A pre-bifurcation horizontal segment middle cerebral artery (M1) saccular aneurysm at a right-angle bend in the M1 with an arachnoid band.

Background: Pre-bifurcation horizontal segment of the middle cerebral artery (M1) saccular aneurysms were rare and mostly in the mid-portion of M1 - the common location of the origins of the lateral lenticulostriate perforators, or at the branching point of the anterior temporal artery. Pre-bifurcation M1 saccular aneurysm not associated with a branching point is exceptionally rare.

Case description: The authors report a 68-year-old female with a ruptured saccular aneurysm in the M1, which was not associated with an arterial side branch or a bifurcation. The formation of the saccular aneurysm was attributed to the increase in hemodynamic stress at a right-angle angulation in the M1, which was due to the presence of a tight arachnoid band. The patient underwent a craniotomy with clipping of the aneurysm. Her postoperative course was uneventful.

Conclusion: This is the first report of an association of an arachnoid band with a cerebral aneurysm, and it supports including arachnoid bands in the list of contributing factors in cerebral aneurysm formation.