Aetiology of intellectual disability--the Finnish classification: development of a method to incorporate WHO ICD-10 coding.

The present authors made an attempt to ease the diagnostic work of physicians who have patients with intellectual disability by creating an aetiological classification system based on the time and mechanism of injury to the central nervous system (CNS). The current paper presents the work-up needed for understanding at least the timing of the causative factor/factors. The timing principle opens a direct course to family counselling. This method has been very well accepted during its 18 years of use in Finland, and therefore, it was felt that it would be helpful to organize the relevant ICD-10 diagnoses according to the timing principle. This method has been published as a manual. An image of a tree became the obvious metaphor for this system. The genetic category forms the main root and stem, from which multiple branched roots and limbs emerge. The individual diagnoses appear as root nodules and leaves. The system is flexible, making it possible to add new branches for groups of diagnoses when improved diagnostic methods create these options (e.g. microdeletions). The aetiological diagnoses change accordingly. Over the course of further development, new incidents damaging the CNS may affect the functional level of an individual and require additional diagnoses. It is a constant challenge for physicians to keep the diagnoses of their patients up to date. The image of the tree helps professionals to think in terms of timing, and thus, makes family counselling easier. It is also helpful in the education of medical professionals.