Looking back on the history of cerebral aneurysm treatment, surgical managements have improved since Victor Horsley’s ligation of the common carotid artery for an ipsilateral cerebral aneurysm in 1855. Normann Dott first wrapped a cerebral aneurysm in 1933¹¹⁾. Walter Dandy performed the first direct neck clipping with a V-shaped silver clip in 1935^8), and Gazi Yasargil started to use microscopy in the 1960s²⁰⁾. The era of endovascular treatment began when Guido Guglielmi invented detachable coils in 1991^17), and endovascular intervention is accepted as a primary treatment modality for cerebral aneurysms with lower complication rates and satisfactory outcomes compared to surgical clipping^23), To reduce surgical morbidities, much effort has been recently made, such as intraoperative cerebral angiography, intraoperative indocyanine green (ICG) angiography, intraoperative physiologic monitoring, neuroendoscopy, the keyhole approach, and new instrument designs.

The keyhole approach is not so long that Donald Wilson and Mario Brock first attempted a limited craniotomy as a premature step in 1970s^3,37), and John Jane described a modified supraorbital approach with minimal brain retraction in 1982¹⁹⁾. Coming into the late 1980s, the concept and indications of a keyhole approach were established by Josip Paladino and Axel Perneczky^27,30,36). A standard craniotomy supplies a sufficient space for inspection and instruments as well as room for diverse angles of approach towards the target lesion. Meanwhile, a keyhole craniotomy provides an essential space for microscopic viewing and 2 or 3 instruments. A wide range of areas can be approached by varying the microscopic angle of the view; however, only a fixed and limited approach angle towards one target is permitted via the keyhole. According to the Axel Perneczky’s report in 1998^36), the keyhole approach is not limited via a limited corridor but is rather a tailored and adjusted method for minimal exposure and retraction of the brain. Its advantages include minimal brain exposure, excellent cosmetic results, preservation of the surrounding structures, and short procedural time. It also has some disadvantages: less opportunity for changing a plan, weak microscopic illumination, and difficulty in proximal control over the parent arteries. However, these types of disadvantages can be overcome via preoperative planning with imaging modalities, intraoperative manipulation with instruments such as an endoscope and specific devices, and specific facilities such as a hybrid operating system⁹⁾. Keyhole approaches may be still inappropriate in cases with a high probability of massive bleeding and brain swelling; however, the proportion of keyhole surgeries appears to be increasing.

Keyhole approaches include supraorbital, subtemporal, suboccipital, interhemispheric, and transcortical³⁰⁾. The symbolic and representative approach is supraorbital, which is applicable for most anterior circulation aneurysms. A supraorbital keyhole craniotomy exposes the anterior cranial fossa just above the orbit and sphenoid ridge (Fig. 1). It can be classified into 3 types: (1) superciliary (or eyebrow) supraorbital craniotomy (SSO approach)^6,28,36); (2) frontolateral supraorbital craniotomy (FL approach)^3,38); and (3) lateral supraorbital craniotomy (LSO approach)^5,18). When performing the SSO approach, surgeons should consider surrounding structures: supraorbital and supratrochlear nerves at the medial side of the incision, the frontal branch of the facial nerve at the lateral side, and the extent of the frontal sinus. Muscle injury and bleeding are minimal because peeling the temporalis muscle to make the keyhole is sufficient and does not destroy the sphenoid ridge. However, a small craniotomy sometimes makes it hard to retract the frontal lobe and secure a sufficient space for the procedure, forehead hypaesthesia and frontalis palsy temporarily but infrequently occur, and olfactory nerve injury during clipping of the anterior communicating artery (ACoA) aneurysms is not rare. The FL approach is fundamentally the same as the SSO approach; however, FL approach has a lower risk of nerve injury, a higher cosmetic satisfaction and a chance to make a larger bone flap because the skin incision is made just behind the hairline and an interfacial dissection is performed. The LSO approach includes a skin incision just behind the hairline, incision and retraction of the temporalis muscle, and a more laterally located supraorbital craniotomy. Therefore, there exists a risk of muscle problems such as pain, bleeding, swelling and atrophy. However, it is advantageous in cases with inferiorly directed aneurysms and large frontal sinus, and it has a lower risk of olfactory nerve injury. If necessary, the craniotomy can be extended into the middle cranial fossa crossing the sphenoid ridge such that the LSO approach has a wider range of application through medial and lateral sylvian dissection; compared to the SSO and FL approaches, the medial sylvian dissection is the only window towards the aneurysms. Meanwhile, in cases with intradural bony drilling of sphenoid ridge and anterior clinoid process, endoscopic assistance, and bilateral approach via the unilateral corridor, LSO and FL approaches are more advantageous than SSO approach because of the flexibility of craniotomy size. Finally, there are mini-pterional (MP) and mini-interhemispheric (MI) approaches. The MP approach is mainly used for middle cerebral artery aneurysms by focally exposing the perisylvian area^24,34). Via the MI approach, aneurysms at the distal anterior cerebral arteries and ACoAs can be clipped³⁰⁾. The most important aspect in selecting MP and MI approaches is the venous structures because damage to major bridging veins can result in painful complications of venous infarction and haemorrhagic conversion. Thus, preoperative imaging of both the arteries and veins is essential to select an appropriate approach and determine a craniotomy area.

Based on the personal experience during the past 3 years, 377 aneurysms in 280 patients were treated via keyhole approaches and composed 87.9% of all the aneurysms treated. The percentage of patients undergoing the FL, LSO, SSO, MP and MI approaches were 43.8%, 23.8%, 15.5%, 13.4%, and 3.5%, respectively; and the treatment results consisted of complete occlusion in 75.4% of aneurysms, residual neck (remnant <1 mm) in 20.7%, and residual sac in 4.8%. Among 325 unruptured aneurysms in 244 patients, the mortality was zero, and the permanent morbidity was 0.6% per aneurysm and 0.8% per patient with excellent cosmetic results and a short operation time of approximately 150 minutes. Occasionally, an endoscope and mirror were needed because of weak illumination and blind spots beyond the line of the microscopic view, and specific instruments such as devices with tubular shaped and bayonet type of shafts, various designs of aneurysm clips and malleable clip applier with a small head were effective.

Modern neuroendoscopy has some advantages over microscopy—strong illumination, clear depiction in a close-up view and a wide viewing angle. In addition, when using the holding arms, bimanual manipulation is possible^14,29), which allows surgeons to safely and precisely clip certain aneurysms beyond the straight line of the microscopic view³⁹⁾. This usefulness can be highlighted when clipping is performed via the small corridor of keyhole approaches. Sometimes, a micromirror is simple to use; however, the image resolution is low, and fog on the mirror can be problematic. Neuroendoscopy was implemented to treat aneurysms in the early 1990s, and its role has become widespread from assisting microscopy procedures to providing the main view. In selected cases, aneurysm clipping has evolved from endoscope-assisted procedures to endoscope-controlled ones^1,14), and neuroendoscopy is progressively used via the keyhole^13,33) and endonasal^12,15,16,35) approaches as well as conventional approaches. Endoscopic endonasal approaches are thought to be a good alternative for aneurysms located in the medial paraclinoid segment of the internal carotid artery (ICA) and posterior circulations such as basilar bifurcation, as well as those in cerebellar and vertebral arteries, which are limited with conventional surgical approaches³⁵⁾. To date, the smallest endoscope diameter is 2.7 mm, and a rigid endoscope produces the best imaging quality. Neuroendoscopy with softer (fiberoptic and steerable) shafts that have a smaller diameter would be more useful in the near future.

Personally, the author started to use neuroendoscopy after a painful experience during an incomplete inspection around the aneurysm and blind clipping using a keyhole approach. Neuroendoscopy is mainly used in selected cases such as inferiorly directed aneurysms at the distal ICA, superoposteriorly directed aneurysms at the ACoA and middle cerebral arteries, and ICA bifurcations. In total, 9.0% (34 of 377 aneurysms) were clipped using either endoscopic-assisted or controlled keyhole approaches.

ICG fluorescence angiography (ICGA) is a prominent techniques in the surgical treatment of cerebrovascular diseases. Since Raabe et al. introduced this technique to the field of cerebrovascular surgery in 2003^32), microscopic ICGA has been widely used in aneurysm clipping, bypass surgery, arteriovenous fistulas and malformations in the brain and spine. It is especially used in aneurysm surgeries in order to detect and correct any compromise of the perforators and parent arteries and/or incomplete clipping. However, microscopic ICGA systems have some shortcomings for structures beyond the line of the microscopic view and deep-seated structures with weak illumination. Such limitations can be profound during the procedures utilizing the keyhole craniotomy. Therefore, some neurosurgeons from Japan, Germany and Korea (our system) developed endoscopic ICG angiography (eICGA) systems^4,7,21,25). Compared to conventional endoscopy, endoscopic ICGA has some additional advantages for aneurysm clipping in that it can visualize the ICG fluorescence within small perforators, parent arteries and clipped aneurysms beyond the line of microscopic view, detect even faint fluorescence of deep-seated structures, and obtain ICG fluorescent images for a longer duration. Our system has a unique technical superiority over the commercial endoscopic ICGA systems—simultaneous display and real-time merging of both visible light and ICG fluorescence images. Previous systems can alternatively visualize either the visible light or ICG fluorescence images, which is inconvenient and has the risk of violating unseen structures during the fluorescence mode. Recently, we completed a prospective clinical study with our dual channel eICGA system in treating cerebral aneurysms. Our system showed a higher detection rate of branch orifices and the exact clip position than the commercial microscopic ICGA (Fig. 2).