see also: [[Chest trauma radiology#pneumomediastinum|Chest trauma → pneumomediastinum]] , [[Chest trauma radiology#pneumopericardium vs. pneumomediastinum|Chest trauma → pneumopericardium vs. pneumomediastinum]], and [[Radiology signs with names#Mach effect|Mach effect (pseudo-pneumomediastinum)]] see: [Learning radiology - pneumomediastinum](x-devonthink-item://5B858772-86A7-4299-9E1E-A2CABD5A76C1?page=113) > [!key points] > Pneumomediastinum can occur following blunt or penetrating trauma, often, but not always, major trauma, or can occur “spontaneously,” without evident trauma. > > ***There are three potential sources of mediastinal air:*** > 1. Oesophagus > 2. Tracheobronchial tree > 3. Lung > > (sometimes peritoneum) > > The most frequent source of mediastinal air is the lung. > > Pneumomediastinum occurs in clinical settings associated with *alveolar hyperinflation* and *high intrapulmonary pressure* such as: > - Asthma > - Forceful inhalation followed by breath holding or valsalva barotrauma (as occurs during ==use of illicit drugs such as crack cocaine==) > - Positive pressure ventilation (especially in patients with noncompliant lungs, e.g., ARDS) > - Rapid ascent during [[Diving injuries and Dysbarism#Pulmonary barotrauma|scuba diving]]. > > Alveolar air ruptures into the adjacent interstitial lung tissues and then dissects along the bronchovascular connective tissues to the hilum and then into the mediastinum. This is known as the **Macklin effect**. > > Blunt [[Chest trauma radiology#pneumopericardium vs. pneumomediastinum|trauma]] such as a direct blow to the chest or abrupt deceleration in a motor vehicle collision can also cause alveolar rupture and pneumomediastinum. > [!info]- Causes of pneumomediastinum > - Blunt or penetrating chest trauma > - Oesophageal perforation: (boerhaave syndrome, anorexia with cyclical vomiting or malnutrition emphysema, recent endoscopy, oesophageal cancer) > - Tracheobronchial perforation: (bronchoscopy, tracheostomy, laryngeal fracture) → usually significant trauma > - vigorous exercise: (childbirth ([Hamman syndrome](https://radiopaedia.org/articles/hamman-syndrome-2?lang=us)), valsalva, weightlifting) > - Cough eg asthma or interstitial lung disease > - [[Diving injuries and Dysbarism#Barotrauma|Barotrauma]] : either diving or ventilator eg ARDS, inhaled substance abuse / [[Solvents|chroming]] > - Infection ([[Cavitary lung lesion]] ) **Pneumopericardium** is usually caused by direct penetrating injuries to the pericardium, either caused iatrogenically (during cardiac surgery) or accidentally (from penetrating trauma). Pneumopericardium is ==more common in pediatric patients than adults== and may develop in neonates with hyaline membrane disease. It is rare for air in the pleural space to enter the pericardium, except in those who have a pericardial defect, such as a surgical window incised in the pericardium, which allows free exchange between the pleural and pericardial spaces. Most pneumomediastinum requires no treatment, with the air being gradually absorbed on the following days.  ## Clinical features - chest pain (90% of cases) - dyspnea (50% of cases) - +/- neck pain or dysphagia - cardiac auscultation: crunching or cracking sound synchronous with cardiac contractions, known as *Hammond’s sign*. best heard in left lateral decubitus position. - Air that has migrated into the subcutaneous tissues of the neck and chest wall causes palpable crepitus and swelling, which may be significant. - s/c emphysema usually most prominent at suprasternal notch ## Radiographic features - s/c emphysema - [[#Continuous diaphragm sign]] - [[Chest trauma radiology#pneumopericardium vs. pneumomediastinum|pneumopericardium]] - Gas around pulmonary artery - In paeds may have elevated thymus, gas crossing superior mediastinum > [!tip]- greater detail > - Linear, streaklike lucency associated with a thin white line paralleling the left heart border > - Streaky air outlining the great vessels (aorta, superior vena cava, carotid arteries) > - Linear streaks of air parallel to the spine in the upper thorax extending into the neck and surrounding the oesophagus and trachea ([[#Example 1]]) > - [[#Continuous diaphragm sign]] -- With pneumomediastinum, air can outline the central portion of the diaphragm beneath the heart, producing an unbroken superior surface of the diaphragm, which extends from one lateral chest wall to the other. ***Pneumopericardium features*** - Pneumopericardium produces a continuous band of lucency that encircles the heart, bound by the parietal pericardial layer, **which extends no higher than the root of the great vessels** (corresponding to the level of the main pulmonary artery). - Pneumomediastinum, in contrast, **does extend above the root of the great vessels** into the uppermost thorax. ### Example 1: *Pneumomediastinum*, *pneumopericardium*, and *subcutaneous emphysema*. This patient with asthma developed spontaneous pneumomediastinum, most likely from rupture of an alveolus followed by formation of pulmonary interstitial emphysema. The air tracked back to the hila, then into the mediastinum, where it produced streaky white linear densities (solid white arrows) extending to the neck. In the neck, there is subcutaneous emphysema (dotted black arrows). In adults air does not usually enter the pericardium, except by direct penetration, and so it is somewhat unusual that this patient also developed pneumopericardium (solid black arrows). Notice how the air in the pericardial space does not extend above the reflections of the aorta and pulmonary artery, unlike pneumomediastinum, which does extend above the great vessels. ![[Pasted image 20250427092949.png]] ### Example 2: Pneumomediastinum in a 20-year-old man with an asthma exacerbation. A thin white line representing the medial surface of the parietal pleura is raised from the left heart border and aortic arch (arrowheads). ![[Pasted image 20250427094757.png]] ### Example 3: ![[Pasted image 20250427102208.png]] > [!caption] Massive pneumomediastinum. > A 14-year-old boy with an asthma exacerbation developed marked swelling and palpable crepitus of the soft tissues of his upper chest wall and neck. > - (_A_) The PA view shows extensive subcutaneous emphysema (_arrows_). Air in the mediastinum outlines the normally invisible inferior margin of the heart the [[#Continuous diaphragm sign]] (_arrowheads_). > - (_B_) The lateral view shows subcutaneous air in the anterior chest wall (_arrow_) and air between the heart and left hemidiaphragm—the “continuous left hemidiaphragm sign” (_arrowheads_). ### parietal pleura displacement ![[Pasted image 20250427103625.png]] > [!caption] Note the thin layer of air is adjacent to the left and right heart borders and the fine white line of the **raised parietal pleura** (_white arrowheads_). Also air outlines the lateral margin of the descending aorta (_black arrowhead_) and tracks into the soft tissues of the superior mediastinum and base of the neck (_arrows_). ### Continuous diaphragm sign Sign of pneumomediastinum or pneumopericardium if lucency is above the diaphragm. Sign of pneumoperitoneum if lucency below diaphragm ![[Pasted image 20250427084043.png]] > [!caption] Pneumomediastinum with continuous diaphragm sign ### Pneumopericardium example This patient underwent a procedure to produce a pericardial window for recurrent pericardial effusions. Postoperatively, there is a pneumopericardium, shown by the visible parietal pericardium (white arrows), outlining air around the heart in the pericardial space. Notice how the air *does not extend above the reflection of the aorta* and main pulmonary artery. Pneumopericardium usually occurs from direct violation of the pericardium by trauma. ![[Pasted image 20250427093621.png]] ## pseudo-pneumomediastinum vs. true pneumomediastinum See also: [[Radiology signs with names#Mach effect]], [[Chest trauma radiology#pneumopericardium vs. pneumomediastinum]] ***Mach Bands/***[[Radiology signs with names#Mach effect|Mach effect]] A Mach band is an optical illusion that results from the tendency of visual perception to accentuate or enhance edges between regions of moderately or *slightly different optical density* (No Mach effect occurs between areas of great density difference, i.e., black and white). A dark Mach band appears adjacent to such radiopaque structures as the heart. When the lower lobe pulmonary artery parallels the heart border, this effect is increased. > ==A Mach band can be distinguished from pneumomediastinum== by the absence of the fine line of [[#parietal pleura displacement|displaced parietal pleura]] seen with pneumomediastinum. Extension of air into the neck and abnormalities on the lateral view (air adjacent to the aorta) are also not seen with a Mach band pseudo-pneumomediastinum. **Pseudo-pneumomediastinum**, is radiographic evidence of pneumomediastinum that does not follow the expected time course of a pneumomediastinum and is not associated with the expected clinical signs and symptoms, such as chest pain, labored breathing, crepitus associated with the cardiac cycle, and subcutaneous emphysema. ![[Pasted image 20250427082422.png]] >[!caption] ↑ : Normal CXR with appearance of perceived lucency outlining mediastinum and diaphragm. Is NOT pneumomediastinum; optical illusion ![[Pasted image 20250427102557.png]] > [!caption] **Pseudo-pneumomediastinum** with [[Radiology signs with names#Mach effect|Mach band]]. The edge-enhancement property of visual perception creates an illusory dark band adjacent to the heart borders, particularly the left heart border (_arrows_). ## Management - **Tension pneumomediastinum** is a rare condition, starting with pleuritic chest pain, dysponea, dysphagia, and progressing to severe respiratory distress and cardiovascular collapse due to generalised ↑ intrathoracic pressure causing obstructive shock - needle aspiration in this case , may need a subxiphoid mediastinal drain placed by a thoracic surgeon[^1] > **Emergency Tx for tension pneumomediastinum:** > - ​Incise skin in the suprasternal notch > - Insert finger into superior mediastinum until gas escapes - ***Spontaneous pneumomediastinum*** is usually relatively benign; minimally symptomatic pneumonediastinum usually only requires conservative management, analgesia, and repeat CXR to ensure spontaneous resolution - mediastinal air will be absorbed faster if the patient inspires high concentrations of O2 - resp compromise is rare as the air usually decompresses into the soft tissues of the neck [^1]: See this [case series](https://pmc.ncbi.nlm.nih.gov/articles/PMC9399585/) of surgical treatment for pneumomediastinum in COVID-19 patients