Quantification of dysmorphogenesis: pattern variability index, sigma z.

The pattern variability index (sigma z) is the standard deviation of Z-scored radiogrammetric measurements of an individual expressed relative to norms for age and gender. As applied to standardized measurements of the head and face, a sigma z value of 1.2 approximates the 95th percentile of the normal range of children, adolescents, and adults. Individuals with congenital malformation syndromes tend to exceed 1.2 in the pattern variability index and may achieve values of 1.8-2.0 and above. sigma z thus provides objective indications of dysmorphogenesis and quantification of the degree of departure from the normal appearance.