The aim of this axis is to classify dystonia according to common biological and pathophysiological mechanisms. The causes of dystonia are broad, and range from environmental factors to other hereditary or sporadic neurodegenerative diseases or disorders of altered neuronal function. This axis contains two non-mutually exclusive subgroups:

The main difficulties in the classification of dystonia have always related to classification by aetiology, and many changes have been proposed for the aetiological axis in recent years. A problem with aetiological classifications is that they conflate axis I and axis II, for example, the classification might use terms better used to describe phenomena (“dystonia plus”), and which therefore do not accurately reflect the underlying cause.