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Home Diagnostic Strategies
As mentioned above, healthcare plans, insurance reimbursement and structure of the healthcare system available in some countries for the diagnosis of OSA requires an active search for solutions. Diagnostic studies performed at home with portable equipment have been proposed as an alternative to traditional hospital PSG to improve the diagnostic process and also reduce costs (Table 1).
Clinical Prediction Models
A number of screening clinical questionnaires have been developed to help identify patients at high risk for OSA. The most studied clinical questionnaire is the Berlin Questionnaire. It was validated in primary care, predicting in the high-risk group an AHI greater than 5, with a sensitivity and specificity of 0.86 and 0.77, respectively, identifying patients who are likely to have sleep apnea. The questionnaire with the highest methodological quality is the STOB-BANG questionnaire. It has been developed and validated in surgical patients at preoperative clinics. Their sensitivities with AHI greater than 5, greater than 15 and greater than 30 as cutoffs were 83.6, 92.9 and 100%, respectively. The existing evidence regarding the accuracy of clinical questionnaires is associated with promising but inconsistent results.
Based on these clinical questionnaires, the clinical prediction models seek to establish a reliable diagnosis avoiding the need to perform objective sleep tests. These models are constructed from the analysis of the clinical characteristics of a sample of patients with suspected OSA, who then undergo an objective sleep test to establish the diagnosis and severity of disease. Statistical methods are used to develop a series of mathematical equations involving numerical variables derived from data extracted from the clinical history and physical examination findings. Almost all the proposed models use anthropometric measures such as BMI and neck circumference, as well as sleep disturbances such as snoring, episodes of apnea or asphyxial awakenings. However, the results have not confirmed the objective diagnosis of these tools and their potential use is limited to the prioritization of an objective diagnostic test.
Single-channel Devices
There are several techniques that have only one logging channel. The most widely studied has been pulse oximetry, which quantifies oxyhemoglobin saturation through dual-wavelength spectrophotometers and monitoring of the arterial pulse. Respiratory events are identified indirectly via their impact on the nocturnal saturation curve. Often analysis of the desaturation curve is performed without simultaneous analysis of the finger plethysmography curve, a signal derived from the same sensor that demonstrates validity of the measured desaturation. Absence of simultaneous measurement of the plethysmography curve will always ignore artifact-related saturation drops associated with movement. The result is expressed in episodes of desaturation, usually considered as a drop of 4% from the baseline saturation. Several indices for calculating respiratory involvement have been investigated to reduce the number of PSG necessary. Variability in the reading has also been used, as measured by spectral analysis of the curve (sensitivity: 94%; specificity: 82%; negative predictive value: 92%). However, the overall evidence is not strong enough for this technique to be recommended in the diagnosis of OSA, partly due to the existence of multiple false negatives, especially in young patients, as well as false positives in patients with comorbidities.
Other single-channel devices only analyze the log of the air flow, estimated via changes in temperature or pressure sensors placed in the nose or mouth. Interestingly, some of them have also been validated specifically in an ambulatory setting.
The SleepStrip® (Accutest, CA, USA) is a small, lightweight device worn underneath the nose and above the upper lip comprising flow sensors (oral and nasal thermistors), real-time analysis hardware and software and a miniature display unit. Correlation between AHI in PSG and score in the device was r = 0.73, with sensitivity and specificity values ranged from 80–86% and 57–86%, respectively, at varying AHI thresholds.
The MicroMESAM® (HeimoMed, Kerpen, Germany) is a device that measures respiratory pressure via nasal cannula, allowing automated analysis of apneas, hypopneas and snoring. It correlates highly with PSG (r = 0.98). Sensitivities and specificities were 97.3 and 46%, respectively, at AHI of 5, and 100 and 87.5%, respectively, at AHI of 10.
The ApneaLink™ (ResMed, Abingdon, UK) device measures airflow through a nasal cannula connected to a pressure transducer. Compared with PSG, it has shown the best results at an AHI of greater or equal to 15 (sensitivity: 91%, specificity: 95%).
Although the results are promising, the studies are relatively scarce. They are therefore still not recommended for diagnosis and their usefulness may lie in rapid screening to prioritize more complete diagnostic tests.
Home RP
Devices recommended for unattended home studies by the AASM belong to the so-called type 3 classification of portable devices, which consist of a limited number of channels but no neurological variables. This type includes the home respiratory polygraphy (HRP), which records airflow, respiratory effort and oxygen saturation, identifying the respiratory events and episodes of desaturation necessary for diagnosis.
Several studies have evaluated HRP in the diagnosis of OSA, but there is great variability between them in terms of design, methodology, definitions of respiratory events, cutoff points for diagnosis and the types of device used. These studies provide positive results, but few are randomized. Moreover, few studies assess the costs of these tests and their savings with respect to conventional techniques.
One recent study stands out on account of its larger sample and multicenter design. Masa et al. included 366 patients suspected of OSA in a comparison of the effectiveness of HRP in diagnosis versus hospital PSG, as well as a cost analysis. A total of 348 patients completed the protocol, which included a HRP and a hospital PSG and simultaneous RP. With a cutoff point of AHI <5 events/h, the sensitivity was 96% and the specificity was 57% with a negative likelihood ratio of 0.07. With a cutoff point of >10 events/h, the levels of sensitivity and specificity were 87 and 86%, respectively, with a positive likelihood ratio of 6.25. Critics of this study indicate that the patients were already prescreened and clinically suspected of OSA, and thus results are only valid for patients already clinically suspected of having the syndrome, not a general population study. In the economic evaluation, the HRP cost approximately 50% less than the PSG in the hospital. The authors therefore concluded that it is a valid and efficient diagnostic test for the diagnosis of OSA.
Home PSG
Some studies have chosen to use a PSG in the patient's home to add to the information obtained by including neurological variables, while avoiding the first-night effect often experienced by patients when they go to sleep laboratories. As conventional in-laboratory studies, home PSG devices recorded electroencephalograms, electrooculograms, electromyograms, nasal pressure, oronasal flow, chest and abdominal movement, arterial oxygen saturation, electrocardiograms, body position, leg movements and sound. Thus, Bruyneel et al. compared the results of PSG in a hospital and those produced with a laptop in the patient's home recording the above channels. Although most of the studies proved less efficient in the home (the signals most likely to fail were: respiratory effort band, sound, nasal pressure and oximetry), the differences were not significant. By contrast, there was an improvement in sleep quality, measured by efficiency, latency and total sleep duration, and a reduction in costs of nearly 75%. The authors concluded that a PSG is a feasible and reliable alternative in patients with a high suspicion of OSA. Other studies have obtained similar results, particularly with regards to patient satisfaction, so this option should be more widely evaluated so that its use can become generalized in subjects with high suspicion of OSA.
Critical Summary of Home Diagnostic Strategies
The evidence available seems to endorse the use of portable devices in the home, but two questions need to be asked about home studies. The first is: how should these tests be performed? The AASM recommends the use of type 3 devices along with a formal clinical evaluation by a specialist in sleep medicine. It is possible to correct the reading by performing an automatic analysis with the software that comes with the device. Several studies have evaluated the concordance between the manual analysis of HRP and the PSG results and they have proved comparable when a higher cutoff point in the AHI was needed for diagnosis. Thus, the use of these devices with a manual correction is currently recommended.
The second question is: which patients should use these home tests? According to the recommendations of the AASM, type 3 devices should be used as an alternative to PSG in patients with a high pretest probability of having moderate-to-severe OSA (risk factors such as snoring, sleepiness, obesity and witnessed apneas are often correlated with the severity of the syndrome). Clinical judgment is the tool most used to establish this probability, guided by characteristics such as snoring, observed apneas, EDS and obesity. It is also recommended when a hospital evaluation is not possible (immobility, advanced disease and so on). Patients with significant comorbidities or other suspected sleep disorders must be excluded; these cases call for a PSG. In a case of clinical suspicion and a negative home test, OSA cannot be ruled out and a further hospital test should be undertaken.
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