What is the difference between Eisenmenger syndrome and ventricular septal defect?
They are both related to a hole between the two ventricles, right?
The right side of the heart receives blood from the head and body (via the vena cava) and pumps it to the lungs to be oxygenated.
The left side of the heart receives oxygenated blood from the lungs and returns it to the body (via the aorta). The left side is at a higher pressure than the right.
Heart anatomy and ventricular septal defect
Note that the right side of the heart is on the left side of the image and vice versa.
A VSD results in oxygenated blood being pushed (or shunted) from the higher pressure left ventricle through the defect to the right ventricle, where it will travel to the lungs again (unnecessarily).
Eisenmenger’s syndrome is a complication that can arise from many untreated heart defects (including VSD).
Pathophysiology of Eisenmenger’s syndrome
The left-to-right shunt causes increased blood flow to the lungs, which damages the endothelium (inner lining) of the blood vessels. This results in gradually increasing vascular resistance in the lungs.
Eventually the resistance gets to a point where the pressure in the right ventricle rises above that of the left ventricle across the VSD. As a result, the shunting reverses, so that blood is now moving from right to left.
This is a problem because blood is now bypassing the lungs, resulting in reduced oxygen availability to the body (hypoxia).
This can result in cyanosis (blue discolouration due to hypoxia), heart failure, breathlessness, chest pain, fatigue, haemoptysis (coughing blood), collapse and cardiac arrest.
In summary, a VSD is a congenital defect of the heart, and Eisenmenger’s syndrome is a potential long-term complication that can arise if it is left untreated.
You can read more about the two conditions by following the links.
Images courtesy of the American Medical Association and Mayo Clinic.
Eisenmenger syndrome is a clinical syndrome. A ventricular septal defect (VSD) is an anatomic lesion. They are related in that Eisenmenger syndrome can be caused by a VSD (among other things).
A ventricular septal defect is a (typically) congenital opening between the right and left ventricle, caused by a failure of the ventricular septum, or wall between the two ventricles, to fully develop
Eisenmenger syndrome is a clinical syndrome and disease process in which a (congenital) left to right shunt (from the systemic circulation to the pulmonary circulation) causes the development of pulmonary vascular disease, pulmonary hypertension, and eventually a right to left shunt, leading to cyanosis (a bluish discoloration of the skin caused by inadequate oxygenation of blood). The eventual right to left shunt means that blood returning from the systemic veins goes directly through to the systemic arteries without passing through the pulmonary circulation and the lungs. This prevents gas exchange. Ventricular septal defects are the most common cause of Eisenmenger syndrome, but other initial left to right shunts can cause the same syndrome. Atrial septal defects and a patent ductus arteriosus are also relatively common causes of Eisenmenger syndrome.
I've added links to competent Wikipedia articles about some of the terms here, and you can read about Eisenmenger Syndrome, VSDs, and other congenital cardiac shunts in Lilly's Pathophysiology of Heart Disease, Chapter 16, on Congenital Heart Disease.
Apart from information on differences given in two excellent answers here, there is a major difference in treatment of these two conditions.
Ventricular Septal Defect (VSD) before development of Eisenmenger syndrome can be treated by surgery. Usually the defect is closed using a patch. Closure can sometimes be done without surgery using devices inserted through peripheral arteries or veins.
If the VSD is very small, it may not need any treatment at all apart from preventive measures to be taken at time of other medical and dental procedures. These are required since there may be a risk of infection at the site of VSD from bacteria that may enter the bloodstream during these procedures.
However, once Eisenmenger syndrome (irreversible pulmonary hypertension with reversal of shunt) develops, surgery (or device closure) is no more an option and condition is generally managed by medicines only.
Pregnancy also carries an increased risk in patients with Eisenmenger syndrome while risk is generally not increased in patients who have had VSD successfully closed earlier.
See this American Heart Association (AHA) page: https://www.heart.org/en/health-topics/congenital-heart-defects/about-congenital-heart-defects/ventricular-septal-defect-vsd for more information (search for 'pulmonary hypertension', since the term 'Eisenmenger syndrome' is not used there).
Treatment/management options are best discussed with treating doctors since many factors have to be taken into account before deciding best course of action.