The Haller index (HI), also known as the pectus index, is a simple mathematical way to assess and describe the chest cage on CT of the thorax and is used in the detection and pre/postoperative assessment of pectus excavatum 1,5.
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Measurement
The Haller index is calculated by dividing the transverse diameter of the chest by the anterior-posterior distance on CT of the chest on the axial slice that demonstrates the smallest distance between the anterior surface of the vertebral body and the posterior surface of the sternum 1,5. Some authors have found that both radiographic- and CT-calculated Haller indices are strongly correlated and thus recommend the use of chest radiography instead of CT to minimize the radiation exposure 2.
The Haller index is affected by the vertebral level at which it is measured and is largest cranially 3. For consistency, therefore, it is recommended to calculate the largest Haller index in pectus excavatum patients by obtaining the AP diameter at the deepest point of the sternum 1.
Interpretation
The following values are used:
- normal chest: <2.0
- mild excavatum: 2.0-3.2
- moderate excavatum: 3.2-3.5
- severe excavatum: >3.5 1
Corrective pectus excavatum surgery is considered with a Haller index ≥3.25 5.
Age
Young age also reduces the values, with 0 to 2-year-olds having smaller Haller indices than older children 5.
Increasing age also changes the shape of the chest cage and affects the vertebral levels by various degrees:
- cranial levels show a larger Haller index in older subjects
- mid-thoracic levels remain unaltered
- caudal vertebral levels show only a slight increase 3
Sex
Females have a greater Haller index than males in early childhood (from 0-6 years of age) and in teenage years (12-18 years of age) 5.
History and etymology
The Haller index is named after the American pediatric surgeon J. Alex Haller, Jr of Johns Hopkins University School of Medicine. He first described it in 1987 5.
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