Historical Background

Anatomical hemispherectomy was introduced in 1928 by Dandy1 for the treatment of a diffuse glial tumor of the non-dominant hemisphere in a patient who survived the operation in good clinical conditions before dying from his disease. The technique consisted of the complete excision of the right hemisphere, including the basal ganglia, by means of a complete corpus callosotomy and opening of the lateral ventricle, after the exclusion of the anterior and middle cerebral arteries at the internal carotid bifurcation and coagulation of the veins. In the same year, Lhermitte2 in France published a paper on the physiological features of this operation. A first modification of this procedure was proposed in 1933 by Gardner3 to save the basal ganglia by occluding the middle and anterior cerebral arteries distally to the perforating len-ticulostriate branches and to the Heubner’s artery. Although originally conceived for the treatment of hemispheric gliomas, anatomical hemispherectomy was rapidly considered for the treatment of drug-resistant hemispheric epilepsy. In 1938, anatomical hemispherectomy was used by McKenzie4 in a child with intractable epilepsy and infantile hemiplegia, obtaining the complete disappearance of the seizures. Later on, in 1950, Krynauw5 reported on the favorable results he had obtained in a series of children in whom the procedure was specifically adopted to control seizures. The author, indeed, described 12 epileptic patients affected by infantile hemiplegia who showed not only an excellent epileptic outcome but also experienced a significant postoperative motor and cognitive improvement. On these grounds, during the following two decades, anatomical hemispherectomy gained an increasing popularity among the neurosurgical community because it allowed surgeons to obtain complete or nearly complete seizure control in 70 to 80% of patients with an acceptable surgical mortality rate (approximately 6 to 10%).6 The procedure was performed according to two main techniques: the en bloc excision of the cerebral hemisphere and the piecemeal hemispherectomy, obtained through multiple lobectomies.7–9

During the late 1960s and the early 1970s, the interest in anatomical hemispherectomy rapidly declined because of the occurrence of late complications, as obstructive hydro-cephalus and chronic subdural fluid collections accounted for a significantly high rate of late mortality (up to 25% of the cases).7,10 These complications were interpreted on the grounds of chronic intracranial hemorrhages, secondary to the dislocation of the residual brain, resulting in repeated vascular tearings leading to neomembrane formation in the surgical cavity and chronic iron deposition on the cerebral structures. The acritical acceptance of late hemosiderosis as a frequent and unavoidable cause of late neurological deterioration led to the practical abandonment of the technique in favor of variants aimed at reducing the volume of the residual cavity after the excision of the epileptogenic cerebral hemisphere (subtotal hemispherectomy or functional hemispherectomy). In particular, in 1973, Rasmussen11 introduced a promptly accepted technique consisting of the mere excision of portions of the central and temporal regions and disconnection of the remaining cerebral cortex, which was left in situ. Although functional hemispherectomy, compared with anatomical hemispherectomy, appeared to be weighted by a minor rate of complications while allowing comparable seizures control, some contemporary authors complained of the abandonment of anatomical hemispherectomy by considering its higher efficacy with regard to epilepsy control. In 1973, Northfield12 writing about anatomical hemispherectomy, commented, in fact, “It is a pity that such a beneficial operation should be abandoned.”

Anatomical hemispherectomy was revived in the 1980s when, after the introduction of the computed tomography (CT) scan, it was possible to identify the main cause of late neurologic deterioration, that is, the late occurrence of progressive hydrocephalus, a phenomenon that was difficult to recognize on the mere clinical ground before the wide availability of CT and magnetic resonance imaging (MRI). Silently, the demonstrations of hemosiderosis disappeared from the medical literature in the last decades. Nevertheless, it continued to be indicated in a nearly automatic fashion as one of the main factors in favor of the adoption of the functional hemispherectomy versus anatomical hemispherectomy.13 Nowadays, hemosiderosis continues to be quoted in spite of reports that deny its occurrence in operated on patients observed for considerably long periods of time (definitively longer than the 8–10 years considered to be necessary in the past for the development of the complication).14–17

With the revival of the technique, new variants were introduced, all of them with the goal of counteracting the disproportion between the cranial and the brain volume brought about by the cerebral hemisphere excision and the occurrence of postoperative hydrocephalus. In 1983, Adams18 introduced the Oxford-Adams modification consisting of the plugging of the homolateral foramen of Monro and a narrowing duraplasty. In the following years, other technical variants became available, such as hemidecortication and hemicorticectomy.19,20 In the same time, the advances in neuroimaging diagnosis allowed surgeons to select better candidates for the procedure, namely those patients with hemimegalencephaly and diffuse cortical dysplasia in whom functional hemispherectomy may have some limitations because of the anatomical abnormalities of the midline structures and the huge volume of the malformed hemisphere.

Indications and Preoperative Evaluation

Candidates for anatomical hemispherectomy are those subjects suffering from intractable, catastrophic epilepsy caused by a diffuse lesion of one cerebral hemisphere. The anatomical substrates responsible for the seizure disorder are either congenital or acquired. Among the congenital disorders, the disturbances in neuronal migration and differentiation, namely hemimegalencephaly, plurilobar cortical dysplasia, Sturge-Weber syndrome, and perinatal occlusion of the middle cerebral artery, are the most frequent clinical entities. Posttraumatic and postinfective diffuse unilateral brain injuries and Rasmussen encephalitis account for nearly all the cases of acquired epileptogenic lesions, possibly requiring a resective or disconnective surgical treatment. All the types of unilateral hemispheric cerebral damage actually do respond quite satisfactorily to both anatomical hemispherectomy and functional hemispherectomy, the epileptic outcome being related to the effectiveness of the cortical disconnection and the psychomotor outcome to the “normality” of the contralateral hemisphere.21–28 However, the results associated to the neuronal migration disorders are less rewarding than those observed in subjects with other congenital anomalies or acquired lesions, whichever technique is adopted for the hemispherectomy.

The best candidates for disconnective procedures (functional hemispherectomy, hemispherotomy) are patients with atrophic brains and large ventricular cavities. Conversely, subjects with hypertrophic hemispheres, caused by diffuse migration disorders, and small, distorted cerebral ventricles may be the ideal candidates for anatomical hemispherectomy. Indeed, in these patients the anatomical resection of the malformed hemisphere ensures the complete excision of the epileptogenic substratum as well as the preservation of the integrity of the contralateral midline structures. These structures, in fact, are often dislocated from their normal position, because of the abnormally large malformed hemisphere and distorted so that they can be damaged by disconnective surgical maneuvers performed without an adequate exposure. Furthermore, by completely removing the hyper-trophic hemisphere, anatomical hemispherectomy provides the space necessary to relieve the previously compressed residual healthy hemisphere and to allow its normal volumetric growth.

Several studies demonstrated that early hemispherectomy can give a significant advantage in favoring the psycho-motor development.17,29,30 Accordingly, patients undergoing hemispherectomy currently tend to be operated on in their infancy or young childhood. This trend involves in particular anatomical hemispherectomy because most of the children undergoing this procedure harbor a congenital cortical malformation producing early catastrophic seizures.31 The young age of the candidates for anatomical hemispherectomy requires a careful and accurate preoperative evaluation for correct surgical planning ( Table 25.1 ). To achieve this goal, the following preoperative steps are necessary:

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  Perform a detailed epileptic history centered on the seizures’ onset and semiology as well as their evolution and the development of drug resistance.

  
  
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  Perform a careful neurological examination to verify the presence and the degree of the hemiparesis and of the possible visual field defect (hemianopsia).

  
  
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  Perform neuropsychological assessment based on age-tailored scales. The target of this evaluation is to assess the developmental level in infants or children with severe mentally disabilities and to test the cognitive and behavioral skills in the older children (memory, language, visuospatial and perceptual-motor function, attention, executive function, behavioral features).

  
  
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  Ensure adequate electroencephalographic (EEG) exploration to confirm the diagnosis of epilepsy and define a possible epileptic syndrome (e.g., Ohtahara syndrome, West syndrome, Lennox-Gastaut syndrome), to detect the epileptic foci, to analyze the interictal and the ictal patterns, and to confirm the absence of seizures arising from the contralateral hemisphere. Such a study is usually performed by using a long-term surface video-EEG recording. An invasive recording (stereo-EEG) may be required in case clini-coradiological discordance (e.g., MRI showing diffuse hemispheric alterations and clinical examination and surface EEG demonstrating only localized alterations).

  
  
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  Detailed neuroimaging studies including structural and functional MRI with standard sequences, spectroscopy, diffusion and perfusion imaging, and angio graphic sequences provide excellent anatomical details of the brain and the vascular structures, and they have paramount importance for the correct planning of anatomical hemispherect omy. Actually, in children with hemimegalencephaly, it is mandatory to preoperatively verify the presence of possible anatomical abnormalities, because distortion of the lateral ventricles, contralateral shift of the median vascular structures, hypertrophy of the veins draining into the sagittal sinus/ hypoplasia of the deep venous system, abnormal extension of the sylvian veins up to the sagittal sinus, cortical hypervascularization, and so on. Functional studies (functional MRI, single photon emission computed tomography (SPECT), positron emission tomography (PET), and integration with magnetoencephalography) may be required to complete the preoperative Neuroimaging assessment in selected cases.

Surgical Technique

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15 Implantation of Strip, Grid, and Depth Electrodes for Invasive Electrophysiological Monitoring 18 Transsylvian Selective Amygdalohippocampectomy 22 Surgical Approaches in Cortical Dysplasia 34 Cortical and Deep Brain Stimulation 28 Transsylvian Hemispheric Deafferentation 32 Hippocampal Transection

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