Is an acoustic neuroma an epiarachnoid or subarachnoid tumor?

There are arguments about whether acoustic neuromas are epiarachnoid or subarachnoid tumors.

To retrospectively examine 118 consecutively operated-on patients with acoustic neuromas to clarify this point.

Epiarachnoid tumors are defined by the absence of an arachnoid membrane on the tumor surface after moving the arachnoid fold (double layers of the arachnoid membrane) toward the brainstem. In contrast, subarachnoid tumors are characterized by the arachnoid membrane remaining on the tumor surface after moving the arachnoid fold. Based on this hypothesis, we used intraoperative views and light and electron microscopy to confirm the existence of an arachnoid membrane after the arachnoid fold had been moved.

The tumors were clearly judged to be subarachnoid tumors in 86 of 118 patients (73%), an epiarachnoid tumor in 2 patients (2%), whereas a clear judgment was difficult to make in the remaining 30 patients (25%).

The majority of acoustic neuromas are subarachnoid tumors, with epiarachnoid tumors being considerably less common.