Previous Work

Sleep disorder is a major public health problem, affecting up to 5% of the world population [6], with levels reaching values as high as 4% for men, 2% for women, and 3% for children [7]. The science of sleep medicine has evolved tremendously as a result of the development of tools that enable us to detect and document the activities of various physiological and pathological events that occur in the central nervous system accompanied by changes, which develop in the cardio respiratory, circulatory and autonomic nervous systems during sleep. Polysomnography is used to diagnose, or rule out, many types of sleep disorders including narcolepsy, restless legs syndrome, REM behavior disorder, parasomnias, and sleep apnea.

Fig. 1-1: Normal people and OSA patient [44]

Obstructive sleep apnea syndrome (OSAS) evaluation from PSG data was scored by clinical experts by using the standard procedures and criteria [6]. Sleep Apnea is a condition where a person periodically stops breathing during sleep. This causes the person to wake up dozens of times during the night, but in most cases they are unaware of this disruption. The most common type of sleep apnea is Obstructive Sleep Apnea (OSA), which is caused by closure of the airway. The drawing to the right shows the airway of a healthy individual (left) and an OSA patient (right). The

healthy patient is breathing normally with the airway open, but the OSA patient's airway is closed, with the arrows showing the blockage [Fig. 1-1].

The American Association of Sleep Medicine [AASM] published initial practice parameters regarding the use of Portable Monitor (PM) devices in the assessment of OSA in 1994[8]. Many studies have been carried out for OSAS screening attempting to reduce PSG cost and complexity. Different techniques have been proposed, oximetry-based screening being one of the most widely suggested for both the adult and pediatric population. Although these methods have high sensitivity, they tend to have very low specificity [9]. The society classified sleep apnea evaluation studies based on the number of channels or signals that the monitor employed, categorized from level I to level IV. A minimum of 6 hours of recording time was recommended when using any of the configurations. The levels of portable recording equipment and delineated specifications are shown asTable 1.

Table 1: Sleep-apnea evaluation studies (6-hour overnight recording minimum)

Level I monitoring consists of full overnight polysomnography, with a minimum of two channels each for EEG, chin EMG, EOG, as well as respiratory airflow (with thermistor or pressure-flow transducer), respiratory effort (thoracic and abdominal breathing movements), oximetry, and ECG or heart rate monitoring. These studies are fully attended by a technologist and are typically conducted in a sleep center. The level I system is shown as Fig. 1-2.

Fig. 1-2: Alice 5^® Diagnostic Sleep System and Pediatric Polysomnography patient [10]

Laboratory-based polysomnography forms the framework upon which the field of sleep disorders medicine has been built over the last 40-50 years. For the standard test the patient comes to a sleep lab in the early evening, and over the next 1-2 hours is introduced to the setting and "wired up" so that multiple channels of data can be recorded when he/she falls asleep. The sleep lab may be in a hospital, a free-standing medical office, or in a hotel. A sleep technician should always be in attendance and is responsible for attaching the electrodes to the patient and monitoring the patient during the study. After the test is completed a 'scorer' analyzes the data by reviewing the study in 30 second 'epochs' [11].

When obstructive sleep apnea (OSA) became recognized as a common sleep

disorder, cardiorespiratory variable were added to polysomnography as a standard feature. The high prevalence of sleep-related breathing disorders has highlighted limitations in patient accessibility to diagnostic and therapeutic services. In addition, and the need for studies has increased, less costly but comparable efficacious alternatives to laboratory-based polysomnography are being sought in response to current economic imperatives. Standard sleep laboratory research protocols often require two consecutive PSG nights because of inter-night variability in sleep attributed to first-night adaptation to a novel sleep environment and recording procedures. Finally, home studies may provide a more realistic appraisal of nighttime pathology than can be obtained in the laboratory setting. Patients can now be evaluated outside of the laboratory by using portable devices that can record a single channel such as oximetry, two or more channels that measure only respiratory variables, or multiple channels that allow for sleep staging as well as measurement of respiratory variables.

Portable studies (level II to IV) are summarized as follows: level II consists of an equivalent number of channels as level I, with the singular difference being that the study is not attended by a technician. Like level I studies allow for the identification and quantification of sleep stages. Using the Sleep Heart Health Study (SHHS) methodology and technology, Iber and colleagues [12] recruited 76 participants from the general community to volunteer for recordings both in the laboratory and at home.

Subjects were randomized with respect to recording order and were monitored with the same level II device used for the SHHS cohort (Compumedics; Abbotsford Australia). From this study, Using SHHS methodology, median RDI was similar in the unattended home and attended laboratory setting with differences of small

a rate of disease misclassification that is similar to repeated studies in the same setting.

The level III utilizes at least four channels, including two channels for respiration and one channel for cardiac monitoring. The level III system is shown as Fig. 1-3.

Dingli and colleagues [13] assessed the diagnostic accuracy of a type 3 monitoring.

The study design consisted of simultaneous recording with the PM and traditional in-laboratory polysomnography, followed by an at-home assessment with the same PM. While the in-laboratory RDI and home RDI recorded from the type 3 monitor demonstrated no difference, the AHI generated from the in-laboratory polysomnography was significantly different.

Fig. 1-3: Patient wearing the Embletta (Level III) [13]

Level VI is made up of only one or two channels, typically including oxygen saturation or airflow. Pittman and colleagues tested a novel type4 monitoring device (Watch PAT; Itamar Medical; Caesarea, Israel, Fig.1-4) against traditional in-laboratory polysomnography [14]. The Watch PAT is wrist-worn device that collects peripheral arterial tonometry and oxygen saturation data, coupled with actigraphy.

Fig. 1-4: Watch PAT; Itamar Medical; Caesarea, Israel[45]

The paper [12] objective of this meta-analysis study was to compare the accuracy of home sleep studies with laboratory polysomnography in the diagnosis of obstructive sleep apnea (OSA). Home sleep studies provide similar diagnostic information to laboratory polysomnography in the evaluation of sleep-disordered breathing but may underestimate sleep apnea severity. The lower cost of home sleep studies makes it a viable screening tool for patients with suspected OSA; however, these lower costs are partially offset by the higher rate of inadequate examinations.

The primary end point examined was the ability of PM devices to confirm or rule out disease. The AASM guidelines [15] did allow for the use of PM devices under certain conditions. These include the lack of available polysomnography for patients with severe clinical symptoms consistent with OSA, the inability of the patient to be studied in the laboratory, or to evaluate response to therapy in a patient who has already undergone traditional in-laboratory polysomnography. A number of limited-channel, in home devices for the diagnosis of OSA have been described [16-22]; however, as a group they have not been recommended in the published practice parameters for in-home unattended studies [15, 23]. The primary reason

apnea diagnostic systems into levels of complexity is used to simplify comparisons [23], it has the effect of obscuring the validity of individual devices with acceptable validation studies.

However, there are several limitations of PM devices that must be considered as well. These include the inherent lack of an attendant during the study, which may potentially affect data quality. In addition, the most widely used applications of PM technology do not have EEG channels and are unable to assess sleep architecture of staging. This inability does not allow for the computation of the apnea-hypopnea index (AHI) because total sleep time cannot be calculated.