Mendelson's syndrome

Overview

Mendelson’s syndrome is a form of chemical pneumonitis that occurs as a result of the aspiration of gastric contents into the lungs. It was first described in 1946 by Dr. Curtis Mendelson, an American obstetrician, in the context of pregnant women receiving general anesthesia during labor. Since then, the condition has been recognized as a serious perioperative complication and is now understood to occur in a variety of settings where aspiration risk is increased.

Mendelson's syndrome is considered a medical emergency. It can lead to acute lung injury, hypoxemia, and respiratory failure. The syndrome is most often associated with aspiration under anesthesia or sedation, especially in patients who have not fasted adequately or who have impaired protective airway reflexes.

Causes

Mendelson’s syndrome is caused by the aspiration (inhalation) of acidic gastric contents into the lower respiratory tract. This results in chemical injury to the lungs, inflammation, and impaired gas exchange. The primary contributing factors include:

• General anesthesia: Suppresses the gag and cough reflexes, increasing the risk of aspiration, particularly if the patient has not fasted prior to surgery.

• Pregnancy: Increased intra-abdominal pressure and delayed gastric emptying in pregnant women make aspiration more likely during labor and delivery under anesthesia.

• Emergency surgery: Patients may not have an empty stomach, heightening the aspiration risk.

• Impaired consciousness: Conditions such as head trauma, seizures, stroke, or drug overdose can compromise airway protection.

• Gastrointestinal disorders: Such as gastroesophageal reflux disease (GERD), gastroparesis, or hiatal hernia may increase aspiration risk.

Unlike aspiration pneumonia, which is typically caused by inhalation of bacteria-laden material, Mendelson’s syndrome is due to a sterile chemical burn resulting from highly acidic stomach contents (usually with pH <2.5).

Symptoms

Symptoms of Mendelson’s syndrome typically appear within minutes to hours following the aspiration event. They can vary in severity depending on the volume and acidity of the aspirated contents. Common symptoms include:

• Sudden onset of respiratory distress

• Hypoxemia: Low oxygen saturation levels

• Coughing and wheezing

• Tachypnea: Rapid breathing

• Fever: May occur within several hours

• Cyanosis: Bluish discoloration of the lips or extremities due to low oxygen levels

• Rales or crackles on auscultation

• Reduced breath sounds in affected lung areas

In severe cases, the syndrome may progress to acute respiratory distress syndrome (ARDS), requiring mechanical ventilation and intensive care.

Diagnosis

Diagnosis of Mendelson’s syndrome is primarily clinical and based on a known or suspected aspiration event, rapid onset of respiratory symptoms, and exclusion of other causes. Diagnostic steps may include:

1. Clinical History

• Recent surgery, anesthesia, or sedation

• Vomiting or regurgitation, especially in a high-risk setting

• Impaired level of consciousness or airway protection

2. Physical Examination

• Signs of respiratory distress such as rapid breathing, use of accessory muscles, and cyanosis

• Crackles or rales heard on lung auscultation

3. Imaging

• Chest X-ray: May reveal infiltrates, usually in the right lower lobe due to anatomical predisposition

• CT scan of the chest: Provides more detailed imaging if needed

4. Laboratory Tests

• Arterial blood gas (ABG): Shows hypoxemia and possibly respiratory alkalosis or acidosis

• Pulse oximetry: Continuous monitoring of oxygen saturation

Microbiological testing is usually negative unless secondary infection has occurred. The key differentiating factor from aspiration pneumonia is the absence of an infectious etiology in the early phase.

Treatment

Treatment of Mendelson’s syndrome is supportive and focused on maintaining oxygenation, minimizing lung injury, and preventing complications. Management strategies include:

1. Airway Management

• Supplemental oxygen: Delivered via nasal cannula, face mask, or high-flow devices

• Mechanical ventilation: Required in cases of respiratory failure or severe hypoxemia

2. Pulmonary Support

• Positive end-expiratory pressure (PEEP): May be used to improve oxygenation and prevent alveolar collapse

3. Positioning

• Head elevation: Reduces risk of further aspiration and helps with breathing

• Lateral decubitus positioning: May be used to drain secretions from affected lung areas

4. Medications

• Corticosteroids: Controversial; sometimes used to reduce inflammation but not routinely recommended

• Antibiotics: Not indicated unless secondary bacterial infection develops

• Bronchodilators: Used if there is associated bronchospasm

5. Prevention in High-Risk Settings

• Preoperative fasting (NPO guidelines)

• Use of rapid sequence induction and intubation in emergency anesthesia

• Proton pump inhibitors or antacids preoperatively to reduce gastric acidity

Prognosis

The prognosis of Mendelson’s syndrome depends on the volume and acidity of aspirated contents, the promptness of treatment, and the patient’s underlying health status. Key points include:

• Mild cases: Often resolve within a few days with supportive care

• Moderate to severe cases: May result in prolonged hospitalization or intensive care

• Complications: Include respiratory failure, secondary bacterial pneumonia, and progression to ARDS

With prompt recognition and appropriate management, most patients recover fully. Preventive strategies during anesthesia and in high-risk medical settings remain critical to reducing the incidence of this potentially serious condition.