal, right cranial, right middle, accessory, or right caudal lobe. A—In this dog, a severe alveolar infiltrate (score 6) is identified in the right cranial lung lobe. The subjective radiographic severity score for this dog is 6. B—In this dog, a severe alveolar infiltrate (score 6) is identified in the left cranial-cranial segment, left cranial-caudal segment, and left caudal lobe (3 affected regions). The subjective radiographic severity score for this dog is 18.
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1748
Dorsoventral radiographic views of the thorax of 2 dogs with aspiration pneumonia. For each dog, a score was assigned on the basis of the severity (relative opacity) of interstitial and alveolar infiltrates. An interstitial pattern was scored as 1 (mild), 2 (moderate), and 3 (severe) and an alveolar infiltrate was scored as 4 (mild), 5 (moderate), and 6 (severe). The score was multiplied by the number of apparently affected lung regions to provide a subjective radiographic severity score, with higher numbers reflecting more extensive pulmonary involvement. Because the left cranial lung lobe in dogs is comprised of cranial and caudal segments (and each was considered independently), the location of infiltrates is described as left cranial-cranial segment, left cranial-caudal segment, or left caudal, right cranial, right middle, accessory, or right caudal lobe. A—In this dog, a severe alveolar infiltrate (score 6) is identified in the right cranial lung lobe. The subjective radiographic severity score for this dog is 6. B—In this dog, a severe alveolar infiltrate (score 6) is identified in the left cranial-cranial segment, left cranial-caudal segment, and left caudal lobe (3 affected regions). The subjective radiographic severity score for this dog is 18.
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1748
Dogs with aspiration pneumonia were examined by multiple services throughout the hospital, including the emergency, internal medicine, neurology, and surgical services. Some dogs had aspiration pneumonia at the time of admission to the hospital, whereas others developed aspiration pneumonia during hospitalization. For analysis of specific variables, dogs were initially stratified as discharged from the hospital, hospitalized (regular ward or intensive care unit), or died or were euthanatized at the time of examination. For any dog that was admitted to the hospital, the number of days that it remained in an intensive care unit and duration of hospitalization were recorded.
Statistical analysis—Data are presented as mean ± SD, and median values and ranges are reported where appropriate. Via χ2 analysis,a survival rates were compared between dogs with interstitial and alveolar infiltrates and between dogs with involvement of 1 and > 1 lung region. For some analyses, dogs were grouped into those that were or were not admitted to the hospital. Radiographic severity score was compared between these 2 groups, and for hospitalized patients, the radiographic score was compared between those that survived to discharge from the hospital and nonsurvivors. Days spent in an intensive care unit and duration of hospitalization were compared between dogs that survived and those that died or were euthanized on initial examination or during hospitalization. All analyses were performed by use of a Mann-Whitney U test for nonparametric data.a
For comparison of outcome, dogs were also grouped according to the number of underlying disease processes identified. Among dogs that had 1, 2, or 3 underlying disease processes associated with aspiration pneumonia, duration of hospitalization, days spent in an intensive care unit, radiographic severity of lung infiltrates, and outcome were compared by use of a Kruskal-Wallis comparison for categorical data.a Survival rates were also compared among dogs with specific predisposing conditions for aspiration pneumonia, including neurologic disorders, laryngeal disease, esophageal dysfunction, and postanesthetic aspiration by use of a χ2 analysis.a For all analyses, significance was set at a value of P < 0.05.
Results
Search of the electronic medical database yielded medical records of 105 dogs for which aspiration pneumonia was listed as a final clinical diagnosis. Two cases were excluded because review of the medical records failed to identify supportive clinical evidence for aspiration pneumonia, and 15 cases were excluded because of the lack of radiographs for review. Medical records for the remaining 88 dogs were comprehensively reviewed.
Clinical, clinicopathologic, and radiographic findings in the dogs included in the present study have been reported elsewhere.15 Among the 88 dogs, a single underlying disease process associated with aspiration pneumonia was identified in 60 (68%); 2 disease associations were identified in 23 (26%) dogs, and 3 disease associations were identified in 5 (6%) dogs. The most common underlying disorders included esophageal disease or dysfunction (n = 35), vomiting (34), neurologic disorders (24), laryngeal disease (16), and postanesthetic aspiration (12).
In the 35 dogs with esophageal disease, the cause was determined as megaesophagus (n = 25) or esophageal motility disorders (6) or hiatal hernia (1); causes of regurgitation were unidentified in the remaining 3 dogs. Megaesophagus was considered idiopathic in 13 of the 25 dogs and was associated with diffuse myasthenia gravis (n = 9), focal myasthenia gravis (2), or persistent right aortic arch (1) in the remaining dogs. Esophageal dysmotility was idiopathic in 3 dogs and caused by cricopharyngeal achalasia in 2 dogs and oropharyngeal dysphagia in 1 dog.
In the 34 dogs that had vomiting, the condition was related to primary inflammatory, functional, or neoplastic gastrointestinal tract disease in 8, toxins or foreign bodies in 8, renal disease in 5, pancreatic disease in 4, and liver disease in 2; causes were unidentified in 7 dogs.
In the 24 dogs with neurologic disorders, muscle weakness associated with generalized myasthenia gravis was identified in 9 dogs, all of which had megaesophagus; 8 dogs had spinal cord disease resulting in recumbency, and 7 dogs had a seizure disorder (1 dog had concurrent myasthenia gravis). Four of 7 dogs with seizures were receiving antiseizure medication long term. In dogs with spinal cord disease, the diagnoses included intervertebral disk disease in 5 dogs, meningomyelitis encephalitis (determined via magnetic resonance or computed tomographic imaging and CSF analysis) in 2 dogs, and degenerative myelopathy in 1 dog.
Laryngeal disease was reported for 16 dogs, among which laryngeal paralysis was the most common diagnosis (n = 14). Arytenoid lateralization had been performed 2 months to 3 years prior to development of aspiration pneumonia in 6 dogs and within 24 hours in 3 dogs. Uncorrected laryngeal paralysis was reported for 3 dogs, and a laryngeal foreign body and previous laryngectomy were each reported for 1 dog.
Overall, 68 of 88 (77%) dogs survived the episode of aspiration pneumonia and were discharged from the hospital. Survival rates did not differ among dogs with aspiration pneumonia associated with esophageal, laryngeal, gastrointestinal, or neurologic disease processes or postanesthetic aspiration (Table 1).
Diseases identified in dogs with aspiration pneumonia and survival rates among affected dogs. More than 1 disease process were defined in some dogs.
| Disease category | Underlying disease process | No. of dogs affected | No. (%) of survivors |
|---|---|---|---|
| Esophageal disease (n = 35) | Megaesophagus | 25 | 27 (77) |
| Motility disorder | 6 | ||
| Hiatal hernia | 1 | ||
| Unknown | 3 | ||
| Vomiting (n = 34) | Primary gastrointestinal disease | 8 | 25 (74) |
| Toxin or foreign body | 8 | ||
| Renal disease | 5 | ||
| Pancreatic disease | 4 | ||
| Liver disease | 2 | ||
| Unknown | 7 | ||
| Neurologic disease (n = 24) | Generalized myasthenia gravis | 9 | 15 (62) |
| Seizures | 7* | ||
| Spinal cord disease | 8 | ||
| Laryngeal disease (n = 16) | Laryngeal paralysis | 14 | 12 (75) |
| Previous debarking surgery | 1 | ||
| Laryngeal foreign body | 1 | ||
| Postanesthetic aspiration (n = 12) | 12 | 10 (83) | |
One dog had concurrent myasthenia gravis.
In 60 dogs in which a single disorder was identified, the most common finding was esophageal disease (n = 23 [38%]), followed by vomiting (20 [33%]), neurologic disease (7 [12%]), laryngeal disease (6 [10%]), and postanesthetic aspiration (4 [7%]; Table 2). Twenty-three of 88 (26%) dogs had 2 underlying disorders. The combination of esophageal and neurologic disease was most common (7/23 [30%]); those dogs had generalized weakness and megaesophagus secondary to diffuse myasthenia gravis. Neurologic disease was identified in an additional 7 dogs that had 2 underlying disease processes associated with aspiration pneumonia (4 dogs with concurrent vomiting and 3 dogs with aspiration pneumonia that developed as a postanesthetic complication). In the remaining 9 dogs, various combinations of disease processes were identified. In 5 dogs with 3 identified disease associations, laryngeal disease was most common. Laryngeal disease was detected in 2 dogs that vomited in the postanesthetic period and in 2 dogs with concurrent neurologic disease and either esophageal disease or vomiting. An additional dog with neurologic and esophageal disease developed aspiration pneumonia after a vomiting episode. There was no identifiable difference (P = 0.30) in survival rates among dogs with 1, 2, or 3 causes of aspiration pneumonia.
Survival rates (%) and radiographic scores in dogs with 1, 2, and 3 underlying disease processes associated with aspiration pneumonia.
| No. of disease processes | Disease or disease combination | No. of dogs | No. of survivors | Mean radiographic score* | Range of radiographic scores |
|---|---|---|---|---|---|
| 1 | Esophageal disease | 23 | 19 | 7 | 2–14 |
| Vomiting | 20 | 17 | 7 | 4–20 | |
| Neurologic disease | 7 | 5 | 8 | 1–24 | |
| Laryngeal disease | 6 | 5 | 5 | 3–8 | |
| Postanesthetic complication | 4 | 10 | 10 | 1–20 | |
| Overall | 60 | 50 | ND | ND | |
| 2 | Esophageal and neurologic disease | 7 | 6 | 5 | 2–10 |
| Vomiting and neurologic disease | 4 | 2 | 7 | 2–15 | |
| Vomiting and laryngeal disease | 3 | 1 | 7 | 2–10 | |
| Neurologic disease after anesthesia | 3 | 1 | 9 | 6–10 | |
| Laryngeal disease after anesthesia | 2 | 2 | 7 | 4–10 | |
| Esophageal disease and vomiting | 2 | 1 | 15 | 12–18 | |
| Esophageal and laryngeal disease | 1 | 1 | 5 | NA | |
| Vomiting after anesthesia | 1 | 1 | 5 | NA | |
| Overall | 23 | 15 | ND | ND | |
| 3 | Laryngeal disease and vomiting after anesthesia | 2 | 2 | 7 | 3–16 |
| Laryngeal, esophageal, and neurologic diseases | 1 | 0 | 2 | NA | |
| Laryngeal and neurologic diseases and vomiting | 1 | 1 | 12 | NA | |
| Neurologic and esophageal diseases and vomiting | 1 | 0 | 4 | NA | |
| Overall | 5 | 3 | ND | ND |
*Actual value presented if 1 dog in subgroup. NA = Notapplicable. ND = Not done.
Eleven of 88 (12%) dogs were discharged from the hospital on the day of examination. Seven were treated with a β-lactam antimicrobial, 2 were treated with a β-lactam antimicrobial combined with enrofloxacin, and 1 was treated with enrofloxacin alone. The remaining dog received no antimicrobial treatment because the aspiration pneumonia was considered to be mild. Two of the 11 dogs were euthanatized within 30 days because of continual aspiration, 1 dog was lost to follow-up, and the remaining 8 dogs were known to have survived from 6 to 810 days. The remaining 77 of 88 (88%) dogs were hospitalized; 57 (74%) dogs survived to discharge from the hospital, and the remaining 20 (26%) dogs died (n = 2) or were euthanatized (18) during hospitalization. Reasons for euthanasia included a poor prognosis associated with concurrent neoplasia (n = 9), complex medical conditions (3), sepsis (2), the need for ventilator therapy (2), refractory regurgitation (1), or meningomyelitis (1). The role of aspiration injury in failure to survive could not be fully determined. However, necropsies were performed in 14 of 20 dogs and revealed evidence of aspiration pneumonia in 11; in the remaining 3 dogs, pulmonary congestion, metastasis, or fibrosis was evident.
Duration of hospitalization did not differ (P = 0.76) between the group of dogs that survived (mean ± SD, 5.0 ± 4.5 days; range, 1 to 23 days) and those that died or were euthanatized (5.4 ± 4.5 days; range, 1 to 19 days). In dogs that were admitted to the hospital, there was no significant (P = 0.10) difference in the mean number of days spent in the intensive care unit for those that survived (2.0 ± 0.3 days) or did not survive (3.2 ± 0.7 days). Comparison of data for dogs with 1, 2, or 3 underlying diseases processes associated for aspiration pneumonia revealed no significant (P = 0.53) difference in mean number of days spent in the intensive care unit or duration of hospitalization.
Survival rates did not differ between dogs with infiltration of 1 and > 1 lung region (P = 0.58) or between dogs with interstitial and alveolar infiltrates (P = 0.65). Scores for the severity of infiltrates determined via assessment of thoracic radiograph views ranged from 1 (a mild interstitial infiltrate in 1 lung region) to 24 (severe alveolar infiltrates affecting 4 lung regions). A bimodal distribution of severity (centered on scores of 4 and 10) was evident (Figure 2). There was no significant difference in the radiographic score between dogs that were or were not hospitalized (ie, discharged immediately after initial examination). The mean radiographic severity of infiltration in dogs that were hospitalized and later discharged was 7.3 ± 5.0 (range, 1 to 24); this value did not differ significantly (P = 0.57) from that of dogs that died or were euthanatized during hospitalization (mean, 6.6 ± 4.4; range, 1 to 15). The 3 dogs with the most severe radiographic scores (18, 20, and 24, respectively) all survived to discharge from the hospital.
Distribution of subjective radiographic severity scores in 88 dogs with aspiration pneumonia.
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1748
Distribution of subjective radiographic severity scores in 88 dogs with aspiration pneumonia.
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1748
Distribution of subjective radiographic severity scores in 88 dogs with aspiration pneumonia.
Citation: Journal of the American Veterinary Medical Association 233, 11; 10.2460/javma.233.11.1748
Antimicrobial treatment was not given to 4 of 77 dogs; these dogs were admitted to the hospital but were euthanatized prior to drug administration. Airway sampling was performed in 4 of the 88 dogs. The most common antimicrobial treatment administered was a β-lactam antimicrobial combined with enrofloxacin (50/73 [68%] dogs); of the dogs treated with this drug combination, 39 survived to discharge from the hospital and 11 did not. Two dogs received metronidazole in addition to this combination. Among the 73 dogs receiving antimicrobial treatment, a β-lactam derivative alone was given to 20 (27%), enrofloxacin alone was given to 2 (3%), and amikacin was given to 1 (1%). Nebulization with sterile saline (0.9% NaCl) solution followed by coupage was performed in 30 of 57 (53%) dogs that survived to discharge from the hospital and in 9 of 20 (45%) dogs that did not survive. No specific treatment could be associated with survival or duration of hospitalization.
Discussion
In the present study, the specific underlying disease processes in dogs with aspiration pneumonia were determined and the survival rates associated with the detection of 1 or more of these diseases were evaluated. Vomiting, esophageal dysfunction, neurologic disease, laryngeal disorders, and postanesthetic aspiration were detected (in decreasing order of frequency) in the dogs included in this study. Overall survival rate in these dogs was high, and contrary to our hypothesis, the severity of pulmonary infiltrates (determined radiographically) and the presence of more than 1 disease process associated with aspiration pneumonia did not negatively impact outcome.
Among the 88 dogs included in our study, aspiration pneumonia was most commonly associated with either esophageal dysfunction or vomiting in the absence of additional identifiable disease processes. In a recent study16 in humans, 23% (41/180) of patients that died from medically managed gastroesophageal reflux disease developed aspiration pneumonia, indicating the serious health threat of gastrointestinal dysfunction. The severity or chronicity of vomiting in the dogs included in the present study could not be accurately assessed because of the retrospective nature of the investigation; however, no specific entries in the medical record detailed copious regurgitation or vomiting. It is possible that the dogs may have had subclinical or transient concurrent laryngeal dysfunction that allowed aspiration or that the volume of fluid vomited may have overwhelmed laryngeal capacity. The competence of the larynx was not assessed in all dogs, although none had historical or physical examination findings suggestive of laryngeal dysfunction. Subclinical dysphagia resulting in silent aspiration (ie, aspiration that occurs without any signs of swallowing difficulty) has been reported as a contributor to pneumonia in humans with a variety of disorders17 and may play a role in veterinary patients also.
Megaesophagus can result in aspiration pneumonia because of chronic, recurrent, or massive regurgitation or from excessive pooling of secretions in the oropharynx with overflow into the respiratory tract. In the dogs with aspiration pneumonia in the present study, megaesophagus was related to myasthenia gravis in 11 dogs, of which 9 had generalized weakness and 2 had focal myasthenia involving only the swallowing apparatus. In a clinical study18 of acquired myasthenia gravis, generalized muscle weakness was reported in 64% (16/25) of affected dogs, and the combination of systemic weakness and esophageal dysmotility likely contributed to development of aspiration pneumonia in the current study. Mortality rates of 48% to 60% have been reported for dogs with focal, generalized, or acute fulminant myasthenia gravis, and death is often associated with aspiration pneumonia.1,18 However, in the current study, only 3 of 9 affected dogs failed to survive.
Generalized weakness from myasthenia gravis was the most commonly identified neurologic disorder in the dogs of the present study; however, recumbency associated with spinal cord disease was also common. Muscle weakness and deconditioning from thoracolumbar disease may have increased the likelihood of an aspiration event. Seizure disorders were identified in 7 dogs with aspiration pneumonia, and 3 dogs had no other underlying disorder that could result in aspiration pneumonia. In contrast, humans with epilepsy that are otherwise healthy rarely develop aspiration pneumonia.5 Use of phenobarbital for seizure control could play a role in development of aspiration pneumonia if excessive sedation was present or if liver dysfunction resulted in vomiting; these conditions were not identified in dogs of our study.
Aspiration pneumonia developed in an immediate perioperative period in 12 dogs and in most instances (8 dogs) was associated with the presence of an additional risk factor for aspiration. Aspiration pneumonia following anesthesia is uncommon in people, although the rate in children or during emergency surgeries is higher than that in adults; many affected humans have additional diseases that could contribute to the phenomenon of aspiration.7,19,20 The prevalence of aspiration pneumonia following anesthesia has not been fully examined in veterinary medicine to our knowledge, although the risk is well known clinically. Monitoring of pH at the lower esophageal sphincter during anesthesia has revealed gastroesophageal reflux in approximately 17% (47/270) of dogs,21 indicating the risk for esophageal irritation and subsequent regurgitation that could lead to aspiration injury.
Aspiration pneumonia has been reported as a common complication of laryngeal surgery in dogs.2,22 This likely results from a combination of reduced laryngeal sensation and deficient motor responses, as well as anesthetic-induced depression of reflexes including the gastroesophageal reflex. Surgical technique, experience, and postoperative management probably also have a role in development of aspiration injury. Aspiration pneumonia in dogs has been reported less frequently when arytenoid lateralization is performed by a single surgeon.23 In human medicine, laryngeal dysfunction or surgery is variably reported as a contributor to aspiration pneumonia. Vocal fold immobility contributes to aspiration after esophagectomy, although 40% (8/21) of patients with normal vocal fold function may also aspirate.24 Humans that undergo vocal fold surgery25,26 rarely develop aspiration pneumonia as a complication. In the present study, 6 dogs with laryngeal paralysis developed aspiration pneumonia 2 months to 3 years after surgery; such a delayed occurrence has been previously reported.2 Although some dogs had additional underlying diseases associated with aspiration, 6 dogs with laryngeal disease (4 with laryngeal paralysis) did not have additional identifiable disorders, suggesting that laryngeal dysfunction alone may predispose to aspiration pneumonia, perhaps as a result of silent aspiration. However, full neurologic work assessments were not performed in all dogs of our study, and it is possible that generalized neuromuscular disease was not detected.27
Some findings in the present study were unexpected. First, radiographic evidence of disease severity was not related to duration of hospitalization or with overall survival rate. Similarly, a recent evaluation of radiographic findings in dogs with pulmonary blastomycosis failed to identify an association between a weighted index of radiographic severity and survival,28 indicating that prognosis for survival should not be based on radiographic evidence of disease. The scoring scheme used in the present study has not been used previously. It was deliberately simplistic for ease of clinical application and would not be suitable for evaluation and comparison of cases with other disease processes. For example, a dog with a moderate diffuse interstitial pattern associated with chronic bronchitis would generate a relatively high radiographic severity score (2 × 7 = 14), but that disease process is obviously not comparable to aspiration pneumonia. It is possible that the timing of radiographic view acquisition in relation to disease made it impossible to detect differences. Radiographic views obtained soon after the aspiration event were scored for severity; however, the number of hours that elapsed after the aspiration event prior to radiography could not be accurately assessed. Dogs with radiographically evident interstitial infiltrates may have been in the early stages of the disease or have already progressed into the recovery phase. Progressive changes in infiltrates could not be evaluated because of variation in the number and timing of additional radiographic evaluations. Thus, results may have been influenced by a time delay between the generation of radiographic views and the onset or zenith of aspiration pneumonia. It is also possible that radiographic evidence of pulmonary involvement did not correlate with duration of hospitalization or outcome because radiographic changes lag behind clinical changes in physiologic processes or function. It was not possible to determine whether the dogs' conditions were improving or destabilizing or whether aspiration pneumonia or pneumonitis was present at the time that radiography was performed.
Two other unexpected findings were that survival rate and duration of hospitalization did not differ between dogs that had 1 or had > 1 underlying disease identified and that the specific disease that was associated with aspiration did not impact duration of hospitalization or survival rate in this population of dogs. Despite the fact that many dogs had a disorder that could be associated with persistent or chronic aspiration (eg, laryngeal disease and megaesophagus), there was no difference in duration of hospitalization or outcome, compared with dogs that had aspiration pneumonia associated with gastrointestinal tract conditions or postanesthetic aspiration, which are typically reversible. This lack of influence of underlying disease on duration of hospitalization or survival might explain the high survival rate among dogs in the present study.
Results of our study indicated that multiple diseases are associated with aspiration pneumonia in dogs; however, limitations of the study should be considered. As a retrospective study, diagnostic testing was not standardized and multiple clinicians were involved in many cases. Invasive tests were only performed if a disorder was considered likely to be present in a given dog and if that dog's condition was stable. For example, laryngoscopy was performed only in dogs with evidence of laryngeal dysfunction, including inspiratory respiratory distress, voice change, exercise intolerance, or stridor. Dogs with subclinical disease may not have been identified. In dogs that developed postanesthetic aspiration, specific tests for esophageal or gastrointestinal dysfunction were not performed, and it could not be determined whether a primary disorder such as esophagitis was present or if decreased consciousness was solely responsible for aspiration. Necropsy data were unavailable for most dogs in the study, and the role of aspiration pneumonia in death of dogs could not be accurately estimated with available information.
The data obtained in the present study provide important information regarding the number and type of underlying diseases that may be associated with aspiration pneumonia in dogs and expected outcome. Contrary to our hypothesis, dogs with multiple underlying diseases associated with aspiration pneumonia or dogs with more extensive radiographic evidence of pulmonary involvement did not have prolonged hospitalization or decreased survival rates. Importantly, findings of our study suggest that among dogs with multiple disorders that could result in aspiration pneumonia, the outcome is generally favorable with an overall survival rate of 77%.
GraphPad Prism, version 5.0, GraphPad Software Inc, San Diego, Calif.
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