Why and how to assess the aetiological diagnosis of children with intellectual disability/mental retardation and other neurodevelopmental disorders: description of the Finnish approach.

One of the most important tasks of a physician who has patients with delayed development is to assess the cause of the problem. To help with this work, an aetiological classification based on timing and type of the injury to the central nervous system (CNS), has been created. The main divisions are: genetic causes; CNS malformations and multiple malformation syndromes of unknown origin; external prenatal factors; paranatally acquired disorders (-1 to +4 weeks from delivery); postnatally acquired disorders; and untraceable or unclassified causes. In the classification, the earliest factor injuring the CNS is the primary diagnosis. The first stage, which consists of a quite simple workup, reveals the timing of the injury and allows the possibility for family counselling. The overview of the second stage assessment is also given. The 'aetiological tree' illustrates the classification method and serves as a reference and teaching aid. It can also be used for genetic counselling to demonstrate the situation. This method has been used for almost 20 years and has been proven to enhance diagnostic activity and family counselling.