What is the normal intima-media thickness (IMT)?

Normal Intima-Media Thickness (IMT)

The normal carotid IMT in adults is <0.9 mm, with values varying by age and ranging from approximately 0.5 mm in young adults (20-30 years) to 0.9 mm in older adults (60-70 years). 1

Age-Specific Normal Values in Adults

The European Society of Cardiology establishes that a carotid IMT value >0.9 mm is considered abnormal and indicates increased cardiovascular risk. 1 However, more recent research accounting for novel cardiovascular risk factors suggests these traditional thresholds may be too high. 2

Updated Reference Values (Accounting for Modern Risk Factor Profiling)

Based on high-resolution ultrasound measurements in subjects without cardiovascular risk factors, the 90% upper limits of normal IMT by age are:

Women:

• Ages 18-29: 0.47 mm 2
• Ages 30-39: 0.59 mm 2
• Ages 40-49: 0.67 mm 2
• Ages 50-59: 0.70 mm 2

Men:

• Ages 18-29: 0.47 mm 2
• Ages 30-39: 0.62 mm 2
• Ages 40-49: 0.72 mm 2
• Ages 50-59: 0.80 mm 2

Traditional Reference Values

In Turkish healthy adults, mean CIMT values by decade were: 0.402 mm (ages 20-29), 0.466 mm (ages 30-39), 0.492 mm (ages 40-49), 0.586 mm (ages 50-59), 0.692 mm (ages 60-69), and 0.733 mm (ages 70-79), with CIMT increasing by approximately 0.066 mm per decade. 3

Pediatric and Adolescent Normal Values

The American Heart Association provides normative data for children and adolescents, with common carotid IMT values ranging from 0.38-0.50 mm depending on age and sex:

• Ages 10-12: 0.48-0.49 mm 1
• Ages 13-14: 0.49-0.50 mm 1
• Ages 15-16: 0.49 mm 1
• Ages 17-18: 0.48-0.50 mm 1

Critical Measurement Considerations

The far wall of the common carotid artery 1 cm proximal to the bulb provides the most reproducible measurements, with the best intra- and inter-observer variability. 4 Measurements from the near wall are less reliable due to sonographic artifacts and should be avoided for standardization. 4

Plaque Definition vs. IMT Thickening

The Mannheim Consensus defines plaque as a focal structure that either:

• Encroaches into the arterial lumen by ≥0.5 mm or 50% of the surrounding IMT value, OR
• Demonstrates thickness ≥1.5 mm measured from the media-adventitia interface to the intima-lumen interface 5

This distinction is crucial because plaque presence dramatically increases cardiovascular risk beyond simple IMT thickening. 1

Important Clinical Caveats

The widely cited upper limit of 1.0 mm for normal IMT must be regarded as obsolete when modern risk factor profiling is applied, as this threshold fails to account for novel cardiovascular risk factors including visceral adiposity, metabolic syndrome, and subclinical inflammation. 2

Age is the single strongest independent predictor of IMT in healthy individuals, with each decade of life adding approximately 0.066 mm to carotid IMT. 3 Other significant correlates include visceral fat level, fasting glucose, and LDL cholesterol, though these do not independently predict IMT in truly healthy subjects without risk factors. 3

Computer-based automated measurements provide superior reproducibility compared to manual eye measurements, with coefficient of variation values much smaller than conventional methods and axial resolution of 0.01 mm versus 0.1 mm for traditional B-mode ultrasonography. 6