Fig 2.

Hypothetic mechanism underlying MCA fenestration formation. Anteroposterior schematic representations of the ICA bifurcation into anterior cerebral arteries (ACA) and MCA.

A, Normal MCA development, fetal stage. At this stage, the MCA is constituted by multiple arterial twigs arising from the distal ICA.

B, Normal MCA development, adult configuration. The multiple MCA twigs have been replaced by a single proximal MCA trunk (M1 segment).

C, Fenestrated MCA development, fetal stage. The fetal stage is similar to the normal development illustrated in A. An early branching TPA is shown, which might predispose to the formation of fenestration, either by precluding the fusion of several primitive twigs into a single trunk or by interfering with the regression of some of the twigs.

D, Fenestrated MCA development, adult configuration. Fenestration of the M1 segment is shown, with an early branching TPA arising from the inferior aspect of the fenestrated segment, as observed in our five angiographic cases.