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Current state and future development of intracranial neuroendoscopic surgery Giuseppe Cinalli†, Paolo Cappabianca, Raffaele de Falco, Pietro Spennato, Emilio Cianciulli, Luigi Maria Cavallo, Felice Esposito, Claudio Ruggiero, Giuseppe Maggi and Enrico de Divitiis
Exp ert o pinion
Sinc e the intro duction of th e m o d ern, sm aller end osc o p es in th e 1960s, neuro end osc o py h as b e c o m e a n exp a nding field of neurosurg ery. N euro endosc o py refle cts th e tend en cy of m o d ern n eurosurg ery to aim towards minim alism; th at is, a c c ess a nd visu alization through th e n arrowest pra ctic al c orrid or a nd m a ximum effe ctive a ction at the targ et p oint with minim al disruption of norm al tissue. Tra nsventricular neuro end osc o py allows the tre atm ent of several p atholo gies insid e the ventricular syste m, such as o bstructive hy dro c e ph alus a nd intra - / p araventricular tum ors or cysts, often avoiding the im pla ntation of extra cra nial shunts or m ore invasive cra nioto mic a p pro a ches. End osc opic endon asal tra nsph enoid al surg ery allows th e tre atm ent of p atholo gies of the sellar a nd p arasellar re gion, with the a dva nta g e of a wid er vision of the surgic al field, less tra um atism of th e n asal structures, gre ater fa cility in the tre atm ent of possible re currenc es a nd re duc e d c o m plic ations. However, a n end osc o p e m a y b e use d to assist microsurg ery in virtu ally a ny kind of neurosurgic al pro c e dures (end osc o p e - assiste d microsurg ery), p articularly in a neurysm a nd tumor surg ery. Basic principles of o ptic al im a ging a nd the physics of o ptic fib ers are discusse d, fo cusing on the neuro end osc o p e. Th e thre e m ain ch a pters of neuro end osc o py (tra nsventricular, endon asal tra nsph enoid al a nd end osc o p e - assiste d microsurg ery) are reviewe d, c onc erning o p erative instrum ents, surgic al pro c e dures, m ain indic ations a nd results.
Five -ye ar view
Expert Rev. Med. Devices 2(3), 351–373 (2005)
C O NTENTS Principles of optic al im a ging Equip m ent N euro end osc o pic pro c e dures & indic ations The role of the end osc o p e in pituitary surg ery The role of the end osc o p e c ouple d with the microsc o p e in tra dition al microsurgic al pro c e dures Patholo gies a m en a ble for the end osc o p e - assiste d te chnique
Key issues Referenc es Affiliations
†
Author for correspondence Santobono Children’s Hospital, Via Gennaro Serra n. 75, 80132 Naples, Italy Tel.: +39 335 684 5214 Fax: +39 081 220 5660
[email protected] KEYWORDS: aqueductal stenosis, endoscopeassisted microsurgery, endoscopic third ventriculostomy, hydrocephalus, neuroendoscopy, pituitary surgery
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Endoscopic ventricular navigation has fascinated neurosurgeons since the beginning of the 20th century, especially in the preshunt era, when treatment of the hydrocephalus was largely disappointing. In the 1910s, Lespinasse, a urologist from Chicago [1], and 1920s, Dandy [2], used cystoscopes for choroid plexus fulguration and avulsion, as extreme attempts to control hydrocephalus in their patients. The first report of an endoscopic fenestration of the floor of the third ventricle to bypass an aqueductal stenosis (third ventriculostomy), was that of Mixter [3], who in 1923 used a urethroscope to enter the third ventricle through the foramen of Monro. The opening of the floor into the interpeduncular cistern was performed by puncturing the floor of the third ventricle with a sound.
10.1586/17434440.2.3.351
Since the introduction of valvular shunts in the 1950s to carry the cerebrospinal fluid (CSF) from the ventricles to the venous circulation [4], the treatment of hydrocephalus has become more efficient, with very low short-term mortality and morbidity. Therefore, endoscopy was virtually abandoned, essentially owing to the large size of the available scopes and the poor quality of illumination and magnification. After more than 50 years of widespread use of CSF shunts, the limits of shunt surgery have become well known. Above all, shunt surgery exposes the patient to several complications such as shunt obstruction, fracture, disconnection, migration, malposition, slit ventricle syndrome, craniosynostosis, shunt infection, bowel perforation, subdural
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hematomas, sylvian aqueduct syndrome, chronic tonsillar herniation, renal and cardiopulmonary failure with ventriculo–atrial shunts and nonresorptive ascites with ventriculo–peritoneal shunts [5]. All are potentially lethal and are burdened by high morbidity; in addition they often require surgical revision of the system. Although improvements in the material and hydrodynamic characteristics of the valves have been made, the complications occur so frequently that the possibility that a shunt will still be working 2 years following the insertion has been estimated at approximately 50% in prospective studies [6]. Retrospective studies suggest that the likelihood of it still being functional 10 years after implantation is approximately 15% [7]. Therefore, in the last few decades, the impulse to obtain alternatives to shunting has been renewed. The modern, smaller endoscopes are derived from a solid lens system designed by a British physicist, Harold Hopkins, in the late 1960s. This scope was able to afford improved illumination, resolution and a larger field of view [8]. In 1973, Fukushima and colleagues introduced the modern endoscope for endoscopic biopsy of intraventricular lesions, cyst fenestration and treatment of hydrocephalus [9]. However, only during the second half of the 1990s, with the publication of results in large clinical series [10–14], did neuroendoscopes become widely accepted in the neurosurgical community and are now considered as the first-line treatment of several pathologies of the central nervous system (CNS), such as obstructive hydrocephalus, intracranial cysts, intraventricular septations and, in selected cases, intraventricular tumors. Furthermore, indications for neuroendoscopy have still been evolving, as the result of the tendency of modern neurosurgery towards minimalism [15]. This is based on two concepts: access and visualization through the narrowest practical corridor; and maximum effective action at the target point with minimal disruption of normal tissue [15]. Prin ciples of o ptic al im a ging
The neuroendoscope is the only instrument that allows access to deep anatomic structures in a minimally invasive manner. It allows the illumination of deep, hidden structures of the brain and subsequently to transmit clear images to the surgeon. This is possible due to the physical characteristics of light. In a A
homogeneous medium the light is transmitted in a straight fashion (light ray) at a constant velocity. The velocity of the light is dependent on the medium that it crosses: in the perfect vacuum, the velocity is approximately 3 108 m/s (c), while it decreases in transparent media other than a vacuum, according to the index of refraction of the medium. This is an intrinsic characteristic of each medium (such as air, water and glass) and is defined as the ratio between the velocity in the vacuum and the velocity in the medium (n = c/v). A light ray crossing surfaces separating media with different indices of refraction is deviated, so that the angle of incidence (angle between the penetrating ray and the perpendicular to the surface) is different from the angle of refraction (between the emergent ray and the perpendicular to the surface), according to the law of Snellius–Descartes:
n 1 sin
1
= n 2 sin
Where n1 and n2 are the indices of refraction of the medium 1 and medium 2; 1 and 2 are the angles of incidence and refraction. This phenomenon is known as refraction and forms the basis for the function of lenses and prisms [16]. Endoscopes and telescopes are examples of optical systems composed of multiple lenses. To understand the functioning of these systems some concepts must be explained. In convergent lenses (biconvex lens, planoconvex and convex meniscus), the light rays parallel to the optic axis are brought to a common point on the axis. This is the ‘focal point’ of the lens and the distance between the focal point and the lens is the ‘focal length’ (FIGURE 1A). In divergent lenses (biconcave, planoconcave and concave meniscus), the light rays parallel to the optic axis are deviated away from the optic axis and appear to originate from a common point. This ‘virtual point’ is the focal point of a divergent lens and the focal length is the distance between this point and the lens (FIGURE 1B). Therefore, lenses may differ in size and focal length. Conventionally a positive focal length indicates a convergent lens, while a negative focal length indicates a divergent lens. The simplest device is composed of two elements: an objective lens and an ocular (or eye) lens. The image formed by the first lens (internal image) is the object for the second lens Divergent lens
B
Convergent lens
2
Light rays
Light rays Focal point
Optic axis Focal point
Optic axis
Focal length
Focal length
Figure 1. (A) Example of a convergent biconvex lens. (B) Example of a divergent biconcave lens.
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Intra cra nial n e uro e nd osc o pic surg ery
(FIGURE 2).
In this model the image is magnified, but if a bundle of light reaches the objective lens at a greater angle, it can miss the ocular lens (FIGURE 3A). To increase the field of view, field lenses can be placed exactly on the internal image, to bend the light rays toward the optical axis so that they pass through the ocular lens (FIGURE 3B). However, in the case of endoscopes, the image must be carried a significant distance from the objective lens. Furthermore, the diameter of the lens is limited. Therefore, these devices consist of a series of relay stages between the objective lens and the ocular lens (FIGURE 4). Each relay is composed of a field lens and a relay lens. The number of relay stages depends on the length of the instrument. The power of the field lens is chosen so that it forms an image of the object on the subsequent relay lens, and so on [16,17]. This ‘train of lenses’ mounted in a cylinder was the first endoscope used by Max Nitze in 1879 [16]. However, this device had several disadvantages: logistic difficulties with the optimal designing and mounting of the lenses, especially with the decreasing dimensions of the lenses, required in a neuroendoscope; possible malfunction due to tilting of only one lens and the reduction in brightness due to the presence of multiple relay lenses interspersed with air spaces [16]. Hopkins, who in 1960 described a new lens system, was able to increase the light transmission and has provided the greatest contribution to the technology of neuroendoscopes. Hopkins used a series of glass rods interspersed with small air gaps, which is the exact opposite of the design that has been used since then (FIGURES 5 & 6). This technique forms the basis of most modern endoscopic systems and bears his name [18,19]. This technology became feasible due to the development of a new type of glass that had no uniform index of refraction. The index of refraction varied with the radial dimension of the light. Thus, the rays of light curve towards the region of higher index of refraction, usually in the center of the ‘rod lens’. The light rays are transmitted in a sinusoidal path: a short rod lens functions as the equivalent of a single lens system, while a longer rod can function as a periscope or endoscope with multiple relay stages (FIGURE 5) [16]. The variation of the index of refraction can be achieved by a high-temperature ion exchange process: this effect forms the basis for the Ocular lens
Objective lens Optic axis
Object
Foc Fob
Fob
Internal image
Final image (virtual)
SELFOC® lens (NSG America Inc.) [16]. In the device designed by Hopkins, the conventional field and relay lenses are replaced by SELFOC rods (FIGURE 6). Further reduction in light loss is achieved through the coating of the glass surfaces with an ultra-thin layer of magnesium fluoride. This layer markedly decreases the reflection and improves the optic characteristics of endoscopes and cameras [18–20]. Physics of o ptic fib ers
When light rays travel from a medium with a higher index of refraction towards a medium with a lower index of refraction, the refracted rays are deviated away from the perpendicular of the separating surface according to the law of Snellius–Descartes. By increasing the angle of incidence, the angle of refraction tends to approach 90°, so that the refracted rays travel parallel to the surface. In cases of further increases of the angle of incidence, the light is not refracted, but totally reflected. The angle of incidence at which the refracted angle is 90° is known as the ‘limit angle’ and depends on the indices of refraction of the two media. It can be calculated with the law of Snellius–Descartes, where 2 = 90° (sin 90° = 1) and n1 > n2: sin
1
= n2/n1
When the angle of incidence is higher than the limit angle, the light rays are not refracted but are all reflected. This principle is the basis of fiber optic technology. Each optic fiber consists of a central core of silica glass with a high index of refraction coated by a cladding of glass with a lower index of refraction, so that the light rays are totally reflected at each interface of core-cladding, and can be transmitted in a zigzag fashion for long distances (FIGURE 7). Bundles of optic fibers can be used to transmit light in the surgical field. Optic fibers arranged in a coherent fashion can transmit images to the eye of the surgeon or to a monitor. In a fiber optic endoscope (fiberscope) the images formed by the objective lens are relayed to the ocular lens by a fiber optic bundle. The resolution of fiberscopes is proportional to the number of fibers (pixel-fibers) in the endoscope: most modern fiberscopes contain a minimum of 10,000 pixel-fibers, some as many as 30,000. More fibers provide images with sharper resolution [18]. Due to the nature of the optic fibers that are very thin and can be flexed without breaking, the fiberscopes can be rigid or flexible. However, rigid fiberscopes allow the presence of more pixelFoc fibers than flexible–steerable fiberscopes, allowing for better quality images. Equip m e nt
Figure 2. Two-element optic device. Fob: Focal point of the objective lens; Foc: Focal point of the ocular lens.
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According to their technology, endoscopes are classified as fiber optic endoscopes (fiberscopes) and rod lens endoscopes. The endoscopes specifically designed for neuroendoscopy can be classified into four types:
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A
Bundle of light
Objective lens
Objective lens
Ocular lens
Optic axis
B
Bundle of light
Field lens
Ocular lens
Optic axis
Figure 3. (A) A bundle of light that reaches the objective lens at a too great angle can fail to reach the ocular lens. (B) A filed lens placed on the internal image may bend the rays towards the optical axis so that they pass through the ocular lens.
• Flexible fiberscopes • Steerable fiberscopes • Rigid fiberscopes • Rigid rod lens endoscopes
in cases of multiloculated hydrocephalus, when the ideal position of the catheter should be across the septations. Their limitation is the absence of a working channel. Ste era ble fib ersc o p es
These endoscopes have different diameters. Therefore, optical quality, number and diameter of working channels vary according to size. The choice between them should be made on the basis of indication of surgery and personal preference of the surgeon.
Thanks to the properties of optic fibers, steerable fiberscopes have a tip that can be oriented up- and downwards. The angle of bending varies in the different models; usually it is at least 260° (-100°/+160°) (TABLE 1). The surgeon may modify the orientation of the optical fibers but also of the working channel, allowing the instruments to reach all of the structures visualFlexible fib ersc o p es ized. This is the only system that makes looking and working Flexible fiberscopes have a very small diameter (
