Lennox-Gastaut syndrome (LGS) is regarded as a model of the epileptic
syndrome because of its specific clinicoelectrical manifestation. Howe
ver, a close investigation reveals that its outline is somewhat vague,
having the borderland around it. Precise diagnosis in an individual c
ase is not always easy. In this paper, the diagnostic criteria of LGS
are described. According to these criteria, cases with LGS were subcla
ssified into the typical and the atypical cases, and also cases in the
borderland of LGS were reviewed. On the other hand, our prospective l
ong-term follow-up study revealed that cortical mechanisms played an i
mportant role in the pathophysiology, clinical features and refractori
ness of LGS. Secondary bilateral synchrony (SBS) is supposed to be a m
ode of expression of cortical mechanisms of LGS. A newly developed met
hod with coherence and phase analysis demonstrated that the pathophysi
ology was based on SBS in 33% of the typical LGS cases. This finding i
s not only crucial for the choice of rational treatment including epil
epsy surgery, such as callosotomy, but also contributes to a more refi
ned subclassification of LGS.