In a 5‑year‑old boy, what condition is suggested by a lower oxygen saturation in the left arm compared with the left leg?

Coarctation of the Aorta Proximal to the Left Subclavian Artery

Lower oxygen saturation in the left arm compared to the left leg in a 5-year-old child is highly suggestive of coarctation of the aorta located proximal to the origin of the left subclavian artery. 1

Pathophysiologic Mechanism

This reversed differential saturation pattern occurs when:

• The coarctation is positioned proximal to (before) the left subclavian artery takeoff, creating obstruction between the left subclavian origin and the descending aorta 1
• Blood flow to the left arm is compromised by the proximal obstruction, while the left leg receives better-oxygenated blood through collateral circulation or from flow patterns that bypass the obstruction 1, 2
• This is the opposite of the classic presentation where coarctation is juxtaductal (just distal to the left subclavian), which would show lower leg saturations 1

Typical vs. Atypical Coarctation Location

Standard juxtaductal coarctation (most common - 85-90% of cases):

• Located just distal to the left subclavian artery 1
• Presents with higher arm saturations and lower leg saturations 1, 3
• Blood pressure gradient shows higher readings in arms than legs (>20 mmHg difference) 1, 2

Proximal coarctation (less common):

• Located proximal to the left subclavian artery origin 1
• Creates the reversed pattern seen in this case: lower left arm saturation compared to left leg 1
• The left subclavian artery arises distal to the obstruction, receiving compromised flow 1

Diagnostic Confirmation Steps

Immediate clinical assessment should include:

• Four-limb blood pressure measurements: Check right arm, left arm, and both legs to identify the gradient pattern 1, 2
• Pulse examination: Palpate brachial and femoral pulses simultaneously; expect decreased left brachial pulse with relatively preserved femoral pulses in this atypical variant 1, 2
• Four-limb pulse oximetry: Measure saturations in right arm, left arm, and both legs to map the differential pattern 1, 3

Definitive imaging:

• Transthoracic echocardiography with careful attention to the aortic arch anatomy, specifically visualizing the relationship between the coarctation site and left subclavian artery origin 1
• MRI/MRA or CT angiography provides comprehensive visualization of the thoracic aorta, arch vessels, and collateral circulation, particularly valuable for surgical planning in atypical locations 1

Associated Findings to Evaluate

Screen for common associated anomalies:

• Bicuspid aortic valve (present in 30-40% of coarctation cases) 1
• Intracranial berry aneurysms (present in 10% of cases) 1
• Turner syndrome association (increased prevalence) 1
• Ventricular or atrial septal defects 1

Clinical Pitfall

The key diagnostic error to avoid is assuming all coarctations follow the typical juxtaductal pattern. When you find lower saturation in the LEFT arm compared to the leg, you must think proximal coarctation rather than dismissing the finding as artifact or measurement error 1. This atypical location requires careful surgical planning as the repair approach differs from standard juxtaductal coarctation 1.