by Nic Butkov, RPSGT
The field of sleep medicine has undergone many radical changes during the past decade. Not long ago, few medical professionals were even aware of sleep disorders. Today, almost every medical center has some type of sleep program, even if only to screen for sleep apnea. In fact, it is the ever-growing awareness of sleep apnea that has been responsible for the sudden interest in sleep medicine and subsequent growth of the field.
Although this growth has generally been positive, it has also created some problems. Often in their rush to diagnose and treat sleep apnea, newcomers to the field underestimate the complexity of sleep physiology. Polysomnography, the cornerstone of sleep medicine, is often downplayed as unnecessary and burdensome. Increasingly, sleep laboratories are relying on automated technology, rather than focusing on comprehensive staff training. Equipment manufacturers are competing with each other to sell the latest technological options, often without considering established standards of practice. Consequently, while the number of patients diagnosed and treated for sleep disorders continues to increase, the accuracy of the diagnosis (or appropriateness of treatment) is sometimes less than optimal.
While it may be argued that classic obstructive sleep apnea can readily be identified without performing elaborate polysomnographic studies, there are many reasons why attended polysomnography, conducted by trained sleep professionals, continues to be the gold standard for accurate sleep diagnoses and post-treatment evaluations.
The first and foremost reason is that, sleep-related breathing disorders (and sleep disorders in general) are not limited to sleep apnea. In fact, sleep apnea represents only the more severe end of the sleep disordered breathing continuum. Newcomers to the field are often surprised to discover that classic obstructive sleep apnea is not nearly as common as many of the less-evident forms of sleep disordered breathing. In many cases, these other disorders cannot be adequately evaluated (or even differentiated from normal breathing) without correlating basic cardiorespiratory data with other essential polysomnographic parameters.
Secondly, one must recognize that most respiratory transducers used in sleep apnea studies offer a very primitive assessment of breathing. Most of these devices do not actually measure respiratory airflow or effort; they merely provide indirect, qualitative representations of these activities. Moreover, even those devices that are capable of providing calibrated measurements are not always reliable, nor independently sufficient for differentiating subtle breathing abnormalities from normal variants of NREM and REM sleep physiology.
Thirdly, all sleep recordings are susceptible to artifacts. Artifacts in respiratory channel tracings often resemble breathing abnormalities, which can easily be misinterpreted. A common example of this problem is seen in patients with periodic limb movements in sleep. The movements may cause arousals, which in turn, may cause fluctuations in breathing. Even in the absence of arousals, the movements may cause artifacts in the respiratory channels resembling apneas or hypopneas.
Finally, polysomnography is used to diagnose a wide variety of sleep disorders, some of which are not related to breathing. Examples include: narcolepsy, periodic limb movements, REM sleep behavior disorder, atypical sleep architecture, alpha-sleep, sleep-related seizure disorders, disorders of partial arousal and other pathologies. Many patients with these conditions have clinical symptoms similar to obstructive sleep apnea, Without a comprehensive evaluation, one cannot rule out the possibility of a differential diagnosis or the presence of coexisting pathology.
The primary advantage offered by polysomnography is the precise documentation of the architecture and micro-structure of sleep. This information allows clinicians to evaluate any apparent fluctuations in breathing, heart rate changes, or oxygen desaturations within the context of sleep physiology, while at the same time, documenting the effects of sleep disordered breathing on the quality of the patient's sleep.
For example, patients with partial upper airway obstruction may not necessarily show changes in breath volumes or oxygen desaturation, but the subsequent increase in work of breathing may trigger cyclic activations or micro-arousals recorded by the EEG. This condition, commonly described as the upper airway resistance syndrome, has been demonstrated to cause cardiovascular stress, as well as symptoms of excessive daytime sleepiness, similar to those seen with classic obstructive apnea. Without a comprehensive analysis of the EEG, this condition may be left untreated, leaving patients susceptible to further complications as their condition progresses.
One of the difficulties in evaluating clinical outcomes of patients treated for sleep disordered breathing, is the fact that many of these patients are not fully aware of having a problem in the first place. Thus, they may be satisfied with their treatment, yet unknowingly remain at risk for cardiovascular complications or excessive sleepiness. For example, patients treated for obstructive sleep apnea may still have residual pathology in the form of upper airway resistance syndrome, sustained hypoventilation, or subtle obstructive hypopneas with multiple EEG arousals, without being aware of the situation. Again, these conditions cannot be identified by solely relying on cardiorespiratory data. Only polysomnography can confirm whether or not sleep has been fully restored, or whether residual pathology is present.
Polysomnography is generally defined as the simultaneous recordings of multiple physiological variables relevant to sleep. These include, but are not limited to: central and occipital electroencephalogram (EEG), right and left electrooculogram (EOG), electromyogram (EMG) derived from the chin, and from the right and left anterior tibialis muscles, electrocardiogram (ECG), nasal / oral airflow, breathing effort measured at the chest and abdomen, and pulse oximetry.
In comparing polysomnograms from different sleep centers, one will find many variations in how these parameters are actually recorded. Some sleep centers record only one channel of EEG, while others record as many as six. Some place the right eye channel above the left eye channel, others do it in reverse. Some assign two separate channels for recording leg movements, while others combine the leg EMGs on one channel, etc. Most of these differences are inconsequential; the important question is:
How precise are the recorded signals and how accurately are they interpreted?
In surveying sleep technologists today, one will discover that many do not routinely measure the head for EEG electrode placement. Some do not check electrode impedances before starting the sleep study. Others are not aware of artifacts in their recordings, caused by poor electrode application, improper instrument settings, or faulty equipment. Still others rely on filters to correct obvious recording problems (at the expense of signal accuracy), or indiscriminately use automated scoring functions without validating their performance.
These oversights are not the fault of the newcomer to sleep medicine. They usually result from a gradual breakdown in communication, as more and more individuals are hurled into polysomnography without adequate training, or prior experience. Nonetheless, it becomes everyone's responsibility to strive for improvement.
There is a common misconception today that newer technology (specifically - digital polysomnography) is intrinsically superior to traditional analog polysomnography. This is not always the case. Digital data collection does offer some advantages over pen and paper recordings, but it also has its disadvantages. Many of today’s digital systems lack the quality and recording flexibility of traditional analog polysomnographs. Little effort has been made toward developing high standards in amplifier design, arguably the most important component of any recording system. Instead, the major focus of most contemporary digital systems is their scoring software. Consequently, sleep professionals now have endless ways to display, manipulate and tabulate the digitized data, but they often have less control over its quality and accuracy.
Another common misconception about digital polysomnography is that it can somehow correct problems resulting from poor signal inputs, high electrode impedances, inaccurate electrode placements or excessive signal interference. Although many systems now provide additional (and sometimes excessive) signal filtering capabilities, one must never forget the expression: "GIGO" (a computer term which stands for garbage in = garbage out). In short, a signal that appears "good" may not necessarily be physiologically accurate.
Similarly, one must be careful about relying on any type of automated data analysis functions. No matter how sophisticated they may appear, none are foolproof. Accurate interpretations of polysomnographic data must take into account the numerous subtleties of bio-electrical tracings, as well as differentiate between factual data from various forms of artifact.
In short, the use of digital polysomnography may be helpful for streamlining operations, providing more efficient data storage and generating sophisticated reports. It cannot be expected, however, to act as a substitute for quality recording techniques, reliable signal processing hardware, conscientious recording maintenance, and accurate record interpretations - performed by a well-trained clinical staff.
The most important concept in polysomnography is that the quality and accuracy of any recording are directly related to the quality and accuracy of the input signals. This means that:
1) Electrode sites on the patient must be adequately prepared, ensuring low and relatively equal impedances among all the electrodes. There should be no electrical bridging between any electrodes (caused by over-scrubbing with conductive creams or over-spreading the electrode gel).
2) The electrodes and sensors must be accurately placed and properly secured throughout the duration of the recording. Firm attachments should minimize "popping" artifacts, which can easily be caused by patient movement during a sleep study.
3) The input signals must be processed through high quality amplifiers, precisely calibrated to accurately record each individual parameter. This includes appropriate filter settings and sensitivity adjustments for each channel. Digitized signals must be processed and displayed in a manner consistent with established standards of practice, without excessive filtration or distortion of the input signal.
4) The quality of the data collection must be verified throughout the duration of the recording, making any necessary corrections or sensor re-attachments to eliminate undesirable artifacts.
5) The recorded signals must be authenticated by someone who is adequately trained in scoring or interpreting polysomnographic tracings.
One of the best ways to ensure higher standards in polysomnography is to encourage every clinical staff member to become properly trained in the basics of sleep medicine and technology. This not only benefits the patients, but also creates better job satisfaction. No one likes to mechanically perform the same routine tasks day after day (or night after night) without understanding what he or she is doing. A thorough understanding of the sleep recording process gives sleep professionals better control over the quality and accuracy of the recorded data. It also provides them with a feeling that they are part of a team - professionals who truly know their work and can make a difference.
A variety of educational resources and professional training programs are available to both physicians and technologists. Physicians interested in pursuing sleep medicine are encouraged to contact the American Academy of Sleep Medicine (formerly the ASDA) for information on fellowship programs, board certification and sleep center accreditation. Technologists seeking information about the written registry exam should contact the Association of Polysomnographic Technologists (APT).
The interesting thing about polysomnography is that it continues to reveal new information. Many clues to sleep related pathologies which were once overlooked are now being used to better understand and define sleep architecture, sleep-related breathing disorders, EEG arousals, movement disorders, etc.
Polysomnography continues to represent the gold standard in sleep diagnostics and treatment assessment. As with any technology, its true value depends on the skills and expertise of those who use it. By collectively aiming for higher standards, we will all benefit by gaining more knowledge, providing better care for our patients and adding credibility to our field.
Copyright 1998
Synapse Media, Inc.