Risk factors for the development of aspiration pneumonia after unilateral arytenoid lateralization in dogs with laryngeal paralysis: 232 cases (1987–2012)

David Wilson Blue Pearl Specialty and Emergency Animal Hospital, Skokie, Il 60076.

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 DVM, MS
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Eric Monnet Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523.

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 DVM, PhD

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Abstract

OBJECTIVE To identify risk factors for the development of aspiration pneumonia after unilateral arytenoid lateralization in dogs with laryngeal paralysis.

DESIGN Retrospective case series.

ANIMALS 232 client-owned dogs with a diagnosis of laryngeal paralysis treated with lef-tsided unilateral arytenoid lateralization.

PROCEDURES Medical records were reviewed. Signalment, medical history, surgical complications, and outcome data were collected. Follow-up was performed via review of medical records and by telephone interview with the owner, referring veterinarian, or both.

RESULTS At the 1-, 3-, and 4-year follow-up periods, aspiration pneumonia occurred in 18.6%, 31.8%, and 31.8% of dogs, respectively. The 1-, 3-, and 4-year survival rates for dogs with postoperative aspiration pneumonia were 83.1%, 51.5%, and 25.8%, respectively. None of the dogs with aspiration pneumonia before surgery developed clinical signs of aspiration pneumonia after surgery. Postoperative megaesophagus (hazard ratio [HR], 2.58; 95% confidence interval [CI], 1.56 to 3.93) and postoperative administration of opioid analgesics prior to discharge (HR, 1.69; 95% CI, 1.12 to 2.80) were significant risk factors for the long-term development of aspiration pneumonia in this study. Perioperative metoclopramide administration did not significantly decrease the risk for development of aspiration pneumonia (HR, 0.94; 95% CI, 0.67 to 1.37).

CONCLUSIONS AND CLINICAL RELEVANCE In the present study, aspiration pneumonia was the most commonly reported postoperative complication of unilateral lateralization in dogs treated for laryngeal paralysis; however, preexisting aspiration pneumonia was not associated with an increased risk for development of aspiration pneumonia after surgery. Megaesophagus was identified as an important risk factor for eventual development of aspiration pneumonia. Administration of an opioid analgesic may increase the risk of aspiration pneumonia in dogs treated surgically for laryngeal paralysis.

Abstract

OBJECTIVE To identify risk factors for the development of aspiration pneumonia after unilateral arytenoid lateralization in dogs with laryngeal paralysis.

DESIGN Retrospective case series.

ANIMALS 232 client-owned dogs with a diagnosis of laryngeal paralysis treated with lef-tsided unilateral arytenoid lateralization.

PROCEDURES Medical records were reviewed. Signalment, medical history, surgical complications, and outcome data were collected. Follow-up was performed via review of medical records and by telephone interview with the owner, referring veterinarian, or both.

RESULTS At the 1-, 3-, and 4-year follow-up periods, aspiration pneumonia occurred in 18.6%, 31.8%, and 31.8% of dogs, respectively. The 1-, 3-, and 4-year survival rates for dogs with postoperative aspiration pneumonia were 83.1%, 51.5%, and 25.8%, respectively. None of the dogs with aspiration pneumonia before surgery developed clinical signs of aspiration pneumonia after surgery. Postoperative megaesophagus (hazard ratio [HR], 2.58; 95% confidence interval [CI], 1.56 to 3.93) and postoperative administration of opioid analgesics prior to discharge (HR, 1.69; 95% CI, 1.12 to 2.80) were significant risk factors for the long-term development of aspiration pneumonia in this study. Perioperative metoclopramide administration did not significantly decrease the risk for development of aspiration pneumonia (HR, 0.94; 95% CI, 0.67 to 1.37).

CONCLUSIONS AND CLINICAL RELEVANCE In the present study, aspiration pneumonia was the most commonly reported postoperative complication of unilateral lateralization in dogs treated for laryngeal paralysis; however, preexisting aspiration pneumonia was not associated with an increased risk for development of aspiration pneumonia after surgery. Megaesophagus was identified as an important risk factor for eventual development of aspiration pneumonia. Administration of an opioid analgesic may increase the risk of aspiration pneumonia in dogs treated surgically for laryngeal paralysis.

Laryngeal paralysis causes upper airway obstruction in large-breed dogs,1 and idiopathic polyneuropathy has been identified as the most common cause of laryngeal paralysis.2 Several surgical techniques have been described for the treatment of laryngeal paralysis in dogs, with unilateral arytenoid lateralization currently the most common.1 Complications of arytenoid lateralization include recurrent upper airway obstruction because of surgical failure (eg, suture failure and seroma formation) and vocalization changes.3 Postoperative aspiration pneumonia is also a commonly reported complication that may affect 5% to 24% of cases.3–7 Various technical modifications of the surgical procedure have been recommended to decrease the risk of aspiration pneumonia.8

The objective of the study reported here was to evaluate short- and long-term risk factors for the development of aspiration pneumonia in dogs with laryngeal paralysis treated by means of unilateral arytenoid lateralization. Our hypothesis was that dogs with laryngeal paralysis are at an increased risk for development aspiration pneumonia after unilateral lateralization.

Materials and Methods

Criteria for selection of cases

A retrospective review of medical records was performed to identify dogs that were evaluated for laryngeal paralysis at the Colorado State University Veterinary Teaching Hospital between 1987 and 2012. Dogs were included in the study if a definitive diagnosis of laryngeal paralysis was obtained via laryngoscopy and if unilateral arytenoid lateralization was performed. Dogs lost to follow-up within 14 days after surgery were excluded from the study.

Medical records review

Information collected from medical records included signalment, general physical examination findings, neurologic examination findings, preoperative and postoperative medications, results of diagnostic imaging, and any postoperative complications. Results of a thyroid profile (ie, thyroid-stimulating hormone concentration and thyroxine concentration) for diagnosis of hypothyroidism were also recorded. Preoperative neurologic examination findings were recorded if the dog was evaluated by a neurology, surgery, internal medicine, or critical care specialty service or if several of these specialty services had been consulted. The presence of progressive neurologic disease following surgery was determined on the basis of veterinarian neurologic examination or owner impression of hind limb weakness or generalized weakness in the absence of other known neurologic disease (eg, extradural spinal cord compression or lumbosacral disease). Dogs were recorded as having progressive neurologic disease if the described neurologic findings were evident without signs of pain or reliance on analgesics. If evidence of neurologic disease could not be readily distinguished from orthopedic disease, the patient was recorded as lacking signs of progressive neurologic disease.

Radiographic findings were also obtained from the medical record. Results of preoperative radiography were included if imaging had been performed within 2 weeks of surgery. Radiographic evidence consistent with a diagnosis of aspiration pneumonia included the presence of a focal alveolar pattern within dependent lung lobes evident on thoracic radiographs. Diffuse interstitial disease, diffuse alveolar pattern, evidence of atelectasis, focal soft tissue opacities, or a combination of these findings was not considered to be consistent with a diagnosis of aspiration pneumonia. In addition to pulmonary changes, thoracic radiographs were evaluated for megaesophagus and other evidence of disease of the respiratory tract. For all radiographs performed at Colorado State University, results were confirmed by review of the findings in the medical record (ie, radiology reports) and the original images by a board-certified radiologist. The diagnosis of aspiration pneumonia in this study was made on the basis of results of thoracic radiography because not all dogs with radiographic evidence of aspiration had clinical signs of disease. In cases where radiographic evidence was not definitive (because of mild radiographic changes, difficulty interpreting radiographic images, or both), dogs were recorded as having preoperative aspiration pneumonia if multiple clinical signs of disease (including cough, fever, lethargy, and exercise intolerance) were evident. Dogs that were empirically treated with antimicrobials, or that were reported to have a cough without other clinical signs consistent with pneumonia, were not considered to have aspiration pneumonia.

Postoperative opioid treatment was recorded as any dog that received a parenterally administered pure μ agonist (ie, morphine, hydromorphone, fentanyl, or oxymorphone) via intermittent IM, SC, or IV route of administration; CRI; or a combination. Injectable partial μ agonists (buprenorphine), injectable agonistantagonists (butorphanol), and tramadol administered PO were not recorded as opioids. Administration of metoclopramide as an IV CRI (1 to 3 mg/kg/d [0.45 to 1.36 mg/lb/d]) during anesthesia and during the immediate postoperative period was recorded. Postoperative metoclopramide (10 to 30 mg, PO, q 8 h) was classified as oral treatment dispensed at discharge.

Any complication related to surgery, pertaining to the larynx, or both was recorded. Complications were classified as short term (≤ 14 days after surgery) or long term (> 14 days after surgery). Acute death was defined as intraoperative death or death < 24 hours after surgery while hospitalized. Surgical failure was defined as the return of clinical signs suggestive of laryngeal paralysis and confirmed by a laryngeal examination. Postoperative stridor alone was not considered a postoperative complication.

A death related to arytenoid lateralization was defined as any death or euthanasia secondary to aspiration pneumonia, failure of repair, upper airway obstruction, or other undetermined cause of acute death. Deaths related to progressive neurologic disease were not considered related to unilateral lateralization. As for complications, mortality was classified as short term (< 14 days after surgery) or long term (> 14 days after surgery).

Follow-up information was obtained from the medical record, telephone interviews with clients, and telephone interviews with the primary care veterinarian. Information requested included the dog's current status, description of any complications that had developed, and date and reason for death where applicable.

Statistical analysis

Descriptive data are presented as percentages, mean ± SD, or median (range). Normality of data was evaluated with the Shapiro-Wilk test. Time-to-event analysis was performed by means of a Kaplan-Meier actuarial analysis. Entry time was defined as the date of surgery, and survival times were based on either the date of death or euthanasia or the date of follow-up. The disease-free interval was defined as the time from surgery until the development of aspiration pneumonia. If a dog developed aspiration pneumonia, it was uncensored. Dogs that died or were euthanized for reasons unrelated to the study, were lost to follow-up, or were still alive at the last follow-up were censored in the analysis. Kaplan-Meier curves were constructed to evaluate survival time and disease-free interval. A log-rank test was used to compare distribution of survival. A Cox proportional hazard analysis was performed to identify risk factors influencing development of aspiration pneumonia in the short (≤ 14 days after surgery) and long (> 14 days after surgery) term. From this analysis, HRs were calculated to quantify risk. Age (years) was recorded as a continuous variable and evaluated as a risk factor, as were Labrador breed; presence of neurologic abnormalities; presence of preoperative aspiration pneumonia; perioperative metoclopramide, postoperative opioid, and oral metoclopramide administration; and pre- and postoperative megaesophagus. Except for age, all potential risk factors were evaluated as categorical variables and coded as yes or no. A value of P < 0.05 was considered significant. All analyses were performed with statistical software.3

RESULTS

Two hundred fifty-seven dogs met the initial criteria and were screened for study inclusion. Twenty-six cases were excluded because follow-up was < 15 days. Therefore, 232 dogs that underwent 232 unilateral lateralization procedures were included in the study. Left-sided arytenoid lateralization was performed in each dog. Six dogs subsequently underwent right-sided arytenoid lateralization following failure of the initial procedure; however, data for the second procedure were not included in the study.

Of the 232 dogs, 107 (46.1%) were Labrador Retrievers, 26 (11.3%) were Golden Retrievers, 11 (9.1%) were Brittany Spaniels, 7 (3%) were Australian Shepherds, and 7 (3%) were Newfoundlands. Twenty-one (9%) were classified as mixed-breed dogs. The body weight and age were 34.6 ± 12.0 kg (76.1 ± 26.4 lb) and 10.6 ± 2.8 years, respectively. There were 152 (65.5%) males (118 castrated and 34 sexually intact) and 80 (34.5%) females (73 spayed and 7 sexually intact).

Postoperative injectable pure opioids were administered in 135 of 232 dogs. The dogs that did not receive a pure opioid after surgery received local anesthesia (bupivacaine, 1.5 mg/kg [0.68 mg/lb]) at the completion of surgery. Perioperative metoclopramide was administered to 124 of 232 dogs, and oral metoclopramide was dispensed at the time of discharge from the hospital for administration until 14 days after surgery in 110 of 232 dogs.

Thyroid function was evaluated before surgery in 206 of 232 dogs. Hypothyroidism was diagnosed in 98 (47.5%) dogs. Clinical signs consistent with generalized polyneuropathy were present at the time of diagnosis of laryngeal paralysis in 65 of 232 (28.0%) dogs. Preoperative radiographs were available for review in 205 of 232 dogs (88%). Preoperative megaesophagus was identified radiographically in 7 of 232 (2.9%) dogs. Postoperative megaesophagus was identified in 9 (3.8%) dogs. Preoperative aspiration pneumonia was diagnosed in 21 (9.2%) dogs.

Postoperative complications occurred in 70 of 232 (29.4%) dogs. Five (2.1%) dogs died in the postoperative period because of aspiration pneumonia. Acute complications (≤ 14 days) occurred in 43 of 232 (18.1%) cases. Incisional complications occurred in 18 (7.6%) dogs. Aspiration pneumonia developed < 14 days after surgery in 27 (11.6%) dogs. Failure of the lateralization surgery occurred in 14 (5.9%) dogs at a median of 48.5 days (range, 1 to 1,277 days) after surgery. Persistent stridor was reported in 31 of 232 (13%) dogs. Aspiration pneumonia developed more than 14 days after surgery in 16 (6.8%) dogs.

Median overall time to follow-up was 420 days (range, 0 to 2,483 days). Eighteen of 232 (7.5%) cases were lost to follow-up at a median of 36 days (range, 15 to 804 days). One-, 2-, 3-, and 4-year survival rates were 93.6%, 89.1%, 84.4%, and 75.2%, respectively (Figure 1). Seven years after surgery, the survival rate was 75.2%. Fifty percent survival was not reached. Twenty-two (9.2%) dogs died or were euthanized for reasons related to respiratory disease. Nine of 232 (3.8%) dogs died of or were euthanized for related causes during the short-term period, and 13 of 223 (5.8%) dogs died of or were euthanized for related causes during the long term period. Sixteen of 232 (6.7%) dogs were euthanized for clinical signs of aspiration pneumonia, and 6 of 232 (2.5%) dogs for acute respiratory distress of unknown cause. Eighty-eight of 223 (39.4%) dogs died of unrelated causes, of which 25 (28.4%) dogs were euthanized solely for reasons related to progressive neurologic disease in the absence of other known comorbidities.

Figure 1—
Figure 1—

Distribution of survival for dogs with laryngeal paralysis treated with unilateral lateralization. Dotted lines represent the 95% CI.

Citation: Journal of the American Veterinary Medical Association 248, 2; 10.2460/javma.248.2.188

One-, 2-, 3-, and 4-year rates of aspiration pneumonia were 18.6%, 27.2%, 31.8%, and 31.8%, respectively (Figure 2). The 50% survival time for dogs with postoperative aspiration pneumonia was 1,166 days (3.2 years; Figure 3). The 1-, 2-, 3-, and 4-year survival rates for dogs with postoperative aspiration pneumonia were 83.1%, 63.7%, 51.5%, and 25.8%, respectively. Risk factors for the development of aspiration pneumonia in the short term (< 14 days) included postoperative administration of an opioid (HR, 1.69; 95% CI, 1.12 to 2.80; P = 0.01) and the diagnosis of postoperative megaesophagus (HR, 2.58; 95% CI, 1.56 to 3.93; P < 0.001 [Table 1]). Risk factors for the development of long-term aspiration pneumonia included a diagnosis of postoperative megaesophagus (HR, 3.89; 95% CI, 1.50 to 7.76; P = 0.001 [Table 2]). Diagnosis of aspiration pneumonia before surgery could not be evaluated as a risk factor for the development of aspiration pneumonia in the postoperative period because none of the dogs with preoperative aspiration pneumonia (n = 21) developed aspiration pneumonia in the short or long term, whereas 43 of 211 dogs without preoperative pneumonia developed aspiration pneumonia in the short and long term (P = 0.008). Follow-up time was 468.4 ± 68.4 days for the dogs with preoperative aspiration pneumonia and 532.2 ± 33.0 days (P = 0.401) for the dogs without preoperative aspiration pneumonia. There was no significant difference in the frequency of administration of opioids (P = 0.665) or metoclopramide (P = 0.669) when dogs with or without preoperative aspiration pneumonia were compared. The frequency of postoperative megaesophagus was not significantly (P = 1.00) different when dogs with or without preoperative aspiration pneumonia were compared. Two dogs with preoperative megaesophagus had preoperative aspiration pneumonia (P = 0.179). Three of 7 dogs with preoperative megaesophagus also had megaesophagus after surgery (P = 0.001). Three dogs with megaesophagus before surgery had a history of vomiting and regurgitation. The presence of preoperative megaesophagus was not associated with an increased risk of preoperative aspiration pneumonia (OR, 3.18; 95% CI, 0.62 to 16.30; P = 0.209). The presence of neurologic signs during the clinical evaluation before surgery was not associated with an increased risk of postoperative megaesophagus (OR, 2.19; 95% CI, 0.60 to 8.40; P = 0.209).

Figure 2—
Figure 2—

Distribution of the diagnosis of aspiration pneumonia over time for dogs with laryngeal paralysis treated with unilateral lateralization. See Figure 1 for remainder of key.

Citation: Journal of the American Veterinary Medical Association 248, 2; 10.2460/javma.248.2.188

Figure 3—
Figure 3—

Effect of the diagnosis of aspiration pneumonia on distribution of survival for dogs with laryngeal paralysis treated with unilateral lateralization (P < 0.001). See Figure 1 for remainder of key.

Citation: Journal of the American Veterinary Medical Association 248, 2; 10.2460/javma.248.2.188

Table 1—

Short-term (≤ 14 days) risk factors for development of aspiration pneumonia after unilateral arytenoid lateralization in dogs.

VariableCategoryNo aspiration pneumonia (205 cases)Aspiration pneumonia (27 cases)HR95% CIP value
Age (y)NA10.6 ± 2.911.1 ± 2.61.070.95–1.250.268
Labrador breedYes92121.020.71–1.460.895
 No11315   
Neurologic abnormalitiesYes5770.840.33–1.520.604
 No14820   
Preoperative aspiration pneumoniaYes210NANANA
 No18427   
Perioperative metoclopramideYes110140.990.69–1.040.924
 No9513   
Oral metoclopramide after surgeryYes98120.940.67–1.370.861
 No10715   
Preoperative megaesophagusYes610.990.23–2.110.975
 No19926   
Postoperative megaesophagusYes452.581.56–3.93< 0.001
 No20022   
Postoperative opioidYes114211.691.12–2.800.010
 No916   

Results of Cox proportional hazard analysis. Age is presented as mean ± SD.

NA = Not applicable.

Table 2—

Long-term (> 14 days) risk factors for development of aspiration pneumonia after unilateral arytenoid lateralization in dogs.

VariableCategoryNo aspiration pneumonia (189 cases)Aspiration pneumonia (16 cases)HR95% CIP value
Age (y)NA10.6 ± −2.910.9 ± 2.11.080.97–1.230.123
Labrador breedYes8391.090.81–1.470.531
 No1067   
Neurologic abnormalitiesYes2110.750.37–1.250.316
 No16815   
Preoperative aspiration pneumoniaYes210NANANA
 No16816   
Preoperative megaesophagusYes511.270.51–2.290.533
 No18415   
Postoperative megaesophagusYes223.891.50–7.760.001
 No18713   

See Table 1 for key.

Discussion

In the present study, aspiration pneumonia was the most frequent complication after unilateral lateralization for the treatment of dogs with laryngeal paralysis. The incidence of aspiration pneumonia was 18.6%, 27.2%, and 31.8% at 1, 2, and 3 years after surgery, respectively, in this study and affected short- and long-term survival. Nonetheless, dogs with laryngeal paralysis had an excellent long-term prognosis, with 75.2% of the dogs still alive 7 years after surgery. The age, weight, and breed distribution of dogs in the present study was similar to that reported in previous studies.3,5–7 Laryngeal paralysis is usually diagnosed in large-breed dogs at approximately 10 years of age and commonly affects Labrador Retrievers and Golden Retrievers.3,5–7 Aspiration pneumonia was diagnosed before surgery in 9% of cases in this study. Systemic clinical signs of idiopathic polyneuropathy were present in 9% of the cases before surgery. Hypothyroidism was documented in 47.5% of the dogs tested; however, a relationship between hypothyroidism and systemic neurologic signs could not be documented.

Long-term outcome after unilateral lateralization for the treatment of laryngeal paralysis was excellent in this study, with a 75.2% survival rate at 7 years. MacPhail et al3 documented a 70% survival rate at 4 years for dogs with laryngeal paralysis treated with unilateral lateralization. Hammel et al5 reported a median survival time of 12 months in a study of 39 dogs treated with unilateral lateralization. However, the excellent long-term outcome reported for patients in the present study is in contrast to a study by Stanley et al9 that reported severe deterioration of neurologic status within a year of diagnosis of laryngeal paralysis. Survival rates for dogs were not reported in that study.

Laryngeal paralysis in dogs is reportedly most commonly caused by an idiopathic polyneuropathy.10 Systemic clinical signs of polyneuropathy other than laryngeal recurrent nerve dysfunction were observed in 28% (65/232) of the patients in this study. Similar to Jeffery et al,10 the presence of systemic neurologic signs was not a prognostic indicator in our study. Forty-seven percent of the dogs in our study were determined to have hypothyroidism on the basis of preoperative thyroid function testing. Hypothyroidism can cause polyneuropathy11; however, thyroid supplementation has not been evaluated with respect to its influence on the treatment of laryngeal disease.

Aspiration pneumonia was the most common complication reported in the present study. Nineteen percent of dogs experienced clinical signs of aspiration pneumonia in the postoperative period, with 68% of affected patients free of aspiration pneumonia 3 years after surgery. Aspiration pneumonia was a negative risk factor for short- and long-term survival, with 25.8% of affected patients alive 4 years after surgery. Aspiration pneumonia has been reported to affect 5% to 24% of dogs after unilateral lateralization.3–7 Aspiration pneumonia has also been reported in 30% of cats with laryngeal paralysis treated with unilateral lateralization.12 The lowest rate of aspiration pneumonia previously reported in dogs treated surgically for laryngeal paralysis was by Griffiths et al13 in a study in which patients were followed for 6 weeks after surgery. Because dogs may develop aspiration pneumonia 3 years after surgery, we suggest that it is important to have adequate follow-up to evaluate the outcome when treating laryngeal paralysis. Hammel et al5 reported aspiration pneumonia at 17 and 25 months after surgery in 2 cases. MacPhail et al3 reported a 23.6% overall rate of aspiration pneumonia when the results of unilateral or bilateral lateralization and arytenoidectomy were evaluated. Schofield et al6 reported a rate of 6% in a study in which a combination of bilateral lateralization and ventriculocordectomy was performed through a ventral approach. In the present study, the number of patients developing postoperative aspiration pneumonia may have been increased because we classified dogs as having aspiration pneumonia on the basis of results of thoracic radiography without necessarily requiring obvious clinical signs of aspiration pneumonia to be present. Dogs were followed for 7 years, which also may have increased the number of cases with a diagnosis of aspiration pneumonia because some dogs developed aspiration pneumonia 3 years after surgery. The rate of aspiration pneumonia determined from a Kaplan-Meier actuarial analysis should more accurately reflect the risk for development of aspiration pneumonia because the data were censored if the patients were lost to follow-up or died of causes unrelated to laryngeal paralysis.

The risk of aspiration pneumonia in the postoperative period for dogs in this study was significantly affected by the postoperative administration of opioid analgesics. Patients that did not receive opioids after surgery received local anesthesia by means of a line block with bupivacaine performed at the completion of the procedure, to provide local analgesia until subsequent administration of oral tramadol. This reduction in opioid administration likely permitted these dogs to regain sternal recumbency more rapidly after anesthesia, rather than remaining in lateral recumbency for several hours after surgery. Morphine has been shown to influence gastrointestinal motility and the tone of the lower esophageal sphincter, which can increase the risk of gastroesophageal reflux in the postoperative period, thereby also increasing the risk of aspiration pneumonia.14

Metoclopramide use during and after surgery did not significantly reduce the risk of aspiration pneumonia in the short term in the present study. Metoclopramide administration may increase the lower esophageal sphincter tone, which could reduce the severity of gastroesophageal reflux.15 It is possible that cisapride might be more appropriate than metoclopramide because it may have a more pronounced effect on the lower esophageal sphincter.16

Diagnosis of megaesophagus any time after surgery was a negative prognostic indicator for the development of aspiration pneumonia and for long-term survival in the present study. In a study of 32 dogs with laryngeal paralysis, Stanley et al9 documented esophageal dysfunction with a positive-contrast esophagogram. Esophageal dysfunction in 62% of dogs manifests as gastroesophageal reflux and, as documented with fluoroscopy, was associated with an increased risk of aspiration.9 MacPhail et al3 also documented postoperative megaesophagus as a risk factor for development of complications (HR, 4.81) and survival (HR, 7.40) in dogs with laryngeal paralysis. In that study, dogs were treated with either bilateral or unilateral lateralization or an arytenoidectomy. In the present study, all dogs were treated with unilateral lateralization, which may explain the difference in results.

A diagnosis of megaesophagus before surgery was not a risk factor for the development of aspiration pneumonia or the development of megaesophagus after surgery in this study. We suggest that most of the affected dogs may not have had overt megaesophagus or esophageal dysfunction because 57% of affected dogs in this study did not have clinical signs. A potential explanation for the radiographic evidence of preoperative esophageal dilation may be the presence of aerophagia secondary to dyspnea, reflux esophagitis, or positional changes of the distal esophagus and stomach secondary to exaggerated inspiratory dyspnea. The recommendations proposed by Stanley et al9 to perform an esophagographic evaluation before surgery may be prudent as a means to more accurately evaluate esophageal function, especially if clinical signs related to a megaesophagus are present. If true esophageal dysfunction is present before surgery, a permanent tracheostomy has been recommended.1

The diagnosis of aspiration pneumonia before surgery was not a risk factor for the development of aspiration pneumonia after surgery in this study. Because none of the dogs with aspiration pneumonia before surgery developed documented aspiration pneumonia after surgery, the mechanism of aspiration before and after surgery may differ. Upper airway obstruction has been found to increase the risk for gastroesophageal reflux.17 Esophagitis and gastroesophageal reflux were significantly reduced when upper airway obstruction was corrected.18 Stanley et al9 reported that 62% of dogs with upper airway obstruction due to laryngeal paralysis have some gastroesophageal reflux. Gastroesophageal reflux has been associated with aspiration pneumonia in human patients.19 Therefore, it is entirely possible that dogs with laryngeal paralysis develop aspiration pneumonia before surgery because of gastroesophageal reflux. Postoperatively, the risk of aspiration may be a result of lateralization of the arytenoid cartilage, which increases the diameter of the rima glottidis beyond the limit that can be covered effectively by the epiglottis.8 Because only postoperative megaesophagus was a risk factor for aspiration pneumonia, this suggests different mechanisms for aspiration pneumonia before and after surgery. Esophageal disease has previously been reported as the most important cause of aspiration pneumonia in dogs.20

Idiopathic polyneuropathy is the most common cause of laryngeal paralysis in dogs. In a study of 32 dogs with laryngeal paralysis, Stanley et al9 documented a rapid progression of idiopathic polyneuropathy during the year after diagnosis. Only 11% of dogs in the present study were euthanized because of severe neurologic disease. Presence of neurologic signs on physical examination at the time of diagnosis of laryngeal paralysis was not associated with increased risk of death in the short and long term, and it was not associated with an increased risk for development of postoperative megaesophagus. In addition, it was not associated with an increased risk of aspiration pneumonia. Because a complete neurologic examination was not performed in all 232 dogs of this study at either the time of diagnosis or the time of euthanasia, it is difficult to evaluate the severity or confirm idiopathic polyneuropathy for the patients in this study.

This study has several limitations related to its retrospective nature. Eighteen dogs were lost to follow-up at a median time of 36 days (range, 15 to 804 days) after surgery. Kaplan-Meier actuarial analysis and Cox analysis with censoring of cases lost to follow-up or that died from reasons unrelated to laryngeal paralysis was used in an attempt to correct for this. The definition of aspiration pneumonia was based on results of radiographic examination, clinical signs, or both. We used this approach in an attempt to avoid underestimating the number of cases with pneumonia. As such, we may have overestimated the number of dogs with aspiration pneumonia in this study. This decision was largely based on the observation that postmortem examination of dogs that have undergone unilateral arytenoid lateralization may reveal gross and histologic evidence of chronic bronchopneumonia despite lack of any clinical signs. Additionally, follow-up was performed via telephone interview of owners and referring veterinarians, which may have underestimated the diagnosis of aspiration pneumonia. However, we suggest that dogs with aspiration pneumonia would have required medical attention.

Results of the present study suggested that long-term outcome for dogs with laryngeal paralysis treated with unilateral lateralization is excellent. Aspiration pneumonia was the most common complication after unilateral lateralization and negatively affected long-term survival. Diagnosis of preoperative aspiration pneumonia may not increase the risk for aspiration pneumonia after surgery. On the basis of these results, we suggest that postoperative administration of opioid analgesics should be limited in dogs undergoing unilateral arytenoid lateralization.

ABBREVIATIONS

CI

Confidence interval

HR

Hazard ratio

Footnotes

a.

JMP 11, SAS Institute Inc, Cary, NC.

References

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