输入袢综合征（ALS)

Afferent Loop Syndrome

Introduction

Afferent loop syndrome (ALS) is a purely mechanical complication that infrequently occurs following construction of a gastrojejunostomy. Creation of an anastomosis between the stomach and jejunum leaves a segment of small bowel, most commonly consisting of duodenum and proximal jejunum, lying upstream from the gastrojejunostomy. This limb of intestine conducts bile, pancreatic juices, and other proximal intestinal secretions toward the gastrojejunostomy and is thus termed the afferent loop.

History of the Procedure

The operations most commonly associated with this complication include distal or subtotal gastrectomies for peptic ulcer disease, pancreaticoduodenectomies, and gastrojejunostomies performed to bypass other foregut pathology. The pathophysiology and signs and symptoms associated with ALS result from partial or complete obstruction of the afferent loop.
ALS is included in the constellation of resectional gastric surgical complications known as the postgastrectomy syndromes. The following syndromes are included:

• Early dumping syndrome
• Late dumping syndrome
• Roux stasis syndrome
• Efferent loop syndrome
• Afferent loop syndrome
• Postvagotomy diarrhea
• Chronic gastric atony
• Small gastric remnant syndrome
• Alkaline reflux gastritis
  

Patients with ALS may present with an acute, completely obstructed form or with a chronic, partially obstructed form. The syndrome can manifest at any time from the first postoperative day to many years after surgery. The acute form usually occurs in the early postoperative period (1-2 wk), but it has been described to occur 30-40 years after surgery.

Problem

ALS manifests in acute and chronic forms. Acute ALS represents complete obstruction of the afferent loop and is a true surgical emergency. It must be diagnosed and corrected expeditiously. Chronic ALS is associated with partial obstruction. It is not a surgical emergency but does require corrective surgery.

Frequency

The incidence of this complication decreased dramatically during the final quarter of the 20th century as elective gastric surgery for complications of peptic ulcer disease underwent a logarithmic decline.
Internationally, rates for the development of this complication appear to be similar in other nations.

Mortality/Morbidity:  Mortality is most frequently associated with a delay in diagnosis that leads to bowel infarction or rupture and peritonitis. Patients in whom a timely diagnosis is made or who present with chronic manifestations of the disease can undergo corrective surgery with acceptably low morbidity and mortality rates.

Age: ALS favors no particular age group on a per capita basis.

Sex:  one or more of the postgastrectomy syndromes is more likely to occur in female patients.

Etiology

Postoperative conditions

Each of the following postoperative conditions can cause ALS in a patient with a gastrojejunostomy:

• Scarring due to marginal (stomal) ulceration
• Internal hernia (e.g., through a mesocolic defect)
• Entrapment or compression of the afferent loop by postoperative adhesions
• Recurrence of cancer at or near the anastomotic site
• Volvulus of the intestinal segment
• Enteroenteral or enterogastric intussusception
• Kinking of the afferent limb at the gastrojejunostomy
• Enteroliths in the afferent limb
• Bezoars in the afferent limb or at the anastomosis
• Foreign bodies in the afferent limb or at the anastomosis
  

Surgical technique
Patients have an increased chance of developing ALS if one or more of the following condition is met:

• Mesocolic defects are not properly closed after construction of a retrocolic gastrojejunostomy.
• The jejunal portion of the afferent limb is longer than 30-40cm in length.
• The gastrojejunostomy is placed in an antecolic position instead of a retrocolic position.
  

Pathophysiology

An afferent loop is composed of the duodenal stump. ALS is caused by complete or partial mechanical obstruction at the gastrojejunostomy or at a point along the jejunal portion of the afferent loop.
Passage of food and gastric secretions through the gastrojejunostomy and into the efferent loop triggers release of secretin and cholecystokinin. These enteric hormones stimulate secretion of bile, pancreatic enzymes, and pancreatic bicarbonate and water into the afferent loop.
Symptoms associated with ALS are caused by increased intra-luminal pressure and distention due to accumulation of enteric secretions in a partially or completely obstructed afferent limb. ALS is one of the main causes of duodenal stump blowout in the early postoperative period and is also an etiology for postoperative obstructive jaundice, ascending cholangitis, and pancreatitis due to transmission of high pressures back to the biliopancreatic ductal system.  
Secondarily, prolonged stasis and pooling of secretions with partial obstruction facilitate bacterial overgrowth in the afferent loop. Bacteria deconjugate bile acids, which can lead to steatorrhea, malnutrition, and vitamin B-12 deficiency leading to megaloblastic anemia.

Presentation

History - Chronic ALS

Chronic ALS is caused by partial obstruction of the afferent loop and may be more difficult to diagnose than acute ALS. Approximately 10-20 minutes to an hour postprandially, the patient experiences abdominal fullness and epigastric pain. These symptoms usually last from several minutes to an hour, although they occasionally last as long as several days. Prolonged chronic ALS with stasis and bacterial overgrowth can be further complicated by steatorrhea, diarrhea, and vitamin B-12 deficiency anemia. These effects are primarily due to bacterial de-conjugation of bile salts.

History - Acute ALS

Acute ALS is caused by complete obstruction of the afferent loop. This condition is caused by acute obstruction of the afferent limb due to herniation or volvulus of the afferent loop posterior to the efferent limb. Patients with acute ALS typically present with a sudden onset of epigastric and/or right or left upper quadrant abdominal pain, with associated nausea and vomiting.

Physical findings

Physical examination can reveal one or more of the following findings:

• Jaundice
• Localized mid epigastric or right upper abdominal quadrant tenderness
• An ill-defined mass in the right upper abdominal quadrant may be present in one-third of patients with acute ALS.
• Signs of pancreatitis (e.g., upper abdominal pain radiating to the flank or back
• Peritonitis and/or a rigid abdomen if necrosis or perforation of the bowel wall has occurred
  

Differential diagnoses

• Benign gastric tumors
• Mesenteric tumors
• Bile duct tumors
• Anemia
• Intestinal perforation
• Abdominal abscess
• Mesenteric artery thrombosis
• Abdominal hernias
• Gastric ulcers
• Acute mesenteric ischemia
• Bacterial overgrowth syndrome
• Benign neoplasm of the small intestine
• Bile duct strictures
• Biliary colic
• Biliary obstruction
• Carcinoma of the ampulla of Vater
• Choledochal cysts
• Choledocholithiasis
• Esophagogastroduodenoscopy
• Gastric outlet obstruction
• Gastric sarcoma
• Gastric volvulus
• Gastritis (acute, atrophic, or chronic)
• Mesenteric artery ischemia
• Omental torsion
  

Other problems to be considered

• Mesenteric lymphoid hamartoma
• Bile reflux gastritis
• Cystic metastases
• Pancreatic pseudocyst or cystic tumor
• Mesenteric cyst
• Intra-abdominal abscess
  

Indications

Surgery is indicated in most cases of ALS. The fact remains that ALS is a purely mechanical complication consisting of varying degrees of obstruction of the afferent loop and will not resolve without surgery or other interventional techniques.

Relevant Anatomy

The afferent loop consists of the duodenal stump, the remainder of the duodenum, and the segment of jejunum proximal to the gastrojejunostomy. The clinically relevant portion of the loop pertaining to ALS is the jejunal portion of the afferent limb. The jejunal limb is subject to adhesion formation, internal herniation, volvulus, anastomotic obstruction, and other etiologies of ALS, as described above.

Contraindications

Surgical correction of ALS has no absolute contraindications. Relative contraindications include severe debilitation or extensive intra-abdominal malignancies. Patients with these conditions can be effectively treated with nonsurgical drainage procedures as described in Treatment.