Arrow Sign (Ruptured MCA bifurcation aneurysm).

A) History:
A 54-year-old woman presented to the emergency department with sudden onset severe headache, nausea and vomiting upon waking that morning. On examination, she was alert and oriented, with a mild right facial droop and a right upper extremity pronator drift. A non-contrast computed tomography (CT) of the head revealed diffuse subarachnoid blood, with a prominent triangular focus projecting into the left Sylvian fissure (Figure).

Axial CT shows diffuse sub arachnoid hemorrhage concentrated in the left sylvian fissure

B) Diagnosis:

-Ruptured MCA bifurcation aneurysm.

C) Discussion:

-The middle cerebral artery is divided into 4 major segments.

-The M1 segment (sphenoidal or horizontal) segment originate from the ICA and bifurcate into superior and inferior divisions.

-The M2 (insular) segment begins at bifurcation of middle cerebral artery, coursing laterally and anteriorly to the margin of the insula.

- The M3 (opercular) segment begins at the circular sulcus of the insula, then loop and curves over the frontal and temporal opercula to reach the surface of sylvian fissure.

-M4  (cortical or terminal) segment consists of various branches that course over the cerebral convexity and supply the cortex.

- When MCA aneurysm is rupture, the resulting sub arachnoid hemorrhage can track along ipsilateral sylvian fissure and outline the frontotemporal operculum, producing an "arrow sign".

Short M1 segment (red) with smaller superior division (yellow) supplying the frontal convexity, and larger inferior division (orange) ointo the the temporal lobe (purple, subdividing into black anterior and white posterior temporal  and white parieto-occipital) and parietal lobe (blue) feeders. 

Reference