The diagnosis of Pallister-Killian syndrome (PKS) is confirmed by tissue-specific mosaicism of i(12p). The isochromosome is found in skin fibroblasts and bone marrow, but rarely in peripheral lymphocytes. The nature of the isochromosome loss was evaluated using 2 techniques: micronucleus formation for anaphase lag and in situ DNA hybridization for mosaicism in interphase cells. Cells from serial cultured fibroblasts, peripheral blood lymphocytes, and bone marrow from 4 PKS patients were used for the above analysis. Micronucleus formation was similar for PKS and normal diploid cultures, ruling out loss of i(12p) by anaphase lag as the major mechanism of in vitro mosaicism. In situ hybridization using an alpha satellite DNA probe for chromosome 12 was used to examine the presence of the i(12p) in interphase fibroblasts from 1 patient and lymphocytes from 2 patients (age 8 weeks and 1 day). The i(12p) was present in a significantly higher proportion of interphase nuclei in peripheral lymphocytes than in metaphase, suggesting the initial loss of the isochromosome is exaggerated in metaphase by selective division in vitro. In situ hybridization of peripheral lymphocyte interphase cells with chromosome 12 specific probes may be a useful supplemental procedure for the diagnosis of PKS, at least in the newborn infant.

A retrospective analysis of all fetuses with prenatally detected choroid plexus cysts, identified at our institution between 1988 and 1993, was performed. Cytogenetic data, associated sonographic findings, obstetric outcome and pediatric follow-up was obtained to determine the incidence of aneuploidy and the rate of associated congenital anomalies in second-trimester fetuses with this finding. There were 211 second-trimester fetuses identified with a choroid plexus cyst. Amniocentesis was performed in 175 (83%) and postnatal chromosome analysis was performed in one newborn. Follow-up data are available on 203 (96%). Of the 176 cytogenetically studied fetuses, eight (4.5%) were aneuploid (including four cases of trisomy 18). In four of the aneuploid fetuses, the choroid plexus cyst was the only abnormal sonographic finding identified (including one case of trisomy 18). From this study and a review of the literature, we confirm that choroid plexus cysts are a sonographic marker for trisomy 18, even when identified as an isolated finding in an otherwise normal-appearing fetus. We conclude that the detection of a choroid plexus cyst merits further careful evaluation of fetal anatomy and consideration of cytogenetic evaluation.

Prenatal diagnosis choices were reviewed in 473 women who presented for genetic counselling prior to 11 weeks' gestation for the indication of advanced maternal age. Group A consisted of 336 patients who were unaware of a possible association between chorionic villus sampling (CVS) and limb defects. Group B consisted of 137 patients who were provided this information. Fifty-one per cent of patients in group A and 45 per cent of patients in group B chose CVS. This difference was not significant by chi 2 analysis (P = 0.7). Patterns of prenatal diagnosis procedure utilization from 1987 to 1992 revealed a significant reduction in CVS utilization accompanied by a corresponding increase in amniocentesis after the association between CVS and limb defects was publicized. Referrals for CVS counselling also significantly declined. However, acceptance rates did not change for those patients who received genetic counselling. First-trimester genetic counselling, including a discussion regarding a possible association between CVS and limb defects, helps patients make informed decisions concerning prenatal diagnosis options, and, in our population, resulted in no change in CVS acceptance rates.

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