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Scimitar Syndrome (Pulmonary Venolobar Syndrome)

Source: Fishman's Pulmonary Diseases and Disorders, Chapter 105

1. Definition and Nomenclature

Scimitar syndrome, also termed pulmonary venolobar syndrome, is a rare congenital disorder of the lung and pulmonary vasculature characterized by partial or total anomalous pulmonary venous return of the right lung (most commonly the right lower lobe) to the inferior vena cava (IVC).

The name derives from the appearance of the anomalous pulmonary vein on chest radiograph: a curvilinear shadow along the right heart border that resembles the curve of a Persian scimitar sword.

2. Epidemiology

Prevalence: 1 to 3 cases per 100,000 live births
Female predominance (male:female ratio of 1:2)
Represents only 5% of all partial anomalous pulmonary venous return (PAPVR) abnormalities

3. Embryology and Pathogenesis

During early embryogenesis, pulmonary venous blood drains into the splanchnic venous circulation. As cardiac structures develop, drainage transitions to the left atrium, with regression of the systemic connection. Initially both lungs drain via a common pulmonary vein, which later divides into four separate ostia in the left atrium.

A defect during this transition from splanchnic to cardiac drainage leads to total or partial anomalous pulmonary venous return of the right lung - the fundamental defect in scimitar syndrome.

4. Associated Anomalies

Scimitar syndrome is frequently associated with:

Right pulmonary hypoplasia (small right lung)
Systemic arterial blood supply of the right lung (aortopulmonary collaterals)
Congenital heart disease (most commonly a secundum ASD)
Dextrocardia or mesocardia (cardiac shift to the right due to lung hypoplasia)
Horseshoe lung (rare variant)

5. Clinical Presentation

Infantile Form (Severe):

When pulmonary-to-systemic shunting is severe, patients present in infancy or early childhood with:

Tachypnea
Pulmonary hypertension
Evidence of right heart failure

Adult Form (Mild):

With lesser degrees of shunting, patients may be asymptomatic or present in early adulthood with:

Recurrent pneumonia (31%)
Dyspnea on exertion (19%)
Hemoptysis (6%)

6. Diagnostic Investigations

Chest Radiograph

The classic finding is a curvilinear tubular opacity in the right lower hemithorax coursing from the right hilum toward the IVC, with:

Right lung hypoplasia
Ipsilateral mediastinal shift to the right

Figure 105-19 from Fishman's - Chest radiograph showing the curvilinear scimitar vein (arrow) coursing toward the IVC

Fig. 105-19 (Fishman's): Chest radiograph reveals a curvilinear tubular opacity in the right lower hemithorax (arrow) that courses from the right hilum toward the IVC.

Two-Dimensional Transthoracic Echocardiography (TTE)

Used to assess right ventricular function and estimate pulmonary artery pressure
In the suprasternal short-axis view with color Doppler: the left pulmonary vein can be seen entering the left atrium, but the right pulmonary vein cannot be identified entering the left atrium
Parasternal short-axis view may show a small right main pulmonary artery compared to the normal-sized left PA
Findings can be confirmed on transesophageal echocardiogram (TEE)

CT Chest (Contrast-Enhanced)

The scimitar vein is clearly delineated on CECT as the anomalous vessel draining into the IVC:

Figure 105-20 (Fishman's) - CECT axial image showing the scimitar vein (arrow) draining into the IVC

Fig. 105-20 (Fishman's): Contrast-enhanced CT chest axial image revealing the anomalous scimitar vein (arrow) draining into the IVC, with right lung hypoplasia.

Lung Scintigraphy

Technetium-99m labeled macroaggregated albumin (MAA) scintigraphy can quantify the shunt fraction - an important factor in management decisions

Cardiac Catheterization / MR Angiography

Used for definitive delineation of the anomalous vein, assessment of pulmonary artery pressures, and identification of systemic arterial collaterals

7. Pathophysiology of Hemodynamic Compromise

The anomalous pulmonary vein draining into the IVC creates a left-to-right shunt. The degree of shunting determines clinical severity:

Large shunt → right ventricular volume overload → pulmonary hypertension → right heart failure
Small shunt → may remain asymptomatic for decades
The left-to-right shunt fraction tends to decrease with age in most patients

8. Management

Management depends on the severity of pulmonary-to-systemic shunting and its hemodynamic impact.

Conservative Management:

For small shunt fractions with minimal symptoms: conservative management with follow-up echocardiography is recommended
Life expectancy is normal with small shunts

Surgical Intervention:

Indicated primarily for:

Large shunt with right ventricular volume overload
Pulmonary hypertension

Two surgical techniques:

Traditional approach: Direct anastomosis of the anomalous pulmonary vein to the left atrium - associated with frequent thrombosis or stenosis at the anastomotic site
Novel technique: Anastomosis of the anomalous pulmonary vein to a Gore-Tex vascular graft placed through the right atrial wall, sutured into a surgically created atrial septal defect to direct blood flow into the left atrium (avoids the high anastomotic failure rate of the direct approach)

9. Prognosis

Patients with small shunts and no pulmonary hypertension have normal life expectancy
Surgical outcomes in infantile-type disease (with pulmonary hypertension) are significantly worse
The left-to-right shunt fraction decreasing with age is a favorable prognostic feature

Summary Table for Quick Revision

Feature	Detail
Other name	Pulmonary venolobar syndrome
Core defect	Partial/total APVR of right lung to IVC
Prevalence	1-3 per 100,000; F > M (1:2)
% of all PAPVR	5%
Embryological basis	Failed transition from splanchnic to cardiac venous drainage
CXR finding	Curvilinear opacity right heart border (scimitar sign) + right lung hypoplasia
Common associations	Right pulmonary hypoplasia, ASD, systemic arterial supply to right lung
Infantile presentation	Tachypnea, pulmonary HTN, right heart failure
Adult presentation	Recurrent pneumonia, dyspnea, hemoptysis (or asymptomatic)
Best imaging	CECT chest / MR angiography
Shunt quantification	Tc-99m MAA scintigraphy
Surgical technique	Gore-Tex graft via right atrium to surgically created ASD (preferred over direct anastomosis)

All content sourced from: Fishman's Pulmonary Diseases and Disorders (2-Volume Set), Chapter 105 - Vascular Disorders / Scimitar Syndrome (Pulmonary Venolobar Syndrome), pp. 1867-1869

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