Lung lobe torsion in the dog and cat

The lungs are kept in their physiological position by the thoracic wall, the diaphragm, the heart and the mediastinum; in turn, each single lobe is kept in place by the surrounding lobes which, by insufflating, fill the available space. Lung lobe torsion (LLT) occurs when a lobe rotates around its major axis at the level of the hilum, resulting in the obstruction of the bronchovascular pedicle.^1-5

PREDISPOSING FACTORS 

LLT is a rare pulmonary disorder the aetiology of which is still unknown; the deep interlobar fissures, present in the lungs of dogs and cats, are a probable predisposing factor. Moreover, any condition that increases lung lobe mobility (e.g., trauma, pleural effusion) may alter the spatial relationships with the thoracic wall, the mediastinum and the adjacent lung lobes, causing a certain degree of instability and, therefore, a greater risk of torsion.⁴ The onset of LLT can be primary (spontaneous), as frequently reported in the Pug,⁶ or secondary to a thoracic trauma, pleural effusion, pneumothorax or chylothorax^4,6,7 (Table 1).

Previous thoracic surgery, with manipulation and partial lobe collapse, is also reported as a predisposing cause of LLT. In humans, instead, a predisposing factor is the dissection of the supporting network of lung ligaments performed during surgery, in combination with lobe deflation.^1,3,6,17 Typically, in deep-chested, large-breed dogs the torsion occurs in the right middle lobe (RML), while in small dogs in the left cranial lobe (LCL); however, all lung lobes can potentially be affected.⁶ The greater instability of the RML is apparently related to different anatomic factors, such as the narrow bronchovascular pedicle, the lobe’s elongated and thin shape and its lack of attachment to the mediastinum, the thoracic wall and the adjacent lobes (right cranial lobe and left caudal lobe).^1-8 It has also been suggested that bronchial cartilage dysplasia may potentially increase the mobility of individual lobes, thus becoming a predisposing factor for the disorder.^4,9,10 Torsions of the caudal lobes or of the right cranial lobe are rare, although the latter can be involved by the torsion of the right middle lobe (Fig. 1); of more frequent occurrence is instead the simultaneous torsion of the two segments of the left cranial lobe, as the two segments share a common primary bronchus.³

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PATHOPHYSIOLOGY       

In the presence of LLT the thin venous wall rapidly collapses, whereas the persisting arterial supply causes vascular congestion, hardening of the lung lobe, progressive haemorrhagic engorgement (organ darkening)^1-4 and lung lobe hepatisation, consequent to the flowing of blood into the interstitial spaces and into the airways (Fig. 2). Pulmonary venous hypertension and reduced lymphatic drainage generate pleural effusion which, together with inflammation and the progressive resorption of alveolar gases, contributes to lobe atelectasis (resorption atelectasis).¹¹ Atelectasis, in turn, causes a reduction in the tidal volume, with consequent compensatory tachypnoea as the animal struggles to maintain adequate ventilation.

SIGNALMENT

LLT is rare in the dog and extremely rare in the cat. As previously mentioned, a predisposition is reported in deep-chested, large-breed dogs, in particular in the Afghan Hound, often in association with chylothorax, which is considered both as a consequence and as a predisposing factor.^1-6,12 LLT is less common in small and toy dog breeds, with a greater predisposition in the Pug; the onset is spontaneous, without prior traumas or concomitant disorders.^1-4,6,13 In large-sized dogs, torsion typically occurs in the RML, whereas in small dogs it affects the LCL. In both dogs and cats LLT mostly affects middle-aged or young subjects (mean age 4.5 years); it can however be diagnosed at any age.^1,3,6 The degree of lobe rotation varies from 90° to 580°.¹⁴

CLINICAL SIGNS          

The clinical history is often nonspecific and includes: progressive dyspnoea, tachypnoea, lethargy, anorexia, vomiting, acute or chronic cough and weight loss. Recent traumas, thoracic surgery and concomitant or previous lung disorders should direct the diagnosis to LLT. In the dog, acute or sub-chronic symptoms may be present but the mean duration is of 3-4 days.¹³ At physical examination, the most frequent abnormalities at auscultation are “dull” lung sounds and muffled heart sounds in one area, as opposed to increased murmur in the others, as well as weakness, shock and cyanosis. The more common blood chemistry findings are: neutrophilia, anaemia, increased alkaline phosphatase and creatinine, as well as a possible reduction in albumins and total proteins.¹³ Pleural effusion is often present; cytologically, it is characterised by the presence of erythrocytes and leukocytes; it is rarely of septic nature and it is more often suggestive of a sterile inflammatory process; in one third of cases it is of chylous nature.¹⁵

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DIAGNOSTIC IMAGING     

RADIOGRAPHIC ASSESSMENT. Thoracic X-rays are part of the diagnostic workup and usually allow to detect opacification of the affected lung areas, dorsal displacement of the trachea and pleural effusion⁶ (Fig. 3). In the presence of pleural effusion, to visualise lung lobe consolidation (hepatisation) drainage of the excess fluid may be necessary. Typically, the bronchial branch of the affected lobe appears dislocated in an abnormal anatomical position within the thorax; this unusual air bronchogram can only be seen during the acute phase, as the air slowly dissipates, leaving space for the infiltration of inflammatory fluid and blood^6-16 (Fig.4).

THORACIC ULTRASONOGRAPHY. Thoracic ultrasonography typically shows a hypoechoic lobe, rounded in shape and surrounded by pleural effusion with gas in the centre. As soon as the lung parenchyma consolidates, lung hepatisation occurs. As X-rays and ultrasonography are often nonspecific, when possible, before surgery a tomographic or endoscopic examination should be carried out.^6-13-16

COMPUTERIZED TOMOGRAPHY (CT). In the literature, CT is the most reliable diagnostic investigation, together with virtual bronchoscopy; apart from lung structures in toto, CT also allows to visualise any other underlying lung abnormality. The normal finding is pleural effusion and the sudden interruption of the profile of the bronchus involved, together with the enlargement, consolidation and emphysema of the corresponding lobe¹⁰ (Figs. 5 and 6). Following the intravenous administration of the contrast medium, the twisted lung lobes do not increase in opacity, differently from the adjacent aerated ones (Fig. 7). Recently, a new reconstruction technique of CT images that allows a more accurate diagnosis through internal reconstruction of the trachea and bronchi has been reported.^1,3,16

BRONCHOSCOPY. Bronchoscopy typically allows to detect the torsion of a bronchus, which often appears oedematous, with or without the presence of blood.^13,16

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DIFFERENTIAL DIAGNOSES

Pulmonary thromboembolism, pulmonary contusion, neoplasia, pneumonia, pleural haemorrhage, coagulopathy, diaphragmatic hernia, pyothorax and lung abscesses are all pathological conditions that may cause similar alterations on chest X-rays.

TREATMENT

The first-choice treatment is surgery, specifically pulmonary lobectomy. Spontaneous resolution is unlikely, given the compression exerted by pulmonary effusion and the formation of adhesions. Before surgery it is however crucial to first stabilise the patient by relieving the respiratory distress using oxygen therapy and draining the pleural effusion through thoracentesis or by placing a chest tube if the fluid is persistent or abundant.

SURGICAL TECHNICQUE. If localisation of the torsion is certain, the animal is placed in lateral recumbency and an intercostal thoracotomy is performed; the intercostal space where the incision is made is dictated by the location of the affected lobe (Fig. 8).

Otherwise, a median sternotomy allows the veterinary surgeon to fully explore the thoracic cavity and identify the affected lobe, which usually appears congested, friable and necrotic. In order to avoid the release of toxins and vasoactive substances into the systemic circulation, it is important not to unwind the torsion or to reposition the lobe before removing it^1-4 (Fig. 9). Should this not be possible, the twisted pedicle should be clamped at its base with atraumatic clamps (e.g. Satinsky clamp). To perform the lobectomy the bronchi and vessels can be tied with a monofilament suture using the techniques typically described for pulmonary lobectomy; braided sutures should be avoided in order to reduce the risk of infection.³

The use of a surgical stapler (TA linear stapler 30 or 55) is usually preferred as it facilitates removal during torsion and shortens surgical times; the technique has been shown to be rapid and very effective¹ (Fig. 10). The size and rows of the metal staples are chosen based on the dimension of the vascular-bronchial cord to be closed. Dogs tolerate well up to 58% of total lung volume loss and it has been reported that tolerance to exercise and altitude are not significantly affected until 60% to 75% of the total lung volume has been lost.^1-4 Since the right lung accounts for approximately 58% of the total lung tissue (the left lung about 42%), the removal of a single lobe is a largely sustainable loss for the animal. Before closing the surgical wound and inserting a chest drain (Fig. 11) it is important to verify that the remaining lobes expand normally and are correctly positioned. The excised lobe must undergo histological examination in order to exclude the presence of any predisposing causes such as pulmonary disease or neoplasia; in addition, a sample must be collected for tissue culture.

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PROGNOSIS     

In animals with primary idiopathic LLT, or secondary to trauma, if the treatment is surgical the prognosis is good.^1,4,6 In addition, the prognosis is generally better in small-size dogs, and is reported to be more favourable in the Pug.^1,4,6 However, postoperative recovery is markedly influenced by the duration of the torsion and by the clinical conditions of the animal at the time of surgery. Death may occur due to the systemic effects caused by the inflammatory mediators generated by the decomposition of the ischaemic and necrotic lung lobe. In the presence of chylothorax, present in approximately 1/3 of cases,¹⁵ the long-term prognosis can be uncertain or unfavourable, with high rates of recurrence of chylous effusion in the following 6 months, especially in the Afghan Hound, even after lobectomy for LLT;^4,12 in the presence of other underlying disorders (e.g. lung cancer), the prognosis depends on the type of disease and on the animal’s conditions at the time of diagnosis.

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Bibliografia

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