3.1 Introduction

General principles of indirect bypass surgery are very unique and specific for moyamoya disease. This surgical procedure only requires the attachment of the vascularized donor tissue onto the surface of brain. Gradual, but steady neovascularization occurs between these tissues in moyamoya disease. Nowadays, indirect bypass procedure is indicated almost only for moyamoya disease, although there are a small number of reports demonstrating that indirect bypass is also effective for other disorders, including atherosclerotic cerebrovascular diseases and spinal cord injury. In this chapter, the author describes history, pathophysiology, and concept of surgery of indirect bypass for moyamoya disease.

3.2 History and Initial Description

Fig. 3‑1 demonstrates the history of main surgical procedures for indirect bypass. For these 40 years, various methods for indirect bypass have been reported for patients with moyamoya disease, including encephalo-duro-arterio-synangiosis (EDAS), ¹ encephalo-myo-synangiosis (EMS), ² encephalo-myo-arterio-synangiosis (EMAS), ³ encephalo-duro-arterio-myo-synangiosis (EDAMS), ⁴ encephalo-galeo-synangiosis (EGS), ⁵ and dural inversion. ⁶ These procedures have been developed to provide collateral blood flow mainly to the territory of the middle cerebral artery (MCA) and useful to reduce or resolve ischemic attacks. However, it is well known that a certain subgroup of patients with moyamoya disease does not respond to these surgery and experiences ischemic attacks of the bilateral legs and/or cognitive dysfunction probably due to persistent ischemia in the territory of the anterior cerebral artery (ACA). Several procedures targeting the ACA territory have been reported later. Thus, Kinugasa et al inserted the pedicle of the galea on both sides into the interhemispheric fissure in addition to EDAMS (ribbon EDAMS). Kawaguchi et al developed multiple burr hole surgery to induce neovascularization through indirect bypass, using one to four burr holes. ⁷ Yoshida et al inserted the dural pedicles into the epiarachnoid space to enlarge the revascularized area around craniotomy. For the same purpose, Kim et al also developed EDAS with bifrontal encephalogaleo(periosteal)synangiosis. ⁸ Subsequently, Kamiyama and colleagues developed STA–MCA/ACA anastomosis and pan-synangiosis, which consists of EDMAS and EGS for the MCA and ACA territories through two different craniotomies, respectively. ⁹ Kuroda et al further advanced indirect bypass procedure that can provide collateral blood flow to the whole territory of the internal carotid artery (ICA). For this purpose, a large frontal pericranial flap was employed to widely cover the frontal lobe through one craniotomy. ^{10 ,​ 11}

Alternatively, the omentum has been employed as a donor tissue for indirect bypass and shown an aggressive neovascularization into the brain, but there are no reports on omental transplantation for these 20 years probably because of its invasiveness. ^{12 – 16}