Introduction

Thoracic trauma may be penetrating or blunt with the more common causes listed below.

Many older, small animal publications have looked at fractures with associated thoracic trauma. In our institution, pelvic limb trauma cases always undergo thoracic radiography, but the following data indicate that thoracic radiographs should be made more often.

Small animal pathology elsewhere associated with thoracic trauma according to literature

The presence of thoracic pathology, as seen on CT as a result of severe blunt trauma in man (Oikonomou et al.) and small animals (Simpson et al. - non-imaging stats) is given below. Radiographs are not as sensitive as CT to pick up many of these cases, but it is important to know what to look for and a systematic radiographic examination is thus essential.

Evaluation and interpretation of thoracic radiographs should be performed according to standard principles including assessing technical faults, extrathoracic pathology and intrathoracic pathology. Trauma patients are usually too compromised to administer general anaesthesia. Pain medication and sedation are usually sufficient for imaging when using restraining devices such as bandages, Velcro straps and sand bags. Using only sedation often results in some images being compromised, and in particular due to respiratory motion, and repeat radiographs may be indicated in such patients.

Extrathoracic Abdominal Changes

The visible part of the abdomen should be scrutinised for abdominal and retroperitoneal fluid, as well as the presence of free gas and organ displacement.

Extrathoracic Soft Tissue Changes

Subcutaneous gas and soft tissue swelling, which can include paracostal abdominal content, are readily seen on radiographs. Oblique views may be required to skyline potential pathology not seen on standard views. Bite wounds, usually in intact male smaller breeds, often have extensive subcutaneous changes without appreciable skin involvement due to skin elasticity.

Skeletal Changes

The ribs are most commonly involved and may be fractured or subluxated. Diagnosing rib pathology is not always easy. Several techniques have been devised to optimize fracture evaluation on radiographs with varying success. These include inverting the image or in digital imaging looking at a negative (inverted) image. Fractures of ribs 1–3, which are difficult to see, may be associated with cranial mediastinal and brachial plexus pathology, ribs 4–10 with intrathoracic pathology and ribs 9–13 with cranial abdominal pathology. Widened intercostal spaces are indicative of intercostal muscle tears, which are often seen in thoracic bite wounds including those without skin penetration.

Vertebral fractures and subluxation need specifically to be looked for, as well as sternal pathology, which is quite rare. Trauma-related pneumorachis (air in the vertebral canal) has been described in trauma cases, but has not been seen by us to date.

Intrathoracic - Pleural Cavity

Pneumothorax is a common sequel to trauma, either trauma directly to the thorax (rib fractures or direct impact to thoracic wall or tracheobronchial rupture) or as result of increased thoracic pressure secondary to abdominal trauma. In man, occult pneumothorax (missed on radiographs) occurs in up to 78% of patients, particularly as many of the radiographs are made in an emergency room with the patient in a supine (dorsal recumbency) position. Our experience confirms the fact that many cases of pneumothorax are either missed or the severity underestimated on radiographs.

Routine thoracic views, and in particular DV and VD views, have poor sensitivity for determining small-volume pneumothorax. A recent study by Lynch et al. has shown that horizontal beam VD views markedly increase the sensitivity of radiography to detect pneumothorax, which will be located in the upper hemithorax. Part of the problem with routine views is that not all the free gas may be located at the periphery of the lungs, and often smaller gas pockets are trapped between the lungs and the mediastinum creating strange patterns on lateral radiographs.

It is crucial to detect a small-volume pneumothorax. Many trauma cases end up in theatre for surgical procedures and if positive pressure manual or ventilator respiration is done, terminal tension pneumothorax may result in these patients.

Haemorrhage may take place from the lung, thoracic wall or even herniated liver and spleen or rupture of intercostal arteries secondary to rib fractures. Haemothorax has a much lower incidence than pneumothorax. Horizontal beam VD views also markedly increase the sensitivity of radiography to detect small-volume haemothorax, which will be located in the lower hemithorax.

Intrathoracic - Lungs

Pulmonary contusion is the most common trauma-related change in the thorax with an incidence of about 60%. Contusion occurs as a result of pulmonary compression and tearing with secondary capillary disruption with leakage of blood into the interstitium and alveoli. Contusion usually only becomes visible 4–6 hours post-trauma and progresses to reach maximum lung involvement at 24 hours, when it may be more radiographically visible. In man, clearance of uncomplicated contusion starts at 24–48 hours with complete resolution in 3–14 days. Radiographically, contusion may vary from mild interstitial to advanced alveolar patterns with a varying distribution pattern. Contusion is often evident adjacent to rib fractures and subcutaneous pathology.

Pulmonary laceration and cavitatory lesions are seen occasionally. The lung may tear or disrupt due to compressive forces, shear injuries, rib penetration or adjacent to previous adhesions. The air-filled cavity, which may also contain blood, is usually surrounded by pulmonary contusion. The cavitatory lesions may take weeks or months to clear.

Intrathoracic - Mediastinal Structures

Pneumomediastinum is readily detected with radiographs as increased visibility of mediastinal structures, particularly blood vessels. Haemomediastinum is rare in our experience, but needs to be looked for. Be careful not to misinterpret a normal thymus in young dogs for haemorrhage. Pneumo- and/or haemopericard are rare occurrences. Only pneumopericardium can be reliably diagnosed on radiographs as increased visibility of the auricles. Tracheal (and bronchial) rupture should be looked for, particularly in cats.

Diaphragm

Diaphragmatic ruptures are routinely diagnosed on thoracic radiographs. However, if they are accompanied by pleural effusion or are small ruptures, they can be a radiological challenge. Positive contrast coeliography can assist to make a diagnosis in these cases. However, if the defect is constricted, contrast may not pass into the thoracic cavity. Barium per os may also be useful if gastrointestinal content is suspected of being in the thorax. Note that on occasion only falciform fat may be present in the thorax. Thoracic ultrasonography is also very useful in radiologically challenging cases.

Conclusion

Radiographs, particularly utilising additional horizontal beam views, are an essential diagnostic tool in the evaluation of trauma patients. Severe thoracic trauma is accompanied by multiple pathologies involving the above systems (e.g., pneumothorax, pleural effusion and rib fractures). Should a patient deteriorate clinically after the initial radiographs, follow-up radiographs must be made to look for new pathology or progression of initial pathology. This includes worsening pneumo- or haemothorax or pneumonia developing in a contused lung.

References

1.  Lynch KC, et al. Detection of pneumothorax and pleural effusion with horizontal beam radiography. Vet Radiol Ultrasound. 2012;1:38–43.

2.  Oikonomou A, Prassopoulus P. CT imaging of blunt chest trauma. Insights Imaging. 2011;2:281–295.

3.  Parry A, Lamb C. Radiology of thoracic trauma in the dog and cat. In Practice. 2012;32:238–246.

4.  Simpson A, et al. Severe blunt trauma in dogs: 235 cases (1997–2003). J Vet Emerg Crit Care. 2009;19:588–602.

5.  Risselada M, de Roostyer H, Taemans O, van Bree H. Penetrating injuries in dogs and cats. Vet Comp Orthopaed Traumatol. 2008;5:434–439.