In these cases, the tumor infiltrates and pushs apart important neural structures, and thus these structures fail to function properly.
Malignant lymphoma is notorious for presentation with an epidural mass.
Typically, they are at the junction of the cortex and white matter. Metastases are discrete, and may be multiple. Most important effect - surrounding edema, with subsequent swelling of white matter. This sometimes responds to steroids or hyperosmolar urea. A tumor of arachnoid cells, especially those within the arachnoidal villi.