Filly's 13th Law
Once you set an upper limit of normal I can guarantee two things: abnormal individuals will fall below the cutoff and normal individuals will fall above it. Nonetheless, when it comes to the diameter of the ventricular atrium, I am a 10 mm absolutist.
The standard definition of a reference range for the vast majority of measurements is calculated as the 95% confidence interval. Therefore, if the “upper limit” was correctly calculated then 2.5% of the time a normal individual will demonstrate a value larger than the upper limit of this interval.
But here’s the thing. There are a whole lot more normal individuals than abnormal individuals (at least we hope so). Further, no matter how carefully one sets they “upper limit of normal,” some abnormal individuals will still fall in the “normal range.”
When I did the research project that showed 10 mm to be the upper limit of normal for the transverse, inner-to-inner diameter of the ventricular atrium on a true axial sonogram of the fetal brain I had already established Filly’s 13th Law (1). Ergo, I knew full well that others would disagree. Some would suggest a smaller diameter and some would suggest a larger diameter.
These individuals were, of course, stellar observers and clinicians. However, I can state the following with a certitude rarely experienced in the world of prenatal diagnosis. When it comes to the diameter of the ventricular atrium, I am a 10 mm absolutist. I suggest in the strongest of terms that you become “a 10 mm absolutist,” as well.
1. Cardoza JD, Goldstein RB, Filly RA: Exclusion of ventriculomegaly with a single measurement: The width of the lateral ventricular atrium. Radiology 1988; 169:711-14