Division of Diagnostic Imaging, Faculty of Veterinary Medicine, Utrecht University
Utrecht, The Netherlands
This paper presents the principles of radiographic examination and interpretation of dogs and cats that are presented to the veterinarian following thoracic trauma. Interpretation of radiographic abnormalities is based upon knowledge of normal radiographic anatomy of the thorax and upon understanding of the basic roentgen signs and patterns that develop after trauma. Because of the complexity of radiographic abnormalities in a traumatized thorax, radiographic interpretation follows a systematic approach based on five different categories of structural thoracic damage.
While thoracic injuries are common and important threats to life, the thorax and its contents are not easily injured. The thorax is tough and resilient due to its strong, springlike ribs. The lungs add protection against impact through their air-cushion effects. Due to these protective characteristics, virtually all thoracic injuries are due to high-energy forces generated by violent trauma. Consequently, thoracic injuries are often a part of a constellation of injuries involving several areas of the animal's body. In addition to thoracic changes, also abdominal and skeletal injuries may be present that could easily be overlooked because of the dramatic appearance of the thoracic lesions. Structural damage to the thorax caused by trauma can be divided into five categories:
1. disruption of the bellows apparatus (chest wall and diaphragm)
2. damage to the lung parenchyma
3. disruption of the airways
4. damage to the heart and great vessels (incl. thoracic duct)
5. damage to the esophagus
The basic roentgen signs of thoracic trauma are alterations in density, size, location, shape and margin of structures normally located in the thoracic cavity and thoracic wall, as well as the presence of structures in the thoracic cavity that are normally located elsewhere.
Decreased radiographic density is caused by an abnormal presence of air in the subcutaneous tissues or in the pleural, mediastinal or pericardial spaces, or results from decreased pulmonary vasculature due to hypovolaemic shock or pulmonary thromboembolism.
Increased radiographic density (soft tissue opacity) may occur in the subcutaneous tissues, pleural cavity or mediastinum as a result of fluid accumulation. Atelectasis, edema or haemorrhage induce increased pulmonary density. Intra-pericardial fluid will produce an enlarged cardiac silhouette while the heart itself may be smaller than normal due to reduced circulating blood volume.
Pulmonary arteries may increase in size proximal to an area of arterial embolism or thrombosis but are generally less prominent in hypovolemic shock. Bony changes include fractures and dislocations of ribs, sternebrae, thoracic vertebral bodies, articular facets and spinous processes, and scapulae.
The presence of abdominal viscera in the pleural space, with or without accompanying abnormalities, indicates a diaphragmatic rupture or hernia.
1. Disruption of the bellows apparatus (chest wall and diaphragm)
This type of injury results in diminished efficiency of respiration and restricted expansion of the rib cage. Injuries include open chest wounds, multiple rib fractures incl. Flail chest, rupture of diaphragm and (secondary) lung compression. Radiography may help to define and evaluate the extent of the underlying damage.
With flail chest there are proximal and distal fractures of consecutive ribs resulting in a section of the thoracic wall ("flail segment") that is free to move independently and paradoxically during breathing. A flail segment is usually accompanied by pneumothorax and by damage to the subjacent lung(pulmonary contusion) that are mainly responsible for the severe ventilatory compromise.
The diaphragm is ruptured by forceful impact on the abdomen when the glottis is open. Radiographs will show loss of normal diaphragmatic outline, absence of caudal silhouette of the heart, and increased density in the caudal part of the thoracic cavity. The liver may not be identifiable inside the abdomen, and the stomach and duodenum may be displaced from their normal position. Gas-filled bowel loops may be recognized inside the thoracic cavity. Pleural effusion may be present as result of constriction of a liver lobe by the diaphragm or due to entrapment of the small intestine. When the stomach becomes lodged in the thoracic cavity and the cardia and pylorus become occluded acute gastric dilatation may occur resulting in a life threatening situation.
In cats, the diaphragmatic tear is most of the time at the right side since the central tendon is weakest on that side.
2. Damage to the lung parenchyma
Abnormalities include pulmonary contusion, lung rupture or laceration, pulmonary haematomas and pseudocysts (pneumatocele, bulla, subpleural bleb). Pneumothorax may be an accompanying sign. Most animals with blunt thoracic trauma suffer to some degree of pulmonary contusion, with edema and haemorrhage.
Pulmonary contusion is caused by rapid compression and subsequent decompression of the lungs resulting in disruption of alveolar-capillary integrity. This causes a diffuse bruise of the underlying lung with haemorrhage and edema of the alveolar and interstitial spaces. Trauma of the lung can range from a small localized contusion without clinical signs to massive lacerations that produce rapid atelectasis and exsanguination due to pneumothorax and haemorrhage. Radiographic signs of pulmonary contusion develop in the first 4-6 hours following the traumatic event and may progress in severity in the first 24hours. Characteristic signs are areas of variable patchy density in an alveolar pattern that initially do not follow anatomic patterns, but may eventually include an entire lung lobe (12-24 hours post-traumatically). Radiographs revealing fractured ribs are an indication of pulmonary contusion, even though those radiographs taken early following trauma may not show pulmonary involvement. On the other hand, severe pulmonary contusions maybe present in the absence of rib or sternal fractures. The pulmonary infiltrates will diminish in 3-7 days. However, when animals do not recover in the expected way and diffuse pulmonary infiltrations persist, the possibility of secondary acute respiratory distress syndrome (A.R.D.S.) is very likely.
A.R.D.S. is a form of respiratory failure characterized by progressive dyspnoea, hypoxia, and interstitial and intraalveolar edema, with a delayed onset of signs following the initiating traumatic event. A.R.D.S. is believed to be a silent partner in many causes of death in the critically injured animal.
Pulmonary haematomas may be formed when localized bleedings occur within the pulmonary parenchyma. After an initial silent period of several hours or days, they are radiographically visible as well-circumscribed soft tissue densities that will resolve spontaneously in the following weeks.
Pulmonary pseudocysts are presumed to represent a coalescens of ruptured airspaces within the parenchyma, in combination with pulmonary contusion. They may only become radiographically evident several days following the original injury and usually appear as localized, spherical radiolucent lesions filled with air or with a combination of air and fluid. They resolve spontaneously in a period of weeks to months. Rupture of these traumatic cysts results in pneumothorax.
3. Disruption of the airways
This kind of injury can be caused by penetrating or blunt traumato the thorax while the glottis is closed. Radiographs will show pneumothorax, mediastinal emphysema (pneumomediastinum) and retroperitoneal emphysema (pneumoretroperitoneum), and subcutaneous emphysema.
Pneumothorax is the most common sequela to thoracic trauma and can be found with penetrating chest wall injuries, lacerations, tears and ruptures of lungs and airways, and as an extension of pneumomediastinum or as a result of rupture of a traumatic pulmonary pseudocyst. Pneumothorax has been defined as the collection of free air within the pleural space resulting in a loss of negative intrathoracic air pressure allowing the lungs to recoil away from the chest wall. Normally, a pneumothorax in dogs and cats is bilateral because the thin mediastinum is not closed at all or ruptures easily with the original trauma. Pneumothorax can be open or closed.
A complication is tension pneumothorax that is an immediate life-threatening condition. Due to a one-way valve effect during inspiration intrathoracic air pressure increases exceeding atmospheric pressure. This results in severe compression of the lung lobes, the heart and great vessels. Radiographs should be regarded as being of confirmative value and as a control upon the possible development of tension pneumothorax. Radiographic signs of pneumothorax include increased size and lucency of the thoracic cavity, separation of the cardiac silhouette from the sternum, visibility of the lung borders separated from the thoracic walls while pulmonary vasculature is not extending across the radiolucent pleural space to the thoracic wall, and accumulation of air around the heart. With tension pneumothorax, the lungs are small and collapsed, the heart and large vessels are small, and the mediastinum may be shifted to one side. The caudal thoracic borders may extent far caudally into the abdominal cavity.
4. Damage to the heart and great vessels (incl. thoracic duct)
Traumatic injuries of the cardiovascular system are only seldomly recognized on non-contrast radiographs. Myocardial contusion is caused by impact to the heart and cannot be recognized radiographically. Laceration and rupture of the heart and vessels will result in haemothorax and increased density and widening of the cranial mediastinum. Also, large volume pericardial effusion or even pneumopericardium may be found. However, most of these animals will die fast and never reach the veterinarian.
Haemothorax is the collection of blood in the pleural space and is only a problem when it originates from lacerations of intercostal, bronchio-esophageal or other systemic arterial vessels.
Pulmonary haemorrhage is usually self-limiting because the pulmonary circulation is of low pressure. Chylothorax denotes the presence of intestinal lymphe (chyle) in the pleural space. In the dog and cat, the delicate thoracic duct runs through the left-sided mediastinum and can be disrupted by the force of blunt trauma. Radiographic signs of pleural effusion include a widened, fluid-dense pleural space, prominent interlobar fissures, obscuring of the cardiac and diaphragmatic silhouettes, and rounding of the lung lobe tips in the thoracic periphery.
5. Damage to the esophagus
Esophageal injuries by thoracic trauma are rare. However, a penetrating object (bullet) may perforate the esophagus resulting in pneumomediastinum because of air leakage or in mediastinitis, pleuritis and pyothorax when esophageal contents spill through the leak.
Radiographic control of esophageal integrity should not be performed with barium-contrast esophagography but with one of the modern aqueous iodine contrast media such as iohexol or iopamidol.
As a conclusion, radiography is a very reliable, non-invasive method of establishing the presence, nature, site and extent of a traumatic injury to the thorax of an animal. It is confirmative and supplemental to the clinical examination. Besides that, radiographs form a permanent record that may be used as a basis for checking progress of recovery and are a baseline for further evaluation of respiratory distress syndrome.