Injection of solutes such as Na+ or mannitol, which initially elevate ECF osmotic pressure and then draw water from cells to dilute the load to near isotonicity, causes reliable drinking in all species studied. This osmoregulatory drinking is accompanied by report of thirst in humans and secretion of AVP. The threshold elevation of systemic osmolality to initiate these changes is in the range 1–3%. Regardless of whether drinking is allowed, the osmotic load is excreted by the kidney over the course of2–4 hours. From sham drinking and other studies, osmoregulatory drinking is satiated when osmotic pressure is normalized and cellular water is restored. Many studies have shown that osmoreceptors for drinking and AVP secretion are located in or near the CVOs of the lamina terminalis. In addition, peripheral osmoreceptors in the gut or splanchnic regions monitor the osmolality of fluids absorbed from the gut and are sufficient to stimulate drinking in the absence of systemic hyperosmolality.

Water deprivation or restriction is arguable the most natural of all dehydrations.The magnitude and rate of dehydration depends on the water and electrolyte content of available food, and the extent to which dehydration anorexia mitigates the net dehydrating effect of food.The early stages of water deprivation produce primarily intracellular dehydration, whereas longer durations of deprivation cause progressively greater extracellular fluid losses. Thus, the resulting stimulus to drink is a combination or hybrid of intracellular and extracellular drinking. There are considerable species differences in the rates of postdeprivation drinking and in the mechanisms that terminate such drinking, although in all cases some contribution of preabsorptive controls is evident. The principal structures of the lamina terminalis (OVLT, MnPO, SFO) collectively contribute to postdeprivation drinking, although the relative contribution of these structures may depend on the duration or degree of net dehydration and/or have species differences. Many neurons of the lamina terminalis are excited by both hyperosmolality and ANG II, and so act as integrators, but it has not been established whether the nature of that integration (or threshold) differs between cells within a region, or between regions.

Acute depletion of ECF volume without change in osmolality is sufficient to induce drinking in many species. However, the threshold for drinking appears to be quite large, of the order 10–20% loss of plasma volume. This change may occur without significant drop in arterial pressure, due to effective physiological counter-regulation, including secretion of renin from the kidneys and subsequent generation of ANG II. Under many conditions of actual or simulated hypovolemia, ANG II appears to be involved in the drinking response and probably by action on AT1a receptors in the SFO. However, interference with ANG II production and/or action does not uniformly disrupt all forms of hypovolemia-related drinking, and it appears that afferents from cardiopulmonary pressure receptors may also be involved. There may be strain differences in the relative contribution of these two mechanisms, or others, to extracellular dehydration drinking. Restoration of ECF volume requires ingestion of NaCl as well as water, and mechanisms for this are discussed briefly.

Body water is contained in intracellular and extracellular compartments, in approximately a 2:1 ratio. The ionic concentrations are quite different between compartments and Na+, and the predominant cation in the ECF, Na+, is of particular significance with respect to maintaining the volume of the ECF. Several specialized organs and hormones for maintaining ionic and fluid balance are described.

During pregnancy in humans and rats, plasma osmolality is regulated at a level about 3% lower than in nonpregnant females (or males). This may be, in part, caused by reproduction-related hormonal changes. Late in gestation, fetal sheep show acute swallowing responses to some dipsogens, and the relevant CVOs in the brain appear to have full responsiveness to those dipsogens at this time. Postnatal development of independent drinking in rats shows that responses to hypovolemia are present and vigorous very soon after birth, whereas development of osmoregulatory drinking has a considerably slower ontogenetic trajectory. Further, this trajectory seems to be slower in mice than rats and requires further study. Some aspects of osmoregulatory drinking in rats, as well as food-associated drinking, appear to have an experiential component. Hypovolemia or other circulatory challenges before birth in rats cause alterations in the ontogenetic trajectory of drinking, although the limits and mechanisms of this phenotypic plasticity have not been fully elucidated.

Experimental analysis of drinking in humans has yielded inconsistent results, ranging from no deficits relative to young groups, to substantial deficits. It is possible that cognitive or other factors can account for some of these differences between studies. Age-related dehydration in at-risk human populations may be minimized by ensuring adequate food intake and meal-associated drinking and/or provision of foods with high fluid content. With the exception of hypotension-related stimuli, age-related declines in drinking by rats after about 25% of the life span are small. Increased drinking at younger ages (3–4 mo) seems to be the unexplained anomaly; a generic explanation would be maturation in late adolescence of an as-yet unrecognized inhibitory mechanism.

The definition of combined approaches is broad. This can include multiple portals to the same or different region in the skull base. The “pull-through technique is a versatile, dual-keyhole approach developed to attach tumors that extend between the temporal and occipital poles. This approach is complex, as it entails a knowledge of the intricate anatomy and eloquent cortical and subcortical structures in this region. This combined approach is tailed to minimize peripheral brain damage while providing a direct route to the pathology. This chapter discusses the indications, the anatomy, the patient selection, and the surgical nuances of this approach.

The combined petrosal approach (CTA) is one of the most complicated neurosurgical procedures. It is essential to learn and acquire the micro-anatomical key elements to complete a safe and less invasive CTA. The Fukushima lateral position can provide for an adequate surgical field and reduce the patient’s stress brought on by a long operative time. Three-layer scalp elevation including harvest of a vascularized flap is mandatory to carry out the closure. One should consider the bony landmarks to carry out the craniotomy safely and widely. The tentorial resection and dural incision are very important to secure the wide surgical field and to avoid complications. Closure with fat tissue and a vascularized flap is recommended to avoid postoperative CSF leak, deformity, and infection.

Intraventricular lesions are challenging pathologies in neurosurgery. Walter Dandy had a major impact in advancing our understanding of the management of these lesions. Furthermore, the introduction of the microscope and microsurgical techniques have improved the surgical outcomes of these lesions. Several approaches have been described to address ventricular lesions, and can be classified anatomically as anterior, lateral, or posterior. The operative corridor for each of these approaches transgresses unaffected neural tissues. Therefore, tailoring the approach to individual patient lesion characteristics and anatomy is crucial to maximize exposure and minimize morbidity. The majority of open and endoscopic approaches to the third ventricle use the interhemispheric anterior transcallosal, frontal transsulcal, or frontal transcortical corridor to access the lateral ventricle. Once inside the lateral ventricle, the operative corridors to the third ventricle include working through the foramen of Monroe (transforaminal approach) for small lesions located in the anterior superior part of the third ventricle, or through the choroidal fissure (transchoroidal or subchoroidal) which provide access to lesions located in, or extending into, the middle or posterior parts of the roof of the third ventricle. In this chapter, we will discuss the transchoroidal, subchoroidal, and combined transchoroidal and subchoroidal approaches to the third ventricle.

There is no single optimal approach for all skull base pathologies and locations. Skull base surgeons must tailor their surgery to both the tumor and the goals of care for the patient to optimize conditions for tumor removal while minimizing morbidity. Lateral skull base approaches are an attractive option to access the skull base given the direct surgical access to the lower cranial nerves, the intra- and extratemporal internal carotid artery (ICA), and the venous sinuses. These approaches can be broadly classified as otic capsule-sparing and otic capsule-sacrificing. These approaches are complex in nature, and patient selection on a case-by-case basis with multidisciplinary input is essential. Complex skull base tumors may not be adequately addressed with a single surgical approach and may require a combination or modification of these techniques. In this chapter, we will describe transcochlear approaches and how they can be modified to provide extended skull base access.

Traditionally, two approaches have been utilized to access the rostral basilar artery (BA) for clipping aneurysms. These are: 1) the subtemporal approach pioneered by Canadian neurosurgeon Charles George Drake and 2) the transsylvian or pterional approach popularized by Turkish neurosurgeon Mahmut Gazi Yasargil. Both of these approaches have assets and liabilities, and both have modifications to expand the surgical field of view. Some rostral BA aneurysms and some patients with aneurysms of both the anterior and posterior circulation requiring clip ligation may benefit from a combined transsylvian and subtemporal (half-and-half) exposure. In this chapter, we describe our version of that approach, which provides access to both anterior and posterior circulation aneurysms. We detail neuroanesthesia considerations, patient positioning, bone drilling, subarachnoid dissection, and clip placement. Also included is an illustrative case.

Skull base tumors are often intimately involved with critical vascular structures, which can be a barrier to maximal resection. In these cases, cerebral revascularization, including vascular bypass, can be an important adjunct technique. The feasibility of bypass depends on patient history, the anticipated tumor pathology, and characteristics of the cerebral vasculature. These procedures require considerable experience and expertise, and their indications must be well considered before performing them. A number of imaging and interventional studies can help establish the utility and feasibility of bypass preoperatively. Emerging technologies will help to refine the indications and streamline the performance of these challenging procedures. Here, we discuss the historical basis of bypass in skull base tumor surgery, indications in the modern era, the microanatomy of common bypasses, patient selection, and endovascular options for revascularization.

Combined approaches are often necessary to address large or invasive skull base pathology. A combined suboccipital craniotomy and neck dissection is often utilized for invasive posterior fossa skull base tumors and neck tumors. Management of these tumors often requires the collaboration of multiple specialties. Tumors in this location are often intimately involved with important neurovascular structures. Good pre-operative evaluation, intra-operative monitoring, and closely monitored post-operative care are essential. The extent of resection and goals of care depends on a case-by-case analysis including the patient’s age, the aggressiveness of the tumor pathology, presenting symptoms, and comorbidities. This chapter reviews the indications, anatomy, and surgical nuances of this complex approach.