Lower esophageal sphincter

The lower esophageal sphincter (LES) is a specialized region of smooth muscle located at the junction of the esophagus and stomach. It plays a critical role in preventing gastroesophageal reflux while allowing the passage of food into the stomach. Understanding its anatomy and physiology is essential for the diagnosis and management of esophageal disorders.

Definition and Overview

Definition of Lower Esophageal Sphincter

The lower esophageal sphincter is a functional zone of high pressure at the distal esophagus that acts as a barrier between the stomach and the esophagus. It is not a distinct anatomical ring but a region of tonic contraction that regulates the flow of gastric contents.

General Function

The primary function of the LES is to prevent the retrograde movement of gastric contents into the esophagus. It achieves this by maintaining a baseline tone that increases during gastric distension and relaxes transiently during swallowing to allow the passage of food and liquids into the stomach.

Clinical Significance

Proper LES function is essential for maintaining esophageal health. Dysfunction can result in gastroesophageal reflux disease, esophagitis, Barrett’s esophagus, and other complications. Evaluation of LES function is an important component of managing patients with chronic heartburn, regurgitation, and dysphagia.

Anatomy

Location and Structure

The lower esophageal sphincter is located at the distal end of the esophagus, approximately 3 centimeters above the gastroesophageal junction. It consists of a specialized segment of smooth muscle that forms a high-pressure zone. The sphincter is not a discrete circular muscle but a functional region that works in conjunction with surrounding structures.

Muscle Layers

• Circular smooth muscle fibers: Responsible for the tonic contraction that maintains sphincter tone.
• Oblique and longitudinal fibers: Contribute to esophageal shortening and opening during swallowing.

Relation to Adjacent Structures

• Diaphragmatic crura: Provide additional extrinsic support and contribute to the sphincter’s pressure during respiration.
• Esophageal hiatus: The opening in the diaphragm through which the esophagus passes, influencing LES function.
• Gastric cardia: The proximal portion of the stomach that interacts with the LES to facilitate the passage of ingested material and prevent reflux.

Physiology

Resting Tone and Basal Pressure

The LES maintains a resting pressure higher than the intragastric pressure, creating a barrier against reflux. Basal pressure is maintained by intrinsic smooth muscle tone, augmented by neural and hormonal input.

Relaxation Mechanisms

• Swallow-induced relaxation: During swallowing, the LES transiently relaxes to allow the passage of food bolus into the stomach.
• Vagal control and neural pathways: The vagus nerve mediates inhibitory signals that coordinate sphincter relaxation and esophageal peristalsis.
• Hormonal influences: Gastrointestinal hormones such as gastrin and motilin modulate LES tone and contribute to digestive regulation.

Role in Gastroesophageal Barrier Function

• Prevents reflux of acidic gastric contents into the esophagus, protecting the mucosa from injury.
• Coordinates with esophageal peristalsis to ensure efficient bolus transit and clearance.

Development and Embryology

Embryologic Origin

The lower esophageal sphincter develops from the distal portion of the embryonic foregut during the fourth to seventh weeks of gestation. The muscular layer differentiates into specialized smooth muscle fibers that form the functional sphincter region.

Maturation of LES Function

During early infancy, the LES exhibits lower resting tone, which can contribute to gastroesophageal reflux in neonates. Maturation of neural control and smooth muscle contractility over the first year of life enhances sphincter competence and reduces reflux episodes.

Congenital Variations

Some individuals may have anatomical or functional variations in the LES, such as hypotonic sphincter tone or hiatal abnormalities, predisposing them to reflux or esophageal dysmotility.

Disorders and Pathophysiology

Gastroesophageal Reflux Disease (GERD)

Dysfunction of the LES, including hypotonia or transient relaxations, is a primary contributor to GERD. Inadequate sphincter pressure allows gastric contents to reflux into the esophagus, causing symptoms such as heartburn, regurgitation, and esophagitis.

Hiatal Hernia

Protrusion of the gastric cardia through the esophageal hiatus can impair LES function, exacerbate reflux, and contribute to chronic esophageal irritation.

Achalasia

A condition characterized by impaired relaxation of the LES due to neural dysfunction, leading to difficulty swallowing, regurgitation, and esophageal dilation.

LES Hypotension or Hypertension

• Hypotension: Decreased sphincter tone leading to increased reflux risk
• Hypertension: Excessive tone causing dysphagia and impaired bolus transit

Diagnostic Evaluation

Esophageal Manometry

Esophageal manometry measures the pressure and coordination of the LES and esophageal body. It is the gold standard for assessing sphincter competence, identifying hypotension or failure of relaxation, and diagnosing motility disorders such as achalasia.

pH Monitoring and Impedance Testing

Ambulatory 24-hour pH monitoring evaluates acid exposure in the esophagus, helping to confirm gastroesophageal reflux disease. Combined impedance testing detects non-acidic reflux and provides comprehensive assessment of reflux episodes.

Endoscopic Assessment

Upper endoscopy allows direct visualization of the esophageal mucosa, detection of esophagitis, Barrett’s esophagus, strictures, and hiatal hernias. It also aids in tissue biopsy for histopathologic evaluation.

Imaging Studies

Barium swallow radiography can demonstrate anatomical abnormalities, such as hiatal hernia, esophageal dilation, or impaired LES function during swallowing. CT and MRI may be used in complex cases to assess surrounding structures.

Electrophysiological and Functional Assessment

Emerging techniques, including high-resolution manometry and esophageal motility mapping, provide detailed functional evaluation of LES dynamics and coordination with esophageal peristalsis.

Management

Conservative Approaches

• Lifestyle modifications: Elevating the head of the bed, dietary changes, weight management, and avoiding late meals.
• Medications: Proton pump inhibitors, H2 receptor antagonists, and prokinetic agents to reduce acid exposure and improve LES function.

Surgical and Endoscopic Interventions

• Nissen fundoplication: Surgical reinforcement of the LES to prevent reflux.
• Endoscopic LES augmentation techniques: Minimally invasive procedures that improve sphincter competence and reduce reflux symptoms.

Clinical Implications

Complications of LES Dysfunction

• Esophagitis: Inflammation of the esophageal mucosa due to chronic acid exposure.
• Barrett’s esophagus: Metaplastic changes in the distal esophagus that increase the risk of adenocarcinoma.
• Stricture formation: Narrowing of the esophagus from chronic injury and scarring, leading to dysphagia.
• Increased risk of esophageal adenocarcinoma: Long-standing reflux and metaplastic changes may predispose to malignancy.

Preventive Strategies and Follow-up

Regular monitoring of patients with chronic reflux, lifestyle modifications, and adherence to medical therapy are essential to prevent complications. Endoscopic surveillance may be indicated in patients with Barrett’s esophagus or long-standing GERD to detect dysplasia or early cancer.

References

1. Standring S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 42nd ed. London: Elsevier; 2020.
2. Kahrilas PJ, Pandolfino JE, Ghosh SK. The lower esophageal sphincter. N Engl J Med. 2008;359(15):1503-1515.
3. Fass R, Sifrim D. Management of gastroesophageal reflux disease. Gastroenterology. 2009;137(6):2167-2179.
4. Vaezi MF, Sifrim D. Diagnosis and management of esophageal motility disorders. Am J Gastroenterol. 2007;102(12):2735-2750.
5. Spechler SJ, Souza RF. Barrett’s esophagus. N Engl J Med. 2014;371(9):836-845.
6. Mittal RK, Balaban DH. The esophagogastric junction. N Engl J Med. 1997;336(13):924-932.
7. McColl KE. Physiologic and pathophysiologic functions of the lower esophageal sphincter. Dig Dis. 2010;28(2):188-193.
8. Camilleri M, Katz PO, Malagelada JR. Gastroesophageal reflux: pathophysiology and management. Ann Intern Med. 1992;117(1):38-53.
9. Sharma P, Katzka DA. Barrett’s esophagus: pathogenesis, diagnosis, and management. Gastroenterology. 2017;152(1):34-46.
10. Katz PO, Gerson LB, Vela MF. Guidelines for the diagnosis and management of gastroesophageal reflux disease. Am J Gastroenterol. 2013;108(3):308-328.