岡庭 信司

Shinji OKANIWA

飯田市立病院消化器内科

Department of Gastroenterology, Iida Municipal Hospital

キーワード : Autoimmune pancreatitis, IgG4-related disease, Other organ involvement (OOI), Differential diagnosis, Ultrasound

自己免疫性膵炎（autoimmune pancreatitis: AIP）の診断で最も大切なことは，AIPの可能性を疑うことである．急性期では，びまん性膵腫大は限局性膵腫大に比べ特異度の高いAIPの所見である．感度は低いが，高周波プローブでは被膜様構造（capsule-like rim sign）や膵管穿通徴候（penetrating duct sign）を検出することができる．これらの所見はAIPに特徴的であり，膵癌との鑑別診断にも有用である．限局性AIPでは，造影USにてiso/hypervascularityを示すことや，限局性腫大部のみならず周囲膵実質の硬度上昇を示すエラストグラフィ所見も鑑別診断に有用である．さらに，2週間のステロイド投与後の膵実質腫大の消失または明らかな縮小と，エラストグラフィの平均せん断波速度の低下も，AIPの特徴として重要である．AIPはIgG4関連疾患における膵症状であるため，胆道や唾液腺など他の臓器の評価は，特に限局性AIPの診断に有用である．胆管に特徴的なUS所見は，内部の第2層のエコー層が著しく肥厚した3層構造（高‐低‐高パターン）を呈する壁肥厚である．さらに，USは，胆管造影では異常を認めない胆管の壁肥厚を検出することも可能である．これらの所見は，胆管癌との鑑別診断に有用である．顎下腺の多発性低エコー域は，1型AIPの唾液腺炎に特徴的なUS所見であり，身体所見より感度が高い．高周波プローブの使用に加え，エラストグラフィや造影USを用いることにより，USはAIPの診断にさらに貢献することができる．

The most important thing in the diagnosis of autoimmune pancreatitis (AIP) is to suspect the possibility of AIP. In the acute phase, diffuse pancreatic enlargement is a highly specific finding of AIP compared to focal enlargement. Though the sensitivity is low, high-frequency transducers can detect the capsule-like rim sign and penetrating duct sign. Those findings are characteristic of AIP and useful for differential diagnosis with pancreatic carcinoma. In focal AIP, both contrast-enhanced US showing iso/hypervascularity and elastography showing increased stiffness not only in the focal enlargement but also in the surrounding parenchyma are also useful for differential diagnosis. Furthermore, changes over time after the two-week steroid trial, such as resolution or measurable reduction in parenchymal enlargement and a decrease in the mean shear-wave velocity on elastography, are also cardinal features of AIP. Since AIP is a pancreatic manifestation in immunoglobulin G4-related disease, evaluation of other organs, including the biliary tract and salivary glands, is particularly useful in focal AIP. A characteristic US finding of bile ducts is three-layered (high・low・high pattern) wall thickening with a markedly thickened middle layer. US can also detect wall thickening of bile ducts, which show no abnormalities on cholangiography. These findings are useful for differential diagnosis with cholangiocarcinoma. Multiple hypoechoic areas in submandibular glands are characteristic US findings of sialadenitis in type 1 AIP, and the sensitivity is higher than that of physical examination. US can further contribute to the diagnosis of AIP by employing elastography and contrast-enhanced US in addition to high-frequency transducers.