1Introduction

Sleep apnea in adult patients is a recently recognized disorder of sleep characterized by recurrent apneic and hypopneic episodes. Apnea was typically defined as complete cessation of airflow, but in some studies, a >80 percent reduction in airflow was used. For defining hypopnea, most papers suggested a 50 percent or greater reduction in airflow was used, with or without a coincident O₂ desaturation of anywhere from 2 percent to 4 percent from some average SaO₂ over a preceding interval of time. Most cases are characterized by recurrent airway obstruction (obstructive SA), and a minority of cases are central (i.e., neurologic) in origin. In view of its purportedly high prevalence and serious associated morbidity, SA has recently been described as a major public health concern (Phillipson, 1993). The National Commission on Sleep Disorders Research (1993) estimated that SA may be responsible for 38,000 cardiovascular deaths per year and costs of $42 million annually for related hospitalizations. The cumulative 8-year mortality of untreated SA has been estimated as high as 37 percent for patients with an apnea index >20, where AI is defined as the number of apneic episodes/hour sleep compared to 4 percent for patients with lower AIs (He, Kryger, Zorick, et al., 1988). Patients with obstructive SA need not go untreated, however, since there is a well accepted first line therapy - continuous positive airway pressure (CPAP). A major problem in the field in 1998, however, is diagnosis: whom to test; how to test; and what are the implications of test results regarding the risk of serious clinical sequelae?

SA is a condition where the gold standard diagnostic method, overnight full polysomnography in a sleep laboratory, is intrusive and costly, and the interpretation can be difficult. A standard PSG typically consists of electroencephalogram, submental (± tibialis) electromyogram, electrooculogram, respiratory airflow (usually by oronasal flow monitors), respiratory effort (usually by plethysmography), and oxygen saturation (oximetry). Electrocardiography and body position are also frequently monitored in formal sleep studies and stated to be standard requirements of PSG by some groups. The contribution of each of these components to the PSG diagnosis of SA has not been well substantiated (Pack, 1993).

If the estimated prevalence of SA at 2 percent to 4 percent of middle-aged adults (Young, Palta, Dempsey, et al., 1993) is correct, the costs of full PSGs for all suspected cases would be prohibitive. The development of simpler and less costly alternatives for diagnosis or pre-PSG screening and/or testing would be highly desirable. Diagnostic approaches that might be viewed either as alternatives to PSGs or as screening tests to better select patients for PSG include partial channel PSGs; partial night or daytime PSGs; portable sleep monitoring devices for use at home; radiologic imaging of the head and neck for anatomic abnormalities predictive of SA, including cephalometry; MRI and CT scans; anthropomorphic measurements such as neck circumference; nasopharyngeal and laryngeal endoscopic measurements of upper airway structure and function; and focused questionnaires.

Accepted guidelines for testing are now very different between Europe and the U.S. (Schafer, Ewig, Haspr, et al., 1997), with Europeans promulgating simpler step-wise approaches to reduce testing costs by home-based studies with screening devices. Others have suggested "split-night" studies (Iber, O'Brien, Schulter, et al., 1991), or home studies with new automated CPAP devices (Burk, Lucas, Axe, et al., 1992). The type of sleep evaluation study preferred (and reimbursed) varies widely among physicians, sleep centers, and managed care organizations (Lindblom, 1997).

This lack of consensus may have been facilitated by the fact that SA can be viewed as an orphan condition, shared by many healthcare specialties, yet owned by none. Neurology, psychiatry, dentistry, otolaryngology, pulmonology, and internal medicine all share diagnosis and management of SA, and as a result, the evidence base is uneven and dispersed, and clinical management perspectives are sometimes in conflict. When evidence is scattered, and possibly conflicting, a rigorous and comprehensive assessment of all of the best available evidence is critically important, and in the case of SA, long overdue.

In this study, MetaWorks investigators have developed an evidence base via a systematic review of the literature published in the five major Western European languages pertinent to diagnostic testing and screening in SA. The development of practice guidelines or test recommendations was beyond the scope of this project. This evidence base should, however, be useful to health care providers in the development of evidence-based strategies and algorithms to guide the diagnostic work-up of patients suspected of SA. Furthermore, this evidence base can be efficiently updated as the literature evolves. This work should provide guidance for future researchers to generate new data to fill the information gaps discovered during the review.