US and LS in Context of Arm Span Measurement
US (Upper Segment) and LS (Lower Segment) are body proportion measurements where US is measured from the top of the head to the pubic symphysis, and LS is measured from the pubic symphysis to the heel, with their ratio (US/LS) used clinically to assess skeletal proportions in growth disorders and skeletal dysplasias.
Definitions and Measurement Technique
- Upper Segment (US) represents the distance from the vertex of the head to the pubic symphysis 1
- Lower Segment (LS) represents the distance from the pubic symphysis to the heel, or can be calculated as total height minus upper segment 1
- These measurements are distinct from arm span, which measures fingertip to fingertip with arms outstretched horizontally 1
Clinical Application of US/LS Ratio
- The US/LS ratio changes predictably with age and is used to identify disproportionate growth patterns in skeletal dysplasias 1
- In boys, the mean US/LS ratio decreases from 1.108 at age 3 years to 0.984 at age 10 years, reaching a nadir of 0.922 at age 15 years 2
- In girls, the US/LS ratio decreases to less than 1.0 at age 9 years (one year earlier than boys), with a nadir of 0.946 at age 13 years 2
- Growth charts tracking upper and lower segments are used for ages 2-18 years in skeletal dysplasia assessment 1
Relationship to Arm Span Measurements
- While US and LS measure vertical body proportions, arm span measures horizontal reach and serves a different clinical purpose 1
- Arm span is used to estimate height when standing measurement is impossible, particularly in wheelchair-bound patients or those with skeletal deformities 1
- In patients with Duchenne muscular dystrophy requiring pulmonary function testing, arm span is used to estimate height for predicted FVC values when patients cannot stand 1
Important Clinical Context
- Both US/LS measurements and arm span-for-age are tracked in skeletal dysplasia growth charts, but they assess different aspects of body proportions 1
- The US/LS ratio is particularly valuable for identifying conditions affecting limb length disproportionately, such as achondroplasia or spondyloepiphyseal dysplasia 1
- Standard deviation curves for US/LS ratio vary by sex and should be referenced when evaluating children with short stature 2
Common Pitfalls to Avoid
- Do not confuse US/LS measurements with arm span measurements—they assess different body dimensions and serve different clinical purposes 1, 2
- The US/LS ratio must be interpreted with age-appropriate and sex-specific reference ranges, as the ratio changes significantly during growth 2
- When arm span is used to estimate height for pulmonary function predictions, use appropriate prediction equations rather than assuming a 1:1 ratio, as this relationship varies with age, sex, and race 3