After a long discussion, starting from 1993, a clinical differentiation between classical and mild forms have been proposed.

This distinction was based on clinical criteria (birth weight, post-natal growth, presence/absence of major malformation at the limbs, presence/absence of clinically relevant major malformations at other apparatus).

The positive value of this classification was the demostration of a wide clinical variability of this disease.

 The general idea is that the distinction is not so sharp and that it’s necessary to find prognostic indexes more precise to predict the possible severity of the patient when the diagnosis is made in the first days/months of life.