The polysomnography (PSG), or sleep study, is a non-invasive overnight test used to monitor and record numerous physical activities during sleep, including brain waves, blood oxygen levels, heart rate, and breathing patterns. This monitoring helps physicians diagnose various sleep disorders such as sleep apnea, narcolepsy, and periodic limb movement disorder. While the initial diagnostic study is the first step, a second study is often necessary. This follow-up is a planned step to gather more specific diagnostic information or to fine-tune a treatment plan, ensuring accurate diagnosis and successful long-term care.
When the Initial Study Was Inadequate
The first sleep study, whether performed at home or in a laboratory, may fail to provide the necessary data for a definitive diagnosis, requiring a repeat procedure. Technical malfunctions are a common reason, such as sensor leads detaching during the night, a power outage, or equipment failing to capture sufficient, high-quality data. If the equipment cannot reliably measure breathing effort, brain activity, or oxygen saturation, the study is deemed technically inadequate.
The “first-night effect” is another frequent cause of an insufficient study, where the unfamiliar environment leads to poor sleep quality. Patients may experience increased sleep latency or a lower total sleep time than the minimum required for a valid test. Since many sleep-disordered breathing events occur during deep sleep stages, insufficient time in these stages can lead to an underestimation of the condition’s severity.
A home sleep apnea test is limited and only screens for moderate to severe obstructive sleep apnea. If a home study is inconclusive or invalid, the patient is typically referred for a comprehensive in-lab PSG to examine sleep stages and brain activity. If the initial study was a “split-night study” and the diagnostic portion did not meet criteria for a quick transition to therapy, a full treatment study is then needed.
Determining the Right Treatment Level
The most common reason for a second overnight study is to determine the correct settings for positive airway pressure (PAP) therapy. The initial PSG diagnoses obstructive sleep apnea and determines its severity, but it does not set the treatment pressure. A second study, known as a PAP titration study, is specifically designed to find the optimal air pressure needed to keep the patient’s airway open throughout the night.
During a titration study, the patient is fitted with a Continuous Positive Airway Pressure (CPAP) mask. A sleep technologist manually adjusts the pressure settings while monitoring the patient’s breathing. They seek the lowest pressure that effectively eliminates apneas, hypopneas, and snoring across all sleep positions and stages. This adjustment ensures the patient receives a therapeutic pressure that is effective and comfortable for long-term use.
Patients requiring advanced therapy may undergo a BiPAP (Bilevel Positive Airway Pressure) or ASV (Adaptive Servo-Ventilation) titration study. BiPAP is used for patients who cannot tolerate high CPAP pressures or who have certain lung conditions, as it offers different pressures for inhalation and exhalation. ASV titration is a specialized procedure for patients with complex or central sleep apnea, requiring a sophisticated device to adjust pressure breath-by-breath.
Monitoring Changes in Health Status
A repeat sleep study may be necessary years after the initial diagnosis to ensure the prescribed treatment remains effective as a patient’s overall health changes. Significant weight gain or loss, for instance, can drastically alter the severity of obstructive sleep apnea. An increase in body weight often raises the pressure needed to keep the airway open, meaning the original CPAP pressure may no longer be adequate.
The emergence of a new medical condition can also necessitate a re-evaluation of sleep therapy. A new diagnosis of a cardiac condition, such as congestive heart failure, or a neurological disorder may affect breathing patterns during sleep. This can change a patient’s sleep apnea from obstructive to central. A follow-up PSG assesses the new breathing pattern and determines if the treatment needs to be switched to a specialized machine.
A second study can also evaluate the success of non-PAP treatments. Patients who have undergone surgical procedures, like uvulopalatopharyngoplasty (UPPP), or who are using an oral appliance, may need a follow-up study to objectively confirm the treatment has reduced their apnea events. Additionally, the original treatment pressure may need re-titration every one to two years to ensure optimal long-term efficacy.
Specialized Diagnostic Follow-up
Beyond sleep-disordered breathing, a second type of study is often required to diagnose conditions characterized by excessive daytime sleepiness, such as narcolepsy or idiopathic hypersomnia. These specialized diagnostic follow-up tests are typically conducted during the day immediately following the initial overnight PSG. The overnight PSG is a necessary prerequisite to rule out other causes of sleepiness, like poor sleep hygiene, before proceeding with the daytime tests.
Multiple Sleep Latency Test (MSLT)
The Multiple Sleep Latency Test (MSLT) measures the physiological tendency to fall asleep in a quiet environment. The MSLT consists of a series of scheduled nap opportunities throughout the day. The technician records how quickly the patient falls asleep and whether they enter REM sleep. A short average sleep latency, especially combined with early entry into REM sleep, is highly suggestive of narcolepsy.
Maintenance of Wakefulness Test (MWT)
The Maintenance of Wakefulness Test (MWT) measures a patient’s ability to stay awake. In this test, the patient is instructed to resist sleep for a defined period during four to five trials. The MWT is often used to assess the effectiveness of treatment for excessive sleepiness. It also helps determine if a patient meets safety requirements for occupations like commercial driving, providing an objective measure of alertness.