Despite a systematic diagnostic approach and more
accurate diagnostic tools becoming available in the form
of vascular imaging and hematologic and genetic studies,
stroke of undetermined etiology was the most common
etiology among young stroke patients, with the exception
of two studies.2,23 The high number of young stroke patients
with undetermined etiology may be explained in part by the
insufficient extent and timing of the investigations. On the
other hand, the TOAST classification is not perfect, and can
lead to overestimation of patients with stroke of undetermined
etiology, mainly because patients with two or more potential
etiologies fall into this group. This group also includes
patients with incomplete investigation and those with no
evident cause despite extensive evaluation. Therefore, a newer
classification system for more accurate etiologic subtyping of
ischemic stroke, like A-S-C-O,26 might have an influence on
reducing the high numbers of strokes being identified as of
undetermined etiology. Stroke of other determined etiology
collectively account for 20%–30% of cases of young stroke.
A wide variety of different and uncommon causes of stroke
in young adults is presented in Table 2.