perpendicular to and at the level o f the body o f the hippocampus. HCT2 was measured by placing the largest possible circle as a region o f interest within the hippocampus while avoiding boundaries where partial volume effects with CSF might occur. Mean control HCT2 was 102.4 ± 2.8 msec. The upper limit o f the reference range for the control group was taken as 108 msec i.e. 2 SD above mean control HCT2.
Hippocampal volumetry.
Images for hippocampal volumetric studies were obtained using a IR T1-weighted 3-D Magnetisation Prepared Rapid Gradient Echo (MPRAGE) sequence 10/4/200/1
(TR/TE/TI/NEX), flip angle 12 degrees, matrix size 256x256, and 128 sagittal partitions in the third dimension with partition thickness o f 1.25 mm. The MPRAGE dataset was reformatted into 1 mm thick contiguous slices in a tilted coronal plane that
was perpendicular to the long axis o f the hippocampus. The reformatted images were transferred to a SUN workstation and analysed using Xdispim (Plummer, 1992)^^\ Hippocampal boundaries were as described by Watson et al (1992/^^. The first slice was defined as the one in which the fornix was seen in its full profile. A one-in-three sampling strategy was used, whieh was random and systematic, i.e. from the first three slices, one was chosen randomly and from that slice, every third slice was measured systematically. The hippocampal volume (HCvol) was calculated by summing the hippocampal cross-sectional areas and multiplying this figure by the distance between two slices, i.e. 3 mm (Cavalieri’s principle, Gundersen & Jensen 1987)'^^. Mean control HCvol corrected for intracranial volume was 5180 ± 416 mm^. The lower limit o f the corrected control HCvol reference range was defined by 2 SD below the mean control value and was 4348 mm^. Mean (±SD) hippocampal volume ratio (HCVR) was 0.96 ± 0.03. The smallest HCVR o f the controls was 0.92. A cut-off value o f 0.87 was used which is 3 SD below mean control HCVR.
Surgical Technique
Surgical strategies in patients with an epileptogenic region in the temporal lobe range from anatomically standardised operations involving the removal o f 4-4.5cm in the dominant hemisphere and 5-5.5cm in the non dominant hemisphere (Olivier, 1996)^®^, to amygdalo-hippocampectomy (Yasargil et al, 1985)^^^ and radical hippocampectomy (Spencer et al, 1984,1985)^^°'^^* to resections tailored to the extent o f the
epileptogenic zone in the individual patients determined by interictal epileptic discharges from the intra-operative electrocorticogram (ECoG) (Penfield & Jasper,
M ethods Chapter 8
al, 1987)^^^. This diversity stems from variability between patients in the extent o f the epileptogenic region and its proximity to eloquent regions and areas essential for motor and other language functions. When epileptogenic zones and eloquent areas are considered to have little variation between cases (such as cases with hippocampal sclerosis in the non language dominant hemisphere) a resection can be standardised and defined entirely by anatomy. However when the boundaries o f the epileptogenic region are unclear, and possibly involve an eloquent area, the resection is usually tailored to these physiologic findings. Information obtained extra-operatively prior to the surgery such as MRI findings and assessment o f language dominance via the sodium amytal test and intra-operatively, for example the ECoG data, usually combine to determine the type and extent o f the resection in these cases. The majority o f the patients who participated in the studies presented here underwent a standardised temporal lobe resection.
In the standard temporal lobectomy at the National Hospital for Neurology &
Neurosurgery the patient is anaesthetised, intubated, ventilated and positioned supine in the table with his/her head turned to the left for a right temporal lobectomy (vice versa for a left temporal lobectomy). A scalp flap is cut and the temporal muscle is raised and turned down in two separate portions exposing the cranium. A free bone flap is removed and a craniotomy is taken down to the medial cranial fossa. The dura is then exposed and opened and the cortical surface is inspected. Corticography is then carried out. The findings o f the ECoG determine the extent o f the resection (See Figure 8.2). Typically a standard temporal lobe dissection is performed with a
posterior margin o f approximately 5cm for a right temporal lobe resection and 4cm for a left temporal lobe resection. The temporal horn is identified and the lateral
Figure 8.2: Intraoperative view o f the ECoG procedure prior to the surgical resection
\
A hippocampal specimen is usually taken with 2cm o f the body as well as the pes and sent to histology (see below). The dissection may then be extended more posteriorally medially to ensure an adequate resection o f the hippocampus and mesial structures (see Figure 8.3). The dura is then closed in a water tight feshion, the bone flap is replaced and secured with plates. The scalp is then closed in layers with a drain.
M ethods C h ap ters
Figure 8,3: Post resection view o f the cortical surface following an en-bloc temporal lobectomy.