12 Posterior Quadrant Resections

10.1055/b-0040-177293

12 Posterior Quadrant Resections

Marcelo Volpon Santos and Helio R. Machado

Abstract

Posterior quadrant epilepsy represents a significant subset of epilepsy patients who can improve greatly if treated surgically, both regarding seizure control and neurocognitive outcome. Alongside other disconnective procedures, posterior quadrant resections can be offered to such patients, especially if they have a localized lesion and a posterior focal electroencephalographic pattern. This chapter will describe the techniques and surgical anatomy involved in cortical excision in the posterior quadrant, which includes the temporal, parietal, and occipital lobes.

12.1 Introduction

The term “posterior quadrant epilepsy” refers to a number of epileptogenic pathologies that affect the occipital, parietal, and temporal lobes in conjunction. It is clearly not unusual in pediatric epilepsy patients: up to 22% of them have some form of multilobar pathology, and lesions involving the posterior quadrant comprise 57% of this population. ¹ Refractoriness to medical therapy will ensue in a significant subset of these patients, who, thus, become candidates for surgical treatment. ² Although hemispherectomy is the surgical procedure of choice in patients with clear hemispheric disease and well-defined motor deficits (hemiplegia), ³ posterior quadrant surgery sparing the functional motor cortex is an option that needs to be considered in patients with residual or even good motor function with subhemispheric involvement. ⁴

Our own experience demonstrated that this surgical modality (posterior quadrantectomy) yields excellent results concerning seizure control: of 44 patients referred for posterior quadrant surgery, 28 (65.1%) became seizure free postoperatively. ⁵ Recent studies published in the medical literature further corroborate these data, including neurodevelopmental outcomes. ⁶ , ⁷ , ⁸ Therefore, correct identification of the epileptogenic zone in the posterior cortex and a meticulous surgical technique are of paramount importance for the success of these procedures.

Currently, there is a trend toward disconnective surgery overresective procedures, as the former seems to reduce complication rates while achieving equivalent seizure outcome. ⁹ The presence of a large cavity postoperatively predisposes the accumulation of blood degradation products and thus increases the risk of hemosiderosis and hydrocephalus, due to impaired cerebrospinal fluid resorption, along with other complications. On the other hand, posterior resections yield wider exposure of the affected area and better understanding of the intraoperative anatomy, which can be quite distinct and challenging. For instance, the occipital pole is usually abnormally bigger and distorted, and might even invade or extend to the contralateral side. Likewise, the temporal and parietal lobes are enlarged; vascularization of these regions may be aberrant, which poses a greater risk of intraoperative hemorrhage.

It is important, therefore, to underline some technical differences between posterior quadrant resections and its disconnective counterpart, which is described elsewhere on this book. For resective operations, a larger skin incision is required, alongside a larger craniotomy, which needs to expose the whole extent of the midline, occipital pole, and temporal base. In these regions, there is usually a great number of bridging veins connecting the uppermost limits of the parietal and occipital lobes with the sagittal sinus; the base of the temporal and occipital lobes can also harbor such venous structures, draining into the transverse and sigmoid sinuses, whose dissection must be careful and meticulous.

12.2 Indications for Surgery

Patients with drug-refractory epilepsy who have focal seizures with unilateral temporo-parietal-occipital (TPO) onset or generalized seizures with asymmetrical clinical or electroencephalographic (EEG) features suggesting a focal origin in the TPO region are suitable candidates for TPO resection. ² In addition, factors that strongly favor TPO surgery are the presence of a unilateral posterior quadrant lesion identifiable on magnetic resonance imaging (MRI) scans or functional imaging and absence of hemispheric lesions (which would require hemispherotomy). ² , ¹⁰ , ¹¹ In cases without temporal lobe involvement (pure parieto-occipital dysplasia, for instance), there is no need to approach the temporal structures, or to continue surgery with the temporal steps described below. In such patients, a single parieto-occipital resection is sufficient.

12.3 Surgical Technique

12.3.1 Temporo-Parieto-Occipital (Posterior Quadrant) Resection

The area to be resected must encompass the temporal, parietal, and occipital lobes, so that the entire epileptogenic zone or lesion is excised. In order to achieve this goal, strict adherence to the following steps is quintessential.

Electrocorticography (ECoG) and neuronavigation are valuable surgical adjuncts and are routinely used. Electrical mapping of the posterior cortex often does not change the initial surgical strategy but helps in defining resection limits and confirms information obtained from preoperative noninvasive methods; it also has prognostic significance. In addition, both ECoG and navigation allow for precise identification of the central lobe, which consists of the motor and sensory gyri, and is a paramount landmark that must be spared. Correlation of the MRI scans provided by stealth systems can help overcome the difficulty in identifying the correct anatomy in such patients, who might have abnormal gyral patterns. Also, the motor cortex can be better delineated intraoperatively by electrical stimulation with increasing pulses of 0.5 up to 12.5 mA.

The procedure is performed under general anesthesia, with the patient’s head fixed on a three-pin clamp, and rotated to the contralateral side. Small children might need to be positioned on a padded horseshoe headrest; further fixation with tapes is recommended. A soft pad is placed under the ipsilateral shoulder to avoid excessive stretching of the cervical muscles. The skin incision is a barn-door–shaped one, starting just anterior to the tragus and extending perpendicularly along the coronal suture to the midline, where it turns backward to reach the area of the superior occipital protuberance (▶Fig. 12.1). Elevation of the skin flap can be performed subperiosteally, including the temporalis muscle, and maintaining its vascular supply. A large craniotomy must be fashioned, using three to four burr holes: one in the temporal basal region, another one on the lower and posterior aspect of the lambdoid suture, and one or two burr holes just lateral to the sagittal suture. Additional burr holes might be performed at the surgeon’s discretion; it is extremely important to avoid injury to the sagittal and transverse sinus and consequently copious bleeding, which can be done by following strictly the anatomical and neuronavigation parameters.

**Fig. 12.1** A case of a 6-year-old girl with seizure onset at 1 year and 6 months of age, characterized by infantile spasms. She also had a left facial angioma and was diagnosed with Sturge–Weber syndrome. She had no motor deficits and was refractory to antiepileptic medications. (**a,b**) Preoperative T2-weighted MRI scans showing a right parieto-occipital leptomeningeal angioma (white circles). (c) Skin incision (“barn-door” shape). The extensions of the electrodes have been externalized anterior to the coronal incision. (**d,e**) Intraoperative photographs depicting the posterior angioma and the position of subdural grids. (f) Skull X-ray and (g) cortical map after grid implantation, showing the ictal-onset zone (*red rectangle* on f) and its relation to the motor cortex. (h) Intraoperative picture and (i) postoperative T1-weighted MRI scan after resection of the posterior quadrant without including the temporal lobe which was not involved. The patient recovered well and is classified as Engel I over a 5-year follow-up.

Dural opening must provide wide visualization of the temporal, parietal, and occipital lobes, including the floor of the middle fossa and the occipital pole. Identification and preservation of the central lobe, consisting of the precentral (motor) and postcentral (sensitive) gyri on the lateral surface, is fundamental. Likewise, the sylvian superficial venous complex should be properly identified to avoid inadvertent venous injury. If any of these veins crosses the cortical incisions (which is frequently the case for the inferior anastomotic vein of Labbé), they should not be coagulated or sectioned, but rather preserved; dissection needs to be performed underneath and around them. Cortical incision and resection starts either on the temporal or the parietal area. It is our practice, in accordance with other authors, ⁴ to start with a temporal lobectomy. A cortical incision over the superior temporal gyrus (T1) is fashioned, 5 cm posterior to the temporal pole, parallel to the sylvian fissure, up to the vein of Labbé. Subpial dissection is taken down to the limen of the insula anteriorly and exposing the circular cistern and lower aspect of the insula.

At this point, a posterior diagonal incision is made from the posterior tip of the T1 incision down to the temporal base, across T2 and T3 and reaching the collateral sulcus inferiorly. White matter dissection is deepened until the temporal horn of the ipsilateral ventricle is entered; this dissection is then extended anteriorly and inferiorly, resulting in excision of the neocortical temporal lobe. Further aspiration of the white matter underneath the collateral sulcus allows for resection of the parahippocampal gyrus and is important is isolate the hippocampus, which will later be excised en bloc. To avoid injury to the third cranial nerve and branches of the posterior cerebral artery, the surgeon must visualize the free edge of the tentorium and protect it with a cotton Pattie.

After the subpial dissection of T1 at the sylvian fissure has already identified the temporal stem and middle cerebral artery, the amygdala can be seen in the anteromedial part of the opened temporal horn and resected, along with subpial aspiration of the uncus. The roof of the temporal horn must be preserved to avoid injury to the basal ganglia, and this is achieved by following the orthogonal plane of the sylvian fissure dissection. The fimbria of the hippocampus is identified and dissected in an anteroinferior orientation, allowing for resection of the hippocampus until the level of the trigone.

The next step is the parieto-occipital resection, which starts with an incision just posterior to the central lobe, from the posterior and uppermost end of the temporal resection, up to the midline. Excision of the posterior parietal and temporal opercula ensues, and dissection is taken down to the lateral ventricle, centered at the atrium, at a plane behind the thalamus. The falx must then be found superiorly and followed until the posterior corpus callosum is seen, when disconnection of the splenium is performed. From this point, any posterior parts of the hippocampal formation or fornix can be removed, and complete disconnection is confirmed by visualization of the contours of the tentorial incisura. Lastly, all bridging veins, especially over the upper medial parieto-occipital aspect, and branches of the posterior cerebral artery in the medial surface are cautiously coagulated, completing isolation of the parieto-occipital bloc, and the specimen is removed.

In cases which do not require removal of the temporal lobe, pure parieto-occipital removal is achieved using the same steps mentioned earlier, with the exception that the lateral cortical incision, instead of being continuous with the temporal one, continues straight downward to the temporo-occipital fissure (▶Fig. 12.2). Once the inferolateral aspect (below the level of the sylvian fissure) is reached, the basal dissection is extended perpendicularly, with the free edge of the tentorium as a reference. Large temporo-occipital veins draining into the transverse and sigmoid sinuses are frequently found, and must be carefully coagulated.