Lung Lobe Torsion

Anatomy and Etiology

The lungs of the dog and cat are divided into specific lobes by deep fissures, which allow the lungs to alter position in response to movement of the diaphragm or spine [1]. The extent of movement of each lung lobe is usually limited by the presence of adjacent lobes, as well as the surrounding structures such as the heart, mediastinum, diaphragm, and thoracic wall. Pulmonary ligaments are thin sheets of pleura that extend from the caudal lobe of each lung to the mediastinum and provide further stabilization of these lobes [2]. The accessory lobe is isolated between the mediastinum and the plica vena cavae, providing significant support for this lobe. The space between the parietal and visceral pleura contains only a thin film of pleural fluid, which eases a sliding movement of the lung lobes, but the normal negative pressure within this pleural space helps to minimize excessive movement of the lobes [3].

Development of a lung lobe torsion requires the lobe to overcome the stabilizing forces of these anatomic features in order to twist around its longitudinal axis. Various theories have been considered, and any condition that may increase the mobility of a lung lobe might predispose it to developing a torsion. Review of the human literature indicates lung lobe torsion to have occurred in association with pleural effusion, pneumothorax, and accidental or surgical trauma. The most common predisposing cause in humans appears to be surgical manipulation of the lung lobes. It is thought that the division of pulmonary ligaments during a thoracic procedure, in combination with deflated lung lobes, may predispose a consolidated lobe to twist about its axis [4]. Similarly, reports of lung lobe torsion in the dog have proposed that suspension of a consolidated or atelectatic lung in fluid (pleural effusion) or air (pneumothorax) could predispose the lobe to rotational movement [5,6]. Lung atelectasis can result from a variety of causes such as trauma, pneumonia, pleural effusion, pneumothorax, or surgical manipulation, although surgical trauma is rarely the predisposing cause of lung lobe torsion in dogs or cats. There have also been reports of lung lobe torsion occurring without any history of predisposing factors; therefore, spontaneous torsion may occur [7-9]. Reports of lung lobe torsion in cats are rare, and have all been associated with underlying disease such as chylothorax, pyothorax, or chronic asthma [10,11].

As a lung lobe rotates about its axis, the pulmonary vessels and bronchus become occluded by the rotation at the hilus, leading initially to obstruction of the thin-walled pulmonary vein. Because some blood continues to flow through the pulmonary artery, the lobe becomes congested. Fluid leaks from the blood vessels into the interstitial tissues and airways, leading to consolidation of the lobe. The parietal pleura is eventually breached, leading to the accumulation of pleural effusion in the majority of cases [6,9].

The thin, narrow right middle lobe of the lung is affected most commonly in dogs and cats, although the left cranial lobe is affected almost as often in the dog. Torsion of each lung lobe has been reported, as have occasional episodes of multiple lobe torsions. The right middle and left cranial lobes may be predisposed to torsion because they have less extensive attachments and, therefore, increased mobility compared with the other lobes [5-11].

Clinical reports of lung lobe torsion are dominated by descriptions of large, deep-chested dogs, and it is thought that this thoracic shape might predispose to rotation of a lung lobe. The Afghan hound was found to be 133 times as likely to develop lung lobe torsion as other breeds in one study [6]. More recent reports have noted a trend for lung lobe torsion to occur in the pug, and the barrel-shape of the chest has been suggested as a predisposing factor in these dogs [7-9].

Clinical Signs and Investigations

Clinical history commonly includes dyspnea or tachypnea, coughing, and lethargy, often accompanied by anorexia and vomiting. Physical examination usually reveals varying degrees of respiratory distress, often accompanied by dull cardiopulmonary sounds on thoracic auscultation. Absence of breath sounds may be extensive if significant pleural effusion is present, or there may be a focal absence over the affected lung lobe. Other signs such as pyrexia, cardiovascular instability, vomiting, and depression have also been reported [5-11].

Analysis of a complete blood count and serum biochemistry panel usually reveals mild non-specific results in these patients, but may be influenced by the presence of an underlying thoracic disease process [6,9].

Imaging of the thorax will reveal pleural effusion in the majority of patients, and thoracocentesis may be required before radiographic details of the lungs can be revealed [5-11]. Lung lobe consolidation is typically seen, and occasionally more than one lobe is consolidated [6,9]. One severely consolidated lung lobe with minimal abnormalities of the other lung lobes is highly suggestive of lung lobe torsion, especially if accompanied by abnormal bronchial positioning. Air bronchograms or air alveolograms may been seen if the lobe has twisted recently, but as fluid or blood moves into the airways, the lobe will become opacified [5-11]. The differential diagnosis for the opacified lung lobe includes atelectasis, neoplasia, pneumonia, edema, and pulmonary contusions.

Thoracic ultrasonography may confirm the presence of pleural effusion and can provide further details of the underlying reasons for opacification of the affected lung lobe, as well as revealing any fluid accumulation in the bronchi [6,9]. Ultrasound-guided biopsy may be performed to determine whether any underlying disease process is present [7]. Confirmation of bronchial occlusion using fiberoptic bronchoscopy has been reported [9,12]. Thoracoscopy would allow confirmation of lung lobe torsion, but has not been reported in any clinical case reports to date in the author's knowledge.

Pleural fluid collected from these patients is usually serosanguinous or hemorrhagic, although chylous effusion has been reported in many cases [5-11]. Cytologic findings often indicate an inflammatory cell population with high levels of neutrophils, lymphocytes, and some erythrocytes [6]. Underlying disease processes may complicate interpretation of the pleural fluid. Reactive mesothelial cells may be seen as a result of pleural inflammation: these should not be over-interpreted because mediastinal mesothelioma is rare [6]. Bacterial culture may yield growth in some cases, although pyothorax is rarely reported to accompany lung lobe torsion [5,6,9]. Identified bacteria have included Escherichia coli, Staphylococcus spp., Proteus spp., and Pseudomonas spp [6,9]. Chylous effusion is suspected when a milky-white opaque fluid is obtained, and confirmed by demonstration of a triglyceride concentration greater than that of the serum.

Treatment

Medical stabilization is required before surgical intervention. Provision of oxygen via a mask, nasal catheter, or oxygen cage may be necessary. Removal of all pleural effusion by thoracocentesis will improve ventilation of the unaffected lung lobes, relieving signs of respiratory distress. Many patients require fluid resuscitation prior to induction of general anaesthesia. Administration of intravenous antibiotics is recommended as intraoperative contamination from the respiratory tract is likely to occur.

Exploratory thoracotomy is performed via a lateral intercostal approach at the appropriate intercostal space. The affected lung lobe usually appears dark and consolidated and may be friable. Necrosis may have already begun, increasing the fragility of the lobe. Lung lobectomy without untwisting of the pedicle is recommended to minimize the risk of cytokine release into the circulation and potential reperfusion injury. The bronchus should be clamped prior to lobectomy, and the pedicle can then be ligated by hand or with an appropriate stapling device. A portion of the lung lobe should be submitted for bacterial culture, and the rest of the lobe for histologic examination. Evaluation of the other lung lobes and the rest of the thoracic structures should be performed. Any abnormal tissue is biopsied to allow submission for histologic study and bacterial culture. If chylothorax is present, thoracic duct ligation and pericardectomy might be performed. This treatment decision may be influenced by the breed of the patient. Although chylothorax has been reported to resolve following lung lobectomy in many patients, Afghan hounds appear to be predisposed to development of chylothorax and are likely to require further surgical treatment of the condition following the initial lung lobectomy [5,6]. Prior to closure of the thorax, a chest tube should be placed and adequate inflation and correct orientation of the other lung lobes should be confirmed.

Postoperative care involves analgesia, antibiotics, and intravenous fluid therapy as indicated by regular assessment of the patient. Oxygen supplementation is sometimes required. Drainage of the chest tube takes place at regular intervals until fluid production is less than 5 ml/kg/day and the chest tube can be safely removed.

Histopathologic examination of the excised lung lobe usually reveals bronchi filled with hemorrhagic fluid, thrombosis of blood vessels, plasma cell and lymphocyte infiltration of the tissues, and necrosis [6,9]. Occasionally, an underlying disease is revealed such as pneumonia or neoplasia that may have predisposed the lobe to twisting [6,11].

Prognosis

The majority of patients treated for spontaneous lung lobe torsion without an underlying disease process will recover quickly without significant complications [5-9]. If neoplasia is responsible for the development of pleural effusion and subsequent lung lobe torsion, the prognosis is likely to be poor [6,9]. Some patients will develop systemic inflammatory response syndrome pre- or postoperatively, leading to death or euthanasia. This is believed to result from cytokine release by necrotic lung tissue and may be complicated by cardiovascular compromise, disseminated intravascular coagulation, or acute respiratory distress syndrome [6,9].

Persistent pleural effusion may occur in these patients, and persistent chylothorax is reported most often [5,6,9]. It is thought that disruption or impedance of the thoracic duct or thoracic lymphatic vessels leads to lymphangiectasia, resulting in chylous effusion into the pleural cavity [13]. Although chylothorax has been detected at the time of diagnosis of lung lobe torsion, it is usually reported to develop following surgical correction of lung lobe torsion [5,6,9,10]. Resolution of the chylothorax will occur within 7 days in most patients, but persistent chylothorax has been reported in Afghan hounds and a cat [5,6,9,10]. It has been suggested that Afghan hounds may have a thoracic lymphatic system that has a lower tolerance for any degree of trauma, predisposing them to development of chylothorax [6]. The prognosis for Afghan hounds who developed chylothorax in association with lung lobe torsion has been reported to be poor [6,9], but reports of surgical treatment for chylothorax have revealed a more favorable outcome for dogs and cats than has been reported in the past. [13-15].