Biometry of the fetal cisterna magna: estimates of the ability to detect trisomy 18

Abstract

Measurements of the fetal cisterna magna were obtained from a prospective sample of patients undergoing diagnostic obstetric ultrasound examinations. These normal measurements were then compared to a retrospective sample of ultrasound scans from fetuses with the diagnosis of trisomy 18 by amniocentesis. The fetal cisterna magna increases in size throughout pregnancy. The sample from fetuses with trisomy 18 was different and had a higher rate of small and large cisterna magnas. After the two samples were mixed, it was determined that a cut-off of 2.5 standard deviations for the fetal cisterna magna would give a sensitivity of 26.3%, specificity of 99.1%, positive predictive value of 50.0% and negative predictive value of 97.6% for trisomy 18. The probability that a fetal cisterna magna of abnormal size will predict trisomy 18 in an early second-trimester ultrasound examination with the 2.5 standard deviation cut-off was estimated at 0.019. A fetal cisterna magna that is of abnormal size appears to be of diagnostic value in identifying the fetus with trisomy 18, but its sensitivity is low.

Ratio of fetal anteroposterior to transverse cerebellar diameter for detection of the cerebellar hypoplasia in the second trimester and comparison with trisomy 18

“…An enlarged cisterna magna is also considered an abnormal cerebellar finding in trisomy 18, implying hypoplasia of the vermis. However, it has been reported that fetal cisterna magna had low sensitivity (26.3%) for identifying trisomy 18 . Therefore, we paid particular attention to the biometry of cerebellar hemispheres in order to detect hypoplasia.…”

The Normal and Pathological Fetal Brain

“…The actual measurement is made on the midline between the posterior surface of the cerebellar vermis and the internal surface of the occipital squama (Snijders and Nicolaides 1994 ;Steiger et al 1995 ). …”

Enlarged Cisterna Magna in the Third Trimester as a Clue to Fetal Trisomy 18

“…Recent technical improvement in ultrasound equipment has allowed more precise measurements of the fetal cisterna magna and transverse cerebellar diameter at 14-40 weeks [1][2][3]. However, the sonographic measurement of the cisterna magna in early pregnancy and in midgestation appears to have a low sensitivity for the detection of fetal trisomy 18 [3][4][5].…”

“…However, the sonographic measurement of the cisterna magna in early pregnancy and in midgestation appears to have a low sensitivity for the detection of fetal trisomy 18 [3][4][5]. In contrast, most reported cases of fetal trisomy 18 associated with an enlarged cisterna magna have been described in the late gestation [3,[6][7][8][9][10][11].…”

“…We reviewed 33 reported cases of trisomy 18 fetuses with an enlarged cisterna magna detected in the late gestation [3,[6][7][8][9][10][11] (table 2). All 33, plus our 6 cases, were detected between 24 and 40 gestational weeks.…”