36 Use of Fluorescence in Neuroendoscopy
Little is available in the literature on fluorescence visualization of normal intraventricular structures,1,2,3 and the relevance of additional data for safer endoscopic procedures as yet remains an unexplored field. In fact, although the international anatomical nomenclature committee identifies ~ 70 structures, only ~ 15 of these are usually considered by neurosurgeons for safely navigating within the cerebral ventricles during the most common neuroendoscopy procedures. In most cases of hydrocephalus, the anatomical landmarks of the third ventricle floor are well defined and immediately recognizable. However, in complex situations caused by tumors, hemorrhages, or meningitis, ependymal reactions may affect normal anatomy.4,5 Accordingly, additional anatomical landmarks would be more than desirable. In this perspective, fluorescein-enhanced visualization of otherwise not visible normal intraventricular structures can certainly be considered a new way to approach neuroendoscopic procedures. Fluorescein sodium, due to its specific properties, is a well known tracer of blood–brain barrier (BBB) disruption in animal models.6 It is largely applied in retinal fluorescein angiography, a consolidated ophthalmologic procedure that has been in use for more than 70 years; since the early 1960s, it has become a common investigation technique, mainly used for the diagnosis and treatment of retinal disorders.7 Fluorescein was adopted in neurosurgery as early as 1947, as a localizing tool in the surgical removal of high-grade gliomas.8,9,10,11 Its application was soon abandoned since the fluorescence enhancement could extend far from the tumor in the edematous normal tissue deprived of its BBB. This issue has been recently revisited in microsurgery,12,13 while in endonasal and ventricular endoscopy attempts to differentiate normal from pathologic tissue have been done using indocyanine green (ICG) fluorescence14,15 and 5-aminolevulinic acid (5-ALA)-induced fluorescence.16 Application of angiofluorescein in ventricular endoscopy is, instead, quite recent, and also allows for enhancement of normal intraventricular structures, differently from ICG and 5-ALA: in this way Kubo in 2004 described for the first time the visualization of median eminence (ME) and organum vasculosum laminae terminalis (OVLT) under fluorescein enhancement during an endoscopic procedure for hydrocephalus (Video 36.1).1
36.2 Operative Technique and Anatomical Description
Angiofluorescein-enhanced neuroendoscopy can be prospectively applied in all ventricular procedures, and is generally performed under general anesthesia. A steerable fiberscope (external diameter 4 mm or less) must be equipped with a dual observation mode for both white light and fluorescence under blue light excitation.2,3 The fiberscope is managed with a freehand technique. The surgical approach is generally precoronal through a 12-mm bur hole 2 cm lateral to the midline. Lateral ventricle cannulation is achieved through a semirigid 14-Fr peel-away cannula. In all endoscopic procedures, after the preliminary inspection of the ventricular cavities with the standard white-light mode, the endoscope is kept outside the ventricle; a 10 mg/kg dose of fluorescein sodium, according to the protocol used by ophthalmologists for retinal angiography, is administered intravenously to the patient by the anesthesiologist. The ventricles are then endoscopically inspected again using the fluorescence mode (blue light). Every time an adjustment of the position of the endoscope is required, white-light mode is set to avoid any damage to the ventricular walls.
The use of angiofluorescence in ventricular endoscopy allows the identification of additional landmarks and structures that are not visible in white light, in particular, the circumventricular organs (CVO) (Video 36.1), subependymal microvascular network (SMV), special characteristics of choroid plexus, and intraventricular-tumor features.
36.2.1 Choroid Plexus
Approximately 20 seconds after intravenous injection of fluorescein sodium, the choroid plexus of the lateral ventricle glows green when visualized in blue light (Fig. 36.1). Plexus fluorescence is very high and uniform along the entire structure. A few minutes later, the dye permeates through the plexus barrier and secretion of fluorescein into the cerebrospinal fluid (CSF) is clearly visible with a faint cloud-like appearance in the CSF (Fig. 36.2).
36.2.2 Circumventricular Organs
Circumventricular organs are the most relevant findings during endoscopic navigation in blue light. They are highly vascularized structures located around the third and fourth ventricles and characterized by the lack of an intact BBB.6,17,18 These midline specialized areas are strategically positioned at the entrance and exit of cerebral ventricles and are points of communication between the blood, brain parenchyma, and CSF. Common features described by Duvernoy and Risold6 are the presence of fenestrated capillaries and loosely apposed astrocytic processes that give rise to relatively large perivascular spaces allowing the passage from the blood of large molecules such as peptides and polar substances without the need for specialized transport. Neurons located in CVOs are directly exposed to peripheral signals and can sense the concentrations of different compounds. They play a critical role in sodium–water balance, cardiovascular regulation, and energy metabolism.6,17 They are also involved in complex mechanisms and reflexes such as vomiting,19 fever, and other responses to potentially noxious stimuli. CVOs include the pineal gland, median eminence (ME), subfornical organ, area postrema (AP), subcommissural organ, and organum vasculosum laminae terminalis (OVLT) (Video 36.1). The choroid plexus of the lateral, third, and fourth ventricles share the same properties but are considered secretive rather than sensory organs. The ME seems to be mainly involved in the regulation of endocrine functions,3,17,18 while the OVLT is potentially involved in the systemic homeostasis of fluids and ionic balance, in blood pressure and, perhaps, in inflammation processes.17,18,20 The functions of the AP, mainly demonstrated in animals, include fluid and electrolyte balance, cardiovascular regulation, energetic homeostasis preservation, and triggering of emesis.21,22,23,24,25,26,27,28,29,30
Once the endoscope enters the third ventricle through the foramen of Monro, the ME is clearly visible in all patients. It glows green ~ 30 to 40 seconds after fluorescein injection. The ME is part of the tuber cinereum but circumferential to the infundibulum. Four different patterns of fluorescence can be identified, suggesting an unexpected anatomical variability2: pentagon, keyhole, intermammillary arrowhead, and upsilon (Fig. 36.3).
Organum Vasculosum Laminae Terminalis and Subfornical Organ
When the endoscope is directed through the foramen of Monro toward the most anterior part of the third ventricle and lamina terminalis (LT), the OVLT is always clearly visible in the fluorescent mode (Fig. 36.4). The OVLT presents a pattern of constant bright fluorescence in the center of LT because of its specific type of vascularization. At higher magnifications, small capillaries around the area of the tuber cinereum and lamina terminalis can be detected in the fluorescent mode. Conversely, the subfornical organ cannot be reached, even when the flexible endoscope was positioned ventrally to the fornix, corresponding to the anterior triangle formed by the columnae fornicis and anterior commissure, and therefore nothing could be so far inferred on its capability to enhance after fluorescein injection (Video 36.1).