An Objective Study of Anatomic Shifts in Intracranial Hypotension Using Four Anatomic Planes

Figure 2

Grading downward central incisural herniation. Technical note. A symbol (rather than a line) has been used in the figures when a measured distance is close to or equal to zero.

(a) Grade 1 or early (minimal) central incisural herniation. Findings include caudal shift of the mammillary bodies from their normal position located above the tuberculum-venous confluence line (white reference line) to a position interposed between the tuberculum-venous confluence line (white reference line) and the incisural line (black reference line). Concurrently, the interval between the optic chiasm and the plane of the diaphragm sella (black triangle line) is shortened beyond normal range. Additionally, the long axis of the chiasm/visual tract (gray line) rotates from a positive upward angle relative the tuberculum-venous confluence line (white line) into an orientation which is either parallel or at a slightly negative angle. The splenium of the corpus callosum (white circle) to incisura line (black reference line) distance is essentially zero

(b) Grade 2 or moderate central incisural herniation. Findings include progressive caudal shift of the mammillary bodies from their grade 1 position just below the tuberculum-venous confluence line (white reference line) to a position at the incisural line (black reference line). The distance interval between the mammillary body and the belly of the pons is now noticeably diminished and its shape is altered from a quadrangular to a triangular appearance. The optic chiasm/diaphragm sella interval continues to decrease. The tuber cinereum changes from a straight line into a caudal sagging shape. The long axis of the chiasm/visual tracts is now oriented at a negative angle (gray reference line), another characteristic sign of advanced central incisural herniation, relative to the tuberculum-venous confluence line (white reference line). The iter (black snowflake) and splenial (white circle) positions, formerly supratentorial, now approach the incisural line (black reference line)

(c) Grade 3 advanced central incisural herniation. Findings include progressive caudal shift of the mammillary bodies from their grade 2 position at or minimally below the incisural line (black reference line) to an infratentorial position (black triangle distance). The interval distance between the mammillary body and the belly of the pons is now significantly diminished and the interpeduncular fossa is clearly triangular in shape. The optic chiasm has nearly reached the entrance of the sella. The long axis of the chiasm/visual tracts (gray reference line) is now at a significantly negative angle relative to the tuberculum-venous confluence line (white reference line). The iter position and the splenium have crossed beyond the incisural line into the posterior fossa. The iter to tuberculum-venous confluence line is depicted by the black snowflake line. The splenium to incisural line is depicted by the white circle line. The tuber cinereum clearly sags inferiorly and begins to buckle over the dorsum sella (another characteristic sign of advanced central incisural herniation). The interpeduncular fossa is close to being completely effaced as the mammillary bodies approach the belly of the pons