Patient Evaluation before Selective Amygdalohippocampectomy

Patient selection for selective amygdalohippocampectomy is important in terms of cognitive outcome and freedom from seizures. Patients with mediobasal temporal lobe epilepsy with hippocampal sclerosis and intractable seizures should be clearly identified through a defined underlying hippocampal pathology shown on MRI, clinical seizure types, and electrophysiological and functional imaging techniques. Initial precipitating incidents, including febrile seizures, trauma, hypoxia, and intracranial infections before the age of 5 and before the onset of habitual nonfebrile seizures are very common.18 Habitual seizures begin earlier for medio-basal temporal lobe epilepsy with hippocampal sclerosis, with the majority occurring in patients between the ages of 4 and 16 years; however, these seizures can begin earlier or much later, and the patient can still show the same pathological changes and excellent response to surgery. Focal seizures occur in more than 90% of patients but secondary generalized seizures are rare and may correlate with the extent of the lesion. Auras and automotor seizures, sometimes with impaired consciousness, are characteristics of mediobasal temporal lobe epilepsy with hippocampal sclerosis. Auras are mainly characterized by an ascending epigastric sensation, gradual impairment of consciousness, and are typically associated with oroalimentary automatism in approximately 70% of patients.18 Dystonic posturing occurs in 20 to 30% of patients and is contralateral to the side of seizure onset. Specific baseline and follow-up neuropsycho-logical testing is important. MRI is the most important investigational tool. Improvements in MRI techniques, especially 3-Tesla MRI scanners, contributes to diagnosis of hippocampal sclerosis significantly. MRI volumetric investigations and spectroscopy give more information on T1, T2 and fluid attenuation inversion recovery (FLAIR) findings. 18F-de-oxyglucose positron emission tomography (FDG-PET) demonstrates ipsilateral hypometabolism interictally. However, 11C flumazenil PET is more sensitive than FDG-PET. Ictal and postictal single photon emission computed tomography (SPECT) are preferred over interictal SPECT. In one third of patients, interictal epileptiform anomalies are lateralized and localized to the lesion. In the other two thirds, bilateral dependent or independent (or both) epileptic activity is detected. Sphenoid electrode recordings also disclose more information about lateralization. The ictal onset is not always detected by scalp electroencephalography (EEG) video recording, and seizures are lateralized in 80% of patients.18 Invasive EEG recordings with depth and subdural electrodes are needed in patients with discordant findings on MRI, semiology, functional imaging exams, and electrophysiology.

Surgical Technique

An exact knowledge of the topographical, white matter, and arteriovenous anatomy of the region is crucial for a successful outcome after selective amygdalohippocampectomy ( Figs. 18.1 and 18.2 ).10–12,19–29

A pterional craniotomy is performed in the usual way, and the posterior ridge of the greater wing of the sphenoid bone is removed with a high-speed electric drill down to the level of the superior orbital fissure.1,3,4,6 The dura is opened in a semicircular fashion above the sylvian fissure, and the incision is arched toward the sphenoid ridge and orbit.

Chiasmatic and carotid cisterns must be explored through the fronto-orbital aspect of the frontal lobe before opening the sylvian fissure. The arachnoid between the optic nerve and the internal carotid artery (ICA) must then be opened. These procedures release large amounts of cerebrospinal fluid, which relaxes the brain and facilitates further dissection. Next, the proximal part of the sylvian fissure is opened medially or laterally to the superficial sylvian veins, depending on the variations in the venous anatomy. A simple spreading action with fine bipolar forceps is usually adequate to dissect the fissure. As the sylvian fissure is dissected more deeply, longer fine-tipped forceps are needed. The thickened arachnoid bands must be divided with microscissors where necessary, and dissection continues with a fine suction tip on a moist Cottonoid sponge. Dissection continues to expose the area from the bifurcation of the ICA to 10 to 15 mm beyond the bifurcation of the middle cerebral artery. It also encompasses the anterior third of the insula and the M2 segment of the middle cerebral artery. At this point, the arachnoid fibers between the temporal and fronto-orbital areas are well separated, as are the vessels along the proximal sylvian fissure down to the ICA and its branches. These structures, as well as the position of the oculomotor nerve, the tentorial edge, and the medial basal areas of the temporal pole, may be inspected. The lateral branches of the internal carotid artery (ICA) (posterior communicating artery [PCoA], anterior choroidal artery [AChA] and striocap-sular arteries) and the cortical branches of the M1 segment (temporopolar, anterior, and middle temporal arteries) and its variations are identified, as are the number, position, variation, and courses of the lenticulostriate arteries. The limen insula and the inferior trunk of the M2 segment are observed. The M2 segment curves slightly laterally in the inferior peri-insular sulcus and lies just over the inferior insular vein.

As in many procedures using the pterional approach, the dissection is performed medial to the sylvian vein (on the frontal lobe side) while the surgeon opens the sylvian fissure. However, anatomical variations, such as a large fronto-orbital vein and too many major branches of this vessel, make sacrifice necessary to carry out medial dissection. In such cases, dissection must proceed in an epipial plane, lateral to the sylvian vein along the medial surface of the superior temporal gyrus, until the inferior peri-insular sulcus and the inferior insular vein are reached. Variations of the M1 segment and its lateral branches are also frequently encountered.1 In such cases, the surgeon must find sufficient space to make an incision in the piriform cortex between the temporal arteries by mobilizing them if needed. Significant variations exist among patients regarding the major vascular supply to the amygdala, uncus, hippocampus, and paraHippocampal gyrus.1,4,19,20,24,25,27

The resection of the piriform cortex just anterolateral to the M1 segment and anteroinferior to the limen insula enables the surgeon to reach the amygdala. The superior part of the amygdala is identified a few millimeters under the incision line by its hazelnut color in the white matter. The amygdala must first be removed piecemeal with both a tumor forceps (to gain histological specimens) and gentle suction. During the removal of amygdala, the temporal horn of the lateral ventricle must be entered, allowing a clearer orientation of the hippocampus and the extent of the superior, posterior, and lateral aspects of the amygdala. Removing the amygdala is not usually a bloody procedure. While approaching the ventricular wall, however, particularly in the medial plane, the surgeon must keep in mind the presence of subependymal veins returning from the amygdala. These vessels run subependymally to the atrial vein of the temporal horn, which runs through the choroidal fissure to the basal vein of Rosenthal. Any damage to these veins and their branches may cause torrential, retrograde venous hemorrhage from the basal vein and the vein of Galen. The surgeon should also note that the basal vein may not run in a semicircular fashion around the cerebral peduncle to drain into the vein of Galen but instead may course diagonally over the cerebral peduncle from anteromedial to posterolateral in the direction of the tentorial incisura, draining into the superior petrosal sinus or tentorial veins. The variations in the venous drainage of the insular mediobasal temporal structures, cerebral peduncle, optic tract, and thalamus to the basal vein have been described comprehensively elsewhere.22,29

At this stage of the operation, it is of utmost importance that the optic tract is clearly identified. Thus, great caution must be used when removing the lateral, basal, and cortical nuclei of the amygdala. Care must also be taken not to resect the most medial parts of the amygdala (especially medial and central nuclei) lying superolateral to the optic tract and projecting to the claustrum, putamen, and globus pallidus. After these sections of the amygdala are taken out, the rest of the piriform cortex and the anterior part of the paraHippocampal gyrus are removed subpially. The transparent curtain of pial and arachnoidal membranes near the lateral part of the carotid cistern and the anterior part of the crural and ambient cisterns may be identified readily anteroinferiorly, after subpial resection. After the pia is opened, important anatomical details can be identified, such as the entrance of the AChA to the choroidal fissure along the crural cistern and the optic tract and the basal vein of Rosenthal, which lie medial to the AChA, the cerebral peduncle, the P2 segment of the posterior cerebral artery, and the oculomotor nerve.

To remove the uncus, hippocampus, and parahippocampal gyrus, the surgical microscope must be angled postero-inferiorly. This avenue provides access from the tip of the temporal horn to the trigone and supplies an excellent view of the choroid plexus and of the pes hippocampus. The tela choroidea, the transparent membrane from which the choroid plexus arises, can be isolated by displacing the choroid plexus medially over the choroidal fissure. Through the tela choroidea, important structures such as the AChA, the Hippocampal vein, and ventricular tributaries of the basal vein of Rosenthal can be identified. Subsequently, fine forceps are used to reflect the choroid plexus medially and open the tela choroidea between the choroid plexus and the tenia fimbria. At this point, the hippocampal and uncal branches of the AChA must be coagulated and divided. However, great care must be taken not to injure the main stem of the AChA and its medial branches to the peduncle, optic tract, pallidum, internal capsule, thalamus, lateral geniculate body, and choroid plexus. The surgeon may encounter anatomical variations of the branches to the uncus and amygdala, which may arise separately and proximally from the AChA or even separately from the lateral wall of the ICA or from the M1 segment. Occasionally, these variations arise from the temporal and anterior temporal arteries.4,19,20,24,25,27

As the choroidal fissure along the tenia fimbria is opened, the medial part of the parahippocampal gyrus (subiculum) within the lateral wing of the transverse fissure can be identified. Hippocampal and parahippocampal veins, which run over the subiculum direction, exit the hippocampal sulcus and drain into the basal vein of Rosenthal. They should be isolated from the arachnoidal membranes and meticulously preserved. The hippocampus is supplied by the hippocampal arteries, which lie beneath the veins described previously and enter the hippocampus most often by penetrating the hippocampal sulcus. They usually originate from the P2 segment just proximal to the P2-P3 junction, or from the P3 segment itself, or from branches of the P3 segment and occasionally from the AChA.4,19,20,24,25,27 At this stage of the operation, the hippocampal arteries are coagulated and divided.

The head of the hippocampus-parahippocampal gyrus is transected at the level of the proximal portion of the fimbria and en bloc resection is done. The middle and posterior portions of the hippocampus and parahippocampal gyrus are removed with suction or the ultrasonic aspirator. We prefer this technique instead of en bloc resection of the whole hippocampus-parahippocampal gyrus because it preserves the anterior temporal stem ( Figs. 18.3, 18.4 , and 18.5). The posterior limit of the resection of the hippocampal tail is just at the level of the posterior rim of the cerebral peduncle, some 10 to 15 mm before and inferior to the isthmus cinguli. The resection is performed inferolaterally through the posterior part of the hippocampus-parahippocampal gyrus, in the direction of the collateral sulcus and tentorial edge. Resection continues with forceps and suction along the sulcus in a semicircular fashion within the temporal horn lateral to the hippocampus, then enters the collateral and rhinal sulci. This semicircular resection, 4 to 5 cm long and 5 to 10 mm deep, extends down to the free edge of the tentorium, leaving the fusiform gyrus untouched laterally.

The loops and branches of the temporooccipital trunk arising from the P2-P3 junction can be identified within the collateral sulcus. The branches that supply the parahippo-campal gyrus are coagulated and divided. As the limbic areas are resected, the hippocampal veins are again exposed and coagulated and divided at a proper distance from the basal vein of Rosenthal. Occasionally, bleeding may occur from the pial bed of the resected limbic structures in the cavity; these areas require coagulation with bipolar forceps. Generally, opening the extension of the collateral sulcus provides access to the tentorium some 2.5 cm from its free edge and in its anterior half. In patients with herniation of the medio-basal temporal structures, the mediobasal dissection must be done meticulously because of the potential for damage to the underlying structures. In such cases, staying in the subpial plane allows removal of the parahippocampal gyrus, whereas the P2 segment with its branches, the superior cere-bellar artery, third nerve, and fourth nerve (lying below the tentorial edge) are protected by the pia and a double layer of arachnoid.

During this procedure, retractors should never be placed in the small entrance and the tip of the suction tube, covered with a moist Cottonoid sponge, can be used as a gentle temporary retractor. After careful hemostasis is achieved in the resection cavity and around the middle cerebral artery, the ICA, the AChA, the PCoA, and their branches, the dura is closed with a running suture and the bone flap is replaced in the usual fashion.

After the amygdala and uncus-hippocampus-parahippo-campal gyrus are removed, the neighboring structures, including the superior, middle, and inferior temporal gyri, and the fusiform (lateral temporooccipital) and lingual (medial temporooccipital) gyri remain undamaged. Furthermore, removal of the anterior one third of the hippocampus-parahippocampal gyrus enables the pathologist to carry out scientific studies on resected structures. Because it is less complex and allows the surgeon to preserve the anterior temporal stem, piecemeal removal or subpial suction of the rest of the hippocampus-parahippocampal gyrus is a preferable approach compared with en bloc resection.