**1. Introduction**

The modern endoscope was first introduced by German urologist Maximilian Carl-Friedrich Nitze (1848–1906); in 1879, he introduced the Nitze-Leiter cystoscope, which opened the door for neurosurgeons to adapt endoscopy to their procedures [1]. However, neuroendoscopic techniques for treating hydrocephalus date back to the early twentieth century when, in 1904, Lespinasse, a urologist in Chicago, used cystoscopic cauterization of the choroid plexus to treat two infants with hydrocephalus; one infant died postoperatively, and the other died 5 years after surgery, and the cases were reported publicly at a medical conference in 1910 [2, 3]. In 1922, the neurosurgeon Dandy used endoscopic techniques to treat two patients with hydrocephalus, during which he successfully cauterized and removed the choroid plexus; in an article published in the same year, he described in detail the anatomy of the lateral ventricle, interventricular foramen, transparent septum, and choroid plexus. On this basis, Dandy first proposed the concept of ventricular inspection using an endoscope and the term "ventriculostomy" [4].

The first doctor to complete an endoscopic third ventriculostomy (ETV) was William Jason Mixter [4, 5]; in 1923, he published an article on his successful use of cystoscopy to enter the third ventricle *via* the lateral ventricle foramen of Monro to perform ETV. Subsequently, ETV has been widely used to treat patients with obstructive hydrocephalus. In the same year, Dr. Temple Fay and Dr. Francis Grant developed a method to capture clear black-and-white images of the ventricles using a cystoscope. Their case report of a 10-month-old Italian boy illustrated an attempt to fenestrate the corpus callosum to treat hydrocephalus; however, they were unable to divide the corpus callosum due to a malfunction in the cystoscope they were using. Although the procedure did not go as planned, they concluded that it was safe to visualize the ventricle using an endoscope without causing ventricular hemorrhage or other complications as follows [5–7]:


Subsequently, this technique was widely used in the treatment of hydrocephalus. In 1934, Tracy J. Putnam reported on the technique of neuroendoscopic resection of the choroid plexus [8]. Additionally, he invented the electrocoagulation endoscope and designed a 7-mm ventriculoscope with a rod of optical glass, resulting in a wider optical aperture.

For some time, ETV, or endoscopic coagulation of the choroid plexus, was the mainstay of surgical treatment for hydrocephalus despite the associated high complication rate. In 1935, John Scarff (1898–1979) adapted an enhanced version of Putnam's ventriculoscope with an irrigation system to maintain intraventricular pressure, thereby preventing ventricular collapse [2, 9]. In 1947, H. F. McNickle made a significant contribution to the principle of ventriculostomy itself rather than the technique; he was unconvinced that the procedure should be limited to non-communicating hydrocephalus. Additionally, he argued that choroid plexectomy was not the optimal treatment for hydrocephalus since decreasing the production of cerebrospinal fluid (CSF) would not clear the obstruction itself [10].

At this time, neuroendoscopic technology was imperfect, the technology was limited, and patient disability and mortality rates were high. In view of these, the equipment was not popular in neurosurgery due to surgical discomfort, the risk of eye infection from the endoscope, burns from the heat of the lamp in the endoscope tip, and lower optical quality compared with that of a stereoscopic microscope.

From the 1950s to the 1970s, with the advent of magnetron pressure-regulating drainage tubes, the ventriculoperitoneal (VP) shunt was widely recognized by the medical community. After Putnam's initial success with a glass rod instead of lenses, Hopkins' patents, which used several glass rods to fill the former air spaces between the lenses, enabled the development of today's superb-quality rod-lens endoscopes. In 1959, Hopkins, a professor of physics at the University of Reading in the United Kingdom, made a modern optical fiber endoscope, and with the assistance of Karl Storz, they applied the column-lens system to the endoscope and combined it with optical fiber technology to greatly improve the illumination and resolution of the image [11]. In the subsequent 30 years, neuroendoscopic equipment continued to

improve, and coupled with the promotion of the concept of minimally invasive treatment, neuroendoscopic technology was widely used in the field of neurosurgery, and many new neuroendoscopic techniques were developed.

A breakthrough occurred in Paris with the development of the cold-light generator by Vulmiere and colleagues, which Guiot subsequently used as part of a "universal endoscope" [2, 12]. The first modern description of intraventricular endoscopic biopsy using a ventriculofiberscope was provided by Fukushima et al. in 1973 [13]. Around the same time, in England, Hugh Griffith recommended the endoscopic procedure as a first-line treatment for childhood hydrocephalus. He used Hopkins' rigid endoscope to perform ETV as well as choroid plexus coagulation (CPC) to treat hydrocephalus [14].

Advances in illumination and magnification and the refinement of endoscopic tools coupled with improved anatomical knowledge furthered the development of neuroendoscopy [15]. In 1980, Hoffman et al. reported a series of ETVs using stereotactic guidance, noting that the percutaneous stereotactic-guided approach yielded better results than with open craniotomy [16]. In 1991, Heilman and Cohen conducted endoscopic septostomy [17]; then, in 1993, K. Oka et al. performed both aqueductoplasty and ETV using flexible fiberscopes [18]. Finally, in 1996, Mohanty et al. reported the first foraminal plasty of the foramen of Monro [19].

From the 1960s to the 1990s, numerous articles were published that retrospectively described and summarized ventriculoscopic techniques. Scarff reviewed many cases of hydrocephalus in 1966 and 1970 [20, 21], comparing ventriculostomy, choroidectomy, and shunt surgery and highlighting the many advantages of neuroendoscopic techniques. In 1998, Duffer et al. comprehensively described the application of neuroendoscopic techniques on the treatment of hydrocephalus [22], including indications, the choice of surgical methods, complications, and prognosis. Since then, neuroendoscopic technology for the treatment of hydrocephalus has been widely accepted and popularized in academic circles, and neuroendoscopic technology and its theoretical basis have been progressively improved.

Another development came in 1996 and 2002. Endoscopy was respectively used in conjunction with ultrasound and stereotactic navigation to decrease vascular injury during the procedure, and in 2004, the first instance of a robot being used in ETV was reported [23].

At present, neuroendoscopic technology is widely used in neurosurgery, especially in the management of hydrocephalus, as it facilitates a wide range of possibilities for creating alternative CSF pathways (ETV), reducing CSF production, and restoring physiological CSF pathways. The advantages of endoscopy include minimal invasion, the avoidance of brain retraction, low blood loss, fast operation time, and reduced length of hospitalization. Neuroendoscopy provides a magnified view of the ventricular system from the inside and enhances the resolution of the surgical field. Additionally, it avoids the need to implant foreign bodies and reduces the demand for re-intervention, commonly observed in patients with shunts, with the potential to prevent shunt dependency.
