**5. Cor triatriatum sinister**

The CTS is a relatively rare congenital cardiovascular disease that has been first reported in 1868 [16]. In an autopsy research, it was accounted for 0.1% to 0.4% in human patients with congenital heart disease [17]. In veterinary medicine, the true prevalence is hard to know because this abnormality is not always producing heart murmur and develops clinical signs that can be observed by the owner and the veterinarian at the general practice. By reviewing case reports, naturally-occurred CTS is identified more frequently in cats [18–23] than in dogs [24–26].

The embryonic cause of CTS is still controversial, but the theory of pulmonary venous abnormality is the most popular. In the development of pulmonary veins, they should incorporate with left atrium and form four ostia on the smooth part of the dorsal left atrial wall. If certain degree of failure in this process occurs, the left atrium could be separated by the remains of the pulmonary veins, most of the time is a fibromuscular membrane. The left atrium is therefore divided to a proximal chamber that locates between the atriopulmonary junction and the fibromuscular membrane, and a distal chamber that extends from the fibromuscular membrane to the mitral valve

annulus. The molecular cause of CTS was first reported in experimental mice without hyaluronidase 2, which is an enzyme required for the degradation of hyaluronan that is the major extracellular matrix component of the heart [27]. Later, the similar result was obtained by genetic studies in affected human families and mice [28].

Anatomic variation of the membrane exists and whether or how much of the blood flow would be impeded depends on the three-dimensional relative position between the membrane and left atrium. This intra-atrial septum can be complete, incomplete or fenestrated, and its size, shape, thickness and location can be varied among affected patients. Types of diaphragmatic, hourglass and tubular has been used to describe the variations [29]. In a retrospective study, the histopathology of the membranous tissue was investigated. Elastin fibers were found to be presence in the top and bottom side and was absent in the middle layer of the diaphragm. Cardiomyocytes with positive staining of cardiac troponin C were located in the peripheral region, more on the side that near the diaphragm and atrial septum than on the side that near the diaphragm and the atrial free wall. The remanent area was mostly made up by the fibrous collagen and other mesenchymal cells. These specimens were collected from human patients that undergo surgical repair of the Cor triatriatum sinister, without surgical death in this cohort [30].

Impendence of the blood flow in the left atrium could cause turbulence, but the pressure gradient between two chambers may be not large enough for the heart murmur to be heard. Elevated pressure in the proximal chamber of the left atrium could raise the intravascular pressure of the pulmonary veins, and signs of left-side congestive heart failure may occur. However, the natural progression of the CTS in human patients is generally stable, with more than half patients were diagnosed in adulthood. In patients that need surgical correction using cardiopulmonary bypass, the surgery is safe and effective [31].

Transthoracic echocardiography is usually helpful in making diagnosis [32]. Except for detecting Cor triatriatum sinister, the echocardiography can also identify concurrent lesions. High proportion (58%) of affected human patients had associated abnormalities, and atrial septal defect and anomalous pulmonary venous connection were the most common and should be always keep in mind [30, 31, 33]. Two feline cases had been published that one kitten had CTS combined with persistent left cranial vena cava [20], and the other was diagnosed CTS with incomplete atrioventricular septal defect [21]. Some conditions can mimic the CTS under two-dimensional imaging mode, including supramitral ring or pulmonary stenosis [34]. In cases that the echocardiographic result alone is controversial or is suspicious of having multiple cardiovascular developmental diseases, additional imaging tools should be considered. A special case that was diagnosed as CTS with TAPVC by echocardiography combined with saline contrast technique was report in 2020 [35]. In some conditions especially when our target area is located near the heart base, the transesophageal echocardiography can provide better image resolution and details than the transthoracic echocardiography. Cardiac catheterization angiography has its advantages that it can measure the true intra-lumen pressure, which is always an estimated value if only echocardiography is performed. However, its clinical utility is limited in the veterinary field because deep sedation to generalized anesthesia is usually required in veterinary patients. Other imaging tools like computed tomography angiography and magnetic resonance imaging can provide multiplaner image reconstruction and assist with the diagnosis process [29].

Early in the 1998, a kitten presented signs of respiratory distress and diagnosed with CTS was successfully surgically managed. The membrane was torn by a dilator *Pulmonary Vein: Embryology, Anatomy, Function and Disease DOI: http://dx.doi.org/10.5772/intechopen.100051*

introduced from an opened left atrium [18]. Procedure that combining thoracotomy and cardiac catheter guided cutting balloon was performed in a cat that signs of congestive heart failure resolved completely after the hybrid technique [22]. Surgical correction under cardiopulmonary bypass was also feasible in feline patient with appropriate body size and weight [23]. In canine, the first case was published in 2012, and the patient was doing well only by internal medical treatment for the congestive heart failure [25]. A poodle case was presented with acute dyspnea and cyanosis, and was unfortunately made its definite diagnosis in postmortem examination [26]. Recently, Toaldo et al. reported a 6-year-old intact male French bulldog was accidentally diagnosed as CTS [24].

By reviewing veterinary literature, we can find that cats are more frequently presented, and their age at diagnosis is generally younger (8 weeks old to 4 years old, mostly <1 year old) than dogs (3, 5 and 6 years old). Although most of affected cats had congestive heart failure at admission (this result can be biased in veterinary patients), the surgery is usually tolerable and the patient can be free of heart failure after procedure. Medicine for controlling congestive heart failure is an alternative option if surgery is not performed. Weather the surgery is also benefit and recommended in patient without heart failure is not conclusive.
