Diagnosis of Obstructive Sleep Apnea

How is OSA diagnosed?

Unfortunately, many people with OSA are not aware they have it and thus go untreated.

Polysomnography

The primary method to diagnosis sleep-disordered breathing is polysomnography or PSG. PSG is a multi-parametric test. A PSG records physiological changes that occur during sleep such as electrical activity in the brain measured by electroencephalography (EEG), eye movement measured by Electrooculography (EOG), muscle activity and/or skeletal muscle activation measured by electromyography (EMG), and heart rhythm determined by electrocardiography (ECG). Most importantly respiratory airflow, respiratory effort and oxygen saturation are also examined. A PSG involves recording at least 12 channels using at least 22 wires connected to the patient while sleeping. Snoring may also be recorded using a sound probe placed across the neck.

Apnea–Hypopnea Index

While there are several outcomes to these tests, diagnosis primarily depends on the Apnea–Hypopnea Index or Apnoea–Hypopnoea Index (AHI). The level of severity is determined by the AIH. The AHI index is a number determined from the number of apnea (cessations in breathing lasting at least 10 sec accompanied by decreased blood flow) and/or hypopnea (Shallow breathing) events that occur during sleep, and the decrease in oxygen saturation. AHI is determined by dividing the number of apnea events that occur during sleep by the number of hours slept. Severity level falls in the following categories:

Normal: AHI < 5
Mild OSA: 5 ≤ AHI < 15
Moderate OSA: 15 ≤ AHI < 30
Severe OSA: AHI ≥ 30

For children, an AHI above 1 is considered irregular.

AHI does have its limitations in that it does not consider extreme situations in which a patient may or may not experience low levels of oxygen in the blood (hypoxemia). AHI also does not consider alterations in breathing that do not meet the AHI criteria, but still have an impact on OSA symptoms such as non-apneic respiratory events and breathing disruptions that do not cause major hypoxemia. AHI also cannot be used to pinpoint the cause or personalized symptoms of the disease.

PSG does require a great deal of time, attention, and cost. Thus, other at home monitoring methods have been developed. These at home devices may take the same measurements as those taken in the lab or only a few major measurements. At home devices are used to diagnose severe cases.

Pharyngeal critical closing pressure

Upper airway collapsibility during sleep is determined using the pharyngeal critical closing pressure technique or Pcrit. Pcrit is performed by placing the airway under conditions of reduced neuromuscular input. CPAP is used at a level low enough to just prevent airway obstruction. Then brief reductions in holding pressure are applied. These reductions are repeated at various mask pressure levels until airflow is blocked. They then determine the relationship between the peak inspiratory (breathing in) flow during limited breaths and the corresponding mask pressure. These data points are used to determine Pcrit, mask pressure at zero airflow. Pcrit can range from -5 to +5 cm H2O in OSA cases, with Pcrit at +5 indicating a highly collapsible airway, while a Pcrit below 0 (atmospheric pressure) indicate a stable upper airway. However, nearly 20% of people with OSA have a low Pcrit similar to those without OSA. In these individuals, the combination of anatomical and non-anatomical contributors are is the likely cause of OSA.

While beneficial, the Pcrit technique is not realistic for clinical use. The procedure requires high technical skill, is partially invasive, time-consuming, and requires a skilled individual to collect, analyze, and interpret the data. Thus, efforts are being made to generate a more simplified procedure. One procedure involves placing a nasal breathing mask on the patient and applying brief periods of negative pressure during early expiration (breathing out) to temporarily increase expiratory airflow based on the level of upper airway collapsibility. The outcome measure is the ratio of the exhaled volumes prior to negative pressure application to expiratory pressure for 10 replicated trials. An increased ratio suggests a collapsible airway. The application of this technique with others simple measures may inform treatment strategies.

Undiagnosed OSA

Most people with OSA have not been diagnosed. In the clinical studies of individuals with OSA, nearly 80% of people with moderate to severe OSA had not been diagnosed by their primary care physicians after having the condition for 10 years. Individuals with OSA use more healthcare resources (almost twice as much, particularly among women) than those without OSA. However, the costs of treating related health consequences is significantly reduced by treating OSA itself. This emphasizes the need to identify, diagnose and treat OSA early. This would not only prevent or slow down the progression of OSA, but also reduce the development of several related cardiovascular and metabolic conditions.