Spontaneous Pneumomediastinum

00Overview

Spontaneous pneumomediastinum (SPM) 是指沒有外傷、手術或 iatrogenic 原因下發生的 mediastinal free air。影像任務核心是：(1) 辨識 mediastinal air 的來源與分布；(2) 排除 must-not-miss causes（esophageal perforation / Boerhaave syndrome）；(3) 區分 benign SPM（Hamman syndrome，self-resolving）與需要手術介入的 secondary pneumomediastinum。最容易出錯的地方：把所有 pneumomediastinum 當成 benign，忽略了 esophageal rupture 的 possibility。

01Critical concepts

• Macklin effect 是 SPM 的病理機轉：alveolar rupture → air dissects along perivascular / peribronchial interstitium → tracks centrally to mediastinum
• SPM 最常見於 young males（15-30 歲），誘因包括 Valsalva maneuver（coughing、vomiting、straining、exercise、marijuana / inhaled drug use）
• SPM 90% 以上是 self-limiting + benign，不需要 intervention
• 必須排除 esophageal perforation（Boerhaave syndrome）：left-sided pleural effusion + mediastinal air + post-forceful vomiting → until proven otherwise = Boerhaave → CT with oral contrast / water-soluble contrast swallow study

01正常 anatomy / 常用 modality

Mediastinum 是一個 partially enclosed fascial space，air 可沿 fascial planes 擴展至 cervical soft tissues（subcutaneous emphysema）、pericardium（pneumopericardium）、peritoneum（pneumoperitoneum through diaphragmatic hiatus）、或 epidural space。Cervical 與 mediastinum 之間透過 retropharyngeal space / danger space / prevertebral space 相通，這也是 descending necrotizing mediastinitis (DNM) 的傳播路徑。

常用 modality：

• CXR：首選 screening — mediastinal air outlining mediastinal structures（continuous diaphragm sign、ring-around-artery sign、tubular artery sign、thymic sail sign in children、Naclerio V sign、Spinnaker sail sign、extrapleural air sign）
• CT chest：definitive — precise localization of air + evaluation of underlying cause（esophageal thickening、pleural effusion、pneumothorax）
• CT with oral contrast：如果懷疑 esophageal perforation → water-soluble oral contrast → look for extraluminal contrast leak
• Fluoroscopic esophagogram (water-soluble contrast swallow)：classic test for esophageal leak

CXR signs — 外觀與機轉

• Continuous diaphragm sign：air 介於 pericardium 與 diaphragm 之間，使原本被心臟陰影遮蔽的中央橫膈在 frontal view 上呈現一條連續的細線；機轉為 mediastinal air 沿 pericardiophrenic 反射面分布。
• Ring-around-artery sign：air 環繞 right pulmonary artery（lateral view 最明顯），形成一圈 lucency。
• Tubular artery sign：air 沿 aorta 與其主要分支外緣分布，使血管壁在 frontal view 上呈現 sharply outlined tubular structure。
• Thymic sail sign (Spinnaker sail sign)：兒童常見；mediastinal air 將 thymus lobe 從 superior mediastinum 抬離心影，呈現如帆船船帆般向外上方展開的影像。
• Naclerio V sign：左側橫膈內側段上方出現 V 形 lucency（由 mediastinal air 與 left medial diaphragmatic pleural reflection 形成），常與 esophageal perforation 相關，是 Boerhaave 的高度提示 sign。
• Extrapleural air sign：air 介於 parietal pleura 與 mediastinal structures 之間，常沿 descending aorta 外緣呈線狀。
• 其他：subcutaneous emphysema along neck / chest wall、aortic knob 周圍 lucent line、left paraspinal stripe displacement。

02常見 pattern 分類

Perivascular / peribronchial air tracking pattern

• Definition：air 沿著主要血管與支氣管周圍擴散，CT 見 thin strips of air outlining aorta、pulmonary arteries、main bronchi
• Why it matters：最典型的 SPM distribution（Macklin effect pathway），支持 benign alveolar rupture origin
• What it points toward：SPM from alveolar rupture（asthma exacerbation、forceful cough / vomiting、exercise、mechanical ventilation with barotrauma）
• Common trap：perivascular air can also come from esophageal perforation — must check esophageal wall integrity + pleural effusion

Cervical subcutaneous emphysema extension

• Definition：mediastinal air 沿 fascial planes 向上延伸至 cervical / supraclavicular soft tissues
• Why it matters：大量 subcutaneous emphysema 可壓迫 airway（rare）；主要作為 pneumomediastinum 的 confirmatory sign
• What it points toward：SPM with decompression through thoracic inlet（"safety valve mechanism" — air exits mediastinum → cervical soft tissues → prevents tension physiology）
• Common trap：isolated cervical emphysema without mediastinal air → consider direct cervical injury（pharyngeal perforation、dental procedure complication）

Left-sided pleural effusion with mediastinal air

• Definition：pneumomediastinum + left pleural effusion（especially if rapidly accumulating or has high attenuation content）
• Why it matters：==left-sided effusion + mediastinal air + vomiting history = Boerhaave syndrome until proven otherwise==
• What it points toward：esophageal perforation（Boerhaave typically at left posterolateral distal esophagus）；mediastinitis
• Common trap：Boerhaave 的 mortality > 30% if diagnosis delayed > 24 hours → 不能用 "probably benign SPM" delay investigation

Associated pneumothorax / pneumopericardium

• Definition：pneumomediastinum 合併 pneumothorax（usually small, left-sided）or pneumopericardium
• Why it matters：pneumopericardium with significant volume → cardiac tamponade（rare but life-threatening）；pneumothorax 需 chest tube if > 2 cm or hemodynamically significant
• What it points toward：extensive Macklin effect with air rupturing into pleural / pericardial space；or direct pleural / pericardial involvement by underlying cause
• Common trap：tension pneumopericardium is extremely rare but 需 emergent pericardiocentesis 或 needle decompression

03Top common diagnoses

• Primary (idiopathic) SPM / Hamman syndrome：young male + Valsalva trigger → benign + self-resolving → conservative management（pain control + rest + oxygen for reabsorption）
• Asthma-related SPM：severe asthma exacerbation → air trapping → alveolar rupture → Macklin effect
• Boerhaave syndrome：post-forceful vomiting → full-thickness esophageal perforation（usually left posterolateral distal esophagus）→ surgical / endoscopic emergency
• Drug-use related SPM：marijuana / cocaine / methamphetamine inhalation with forceful Valsalva → alveolar rupture
• Diabetic ketoacidosis (DKA) related SPM：Kussmaul breathing → hyperventilation → alveolar rupture（uncommon but documented）

04Cannot-miss diagnosis / emergency

05高頻 mimics 與 discriminators

Benign SPM vs Boerhaave syndrome

• Why they get confused：both have pneumomediastinum + may have vomiting trigger
• Most useful discriminators：(1) pleural effusion — Boerhaave has left pleural effusion；benign SPM usually no effusion or minimal；(2) oral contrast leak on CT/swallow → Boerhaave confirmed；(3) clinical severity — Boerhaave has disproportionate pain + sepsis；SPM has milder presentation；(4) esophageal wall thickening / peri-esophageal fluid on CT → Boerhaave；(5) Mackler triad（vomiting + chest pain + subcutaneous emphysema）→ Boerhaave
• Common trap：Boerhaave 可以 sealed perforation → initial contrast study negative → repeat if clinical suspicion high

Pneumomediastinum vs pneumopericardium

• Why they get confused：air around heart can be in either space
• Most useful discriminators：
  • (1) 上界：pneumomediastinum air 可延伸至 neck、great vessels 周圍與 thoracic inlet 之上；pneumopericardium air 被 pericardial reflection 限制，通常止於 pulmonary artery / ascending aorta 反折處——但注意 supine film 上 air 可沿 pericardial reflection 上升至看似較高位置，故位置判讀需結合 lateral / decubitus view 或 CT，不可單以高度絕對判定
  • (2) 形態：pneumopericardium air 緊貼心緣呈 halo 狀，且不會延伸進入 superior mediastinum 軟組織間隙；pneumomediastinum air 沿 fascial planes 分布、常見 continuous diaphragm sign 與 cervical extension
  • (3) 體位變化：pneumopericardium 隨體位改變（decubitus 時 air 移至 non-dependent side）；pneumomediastinum 不會
  • (4) CT：直接判斷 air 是否被 pericardium 包覆
• Common trap：both can coexist → if pneumopericardium present, assess for hemodynamic significance (tamponade physiology)

SPM vs mediastinal abscess / necrotizing mediastinitis

• Why they get confused：both can have mediastinal air on CT
• Most useful discriminators：(1) mediastinitis has fluid collections + fat stranding + gas；SPM has air only without fluid or inflammatory change；(2) clinical — mediastinitis has fever + sepsis + leukocytosis；SPM is relatively well；(3) history — recent dental / pharyngeal procedure → descending necrotizing mediastinitis
• Common trap：delayed diagnosis of DNM → mortality > 40% → any mediastinal air + fever + pharyngeal symptoms = aggressive workup

06Next step / protocol / appropriateness

影像 protocol 選擇：

• Suspected SPM — low clinical concern：CXR（PA + lateral）→ if mediastinal air confirmed + no pleural effusion + benign presentation → conservative management
• SPM with any red flags（effusion, severe pain, sepsis, vomiting trigger）：CT chest with IV + oral (water-soluble) contrast → rule out esophageal perforation + evaluate mediastinal integrity
• Boerhaave suspected：CT with oral contrast → if negative but high suspicion → fluoroscopic water-soluble contrast swallow → if still negative → use thin barium swallow（more sensitive for small leaks）
• Follow-up：repeat CXR in 24-48 hours → air should be decreasing → if persistent or increasing → re-evaluate for ongoing air leak

07Pitfalls / normal variants

• Mach band artifact：edge-enhancement artifact at lung-mediastinum interface on CXR can simulate thin line of mediastinal air → CT for confirmation if uncertain
• Thymic rebound after chemotherapy：thymic enlargement can create mediastinal widening that mimics pneumomediastinum on CXR → CT resolves
• Normal mediastinal fat：low-attenuation mediastinal fat on CT should not be confused with air → air = very low HU（-1000）vs fat（-80 to -120 HU）
• Recurrent SPM：occurs in 1-5% of cases → usually also benign and self-resolving
• SPM in pregnancy / labor：Valsalva during pushing → SPM → usually benign → but must still exclude esophageal pathology
• Supine CXR 上 pneumopericardium air 位置：仰躺時 air 可沿 pericardial reflection 上升，看似超出常見 teaching 中的 pulmonary artery 高度——不要僅憑高度排除 pneumopericardium，需 lateral / CT 確認