Historical Background
Until the 1960s, neurosurgical procedures for brain tumours were performed with the naked eye and head-mounted lights or glasses with magnifying lenses. The introduction of the surgical microscope in neurosurgical practice provided magnification and coaxial illumination, even in the depth of the brain. This new standard of vision and the evolution of instruments and techniques of haemostasis allowed the development of less invasive surgical approaches through smaller craniotomies. However, the microsurgical operative field is limited to structures directly in line with the microscope, which can be angled to a limited degree to uncover hidden corners and craniotomies. Despite limitations of microscopes per se, in the 1960s Guiot performed endoscopic explorations of the ventricular cavities and of the sellar area,¹ and in the 1970s Apuzzo instigated the use of a side-viewing telescope during conventional microsurgical procedures to highlight hidden intracranial structures.² A new dimension in the field of intra-operative visualisation had unfolded. However, at that time the concept of multimodality imaging was in its infancy and, due to technical limitations, was performed by only a few neurosurgeons. Consequently, this exciting field of research was developed and superior optical systems provided superior illumination and image quality.

The 1980s saw the dawn of a new era in diagnostic imaging: computed tomography (CT) and magnetic resonance imaging (MRI), which could encounter smaller lesions and better differentiate between tissues, thus encouraging the reduction of the conventional surgical approaches to their essential parts. Indeed, the term ‘minimally invasive neurosurgery’ refers to the technique of exposing the surgical target with minimal exposure and manipulation of the surrounding brain tissue, which can also include a keyhole craniotomy. Perneczky made major contributions to this concept, which would guide further developments until the present day. In the 1990s, profound changes took place: the limits of microscopic visualisation were overcome when Perneczky proposed the endoscope as a further visualising tool during microsurgical procedures and reported encouraging early results of endoscope-assisted approaches to intracranial lesions.^3,4 The introduction of the endoscope through limited surgical corridors enhances the visualisation of structures that otherwise would be hidden by the operating microscope (endoscope-assisted technique); alternatively, the endoscope can be be used as the sole visualising tool during the whole procedure (endoscope-controlled technique). Since the 1980s, several ear, nose and throat (ENT) surgeons have pioneered functional endoscopic sinus surgery,^5–7 thus encouraging the pure endoscopic endonasal approach to the sella. Jho and Carrau developed this technique,⁸ followed by another team.⁹ These experiences supported a continued structured evolution of the endoscopic technique: new skills have been transferred across subspecialities,⁹ new instruments have been designed,¹⁰ new image-guidance systems have been developed^11,12 and new surgical corridors have been defined,¹³ thus revealing the way to the pure endoscopic approach to lesions affecting the skull base, the cerebellopontine angle and the ventricular and paraventricular area.