When one half of the brain is removed, what happens regarding the space? Does it fill with CSF? Is something man-made put in there to stop the other hemisphere from moving? Is the other hemisphere centred or does it stay where it normally is?
Once hemostasis was achieved, the hemispherectomy cavity was copiously irrigated until the fluid was crystal clear. While keeping the cavity dry, a layer of Gelfoam and Surgicel was placed over the residual raw surface. This allows these materials to adhere and, hopefully, form the basis of an adhesive layer that will preclude subsequent hemorrhage.We have discovered no adverse effects from leaving the hemispherectomy cavity unfilled with saline. The dura was closed in a water-tight fashion and was not stitched medially to the falx and tentorium or laterally to the inner table of the skull. Eventually the hemispherectomy cavity fills with CSF
The Encyclopedia of Surgery also does not mention the cavity being filled during surgery.
The hemisphere that is left does not move towards the center, as can be seen in MRI scans, for example in this paper: Hemispherectomy in the treatment of seizures: a review