HYPOPLASIA OF THE MIDDLE PHALANX

AND CLINODACTYLY

• 60% of children with Down syndrome have hypoplasia and clinodactyly of the middle phalanx of the fifth digit.
• Normal ratio of size of middle phalanx of 5th digit/4th digit = 0.85
• An arbitrary cut-off of 0.70 for the ratio resulted in a 75% detection rate in fetuses with Down syndrome, but 18% of normal fetuses had this finding.
• Not therefore used as a general screening tool but rather as a useful adjunct.
• Normal values for the entire fifth digit have been described by Goldstein and colleagues (3). They recommend measurement of the entire fetal fifth digit due to the limitations that one encounters when trying to measure the ossification of the middle phalanx. The difference in size of the middle phalanx between normal and abnormal fetuses is only 1-2mm which is at the resolution limit of current ultrasound equipment. Difficult fetal position, maternal body habitus and time constraints resulted in a failure to measure the middle phalanx in 31% of patients.

REFERENCES

1. Hall B. Mongolism in newborn infants. Clin Pediatr 1966;5:4
2. Benacerraf BR, Harlow B, Frigoletto FD. Hypoplasia of the fifth digit of the middle phalanx, a feature of the second trimester fetus with Down syndrome. J Ultrasound Med 1990;9:389-394
3. Goldstein I, Gomez K, Kopel JA. Fifth digit measurement in normal pregnancies, a potential sonographic sign in Down's syndrome. Ultrasound Obstet Gynecol 1995;5:34-37