The use of cerebral hemispherectomy for seizure control implies that the seizure focus is lateralized, though diffusely localized to one hemisphere. In that respect, hemispherectomy may be considered as the most radical focal brain excision. The preoperative evaluation should establish as clearly as possible the degree of damage to the diseased hemisphere and the degree of integrity of the good hemisphere. Classical indications for cerebral hemispherectomy include a severe, unilateral, structural lesion that is responsible for the seizures and produces a contralateral neurologic deficit characterized by hemiparesis and hemianopsia. From the standpoint of producing a neurologic deficit, the hemispherectomy often has already been achieved by the disease process. In a progressive condition, such as progressive chronic encephalitis (Rasmussen’s), extensive Sturge-Weber syndrome, and infantile spasms, in which continued clinical deterioration leading to a maximum contralateral neurological deficit is expected, early hemispherectomy may cause an acute worsening of the neurological deficit, but not worse than that to be produced by the disease process. The improved seizure control and psychosocial improvement following successful surgery outweigh the poor prognosis associated with the natural history of these disease processes that are progressive and uniformly refractory to medical therapy, particularly as mentioned earlier, the likelihood of improved motor function with recovery. It has also been observed in many cases where the motor neurologic findings are actually improved after surgery since the damaged hemisphere was not providing any input to normal function but was impeding the relatively normal functioning hemisphere.

The surgical techniques of hemispherectomy have been historically classified in two main categories: the classic "anatomic hemispherectomy" with its variations, and the functional hemispherectomy. The anatomic hemispherectomy has had multiple modifications to decrease the complication rate, more specifically from superficial cerebral hemosiderosis and hydrocephalus. The functional hemispherectomy, initially described by Rasmussen and more recently modified by Villemure, minimizes the resection of tissue and has been shown to likewise further reduce the long-term complications.

The anatomic hemispherectomy simply consists of the removal of the cerebral hemisphere without the basal ganglia. The procedure may be done either en bloc or in multiple lobectomies, depending on the surgeon’s preference. All vascular input and output must be interrupted, a complete callosotomy, and the corona radiata sectioned superior and lateral to the thalamus. The remaining posteroinferior frontal cortex as well as the medial temporal structures are also excised. Complications encountered following this method of hemispherectomy have led many centers to abandon it in its original form and have attempted multiple modifications.

"Hemidecortication" consists of removal of the whole cerebral cortex, with sparing of the white matter, thus avoiding opening of the lateral ventricle. This maneuver reduces the size of the hemispherectomy cavity created, minimizing the mixing of bloody material and debris from the surgery with the ventricular CSF. Some surgeons have undermined and removed large slabs of cortex, rather than proceeding piecemeal with suction or ultrasonic aspirator , while others have attempted to reduce the volume of the hemispherectomy cavity further by plicating the dura and morcellating the skull. The "modified hemispherectomy," as described by Adams, consists of an anatomic hemispherectomy followed by occlusion of the ipsilateral foramen of Monro with muscle to prevent communication between ventricular CSF and the hemispherectomy cavity. Additionally, the volume of the hemispherectomy cavity was reduced by tacking the convexity dura to the falx, the basal dura and the tentorium, thereby creating a large extradural space. Presently, these methods are used minimally and are usually center-specific. Most centers have converted to the functional hemispherectomy and its modifications.

"Functional hemispherectomy" is more widely used presently and consists of an anatomically subtotal, but physiologically complete, hemispherectomy. The operation is based on principles of disconnection rather than excision. The originally described method requires the excision of the central frontoparietal cortex, including the parasagittal tissue from the level corresponding to the genu of the corpus callosum to the splenium, a temporal lobectomy with an excision of amygdala and hippocampus, disconnection of the residual frontal and parietooccipital lobes medially, a corpus callosotomy and an insulectomy. Modifications of this technique, for the most part, markedly reduce the volume of tissue excised in the central region and perform a hemispheric disconnection with minimal resection. Villemure et al. most recently described a technique where a suprasylvian window is created, allowing access to the ventricle, through which the suprasylvian portion of the hemisphere can be disconnected prior to the completion of the residual disconnections of the hemisphere. Intraoperative complications involve fluid and electrolyte imbalances and bleeding. Significant blood loss is frequent, which can be problematic in young children, not only for perfusion, but for clotting as well. Aggressive fluid, electrolyte, and hemostatic management is absolutely necessary during these procedures. A frequent postoperative complication following hemispherectomies is aseptic meningitis, which is likely secondary to the blood products entering the CSF. Early postoperative brain shift, with herniation and death, has been reported following anatomic hemispherectomy, and, though rare, may have been due to the development of hydrocephalus of the good hemisphere combined with the displacement of the residual hemisphere toward the hemispherectomy cavity. Late hydrocephalus, not related to superficial cerebral hemosiderosis has been reported in up to 30% of patients following an anatomic hemispherectomy and up to 18% in the patients following functional hemispherectomy.

Other late postoperative complications include infections and spontaneous and posttraumatic hemorrhages. Though uncommon, the presence of a large empty space following anatomic hemispherectomy has been suspected to favor the development of a hematoma even after minor head injury. Historically, the main late complication has been superficial cerebral hemosiderosis, though its occurrence has been markedly reduced by minimizing blood and blood by products access to the ventricular system, early drainage using a ventricular catheter, and decreasing ventricular exposure by decreasing the volume of the hemispherectomy cavity (e.g.) functional hemispherectomy. The range of incidence of postoperative hydrocephalus is approximately 10-20% for functional hemispherectomy and 30% for anatomic hemispherectomy.

The mortality rate following hemispherectomy can be significant, ranging from 4 to 39% though these for the most part have been in earlier series’ and have markedly decreased with improvements in intraoperative and fluid management. More recent experiences have reported mortality at less than 2%. Late mortality also may occur though it has been reported secondary to complications related to CSF pathways, (e.g.) shunt malfunction or superficial cerebral hemosiderosis. The first presently unavoidable and the latter, particularly uncommon.

In principle, whichever method of hemispherectomy is utilized based on the same indications, the resulting seizure control should be identical. These techniques, either by removal or disconnection, completely eliminate the epileptogenic influence of the diseased hemisphere to result in a seizure free or virtually seizure free state. Failures in seizure control are due either to incomplete disconnection or bilateral foci. Long-term improvement in seizure control following hemispherectomy is anticipated in 90-95% of patients. This benefit can be further divided into two categories: those who become and remain seizure-free (70-85%) and those who continue to have some seizures but benefit from at least an 80% reduction in seizure frequency (10-20%).

While the control of seizures represents the first and ultimate objective of hemispherectomy, there are secondary benefits from improved seizure control in the sphere of psychosocial development, including improvement in behavior. Beardsworth and Adams have documented not only intellectual stabilization following hemispherectomy but also continued intellectual improvement. These benefits are believed secondary to better seizure control and possibly, as well, to the reduction or elimination of anticonvulsant medication.