Hemothoraces are usually caused by an injury, but they may occur spontaneously due to cancer invading the pleural cavity, as a result of a blood clotting disorder, as an unusual manifestation of endometriosis, in response to pneumothorax, or rarely in association with other conditions.

Hemothoraces are usually diagnosed using a chest X-ray, but they can be identified using other forms of imaging including ultrasound, a CT scan, or an MRI.

In most healthy people, these two layers are tightly apposed, separated only by a small amount of pleural fluid.

Signs and symptoms include anxiety, rapid breathing, restlessness, shock, and pale, cool, clammy skin.

[7] Minor chest trauma can cause hemothorax when the blood's ability to clot is diminished as result either of anticoagulant medications or when there are bleeding disorders such as hemophilia.

The most common iatrogenic causes include subclavian venous catheterizations and chest tube placements, with an occurrence rate of around 1%.

Nontraumatic hemothoraces most frequently occur as a complication of some forms of cancer if the tumour invades the pleural space.

Significant hemothoraces can occur with spontaneous rupture of small vessels when the blood's ability to clot is diminished as result of anticoagulant medications.

[8] Bone growth in exostosis can create sharp edges, which can result in hemothorax by damaging adjacent arteries.

Rarely, a rupture of the thoracic aorta can result in a hemothorax, but the bleeding usually occurs in the pericardial space.

Rarely, hemothoraces can arise due to extrapelvic endometriosis, a condition in which tissue similar to the lining that normally covers the inside of the uterus forms in unusual locations outside the pelvis.

[19] Following the initial loss of blood, a small hemothorax may irritate the pleura, causing additional fluid to seep out, leading to a bloodstained pleural effusion.

[22] In cases where the nature of an effusion is in doubt, a sample of fluid can be aspirated and analysed in a procedure called thoracentesis.

On an erect chest X-ray, a hemothorax is suggested by blunting of the costophrenic angle or partial or complete opacification of the affected half of the thorax.

On a supine film the blood tends to layer in the pleural space, but can be appreciated as a haziness of one half of the thorax relative to the other.

[5] A small hemothorax may be missed on a chest X-ray as several hundred milliliters of blood can be hidden by the diaphragm and abdominal viscera on an erect film.

[3] Computed tomography (CT or CAT) scans may be useful for diagnosing retained hemothorax as this form of imaging can detect much smaller amounts of fluid than a plain chest X-ray.

[5] Hematocrit can be roughly calculated by dividing the red blood cell count of the pleural fluid by 100,000.

While small hemothoraces may require little in the way of treatment, larger hemothoraces may require fluid resuscitation to replace the blood that has been lost, drainage of the blood within the pleural space using a procedure known as a tube thoracostomy, and potentially surgery in the form of a thoracotomy or video-assisted thoracoscopic surgery (VATS) to prevent further bleeding.

Additional treatment options include antibiotics to reduce the risk of infection and fibrinolytic therapy to break down clotted blood within the pleural space.

Inadequate drainage may lead to a retained hemothorax, increasing the risk of infection within the pleural space (empyema) or the formation of scar tissue (fibrothorax).

If VATS is unavailable, an alternative is fibrinolytic therapy such as streptokinase or urokinase given directly into the pleural space seven to ten days after the injury.

While small hemothoraces may cause few problems, in severe cases an untreated hemothorax may be rapidly fatal due to uncontrolled blood loss.

If left untreated, the accumulation of blood may put pressure on the mediastinum and the trachea, limiting the heart's ability to fill.

However, if treated, the prognosis following a traumatic hemothorax is usually favourable and dependent on other non-thoracic injuries that have been sustained at the same time, the age of the person, and the need for mechanical ventilation.

[37] It is more likely in people who develop shock, had a contaminated pleural space during the injury, persistent bronchopleural fistulae, and lung contusions.

If extensive, this scar tissue can encase the lung, restricting movement of the chest wall, and is then referred to as a fibrothorax.

After the chest tube is removed, over 10% of cases develop pleural effusions that are mostly self-limited and leave no lasting complications.

[41] It can result from any injury that involves the pleural, intercostal, intervertebral, cardiac,[38] or thoracic wall muscle.

Drainage is not always required,[43] but can be performed in case of infection or fluid levels resulting in respiratory compromise.