CHAPTER 159 Evidence-Based Approach to the Treatment of Facial Pain

It does not require a thorough review of the history of surgery to note how the evolution of surgery has rested on an empirical approach to problem solving, with methodologies developed from a series of trials and errors and their ultimate value judged by their rise in popularity within the surgical community rather than by scientific scrutiny. The evidence for the usefulness of a surgical procedure has until recently come from case series, which in turn have reflected the surgeon’s understanding of the anatomy, physiology, and pathophysiology of disease. It is no surprise that for common problems (e.g., inguinal hernia), many different surgical techniques are described, none of which can be shown to have superiority over others. The evolution of surgical practice has partially hinged on technical developments, such as the operating microscope or endoscopy, whereas some procedures have been abandoned because of advances in pharmacotherapy, radiotherapy, or endovascular therapy. Although such developments are welcome, the surgical community’s traditional way of adopting or rejecting new treatments without subjecting them to critical assessment compels one to question whether in this way optimal progress is secured.

Evidence-based medicine (EBM) is a concept introduced in the 1970s to improve the standard of health care, and it has been successfully implemented since the last decade of the past century in many fields of medicine. Its principal aim is to ensure that a given diagnostic procedure or treatment is as accurate, effective, and unbiased as possible. In practice, EBM is the conscientious, explicit, and judicious use of current best evidence in making decisions about the care of individual patients.¹ The strength of evidence can understandably vary, and several hierarchic scales have been developed to measure it.²^–⁶ The strongest evidence comes from adequately powered randomized controlled trials with a design that limits bias and ensures the objectivity of measurement of outcome. The weakest evidence comes from uncontrolled case series, case reports, and expert opinion.⁴^,⁵ Large databases exist to show that EBM has become the “gold standard” in drug development. For clinicians, evidence-based guidelines abound, with regular contributions from practicing clinicians, such as the Cochrane Library. There is an increasing drive to ensure that novel diagnostic techniques are adopted following validation based on strong evidence. Many nondrug, noninterventional treatments have similarly been subjected to critical EBM appraisal. The surgical community seems to have adopted a more circumspect approach.⁷^,⁸ There are, however, good reasons to proceed with EBM, and in no other field of medicine more than the surgical management of chronic pain.⁶

There are three major reasons why any treatment for chronic pain should be subjected to as meticulous scrutiny as possible. First, chronic pain is notoriously fluctuating, with many conditions varying over time significantly in intensity, and with some conditions known to go into remission. Second, for chronic pain, many easily accessible treatments exist, making it virtually impossible to ascertain without a controlling arm from case series which intervention ultimately has been effective. Third, in chronic pain, the placebo reaction is greater than in any other condition, except depression, explaining up to 44% of the treatment response.⁹

The resistance toward application of EBM to clinical practice stems from doubts about its suitability to surgical research, especially because of the obvious difficulty in designing trials that would allow random allocation of patients to two different procedures (active or sham) with an independent, blinded assessment of outcome. Yet, such studies have been done, with results that have immediate clinical applicability. Good examples include controlled trials of arthroscopic débridement of the osteoarthritic knee ¹⁰ and adhesiolysis in abdominal pain¹¹—both unequivocal in showing the limitations of these widely used treatments. In trigeminal neuralgia (TN), a controlled trial showed lack of efficacy of pulsed radiofrequency compared with conventional radiofrequency lesioning of the trigeminal ganglion.¹² Studies on laminectomy in sciatica or spinal cord stimulation in failed back surgery syndrome show that even when sham controlled trials are not possible, well-designed comparative trials are, and they yield results that generally are helpful to the practicing clinician.¹³^,¹⁴

It is obvious that there are clinical research questions that cannot be addressed with studies attaining the highest rank in the hierarchy of EBM. However, quality improvement is almost always possible from the routine uncontrolled case series presentations. Hierarchic assessment of strength of evidence is mostly helpful in only what it is supposed to do: to outline the limits and possibilities of reduction of bias and assessment of effect sizes; but it should not be considered as ultimately decisive.³ If the signal-to-noise ratio is sufficiently high, case studies may well suffice.¹⁵ In chronic pain, there are very few such circumstances, although hip replacement for osteoarthritis probably comes close.

Systematic reviews remain the strongest tool for those employing EBM to answer a clinical question posed.³ In this chapter, I summarize results from such a systematic review, published in 2008, and comment on issues not raised while highlighting some further targets for research.

Implementation of Evidence-Based Medicine in Trigeminal Neuralgia

In 2003, the American Academy of Neurology (AAN) and European Federation of Neurological Societies (EFNS) set up a Task Force consisting of practicing clinicians with a track record of research in facial pain. They were invited to conduct a systematic review of the medical literature on TN, using established criteria for assessment of the strength of evidence supporting the use of diagnostic and therapeutic measures.⁴^,⁵ In accordance to the AAN Practice Parameter, the Task Force independently chose the issues related to TN it thought had relevant clinical importance. The strength of evidence was assessed using criteria shared by both organizations (Table 159-1). The results of the review were translated into a set of practice recommendations that were accepted by the scientific committees of both societies.¹⁶^,¹⁷ Review consisted of published research on the topic up to 2006.

TABLE 159-1 American Academy of Neurology (AAN) and European Federation of Neurological Societies (EFNS) Guidelines for Classification of Strength of Evidence for Therapeutic Interventions

CLASS	AAN CLASSIFICATION	EFNS CLASSIFICATION
I	Prospective, randomized, controlled clinical trial with masked outcome assessment, in a representative population. The following are required: a Concealed allocation b Primary outcomes are clearly defined c Exclusion/inclusion criteria are clearly defined d Adequate accounting for dropouts or crossovers with numbers sufficiently low to have minimal potential for bias e Relevant baseline characteristics are presented and substantially equivalent among treatment groups, or there is appropriate statistical adjustment for differences	An adequately powered, prospective, randomized, controlled clinical trial with masked outcome assessment in a representative population or an adequately powered systematic review of prospective randomized controlled clinical trials with masked outcome assessment in representative populations. The following are required: a Randomization concealment b Primary outcomes are clearly defined c Exclusion/inclusion criteria are clearly defined d Adequate accounting for dropouts and crossovers with numbers sufficiently low to have a minimal potential for bias; and e Relevant baseline characteristics are presented and substantially equivalent among treatment groups, or there is appropriate statistical adjustment for differences
II	Prospective matched-group cohort study in a representative population with masked outcome assessment that meets a-d above or a randomized controlled trial in a representative population that lacks on criterion a-d.	Prospective matched-group cohort study in a representative population with masked outcome assessment that meets a-e above or a randomized, controlled trial in a representative population that lacks one criteria a-e
III	All other controlled trials including well-defined natural history controls or patients serving as their own controls in a representative population, in which outcome assessment is independently assessed or independently derived by objective outcome measurement.*	All other controlled trials (including well-defined natural history controls or patients serving as own controls) in a representative population, where outcome assessment is independent of patient treatment
IV	Evidence from uncontrolled studies, case series, case reports, or expert opinion	Evidence from uncontrolled studies, case series, case reports, or expert opinion

* Objective outcome measurement: an outcome measure that is unlikely to be affected by an observer’s (patient, treating physician, investigator) expectation or bias (e.g., blood tests, administrative outcome data).

Diagnosis of Trigeminal Neuralgia

For diagnostics, the questions were related to the differential diagnosis of classic (idiopathic) TN and symptomatic TN and to the role of imaging in showing the presence of a structural lesion.¹⁶^,¹⁷ Limited evidence from a handful of class III studies suggested that routine neuroimaging identifies a structural cause (other than a compression by a blood vessel) in 15% of patients with TN. The authors recognize a possible bias that may result from all reports coming from specialized centers. However, because this is the best evidence available, there can hardly be any need to question the practice of neuroimaging of patients with the clinical diagnosis of TN.

Four class II studies and one class I study demonstrated a higher risk for symptomatic TN in young people and in those with bilateral trigeminal involvement or sensory deficits, but not sufficiently to allow a predictive rule to be created. Five class I to III studies demonstrated a relative high sensitivity (94%) and specificity (87%) for the ability of neurophysiologic reflex testing in distinguishing classic TN from symptomatic TN, whereas evoked potentials were less reliable. No recommendation was put forward regarding the superiority of either investigation in the differentiation of classic idiopathic TN from symptomatic TN. However, rapid development in the field and growth of the number of facilities are likely to make neuroimaging the method of choice in the not so distant future.

Whether dedicated magnetic resonance imaging (MRI) should be used to show the presence of vascular compression in a patient with a clinical diagnosis of TN is still debated. The authors reviewed five class I and two class III studies published by 2006 and noted considerable inconsistency in the results (e.g., sensitivity ranging from 52% to 100% and specificity from 29% to 93%, when findings at operation were compared with blinded assessment of the preoperative scan). Because of this, no recommendation could be given. As techniques continue to improve, this recommendation may eventually change. A drawback is that a positive MRI is commonplace in the general population. Whether preoperative MRI influences long-term outcome from microvascular decompression (MVD) is not known at present. It is possible with future improvement in resolution and imaging techniques that MRI may help determine the clinical significance of a compression and its impact on the nerve, resulting in improvement in outcome.¹⁸

No attempt has been made to look for evidence for a gold standard to diagnose TN. There remains a general agreement among clinicians that no specific test is capable of it—the diagnosis of TN remains strictly clinical and is based to a great extent on the verbal description given by the patient. The criteria for the clinical diagnosis have been agreed on by the International Headache Society ¹⁹ and are similar but not identical with the current International Association for the Study of Pain definition. Although several validated questionnaires exist for differential diagnosis and characterization of neuropathic pain, they have not been shown to be useful in TN.

Equally, the concept of “atypical” TN was not included in the deliberations. Although some studies suggest that TN with atypical features is less responsive to therapeutic interventions,²⁰^,²¹ and may even be considered an extension of the “typical” form, there is insufficient agreement of what criteria should be used to distinguish the two conditions.

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