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Section III. Sylvian Fissure Overview
The Sylvian fissure can be considered a large sulcus, the first to form during development. Therefore, the presence of the Sylvian fissure is very constant. There is even a hint of it with the condition classic lissencephaly (agyria/pachygyria complex) even though other major sulci are essentially absent.
The Sylvian fissure is complex with multiple components and rami (branches). Overall, there is a lateral portion and a medial stem. The main component of the lateral portion is the posterior horizontal ramus, separating the frontal and parietal lobes superiorly from the temporal lobe inferiorly. The lateral portion has two key smaller rami, the anterior horizontal ramus and anterior vertical ramus. These two rami extend into the inferior frontal gyrus and generally divide it into three subcomponents. The anterior vertical ramus is essentially always one sulcus anterior to the precentral sulcus (ascending sulcus sign).
The medial stem extends inward to the lateral aspect of the suprasellar cistern, separating the basal frontal lobe from the basal temporal lobe. The suprasellar cistern has a star-shape and the Sylvian fissure arises from the superolateral points of that star. The medial stem contains the 'Sylvian cistern' or 'cistern for the middle cerebral artery.'
The Sylvian fissure is complex with a medial stem, a lateral portion, and important rami (branches).
Medial Stem
The medial stem of the Sylvian fissure divides the basal frontal and basal temporal lobes, extending medially to terminate as one of the points of the 'star-shaped' suprasellar cistern. Note the M1 segments of the MCA (hyperintense vessels) in the MCA cisterns on this T1 MPRAGE image.
Lateral Portion
The lateral portion of the Sylvian fissure consists largely of a long horizontal ramus. The anterior ascending and anterior horizontal rami divide the inferior fontal gyrus into important named subcomponents (ant -> post: pars orbitalis, pars triangularis, pars opercularis). The posterior termination of the posterior horizontal portion frequently has an upswing into the supramarginal gyrus of the parietal lobe.
In addition to the clinical correlate cases presented already, we will see that the patterns of the rami of the Sylvian fissure are important for identification of other signs, such as the 'M-sign' defining key parts of the inferior frontal gyrus.
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