Introduction
The indications for stereotactic neurosurgical methods have, at one point or another, encompassed all major categories of differential diagnoses. Stereotactic techniques, introduced in the early 20th century, applied instrumentation in a minimally invasive, precise, and reproducible manner for research purposes. The first report of a stereotactic device in the English language literature is the report of Horsely and Clarke, which described a device for accessing the dentate nucleus of the cerebellum in monkeys [1]. Despite this early start, stereotactic methods were not attempted in humans until nearly 40 years later when Spiegel and Wycis inaugurated the era of human stereotaxis for ablative neurosurgical procedures [2]. They developed the paradigm of contemporary stereotactic technique, combining the use of a ste-reotactic device, radiographic imaging, and a quantitative anatomic atlas.
Since then, stereotactic methods have been progressively refined and are now applied for a wide range of indications to accomplish both diagnostic and therapeutic goals. This topic provides a brief outline of the wide indications for stereotaxis in both historical and contemporary neurosurgical practice.
Neoplastic
Diagnostic
Biopsy is the most common indication for the use of stereotactic methods. Before the introduction of modern stereotactic methods, biopsies involved free-hand needle aspiration or craniotomy guided by indirect radiographic procedures, such as angiography or ventriculography. Modern imaging and frame-based stereotactic procedures rapidly obtain diagnostic material with minimal patient mortality and morbidity [3,4]. The vast majority of brain lesions visible on imaging studies can be safely biopsied with high diagnostic yield [5]. Lesions with possible vascular pathology and those with significant associated mass effect, however, should be excluded from consideration.
Radiosurgery
Perhaps no more important method of treating brain tumors has been introduced in the last 20 years than stereotactic radiosurgery. Radiosurgery uses precise delivery of energy in the form of convergent beams of high energy photon or charged particle radiation to tumor tissue on a hypofractionated basis. Devices designed to deliver radiosurgical doses include the Leksell Gamma unit and a number of linear accelerator-based devices. Radiosurgery has been found to yield survival rates comparable to open surgical intervention for metastatic tumors of the brain [6,7]. Radiosurgery may also be used as effective adjuvant or primary treatment of carefully selected extra-axial tumors, such as meningiomas of the skull base or vestibular schwannomas [8,9]. It is imperative that neurosurgeons become familiar with the techniques and indications for stereotactic radiosurgery to maintain leadership in this field.
Brachytherapy
Use of brachytherapy has gradually declined since the introduction of con-formal radiation therapy and radiosurgical methods. Brachytherapy, using the stereotactic implantation of radioactive seeds into the mass of tumor tissue, has the advantages of allowing delivery of highly concentrated radiation energy with a tighter dose fall-off than radiosurgical methods [1012]. In practice, however, brachytherapy has several disadvantages that have contributed to its disuse. These include hazards incumbent on the use of radioactive materials in the operating suite and the invasive nature of the procedure. At our center, since the advent of radiosurgical methods, brachy-therapy is no longer used.
Other Indications for Neoplasms
Approximately 3% of gliomatous tumors may be associated with a significant cystic component. Cyst drainage by stereotactic techniques can provide an important means of palliation for these patients. Colloid cysts have been treated with similar techniques, although recurrence rates remain high [13]. Craniopharyngiomas have been treated with stereotactic techniques using aspiration and the instillation of radioactive isotope into the cyst cavity in selected cases [14,15].
STEREOTACTIC CRANIOTOMY
Recent advances in imaging coupled with the availability of high-performance computing has made possible the introduction of stereotactic-assisted craniotomy for tumor, vascular, and other mass lesions. These stereotactic methods were initially introduced by Kelly, using a frame-based stereotactic system [16,17], and have since been moved to so-called frameless systems by a number of other groups [18,19]. Frameless systems differ fundamentally from their frame-based counterparts. Frame-based systems are designed to mechanically constrain instrumentation to a direct path to tumor tissue. Frameless systems are, by design, unable to provide mechanical constrain-ment of an operative corridor. Frameless systems instead report back the location of a freely mobile pointing device. Such frameless systems should, therefore, not be considered as replacements for frame-based stereotactic devices, as their roles are different.
Currently, stereotactic craniotomy is used for a variety of indications and should be a part of every neurosurgeon’s armamentarium. Stereotactic craniotomy is most useful for small, deep-seated lesions where reliance on surface anatomic landmarks can be misleading. Furthermore, for convexity lesions, stereotactic guidance can allow for smaller, more localized crani-otomies than would be possible with the use of surface landmarks coupled with eyeball evaluation of radiographic studies.
Despite the advantages of stereotactic craniotomy, these methods are still not pervasively used. Several factors account for this. First, most devices of the present generation are complex, with poor user interfaces and cumbersome if not outright awkward set-up requirements. The complexity of these devices often necessitates additional operating room staff expressly for the use and care of this equipment. Second, the devices continue to be very expensive, making access to this technology feasible for higher volume services only. This situation, however, may only be transitory, as evolution of these products will likely remedy many of the present shortcomings.
A separate issue from the shortcomings of the devices themselves is that use of these devices demands a different style of operating to minimize the occurrence of intraoperative brain shift and subsequent loss of registration. Experienced users of these devices minimize use of diuretics and dissect tumor tissue out circumferentially rather than internally debulking the mass [17].
The future direction of stereotactic craniotomy will likely see it in combination with intraoperative imaging technologies capable of updating images during the surgical procedure, thereby rendering unnecessary the need to control brain shift [20].
FUNCTIONAL NEUROSURGERY
Access to small, deep-seated targets for the purpose of effecting a change in the function of the brain, whether for treatment of movement disorders, pain, or psychological disorders, represents the earliest indications for stereo-taxy.
Movement Disorders
Movement disorder surgeries were widely practiced in the 1950s. Targets included thalamic nuclei as well as the pallidum. With the introduction of L-dopa, however, surgical interventions for movement disorders fell by the wayside, reaching a nadir in the late 1970s. The eventual development of dyskinesias, on-off phenomena, and other side effects in long-term patients with Parkinson’s disease led to a re-exploration of surgical techniques for the treatment of movement disorders. Furthermore, the discovery of the Methyl-phenyl-tetrahydropyridine (MPTP) model of Parkinson’s disease led to the development of animal models and a re-evaluation of the pathophy-siology of movement disorders, allowing for a rationalized surgical approach to this set of diseases.
Ablative surgeries have been the core of stereotactic surgery for movement disorders since Spiegel and Wycis undertook their procedures. Ablation includes physical methods, such as use of a leukotome; chemical methods, such as alcohol or glycerol injection; radiofrequency coagulation; and freezing methods. Of these methods, radiofrequency ablation methods are generally safer and more reproducible.
Recently, deep brain stimulation (DBS) has revolutionized the field of movement disorders surgery. Deep brain stimulation, utilizing classic targets for movement disorders including the ventralis intermedius (VIM) nucleus of the thalamus for tremor [21,22], globus pallidus for dystonia [23,24], and the subthalamic nucleus of Luys for Parkinsonism [25] has shown significant promise in recent years. Subthalamic nucleus stimulation, in particular, results in long-term amelioration of all the cardinal signs of Parkinsonism. Deep brain stimulation has a number of advantages over ablative surgery, including the ability to adjust stimulation parameters to titrate effect and reversibility of the procedure.
Biological approaches, including gene therapy, stem cell and tissue transplant for movement disorders are a nascent technology that promises applicability to a wide range of neurologic disorders, including degenerative and demyelinating disease. Early results from these approaches, however, have been disappointing thus far. Currently tissue transplant has not yielded results comparable to either DBS or ablative procedures [26,27]. Despite these early disappointments, it is a virtual certainty that these technologies represent the near future of neurologic treatment. Many groups, in both academics and industry, have been actively involved in the development of these methods.
Psychiatric Disorders
After the introduction of classic physiologic methods to the study of the forebrain, interest turned to how the results of these studies could be applied to the clinical realm. Because of the horrid conditions within psychiatric facilities at the time and the primitive nature of nonsurgical therapies (e.g., insulin and electric shock), the advent of psychosurgery held great promise. Stereotactic variants of psychosurgical procedures have included cingulot-omy [28], anterior capsulotomy [29], tractotomy [30], and others.
Evaluation of the psychosurgical literature is difficult, partly because of reporting methods, partly because diagnostic categories in psychiatry have changed greatly over the years. It appears, however, that certain categories of illness do respond to surgical intervention (e.g., obsessive compulsive disease, anxiety), whereas others do not (e.g., schizophrenia).
As with movement disorders, the introduction of effective drug therapy led to the demise of psychosurgery. Furthermore, political trends as well as the lack of a sound theoretic scientific basis for these procedures made continued widespread use of these methods untenable. Nevertheless, a few centers have continued these procedures on a limited basis. Advances in neu-roscience research may, in the near future, provide a more firm basis for the re-exploration of surgical interventions for psychiatric disease.
Seizure Disorders
Stereotactic methods, as applied to seizure disorders, encompass both the diagnostic and therapeutic realm. From the standpoint of diagnosis, implantation of stereotactic depth electrodes for localization of seizure activity in mesial temporal structures is a common technique. In some centers, especially in Europe, arrays of stereotactically implanted depth electrodes are used in preference to the surface grids commonly used in North America.
Ablative stereotactic procedures have been used in the treatment of seizure disorders in highly selected patients [31,32]. It is unlikely, however, that these techniques will replace the current strategies of resective surgery, especially given high rates of success and safety with open surgery. Recently, work has been done with stimulation of deep brain structures for the treatment of seizure disorders. This work, still unpublished as of this writing, appears to be promising. The efficacy of such procedures remains to be investigated in the coming years.
Pain
The current generation of deep brain stimulation electrodes was initially conceived and devised for use in treatment of chronic pain. Currently, DBS is limited to a few specialized centers treating pain syndromes unresponsive to all other modes of therapy. Because of the subjective nature of pain and the pervasive coincidence in these patients of significant psychiatric overlays, objective analysis of the results from surgical intervention is extraordinarily difficult. Most of the published data suggest that modest improvements can be realized in highly selected patients [33]. An emerging modality is cortical stimulation, whereby primary motor cortex is chronically stimulated by implanted strip electrodes placed with stereotactic guidance and electrophysiologic localization [34]. These technologies hold hope for a late recourse for patients with chronic localized upper limb and facial pain. Other modalities used in chronic pain have included cingulotomy and thalamic ablative procedures with mixed success.
OTHER INDICATIONS
Hematoma
Stereotactic aspiration on intracerebral hemorrhage is a method practiced in some centers for removal of both acute and subacute lesions. It has been suggested that superior results can be obtained after aspiration of these lesions [35,36].
Abcess
Intracranial infection can, in many instances, be treated successfully by ste-reotactic abscess aspiration. Lunsford et al. have reported good results with overall bacteriologic identification of 97% and cure rates of 72% for patients presenting with a wide range of diagnoses and underlying illnesses [37,38].
CONCLUSION
In the more than 50 years during which stereotactic brain procedures have been practiced, gifted neurosurgical pioneers have applied these methods to virtually all major diagnostic categories. The reason for this is simple: ste-reotactic devices are the only means by which one can efficiently target and access any arbitary volume within the space occupied by the brain.
As the basic medical science of the brain yields discoveries leading to treatments for currently uncurable disorders of the brain, it is likely that stereotactic methods will be the vehicles of treatment for these therapeutic modalities. Stereotactic methods are already sufficiently powerful to be applied on a daily basis to the majority of neurosurgical practice.
