Utilization of endoscopic-assisted techniques in neurosurgery can be classified into three categories based on the stage of the surgery. In the initial stage, the endoscope can be used as a “look ahead” or “look around the corner” to assess key anatomical features or relationships between neurovascular and pathological structures. This is seen in the initial survey of anatomical features in pathologies such as aneurysms, vestibular schwannoma, CPA tumors, and microvascular compression syndromes. Here, one can take advantage of the endoscope’s ability to provide better illumination, higher magnification, and angled viewing. In the intermediate stage, the endoscope may be utilized as the primary form of visualization while dissecting. In some cases, the endoscope is used only for selected portions of the intermediate stage, by which the endoscope can expand the surgical working area of a given approach. For example, angled endoscopes can be used to dissect the lateral IAC in retrosigmoid [33, 34], the acom complex in supraorbital [15], or the clivus in transoral approaches [48–50]. Alternatively, the endoscope may be the only mode of visualization such as in keyhole approaches for intraventricular tumors [54–56], fully endoscopic minimal-access retrosigmoid craniotomy for microvascular decompression or vestibular schwannoma resection [40, 42, 57], or freehand posterior fossa endoscopic dissection [58]. In the final stage, the endoscope can be used as a visual tool prior to closure to assess treatment of the pathological disorder: aneurysm clip position, adequacy of microvascular decompression, or inspection for tumor remnants.

One of the keys to maximizing efficiency and effectiveness in endoscopic-assisted microsurgery is integrating visual information from the microscope and the endoscope such that the transition is seamless and does not disrupt the flow of the operation. The ideal technology is easy to set up, slim, maximizes image quality, and allows simultaneous visualization of both endoscopic and microscopic information with minimal effort from the surgeon. Existing options include separate endoscope stand-alone monitors, mounted endoscopic displays on the wall or the microscope [59], and head-mounted liquid crystal display screen [4]. Various picture-in-picture systems exist that display two-dimensional image feeds onto a single screen that is either stand-alone or head-mounted [6]. Newer systems inject endoscopic information into the microscope oculars with the benefit of maintaining stereoscopic vision in the microscope and maintaining the flow of the operation [60, 61].

Various endoscopes have been developed over the last 50 years. Rigid rod lens endoscopes are available in various lengths, diameters, and viewing angles. Standard viewing angles include 0°, 30°, and 45°. Increasing angles can result in disorientation and require careful manipulation under microscopic visualization since the insertion trajectory is not directly seen [7]. The ideal endoscope is thin, lightweight, sturdy, non-heat generating, and self-irrigating and can be easily integrated with a holder such that the surgeon can seamlessly transition between static and dynamic movement. Endoscopes now utilize high-definition technology, and some three-dimensional endoscope technology exists as well [62, 63]. Flexible endoscopes less than 2 mm in diameter and additional integrated fiberscope viewing dissector options exist as well [13]. Trade-offs must be made between function and optics. Smaller diameter, flexible fiberscopes rather than rigid rod lens, and some built-in irrigation systems can all decrease optical quality.

Safe endoscopic-controlled surgery requires bimanual control for fine dissection. This requires either a second surgeon if using dynamic freehand endoscope movement [58] or an endoscope holder. Various holders exist including holders secured to retractor arms and pneumatically controlled arms [55, 64].

Lastly, keyhole endoscopic-controlled approaches necessitate modified instruments that can work in smaller working channels without obstructing vision or competing with the endoscope for space. Various tubular shaft instruments exist that take up less space when opening and closing [55].

The main forthcoming advancements will be in the development of better technology to work in the current scope of indications and techniques. We will come closer to more ideal endoscopes, seamless visual integration, and more appropriate instruments. Current technology in visualization exceeds the development of endoscopic working instruments such that sometimes we have great visualization without an equivalent ability to work safely. A wider array of instruments that can function in narrow channels will likely be developed that are tailored to the needs for each indication and/or technique. If this is achieved, outcomes will improve, endoscopic-assisted indications will expand, and neurosurgeons will continue to develop more tailored individualized keyhole approaches that are equally effective and minimally traumatic.