Journal of Ultrasound in Medicine, Vol 9, Issue 7 389-394, Copyright © 1990 by American Institute of Ultrasound in Medicine
Hypoplasia of the middle phalanx of the fifth digit. A feature of the second trimester fetus with Down's syndrome
B. R. Benacerraf, B. L. Harlow and F. D. Frigoletto Jr
Department of Obstetrics and Gynecology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts.
It has been established that 60% of infants with Down's syndrome have
hypoplasia of the middle phalanx of the fifth digit. To determine whether
this would be a useful prenatal sonographic sign for Down's syndrome, we
measured the middle phalanx of the fifth and fourth digits in 1,032 fetuses
between 15 and 20 weeks gestational age at the time of amniocentesis, prior
to any knowledge of the karyotypes. A ratio of the middle phalanx of the
fifth digit over the middle phalanx of the fourth digit was calculated, and
the median ratio for the 1,024 normal fetuses was 0.85. There were eight
fetuses who had trisomy 21 by karyotype and their median ratio was 0.59 (P
= .04). Of the eight fetuses with Down's syndrome, seven had ratios below
the normal population median. If an arbitrary cut-off point is used at a
ratio of 0.70, 6/8 (75%) of those with Down's syndrome would be identified,
as well as 18% of normal fetuses (positive predictive value = 3.2% in this
group). Although we do not suggest that this ratio be used alone as a
screening test for Down's syndrome, these findings confirm the presence of
a small middle phalanx in fetuses with trisomy 21 as early as 15 to 16
weeks and may be a useful adjunct to the several already reported
sonographic signs in the fetus at risk for Down's syndrome.
