Megacolon

Japanese: 巨大結腸症

Concept Megacolon refers to a condition in which the colon is pathologically dilated in the absence of mechanical obstruction. It is broadly classified as acute or chronic.
Classification
1) Acute megacolon:
Acute colonic pseudo-obstruction (Ogilvie syndrome) is clinically important because it requires prompt and appropriate treatment. A similar condition is toxic megacolon, but care must be taken because the treatment is different.
2) Chronic megacolon:
There are two types of disease: adult Hirschsprung's disease and chronic intestinal pseudo-obstruction.
EtiologyAcute colonic pseudo-obstruction can occur for a variety of reasons, but is often secondary to abdominal surgery (De Giorgio et al., 2009). Toxic megacolon is accompanied by severe systemic symptoms in addition to intestinal dilatation associated with acute exacerbation of inflammatory bowel disease or infectious enteritis. Chronic megacolon, Hirschsprung's disease, is caused by loss of ganglion cells in the digestive tract, but in adult cases, the area of ganglion cell loss is often limited to the lower rectum. Chronic pseudo-obstruction causes peristalsis disorder in the small and large intestines, resulting in intestinal obstruction. Causes of digestive tract peristalsis disorder include primary causes and secondary causes associated with scleroderma, amyloidosis, drug-induced, mitochondrial encephalomyopathy, hypothyroidism, etc. (For details, see [⇨8-5-16)-(2)]).
Diagnostic CT scans or abdominal X-rays reveal pathological intestinal dilation, but there is no site of mechanical obstruction. The diagnostic criterion for toxic megacolon is intestinal dilation of 6 cm or more. The absence of mechanical obstruction is confirmed by careful interpretation of CT scans, but in cases of chronic megacolon, gastrointestinal contrast studies using a water-soluble iodine contrast agent (Gastrografin) and lower gastrointestinal endoscopy are also useful. For details on the diagnosis of Hirschsprung's disease, see [⇨8-5-1)].
Differential diagnosis Differentiation from intestinal obstruction due to mechanical obstruction is the most important factor.
Treatment of acute pseudo-megacolon: Correction of dehydration, administration of neostigmine, and endoscopic decompression are effective, but if intestinal perforation occurs, the mortality rate exceeds 50% (De Giorgio et al., 2004). In toxic megacolon, intensive systemic management is performed using intravenous fluids, steroids, and antibiotics, but if the condition worsens, surgery such as total colectomy is performed. Among chronic pseudo-obstruction, adult Hirschsprung's disease is an indication for surgical treatment. For other chronic pseudo-obstruction, bowel movements are controlled by administering intestinal motility stimulants and laxatives, and attempts are made to suppress the abnormal proliferation of intestinal bacteria by administering lactobacillus preparations and antibiotics. However, the disease is often intractable. When enteral nutritional therapy is difficult, total parenteral nutrition is performed. Surgery is often ineffective (Sakamoto et al., 2010).
Contraindications: In patients with toxic megacolon, colonoscopy and barium enema examinations, which increase intestinal pressure, are contraindicated and require caution. [Togashi Kazutomo and Endo Shungo]
■ References
De Giorgio R, Sarnelli G, et al: Advances in our understanding of pathology of chronic intestinal pseudo-obstruction. Gut, 53: 1549-1552, 2004.
Yasunari Sakamoto, Masahiko Inamori, et al.: Pseudo-intestinal obstruction. "How to diagnose small intestinal disease: diagnosis and treatment", pp176-180, Shindan to Chiryousha, Tokyo, 2010.

Source : Internal Medicine, 10th Edition About Internal Medicine, 10th Edition Information

Japanese:

概念
　巨大結腸症とは，機械的閉塞がないのに結腸が病的に拡張している状態を指す．急性と慢性に大きく分類される．
分類
1）急性巨大結腸症：
急性大腸偽性腸閉塞症（Ogilvie症候群）は，早急に適切な処置を行う必要があるので，臨床上，重要である．類似した病態として，中毒性巨大結腸症（toxic megacolon）があるが，対処法が異なるので注意を要する．
2）慢性巨大結腸症：
成人型Hirschsprung病と慢性偽性腸閉塞症がある．
病因
　急性大腸偽性腸閉塞症は，さまざまな原因により生じるが，腹部外科手術後に続発することが多い．（De Giorgioら，2009）．中毒性巨大結腸症では，炎症性腸疾患や感染性腸炎の急性増悪に伴う腸管拡張に加えて，重篤な全身症状を伴う．慢性巨大結腸症であるHirschsprung病は消化管の神経節細胞が消失していることによるが，成人型では神経節細胞の消失領域が下部直腸に限局していることが多い．　慢性偽性腸閉塞は，消化管のうち小腸・大腸の蠕動障害が生じ腸閉塞症状を呈する．消化管蠕動障害の原因としては，原発性のもの，強皮症・アミロイドーシス・薬剤性・ミトコンドリア脳筋症・甲状腺機能低下症などに伴う続発性のものがある（詳細は【⇨8-5-16）-（2）】）．
診断
　CT検査または腹部X検査により腸管が病的に拡張しているが，機械的閉塞部位がないことによる．中毒性巨大結腸症では，6 cm以上の腸管拡張が診断基準となっている．機械的閉塞がないことの確認はCT検査の丹念な読影により行われるが，慢性巨大結腸症では水溶性ヨウ素造影剤（ガストログラフィン）を用いた消化管造影検査や下部消化管内視鏡検査も有用である．Hirschsprung病の診断の詳細は【⇨8-5-1）】．
鑑別診断
　機械的閉塞による腸閉塞症との鑑別が最も重要である．
治療
　急性偽性巨大結腸症では，脱水の補正・ネオスチグミンの投与・内視鏡的減圧が有効とされるが，腸管穿孔に至れば致死率は50％をこえる（De Giorgioら，2004）．中毒性巨大結腸症では，輸液・ステロイドの投与・抗菌薬の投与などにより強力な全身管理を行うが，悪化傾向がみられる場合には全結腸切除などの外科手術を行う．慢性偽性腸閉塞症のうち，成人型Hirschsprung病は外科的治療の適応である．それ以外の慢性偽性腸閉塞症に対しては腸管蠕動促進薬・緩下薬の投与により便通のコントロールを行い，乳酸菌製剤・抗菌薬の投与により腸内細菌の異常増殖を抑えるように試みる．しかし，難治性であることが多い．経腸的栄養療法が困難である場合には，中心静脈栄養を行う．外科的手術は無効である場合が多い（坂本ら， 2010）．
禁忌
　中毒性巨大結腸症では，腸管内圧が上昇する大腸内視鏡検査および注腸造影検査は禁忌であり，注意を要する．［冨樫一智・遠藤俊吾］
■文献
De Giorgio R, Sarnelli G, et al: Advances in our understanding of pathology of chronic intestinal pseudo-obstruction. Gut, 53: 1549-1552, 2004.
坂本康成，稲森正彦，他：偽性腸閉塞．「どう診る？小腸疾患診断と治療まで」pp176-180，診断と治療社，東京，2010.

出典　内科学第10版内科学第10版について　情報

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