9 Hippocampal Transection

10.1055/b-0040-177290

9 Hippocampal Transection

Michiharu Morino

Abstract

Selective amygdalohippocampectomy and anterior temporal lobectomy are well-established standard surgical procedures for medically intractable mesial temporal lobe epilepsy (MTLE) due to hippocampal sclerosis. However, in cases of MTLE without hippocampal atrophy or sclerosis, that is, magnetic resonance imaging–negative MTLE, amygdalohippocampectomy is associated with postoperative memory function deficit. There is also no hippocampal atrophy or sclerosis in cases of tumor-associated temporal lobe epilepsy with acquired secondary epileptogenesis. New surgical methods that preserve memory function with better seizure control are needed for treatment of such cases of MTLE without hippocampal atrophy or sclerosis. Hippocampal transection (HT) has been developed to overcome this problem. In HT, the hippocampus is not removed; rather, the epileptic longitudinal hippocampal circuits are disrupted by transection of the pyramidal layer of the hippocampus. The author encountered 75 patients with MTLE without hippocampal atrophy or sclerosis treated with transsylvian HT (TSHT). This chapter describes this less invasive surgical technique performed through a transsylvian approach (TSHT) and the memory outcome of 24 cases submitted to TSHT.

9.1 Introduction

Intractable mesial temporal lobe epilepsy (MTLE) due to hippocampal sclerosis can be treated effectively by anterior temporal lobectomy (ATL), which comprises resection of mesial temporal structures and amygdalohippocampectomy.

ATL often damages the anterior bundle of fibers from the optic radiation, causing upper quadrant visual deficits. ATL may damage the language areas located at the temporal tip ¹ and temporal base ² in the dominant hemisphere. In addition, ATL is associated with a risk of memory deficit, especially verbal memory impairment after resection of the language-dominant left temporal lobe. ³ , ⁴ Selective amygdalohippocampectomy is an operative procedure originally developed to spare unaffected brain tissue from resection ⁵ and to minimize memory deficits after temporal lobe surgery. ⁶ , ⁷

However, in the case of MTLE without hippocampal sclerosis, also designated as magnetic resonance imaging (MRI)-negative MTLE, ATL and selective amygdalohippocampectomy may be associated with postoperative memory deficits.

Hippocampal transection has been developed for the surgical treatment of MRI-negative TLE, in which memory function related to the hippocampus is normal. ⁸

Hippocampal transection is based on the principle of multiple subpial transection. ⁹ In hippocampal transection, the hippocampus is not excised; the longitudinal hippocampal circuitry is interrupted by transection of the pyramidal layer of the hippocampus and parahippocampal gyrus.

This chapter describes the surgical technique of hippocampal transection via the transsylvian route ¹⁰ for treatment of MRI-negative MTLE.

9.2 Surgical Procedure

The following section discusses important aspects of the transsylvian hippocampal transection surgical technique, including presentation of intraoperative photographs (▶Fig. 9.1) from a representative case of MRI-negative temporal lobe epilepsy (▶Fig. 9.2) and a diagram of the operation (▶Fig. 9.3; also see Video 9.1).

**Fig. 9.1** **(a–f)** Intraoperative photographs of an illustrative case (left MRI-negative mesial temporal lobe epilepsy) treated with transsylvian hippocampal transection. 1: temporal lobe, 2: middle cerebral artery, 3: hippocampus, 4: amygdala, 5: hippocampal fissure.

**Fig. 9.2** A 30-year-old woman had been suffering from intractable complex partial seizures since the age of 21 years. The seizures occurred at a frequency of twice per month. MRI findings indicated no hippocampal atrophy **(a, b)**. Seizure monitoring was performed with intracranial subdural electrodes placed bilaterally over the temporal lobes. Invasive monitoring showed that seizures originated in the left mesial temporal lobe. She underwent left transsylvian hippocampal transection. MRI T2-weighted image **(a)** and MRI FLAIR slice **(b)** showing no atrophy and no high signal intensity in the hippocampal formation. Postoperative MRI T2-weighted image **(c)** showing no cortical damage and FLAIR slices **(d, e)** showing the transected lines in the left hippocampus (*arrows*).

**Fig. 9.3** Schematic illustration of transsylvian hippocampal transection. **(a)** The route for transection of the parahippocampal gyrus in a coronal view. **(b)** Transsylvian route for hippocampal transection. **(c)** Transection of the pyramidal cell layer. **(d)** Transection of the parahippocampal gyrus. is, innominate sulcus; hip, hippocampus; fim, fimbria; CA, cornu ammonis; phg: parahippocampal gyrus. (Used with modification and permission from the Japan Epilepsy Society. Tenkan Senmon Guide Book. 1st ed. Tokyo: Shindan to Chiryo Sha; 2014:276–277.)

9.2.1 Position, Incision, and Extent of Craniotomy

Both patient position and skin incision are similar to those used in the clipping of cerebral aneurysms involving the anterior circulation. Standard frontotemporal craniotomy can be performed, but a slightly wider exposure of the temporal lobe is necessary to allow intraoperative electrocorticography.

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