Sonny, a 12-year-old, male castrated Chihuahua was presented to the Dove Lewis emergency department. Sonny’s owner reports three episodes of syncope that day. Two events occurred after running up the stairs, and one while drinking from the water bowl. During these episodes Sonny acutely fell to his side, vocalized loudly, then appeared limp and unconscious for less than 30 seconds. The owner picked him up during the events, and he regained consciousness and was able to walk normally and exhibited normal behavior within about one minute. He has had a normal appetite and mentation between these episodes, though the owner does note that he has been a bit low energy on walks and has been polydipsic for two days.
Pertinent to his history, Sonny is also a patient of the Dove Lewis Cardiology Department. He was diagnosed with myxomatous degeneration of the mitral valve 14 months prior to presentation. At his cardiology recheck appointment 5 months ago, his echocardiogram showed severe mitral regurgitation and secondary left atrial and left ventricular dilation. He has never has symptoms of his heart disease. Pimobendan 0.25mg/kg PO twice daily was prescribed at this visit due to the significant heart remodeling. His ECG and blood pressure have been normal at his previous cardiology appointments.
On presentation, Sonny was alert and had a normal neurologic exam. His temperature, mucous membrane color and capillary refill time were normal. His heart rate was 150 bpm and his respiratory rate was 40 bpm. His systolic blood pressure was within normal limits at 105 mmHg. The significant abnormalities on his physical exam were increased respiratory effort with normal lung sounds and a grade 5/6 left apical systolic heart murmur.
A chemistry panel showed mild elevations in BUN, ALP and ALT. His CBC was normal with the exception of a mild mature neutrophilia. Thoracic radiographs showed a severely enlarged cardiac silhouette with a normal pulmonary parenchyma and vasculature (Figure 1). Pericardial effusion was found on ultrasound. The patient was placed in an oxygen cage due the increase respiratory effort and an IV catheter placed. A cardiology consult was requested.
Figure 1: Right lateral and ventrodorsal thoracic radiograph, showing significant enlargement of the cardiac silhouette due to left heart dilation and pericardial effusion.
ECG: Normal intervals, amplitudes and mean electrical axis. Sinus rhythm, heart rate 160 beats per minute. Occasional single ventricular premature complexes.
Echocardiogram: Moderate pericardial effusion with collapse of the right atrium. Moderate left atrial and left ventricular dilation. The mitral valve was thickened, consistent with myxomatous degeneration and severe mitral regurgitation was present. No cardiac masses seen (Figure 2).
Figure 2: Echocardiogram images showing pericardial effusion, left heart enlargement, thickening of the mitral valve,
and severe mitral regurgitation.
Diagnosis: Left atrial tear
The most likely cause of pericardial effusion and syncope in this patient is a left atrial tear. This can be a consequence of chronic mitral regurgitation, due to the combination of diseased myocytes, high left atrial pressure, and direct damage from high pressure mitral regurgitation. A full thickness rupture of the left atrial free wall results in rapid hemorrhage into the pericardium and subsequent cardiac tamponade. The symptoms range from mild lethargy to cardiogenic shock to death, depending on the degree of hemorrhage.
Pericardiocentesis is generally recommended in patients with symptomatic pericardial effusion; however, in the setting of a left atrial tear, this poses risk of continued hemorrhage. The pressure in the pericardial space may assist in clot formation and subsequent healing of the atrial tissue. Necropsies of chronic valvular disease patients often show partial or full thickness left atrial tears that have healed, even in patients that never showed apparent symptoms of pericardial effusion. Therefore, if the patient is not in shock, a conservative approach based on supportive care without pericardiocentesis is our general approach.
Sonny was hospitalized in the ICU. He was monitored with continuous ECG, which showed occasional single ventricular premature complexes. His blood pressure was assessed frequently and remained between 100mmHg and 110mmHg systolic. The ICU team was prepared to perform a pericardiocentesis if syncope occurred again or his blood pressure dropped below 90mmHg systolic. Furosemide at 1mg/kg IV q6h and maropitant 1mg/kg IV q24h were administered in addition to his previously prescribed pimobendan. He was kept in an oxygen cage throughout his 48 hour hospitalization and IV butorphanol was administered prn for anxiolysis.
The volume of pericardial effusion was monitored with ultrasound every 6 hours. The distance between the pericardium and left ventricular wall in long axis was measured at each assessment. This distance was 1.6cm at the time of diagnosis, but gradually decreased to 0.4cm during his ICU stay as his pericardial effusion was reabsorbed.
After 48 hours of hospitalization and improvement in the volume of pericardial effusion, Sonny’s ventricular ectopy had improved, he had exhibited no syncope, and his blood pressure was stable. He was eating and drinking normally, had diarrhea but no vomiting. He was discharged on furosemide 2mg/kg po q12h and continued on his pimobendan. His owners were instructed to restrict exercise for one week and advised to monitor his resting respiratory rate at home. At his recheck visit one week later, he was asymptomatic. No pericardial effusion was seen on ultrasound, indicating his left atrial tear had healed. He was continued on the same medical plan and allowed to go back to normal activity. A cardiology follow up appointment was planned in 4 months. Eventually, his chronic valvular disease progressed, and Sonny was euthanized 13 months later for refractory pulmonary edema. His left atrial tear remained healed and no pericardial effusion was documented in this patient after his ICU stay.
Syncope can be induced by several mechanisms in this common canine cardiac disease (see below). Once the cause is determined, medical management can usually control or reduce this symptom. In this patient, cardiac tamponade secondary to a left atrial tear was found. The hemopericardium resolved and the left atrial tear healed completely with medical management and supportive care.
Causes of Syncope in Chronic Mitral Valve Disease Patients
Severe mitral regurgitation increases the pressure in the left atrium and therefore in the pulmonary veins. When the pressure in the capillaries becomes elevated, pulmonary edema results. This pressure increase is translated to the pulmonary arteries, and, in some dogs, this stimulates vasoconstriction. The resulting pulmonary hypertension can cause low cardiac output and syncope. This is diagnosed with echocardiography. Treatment with sildenafil and pimobendan can help improve cardiac output and reduce syncope.
Low output from mitral regurgitation
Severe mitral regurgitation can decrease cardiac output and cause hypotension. The decrease in cerebral perfusion can cause syncope in these patients. This can occur when there is mild congestion, before other symptoms of pulmonary edema. Mild pulmonary edema on thoracic radiographs may be present, and severe mitral regurgitation and elevated left heart pressures are seen on echocardiogram. Many patients have resolution of syncope with a loop diuretic and pimobendan.
A chronic cough is not uncommon in the CMVD patient, and coughing can stimulate vagal response resulting in syncope. This can generally be discerned by the patient history, i.e. an association between coughing and collapse events. Treatment is aimed at optimizing heart failure medical therapy and reducing the frequency and severity of cough.
Cardiac tamponade caused by a left atrial tear is another sequela of CMVD. The chronic damage to atrial myocytes by stretch and strain on the tissue coupled with repeated striking of high-pressure mitral regurgitation against the endocardium (jet lesions) results in a perforation of the tissue. If the tear is full thickness through the free wall of the atrium, hemorrhage results in pericardial effusion. These patients often present with syncope and cardiogenic shock. Treatment is aimed at supportive care, as these lesions often heal with time and the pericardial hemorrhage will be reabsorbed. If persistent hypotension is present, pericardiocentesis is indicated; however, in the author’s experience this often results in continued hemorrhage and rapidly reccurrent effusion.
Chronic myocardial remodeling can result in supraventricular or ventricular tachycardia. These rapid heart rates (>220bpm) can cause low cardiac output and syncope. These arrhythmias can be intermittent, so a Holter monitor is often needed to make the diagnosis and assess the response to treatment.
Nakamura, RK, Tompkins E, et al. Left atrial rupture secondary to myxomatous mitral valve disease in 11 dogs. J Am Anim Hosp Assoc 2014. 50(6):405-8.
Keene BW, Atkins CE, et al. ACVIM consensus statement for the diagnosis and treatment of myxomatous mitral valve disease in dogs. J Vet Intern Med 2019.33(3):1127-1140