Principles of Neurosurgery

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Chapter 46 Principles of Neurosurgery

The basic principles of neurosurgery include but are not limited to:

Although these principles have remained constant over the past century, the practice of neurosurgery has evolved tremendously since the days of Cushing, Dandy, and Penfield.* This dramatic change has occurred as a result of a convergence of advancing technology combined with the exponential growth of neuroscience knowledge and discoveries (Apuzzo, 1996; Kaibara et al., 2000).

No one chapter can even begin to cover the gamut of neurosurgical practice in any meaningful way. This chapter is aimed at the practicing clinical neurologist who interacts with neurosurgeon colleagues. Knowing if, when, and which neurosurgical subspecialists to refer patients to can improve outcome as well as avoid unnecessary utilization of healthcare resources. In the best of both worlds, the neurologist can play a key role in the diagnostic and perioperative management plus the long-term follow-up of patients. This paradigm already exists and works well in many brain tumor programs where the neuro-oncologist is an integral member of the surgical decision-making and postoperative management team (Yasargil et al., 2004).

There are countless “principles” of neurosurgery, ranging from broad to highly specific. Here, the basic principles noted earlier will be individually addressed, with an emphasis on highlighting intrinsic differences between the complementary and sometimes overlapping fields of neurology and neurosurgery.

Establishing the Diagnosis

Inherent to surgical intervention, the vast majority of neurosurgical consultations involve diagnostic neuroimaging. In most cases, referral to a neurosurgeon occurs as a consequence of abnormal imaging results, already creating a unique patient-physician relationship. The neurosurgeon is often confronted with the situation of deciding whether the patient’s symptoms or signs, if any, correlate with the neuroimaging findings. This is in stark contrast to neurological practice, in which certain diagnoses such as headache and seizures may not receive diagnostic imaging initially or even at all. Although the finding of a neuroimaging abnormality is not a prerequisite for neurosurgical intervention (such as movement disorder surgery), neuroimaging is required nevertheless (Britz et al., 1995; Kent et al., 1994; and Nijeholt et al., 1998).

The reliance upon diagnostic neuroimaging has changed neurosurgical residency training, necessitating that neurosurgeons achieve a high level of diagnostic neuroradiological competence. For the neurosurgeon, the diagnostic process (and surgical planning) typically hinges upon the imaging, and therefore a considerable amount of time is devoted to carefully, personally reviewing the imaging studies. As a result, when referring a patient for neurosurgical evaluation, it is important to emphasize to the patient that the actual imaging study be physically available for review at the time of the neurosurgical consultation.

It is important to understand the difference between standard diagnostic imaging and the high-resolution neuroimaging required for neurosurgical decision making. For example, routine diagnostic magnetic resonance imaging (MRI) studies of the brain are performed with relatively thick 5-mm slices, with 2.5-mm skip between slices. Although generally adequate to detect lesions 4 mm in size or larger, partial volume averaging can obscure fine anatomical detail such as the relationship of arteries, veins, and cranial nerves to the lesion. For example, although the diagnosis of hydrocephalus may be obvious on routine diagnostic imaging, establishing the etiology of the ventriculomegaly may require high-resolution constructive interference in steady state (CISS) imaging (Fig. 46.1). In this case, a diagnosis of occult aqueductal stenosis would dramatically change the treatment, with endoscopic third ventriculostomy being the preferred treatment rather than a ventriculoperitoneal shunt.

With routine “diagnostic” imaging, the number of different MRI sequences is often limited by the concept of the study being a screening exam rather than one honing in on a certain differential diagnosis. For example, atypical arachnoid cysts can be differentiated from epidermoid cysts if repeat imaging includes a diffusion-weighted imaging (DWI) study (Tsuruda et al., 1990). In this case, an arachnoid cyst may not require referral to a neurosurgeon, whereas the epidermoid cyst should. Consultation with a neuroradiologist, if needed, could lead to proper imaging studies being performed prior to possible neurosurgical consultation.

In other cases, routine diagnostic imaging may be adequate for establishing a diagnosis but inadequate for neurosurgical decision making. In some cases, additional imaging modalities may be required, such as computed tomography (CT) imaging of bony anatomy to supplement abnormal soft-tissue findings on MRI. In other cases, special high-resolution or unique imaging sequences may be required (Barnett et al., 1993; Curtin and Hirsch, 1992). Although beyond the scope of this chapter, suffice it to state that a brief discussion with the neurosurgeon—prior to referral—may in some cases lead to essential imaging studies being obtained prior to the consultation. This may make for a much more productive and useful consultation, obviating the need to have the patient return for an otherwise unnecessary follow-up visit owing to the lack of key imaging studies at the first visit.

Patient Selection for Surgery

Patient selection for surgery is under the purview of the neurosurgeon in most cases, although for many disorders, the neurologist plays an integral and key role. As noted previously, the increasing sophistication and complexity of surgical intervention mandates intimate collaboration between neurologists, neurosurgeons, and other subspecialists. Knowing when to refer to multidisciplinary specialty centers is obviously a function of patient diagnosis and proximity to such centers. In general, community neurosurgeons are well versed in treating a wide range of neurosurgical disorders, although increasingly, many are limiting their practices to mainly treating spinal disorders. Increasing evidence suggests that for specific diagnoses, surgical outcomes are a function of surgical volume load and experience. For example, dedicated brain tumor neurosurgeons performing over 50 brain tumor operations per year generally have better patient outcomes than those only occasionally performing these surgeries (Cowan et al., 2002; and Cowan et al., 2003; D’Agostino et al., 2009; and Shahlaie et al., 2010). Similar findings have been reported for cerebral aneurysm treatment, epilepsy, movement disorders, and pediatric disorders. Most academic centers and some large group private practices have dedicated subspecialty neurosurgeons (as well as the necessary ancillary team) to comprehensively and optimally manage these patients.

Surgeon experience, however, is only part of the equation. Whereas it is highly possible a community neurosurgeon can perform an adequate surgical removal of a brain tumor, neurosurgeons affiliated with a brain tumor center may be able to offer patients clinical studies they might not otherwise be aware of. For some trials, fresh pathological tissue is required, and therefore the operation has to be performed at the site of the trial. Understanding the practice of the neurosurgeons to whom you routinely refer may therefore indirectly improve the care your patients receive.

Patient selection for surgery has become increasingly complex, and new treatment modalities and alternatives have arisen. Just as surgical volume load and experience varies among neurosurgeons, it may be important that the referring physician understand other important factors in patient selection for surgery. An example is vestibular schwannoma (acoustic neuroma) management. As already noted, surgical experience may play a major role with regard to patient outcome. It is important to recognize, however, that fewer patients are choosing surgery for small acoustic neuromas, and instead are undergoing stereotactic radiosurgery or radiotherapy treatment (Andrews et al., 2001; Flickinger et al., 2001; Lunsford et al., 2005; and Meijer et al., 2000). Referring to a neurosurgeon whose practice is located in an area without focal beam radiation capabilities may inadvertently lead to an emphasis on surgical alternatives. A similar situation exists with intracranial aneurysms, in which endovascular coiling has increasingly become the treatment of choice for selected patients. Typically, high-volume neurovascular surgeons are closely affiliated with (or perform themselves) endovascular treatment, and therefore this issue is a moot point (D’Agostino et al., 2009).

Neurosurgery is increasingly subspecialized (as is neurology), and certain diagnoses may require a higher level of expertise. In addition to skull base tumors and neurovascular disorders, others include intractable epilepsy, adult hydrocephalus, movement disorders, and rare conditions such as colloid cysts. A knowledgeable neurologist making the proper initial referral may be a key factor in outcome.

In some cases, the neurologist is the primary determinant of patient selection for surgical intervention. The management of movement disorders is an obvious example. In only a minority of patients is surgical intervention such as deep brain stimulation appropriate Olanow and Koller, 1998). Referral to a movement disorder neurologist first may be appropriate instead of initial referral to a functional neurosurgeon. Other examples include carotid endarterectomy (versus stenting) and cerebrovascular bypass procedures.

To some extent, patient selection for surgery begins prior to neurosurgical evaluation. There are no hard-and-fast guidelines as to what types of neurological diagnoses should be referred for neurosurgical consultation, but in most cases, the decision to refer for neurosurgical consultation is obvious. Symptomatic patients with obvious abnormalities seen on imaging, such as a large cerebral aneurysm, require neurosurgical consultation. There are other cases, however, that may not require neurosurgical consultation and can be managed well and appropriately by the neurologist. Although it may seem obvious and trite, it is important to understand that a neurosurgical practice is a surgical practice. With the increasing use of neuroimaging in our society, so-called incidentalomas are being noted by radiologists (Fainstein et al., 2004; Molitch, 2008; and Molitch, 2009). In some cases, this has led to reflexive referrals to neurosurgeons based solely on the radiology report, without the personal review of the imaging study. Referral of these nonoperative lesions to the neurosurgeon first raises the expectation of the patient that surgical intervention is advised and/or pending, and second takes up a significant amount of the neurosurgeon’s clinic time explaining why the lesion does not require surgery. Examples follow.

Tiny Intracranial Aneurysms

With the increasing availability and use of MRI, unruptured intracranial aneurysms less than 7 mm in size are being incidentally discovered during diagnostic imaging. Current international guidelines support repeat imaging of these lesions to ascertain whether they are enlarging or not (Asari and Ohmoto, 1993; Juvela, 2002; Mocco et al., 2004; and Wiebers et al., 2003). Because generally they do not require surgical intervention, referral to a neurosurgeon can be premature. A brief telephone conversation with the neurosurgical colleague may be all that is needed, conserving time and cost for both the patient and surgeon.

Incidental Small Cavernous Malformations

Like small aneurysms, these lesions are rarely surgical and can be followed with neurosurgical consultation (Gross et al., 2009; Robinson et al., 1991). Most neurosurgeons will gladly review imaging studies in order to differentiate which patients should be referred for consultation.