What is neurogenic pulmonary edema?
Neurogenic pulmonary edema is a condition where fluid accumulates in the lungs after an injury to the brain, spinal cord and surrounding structures of the central nervous system. It can lead to serious and life threatening complications, and may even increase the risk of pulmonary infections. Pulmonary edema may occur for various reasons and neurogenic pulmonary edema is a specific type diagnosed when there is no other identifiable cause of fluid accumulation in the lungs.
Effects of fluid filling in the lungs
The lung tissue is highly vascularized and has a large surface area to facilitate gas exchange. Normally a small amount of alveolar fluid is present to help facilitate the transfer of gas between the air in the lungs and the bloodstream. In pulmonary edema, there is an increase in the fluid within the tissue spaces and alveolar fluid. This hampers gas exchange meaning that the blood cannot be oxygenated as easily as it normally would leading to low blood oxygen levels. Due to the large surface area within the lungs, mild pulmonary edema does not significantly hamper the blood oxygen levels. However, in severe cases the drop in blood oxygen levels can lead to respiratory distress and may even be fatal.
How common is neurogenic pulmonary edema?
It is difficult to estimate the incidence of neurogenic pulmonary edema as it is often under diagnosed and there may be several variables that can contribute to the development of this condition. Patients with very severe central nervous system injuries may be hospitalized and therefore at a greater risk of nosocomial pneumonia which presents with pulmonary edema.
The likelihood of pulmonary edema is also greater in people who are on mechanical ventilation. There is also an increased risk of ventilator-associated pneumonia (VAP). It is estimated that neurogenic pulmonary edema may occur in about 30% to 70% of cases of acute central nervous system conditions.
Why does neurogenic pulmonary edema occur?
The exact reason why rapid fluid accumulation occurs in the lungs immediately after an injury to the central nervous system is not clear. It is believed to be largely associated with activation of the sympathetic nervous system following injury to the brain or cervical cord. There may also be specific centers responsible for this condition and stimulated in the course of the injury. The significance of these centers and the ensuing pulmonary hypertension is not clear. Neurogrenic pulmonary edema is usually seen in injuries leading to raised intracranial pressure (pressure within the skull cavity housing the brain).
There are various other possible mechanisms by which neurogenic pulmonary edema may arise.
- High blood pressure within the arteries of the lungs (pulmonary hypertension) or in all arteries (systemic hypertension).
- Changes in the movement of fluid within the blood vessel and tissue spaces.
- Alteration of the permeability of the capillaries in the lung tissue thereby allowing fluid to leak out of the bloodstream in an uncontrolled manner.
While these mechanisms are seen with all types of pulmonary edema, in cases other than NPE it is usually explained by injury and inflammation of the lung structures or the heart. With neurogenic pulmonary edema, the lung, its blood vessels and the heart may not be injured or inflamed yet some pathways stemming from the central nervous system triggers these effects.
Signs and symptoms
Neurogenic pulmonary edema presents in much the same way as other types of pulmonary edema with the associated presence of neurologic symptoms related to central nervous system injury. The condition develops within minutes to hours after the injury. It is therefore important for patients who have not been hospitalized to be aware of the possibility of this condition and seek immediate medical attention should these symptoms arise.
- Difficulty breathing or shortness of breath that starts up suddenly (dyspnea).
- Mild cough with blood streaked mucus at times (hemoptysis).
- Signs of respiratory distress like rapid shallow breathing (tachypnea)
- Rapid heart rate (tachycardia)
- Abnormal breathing sounds like crackles
Causes of Neurogenic Pulmonary Edema
Neurogenic pulmonary edema is more likely to occur with certain central nervous system insults. These are classified as the major causes. It includes :
- Subarachnoid hemorrhage is one of the main causes and NPE may occur in about 70% of these cases. A subarachnoid hemorrhage is a condition where there is bleeding between the linings around the brain, specifically between the pia mater and arachnoid mater known as the subarachnoid space.
- Cerebral hemorrhage is where there is bleeding within the brain tissue itself and also known as intracerebral hemorrhage. It can occur spontaneously, may be related to a burst brain aneurysm, is seen with a hemorrhagic stroke or with head trauma.
- Traumatic brain injury occurs as a result of external mechanical trauma. It can be due to blunt impact, penetrating injuries or sudden jerking of the head as is seen with motor vehicle accidents.
- Epilepsy where there is abnormal electrical activity in the brain leading to episodes known as seizures or commonly as ‘fits’. It is after these epileptic seizures that NPE may arise.
It can also be seen in other neurologic conditions, some of which may be chronic.
- Stroke (cerebrovascular accident) – non-hemorrhagic included.
- Brain tumors
- Gas emoblism
- Multiple sclerosis (MS)
- Meningitis (bacterial)
- Spinal cord (cervical) injury
- Iatrogenic – electroconvulsive therapy and intracranial andovascular therapy
Tests and Diagnosis
Respiratory symptoms that develop within minutes to hours after a central nervous system insult should raise the suspicion of neurogenic pulmonary edema. Lab tests may not conclusively identify neurogenic pulmonary edema but are useful in differentiating it from other causes of pulmonary edema. The most important tests for the diagnosis of NPE are imaging studies. Chest x-rays are the most accessible imaging study that is sufficient to diagnose pulmonary edema. Fluid accumulation in both lungs (bilateral) are evident on chest x-ray with a normal sized heart thereby excluding some of the other causes of pulmonary edema.
Picture of a chest x-ray showing pulmonary edema from Wikimedia Commons
Most cases of neurogenic pulmonary edema resolve on its own within 48 to 72 hours. No specific treatment may be required in milder cases.
Fluid in the lungs decreases oxygenation of the blood leading to low blood oxygen levels (hypoxemia). Therefore oxygen therapy is necessary in most cases. Mechanical ventilation may sometimes be required depending on the patient’s ability to breathe on their own.
- Diuretics may be used to increase water loss via the kidneys (in urine). Urinary catheterization may be necessary.
- Drugs known as alpha-adrenergic antagonists cause vasodilation and may speed up the resolution of neurogenic pulmonary edema. It can also be used to prevent NPE.
- Beta-adrenergic agonists increase cardiac output and minimize the capillary permeability thereby reducing fluid accumulation in the lungs.
Other drugs like narcotics that are used to treat some of the symptoms in other types of pulmonary edema should not be used in neurogenic pulmonary edema (NPE).