SHORT HUMERUS

Fetuses with trisomies, especially trisomy 21, appear to have a specific pattern of growth aberration, which affects the long bones, especially the humerus and femur.

• Ratio of expected/measured femur length between 15-21 weeks: 68 % of fetuses with Downs Syndrome had a ratio of <0.91, with a 2% false positive rate. The sensitivity of the sonographic detection of Down syndrome in the second trimester rose to 82% when the findings of a shortened femur length, thickened nuchal fold and major congenital abnormalities were combined (1).
• FitzSimmons and co-workers (2) were the first to show that humeral length shortening may be more affected in aborted fetuses with Downs Syndrome than femoral shortening.
• Benacerraf and co-workers (3) showed that 50% of second trimester fetuses with Down syndrome and 6.25% of normal fetuses had a measured/expected humerus length ratio of <0.90, based on the relationship between the humerus length and BPD among normal controls.
  Combining the nuchal fold findings with the humeral length criteria resulted in identifying 75% of fetuses with Down syndrome in their high-risk population.
• Nyberg et.al. (4) reported a sensitivity of 31.1% and a false positive rate of 7.5% for the detection of trisomy 21 with either a short femur (measured femur length divided by predicted femur length £0.91) or a short humerus (measured humeral length divided by predicted humeral length £0.89). By requiring both a short humerus and femur, the sensitivity was reduced to 17.8% and the false positive rate decreased to 1.6%.
• Platt and coworkers (5) did not find a significant difference in femur length between normal fetuses and trisomy 21 fetuses from 15 to 16.9 weeks gestation; a difference of 3mm at 17 weeks gestation was statistically significant.
• Other Studies see reference (6).
• Femur / foot length ratio of £0.90 has a sensitivity of 71% for the detection of trisomy 21 with a 9% false positive rate (7).
• Humerus / foot ratio of £0.85 has a sensitivity of 47% for the detection of trisomy 21 with a false positive rate of 8.1% (7).

REFERENCES

1. Benacerraf BR, Gelman R, Frigoletto FD. Sonographic identification of second trimester fetuses with Downs Syndrome. N Engl J Med 1987; 317: 1371-1376.
2. FitzSimmons J Driste S, Shepard TH et.al. Long bone growth in fetuses with Downs Syndrome. Am J Obstet Gynecol 1989; 161:1174-1177.
3. Benacerraf BR, Neuberg d, Frigoletto FD. Humeral Shortening in second trimester fetuses with Downs Syndrome. Obstet Gynecol 1991; 77:223-227.
4. Nyberg DA, Resta Rg, Luthy DA et al. Humerus and femur length shortening in the detection of Down syndrome. Am J Obstet Gynecol 1993;168:534-538.
5. Platt LD, Medearis AL, Carlson DE et.al. Screening for Down syndrome with the femur length / biparietal diameter ratio: a new twist of the data. Am J Obstet Gynecol 1992;167:124-128.
6. Benacerraf BR. The second-trimester fetus with Downs Syndrome: detection using sonographic features. Ultrasound Obstet Gynecol 1996; 7:147-155.
7. Johnson MP, Michaelson JE, Barr M et.al. Sonographic screening for trisomy 21: fetal humerus: foot length ratio, a useful new marker. Fetal Diagn Ther 1994;9:130-138.