Pulmonary Arteriovenous Malformation / Right-to-Left Shunt Presentation

00Overview

Pulmonary arteriovenous malformation (PAVM) 是肺動脈與肺靜脈之間的異常直接連通，繞過 capillary bed 形成 right-to-left shunt。影像任務核心是：(1) 在 CXR 或 CT 上辨識 PAVM 的特徵性形態；(2) 評估 feeding artery / draining vein architecture 以決定 embolization 可行性；(3) 排除 hereditary hemorrhagic telangiectasia (HHT / Osler-Weber-Rendu syndrome) 的系統性關聯。最容易出錯的地方：把 PAVM 當成普通 pulmonary nodule 追蹤，忽略其 stroke / brain abscess 風險。

01Critical concepts

• PAVM 約 80-90% 與 HHT 有關，isolated / sporadic PAVM 佔少數；所有 PAVM 患者都應 screen for HHT（Curacao criteria）
• Right-to-left shunt 的臨床後果：paradoxical embolism → stroke / TIA、brain abscess（bypasses pulmonary capillary filter）、hypoxemia
• Feeding artery diameter ≥ 3 mm 是 embolization indication：feeding artery ≥ 3 mm 的 PAVM 有 significant stroke / abscess risk，需 transcatheter embolization
• Simple vs complex PAVM：simple = 單一 feeding artery + 單一 draining vein（80%）；complex = 多條 feeding arteries 或 draining veins（20%），embolization 技術更複雜

01正常 anatomy / 常用 modality

正常肺循環為 low-pressure system，所有血液經過 pulmonary capillary bed 進行氣體交換後匯入 pulmonary veins → left atrium。正常情況下不應有 pulmonary artery 直接連通 pulmonary vein。

常用 modality：

• CT chest with contrast（CT angiography）：gold standard for PAVM detection + characterization；可辨識 feeding artery、nidus、draining vein
• CXR：screening tool — 可見 well-defined round/oval opacity，常在 lower lobes，可追蹤至 feeding vessel
• Contrast echocardiography (bubble study)：screening for right-to-left shunt — saline contrast 在 3-8 cardiac cycles 後出現在 left atrium → intrapulmonary shunt（vs < 3 cycles → intracardiac shunt）
• Catheter pulmonary angiography：diagnostic gold standard + simultaneous embolization
• Nuclear medicine perfusion scan with Tc-99m MAA：quantify shunt fraction（MAA particles > 8 μm normally trapped in lungs；shunt → systemic distribution → kidney / brain uptake）

02常見 pattern 分類

Solitary well-defined nodule with feeding vessel sign

• Definition：CXR/CT 見 well-defined round or oval pulmonary nodule，有 visible feeding pulmonary artery 與 draining pulmonary vein 直接連接
• Why it matters：feeding vessel sign 是 PAVM 最具 pathognomonic 的影像特徵，幾乎可直接診斷
• What it points toward：simple PAVM（單一 feeding + draining）；sporadic or HHT-associated
• Common trap：small PAVM 的 feeding vessel 在 non-contrast CT 上可能不明顯 → contrast-enhanced CTA 是必要的；且 feeding vessel sign 在 CXR 上不一定看到

Multiple bilateral lower-lobe lesions

• Definition：bilateral lower lobes 多發性 well-defined nodules，各自有 feeding artery + draining vein
• Why it matters：multiple PAVMs 幾乎 pathognomonic for HHT — 需要完整 HHT screening（epistaxis history、family history、mucocutaneous telangiectasia）
• What it points toward：HHT type 1（ENG mutation，PAVM prevalence ~50%）> HHT type 2（ACVRL1 mutation，PAVM ~15%）
• Common trap：small PAVMs（< 5 mm）可在 screening CTA 上被忽略 → 需 thin-section CT（≤ 1 mm）+ MIP reconstructions

Diffuse microscopic right-to-left shunt (no visible lesion)

• Definition：contrast echocardiography positive for intrapulmonary shunt，但 CT 上無可見的 discrete PAVM
• Why it matters：hepatopulmonary syndrome（HPS）的 hallmark — 見於 chronic liver disease，diffuse intrapulmonary vascular dilatations（IPVD）太小而 CT 不可見
• What it points toward：hepatopulmonary syndrome in cirrhosis（orthodeoxia + platypnea + positive bubble study + no visible PAVM）
• Common trap：HPS 的 shunt 是 diffuse microvascular dilatation，不適合 embolization → treatment 是 liver transplantation

Post-embolization residual / recurrence pattern

• Definition：embolized PAVM 可見 coils / plugs at feeding artery，但仍有 residual opacification 或 new feeding vessel recruitment（recanalization / reperfusion）
• Why it matters：reperfusion rate 約 5-15%；所有 embolized PAVM patients 需 lifelong CT follow-up
• What it points toward：coil compaction / recanalization；collateral feeding artery recruitment；growth of previously sub-threshold PAVMs
• Common trap：post-coil artifact 可能使 residual patency 的評估困難 → MRA 或 repeat angiography 可為 supplement

03Top common diagnoses

• Simple PAVM：佔 80%，單一 feeding artery + draining vein + nidus，最常位於 lower lobes
• Complex PAVM：佔 20%，多條 feeding arteries +/- 多條 draining veins，embolization 更複雜
• HHT-associated multiple PAVMs：bilateral, multiple, frequently lower lobe predominant
• Hepatopulmonary syndrome (HPS)：diffuse IPVD in cirrhosis，positive bubble study，no discrete PAVM on CT
• Patent foramen ovale (PFO)：most common cause of right-to-left intracardiac shunt → bubble study positive in < 3 cycles（vs PAVM > 3 cycles）

04Cannot-miss diagnosis / emergency

05高頻 mimics 與 discriminators

PAVM vs pulmonary nodule / metastasis

• Why they get confused：small PAVM 在 non-contrast CT 上可呈 solid well-defined nodule
• Most useful discriminators：(1) feeding vessel sign on CTA — PAVM 有 afferent artery + efferent vein 直接連接；metastasis 無此 pattern；(2) MIP reconstructions 可追蹤 vessel connections；(3) PAVM enhances intensely and simultaneously with pulmonary arteries（arterial phase）；(4) clinical context — HHT history, hypoxemia → PAVM
• Common trap：non-contrast CT 上 small PAVM 可被 autodetect software 判為 lung nodule → 放射科醫師需 manual review + contrast study

Intrapulmonary shunt (PAVM) vs intracardiac shunt (PFO/ASD)

• Why they get confused：both cause positive bubble study（right-to-left shunt）
• Most useful discriminators：(1) timing on bubble study — intracardiac shunt: bubbles appear in left atrium within 1-3 cardiac cycles；intrapulmonary shunt: 3-8 cycles（transit time through pulmonary capillary bed）；(2) intracardiac shunts usually seen on echocardiography with color Doppler or on saline study；(3) CTA → PAVM visible vs normal pulmonary vasculature
• Common trap：Valsalva maneuver can increase both intracardiac and intrapulmonary shunts → bubble study should include Valsalva provocation

PAVM vs pulmonary varix

• Why they get confused：both are dilated vascular structures in lung parenchyma
• Most useful discriminators：(1) pulmonary varix is dilated pulmonary vein segment（no arterial feeder, no nidus）；(2) PAVM has arterial feeding + nidus + venous drainage = complete AV circuit；(3) varix opacifies later（venous phase），PAVM opacifies simultaneously with pulmonary arteries
• Common trap：large draining vein of PAVM can be mistaken for pulmonary varix → trace back to feeding artery to differentiate

06Next step / protocol / appropriateness

影像 protocol 選擇：

• Screening for PAVM（HHT patient）：contrast echocardiography (bubble study) → if positive → CT chest with contrast（arterial phase, thin-section ≤ 1 mm）
• Characterization of known PAVM：CTA chest arterial phase → measure feeding artery diameter → if ≥ 3 mm → embolization referral
• Shunt quantification：Tc-99m MAA lung perfusion scan → calculate shunt fraction（normal < 5%; > 5% = significant）
• Pre-embolization planning：CTA or catheter angiography → map all feeding arteries, nidus, draining veins → plan coil / plug placement
• Post-embolization follow-up：CTA at 6-12 months → then every 3-5 years lifelong → check for recanalization, new PAVM growth
• HHT systemic screening：brain MRI（cerebral AVM）、liver imaging（hepatic AVM）、genetic testing（ENG / ACVRL1）

07Pitfalls / normal variants

• Small PAVMs grow over time：sub-threshold（< 3 mm feeding artery）PAVMs 可在 years to decades 內增大 → 需 serial surveillance
• Pregnancy-related PAVM enlargement：hormonal and hemodynamic changes during pregnancy can enlarge PAVMs → pre-pregnancy screening in HHT women recommended
• Pulmonary sequestration can mimic PAVM：both are vascular lesions with systemic arterial supply → but sequestration has systemic artery from aorta, not pulmonary artery
• Artifact from adjacent rib / diaphragm：lower lobe PAVM 可在 CXR 上被 overlying structures 遮蔽 → CT 是 definitive
• Normal prominent lower lobe veins：dependent position 造成 lower lobe veins distended on supine CT → 不是 PAVM → trace to left atrium without nidus