Deepak Shrivastava, MD, FAASM, RPSGT
(A Sleep Specialist)
A ‘Sleep Study’, also known as polysomnography or PSG, has been used for decades to diagnose and evaluate the severity of sleep apnea or reversible cessation of breathing during sleep. Sleep apnea, sometimes called obstructive sleep apnea, is a common health problem that affects millions of men and women. It occurs in children as well. The potential life-threatening effects of sleep apnea include heart and blood pressure problems, anxiety, depression and other mental health problems and difficulty is controlling blood sugar and cholesterol levels. An increasing number of sleep studies are being conducted as more people are becoming aware of the importance of good quality sleep and doctors are evaluating more people with sleep apnea symptoms. The presenting symptoms could be as subtle as daytime fatigue, tiredness, non-refreshing sleep or more intense like snoring, daytime sleepiness, and bed-partner noticing complete cessation of breathing until an awakening (can occur every few seconds to every couple of minutes).
After a complete evaluation in the office, the doctor orders a sleep test according to patient needs. In some cases, patients need a thorough overnight in-lab sleep test in the clinic, while others require a second night if continuous positive airway pressure (CPAP) is required to open up the airway. Some patients may need a split night study whereby after a couple of hours of documenting difficulty in breathing, a CPAP mask is used for the remainder of the night with the idea of accomplishing both the diagnosis and treatment in one night. Depending on the severity of symptoms, doctors may order a sleep test at the patient’s own home with the use of a “Home Sleep Test, or HST, instead of in the sleep laboratory.
Once the sleep study is completed, a detailed sleep study report is sent to the requesting physician with the diagnosis and recommendations for treatment. A copy of the report becomes available to the patient upon request. The intention of this article is to provide a simple and easy method to understand the results of the sleep test.
Components of a sleep study report
The sleep study reports are typically organized into sections containing patient’s demographic information, reason for the test, pertinent findings noted during the pre-test office evaluation that includes sleep related symptoms, and the technical details of how the study was conducted. The technical detail part documents the number of wires attached to the patient’s head, around the eyes, on the chin, legs, chest, and small plastic tubing placed just outside the nostrils and mouth to measure and record the air flow. The chest and abdomen movements are recorded by two belts. The amount of oxygen in the blood is measured without using any needles or syringe; by a band-aid like device wrapped around the tip of the finger called an oximeter. A small microphone is attached to the skin in front of the neck to record snoring sounds. Although the whole thing sounds busy, all the wires are neatly pony-tailed and do not come in the way. One can move around, walk, read, and talk with just a few wires stuck to skin painlessly.
Most reports show data indicating total recording time and/or time in bed along with total sleep time. The total recording time is the total amount of time during which the patient is in bed with recording equipment activated. The amount of time actually spent in bed is an important limiting factor for the total sleep time and sleep stages. A patient who spends only three to four hours in bed cannot reasonably accumulate normal amounts of sleep and may not go to all normal stages and cycles of sleep. Therefore, a low total time in bed may be of clinical significance and may support a diagnosis of insufficient sleep.
Sleep latency is perhaps one of the most important parameters in a sleep study. The duration of time it takes to fall asleep once lights are turned off (also known as lights out) as the patient attempts to sleep, until the time patient actually falls asleep, as evidenced by brain waves and behavioral observation, is reported as sleep latency. Sleep latency is the time in minutes from ‘lights out’ that marks the starting of total recording time to the first evidence of sleep. Sleep latency also indicates if reasonable attention was given to the patient’s habitual sleep schedule and the ‘lights out’ time was close to the patient’s routine bedtime at home. Clearly, if the lights are turned out earlier than the patient’s usual bedtime, sleep latency would be artificially long, and the patient may not fall asleep until his/her usual sleep time is reached. Similarly, if the ‘lights out’ time is later than the patient’s usual bedtime, the patient will be sleepy and erroneous short sleep latency will be recorded.
The total sleep time is the total amount of sleep time scored during the total recording time. This includes time from sleep onset to sleep offset and is distributed throughout the sleep time as minutes of Stage N1 sleep, Stage N2 sleep, Stage N3; all non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. All these times are described in minutes. A low total sleep time may indicate that the patient slept for an insufficient period of time due to non-medical/non-physiological reasons, certain medical or sleep disorders, or as a result of the effect of medications. A high total sleep time may suggest prior sleep deprivation, medical conditions, or effects of medications. High levels of sleep fragmentation, as defined by recurrent awakenings and/or stage shifts may result in complaints of non-restorative sleep even when an apparently normal total sleep time is present. Sleep efficiency is another important parameter that refers to percentage of total time in bed actually spent in sleep. Sleep efficiency gives an overall sense of how well the patient slept, but it does not distinguish frequent, brief episodes of wakefulness. A low sleep efficiency percentage could result from long sleep latency and long sleep offset to ‘lights on’ time with otherwise normal quantity and quality of sleep in between. Many laboratories report total wake time, that is, the amount of wake time during the total recording time in minutes after the sleep onset. The total amount gives a general estimation for overall quality of sleep. Total wake time is the reciprocal of total sleep time. A high total sleep time percent is always associated with low total wake time percent and vice versa.
An important reported number is waking up after sleep onset, also known as ‘WASO’. This value measures wakefulness, excluding the wakefulness occurring before sleep onset. WASO time is a better reflection of sleep fragmentation. Wake time after sleep offset is known as ‘WASF’ and refers to wakefulness that occurs after sleep offset. Long periods of wakefulness following an atypical early morning awakening could be consistent with one of the classic signs of depression. This can be found in elderly patients who have no difficulty in falling asleep, but wake up after three to four hours of sleep and are unable to return to sleep. This pattern may be seen in patients who suffer with depression or anxiety and sometimes due to an effect of medications.
Another crucial reported parameter is rapid eye movement sleep latency also known as REM latency. Rapid eye movement latency is the time from the sleep onset to the first occurrence of REM sleep; therefore, it depends on the patient’s sleep latency. The REM sleep cycles about every 90 to 120 min intervals throughout the night. The changes in REM sleep latency are considered potential biological markers for a number of sleep-related disorders. REM sleep is very sensitive to the effects of medication, sleep deprivation, and circadian rhythm disorders. The knowledge of patient’s current medications and the quality of sleep the night before the sleep study therefore, is extremely important to review. A long or short REM latency time may result from use of or withdrawal of many medications. Sleep apnea and periodic limb movement of sleep can also lead to long REM sleep latency.
Stages of sleep
The normal percentage of each of the four stages of sleep is reported with the number of total NREM-REM sleep cycles recorded overnight.
Non Rapid Eye Movement sleep: In adults, stage N1 is about 5% is; stage N2 50%; and stage N3 is about 20%. The remaining 25% is REM stage sleep. Stage N1 sleep is associated with the transition from wakefulness to sleep and is considered a direct measure of daytime alertness and the subjective refreshing quality of sleep. The quantity and the percentage stage N1 sleep is an estimate of the degree of sleep fragmentation. A high percentage of the stage N1 sleep is generally a result of frequent arousals caused by sleep disorders, like sleep apnea, periodic movement of sleep, or snoring. Other causes of sleep disruption, including environmental disturbances, may also lead to increased amount of stage N1sleep.
Stage N2 sleep tends to dominate sleep with 50% of the total sleep time. It follows stage N1 sleep and continues to recur throughout the night. A low percentage of stage N2 sleep may be a result of sleep fragmentation, increased REM, stage N3 or a result of obstructive sleep apnea-related arousals. An increased amount of Stage N2 sleep may also be noted in age-related changes in sleeping pattern and may be a result of medication effect.
Stage N3 is considered as ‘deep sleep’. It is sometimes referred as slow wave sleep or delta sleep. Stage N3 generally cycles frequently in the first third of the night and begins to reduce towards the second half of the night. A high amount of stage N3 sleep is noted during rebound sleep (such as recovery sleep after sleep deprivation, initiation of nocturnal CPAP treatment, or treatment of periodic limb movement syndrome). Less stage N3 sleep is noted as a side effect of certain medications. Episodes of night terror, sleep walking, sleep talking, and confusion arousals also occur during stage N3 sleep. Stage N3 is known to suppress occurrence of sleep-disordered breathing.
Rapid-eye movement sleep: The exact function of the REM is uncertain. However, it occupies approximately 25% of total sleep time. REM sleep cycles every 90 to 120 min throughout the night with progressively increasing periods of time. REM sleep is associated with more frequent and longer duration apneas, hypopneas, and severe hypoxemia. REM sleep suppresses periodic leg movements of sleep. Certain medications suppress the REM sleep. Many sleep disorders, including sleep apnea, REM behavior sleep disorder, and nightmares occur in REM sleep. A higher amount of REM sleep is noted during recovery sleep after REM sleep deprivation. REM sleep ‘rebound’ occurs after discontinuation of REM sleep suppressing medications, alcohol, and initiation of CPAP therapy.
The report further includes respiratory data summarizing total number of apneas (complete cessation of breathing), hypopneas (reduction in airflow by 30% or more) and amount of oxygen in the blood during these events. The breathing events are reported as an hourly index called ‘Apnea-hypopnea index’ (AHI). The severity of sleep apnea is expressed by range 0 to 4 events per hour of sleep as normal, 5-14 as mild, 15-29 as moderate and 30 and higher as severe. Similarly, information regarding drop in the oxygen saturation is noted throughout the night. Arousals from sleep are recorded and reported as a measure of sleep fragmentation. Cardiac rhythm is analyzed and reported to correlate with the respiratory findings. Unintentional movement of the legs is reported in reference to any associated arousals from sleep. Body position as it relates to the apnea and hypopnea events is reported. If CPAP is applied then extensive data is reported to explain how a certain therapeutic pressure was reached.
Based on the above data and the clinical context of presenting symptoms, the sleep specialist makes the diagnosis and recommendations for the treatment options. These options may include CPAP or oral appliance as non-invasive medical therapy or surgical intervention as invasive therapy.
Sleep hygiene and safety
Doctors generally explain how to get a good quality sleep, so called sleep hygiene. A discussion about a healthy diet, caffeine consumption (and less obvious sources of caffeine such as chocolate, pain relievers, and herbal supplements) and nicotine restriction 6 hours before bedtime, adequate exercise, maintaining a dark and quiet environment suitable for sleeping, coping strategies with shift work, avoiding napping, and having a consistent bed time can be valuable in giving a starting point in improving sleep quality. An assessment of person’s medical conditions (emotional disorders, gastrointestinal disturbances, musculoskeletal pain, and Restless leg syndrome) and use of medications, both prescription and over the counter that may contribute to lack of sleepiness is invaluable. A discussion about the use of alcohol before sleep is important. Although alcohol shortens time to fall asleep, it can lead to many undesired awakenings throughout the night disrupting the continuity of sleep. Recommendation regarding weight management is crucial in the successful control of sleep apnea and its complications. Many safety related warnings are included in the sleep study report. Refraining from drowsy driving, operating heavy machinery and home safety is stressed. Occupational and public health hazards can be prevented with careful attention to the recommendations.
An overall review of the sleep study report provides an excellent account of what was recorded during 6 to 8 hours of sleep test. Patients may also report their post-treatment residual problems and complications related to the treatment to their health care provider. Multiple recommendations can be made based on the observations made in the sleep study report. Clinical management decisions regarding normalizing the long sleep latency may be made by practicing good sleep hygiene, thus avoiding over the counter sleep aids and sleeping pills. Improvement in sleep efficiency can be accomplished with increase in total sleep time in relation to total time in bed, as well as exploring potential causes of poor sleep efficiency. A good example of this is when adequate pain management of chronic pain syndrome results in improved sleep quality. Sleep reports are concluded with a statement of the diagnosis and recommendations for the management. Following specific guidelines patient may require CPAP therapy, or consideration of other options like oral appliances, and surgical interventions. If patient is a candidate for CPAP, management plan includes recommendations for the pressure to be applied, type and size of the mask, and a humidifier for patient comfort and to prevent drying of secretions. ‘Ramp time’, a gradual increase in CPAP pressure over many minutes as the patient tries to fall asleep. Use of Ramp time feature is recommended for patients who may not tolerate high CPAP pressures. A mouth air leak while using a nasal mask is corrected by use of a chinstrap to keep the jaw from falling open.
Multiple variables affect the sleep pattern including the ‘first-night effect’ when the patient cannot sleep well in the sleep laboratory and has a different sleeping pattern than usual. First-night effect can increase both stage N1 sleep and REM latency recorded in the sleep study. The ‘reverse first-night effect’ is when the patient sleeps better in the sleep laboratory compared to their home, as in case of insomnia and frequently observed ‘night to night variability’ in sleep. It is, therefore, important to realize that in a patient with high clinical pre-test probability of sleep-disordered breathing, a negative sleep study may not rule out the condition.
A clear understanding of the sleep study report facilitates appropriate patient participation in decision making. It enables patients and the primary care physician to recognize the need for a follow up appointment with a sleep specialist or the equipment provider. In addition, it empowers the patient to self-address trouble-shooting, and to work on problems that potentially lead to non-compliance. Early recognition and management of problems with CPAP or oral appliance use facilitates efficacious treatment. Such interventions lead to an improvement in patient’s overall sense of well-being and potentially reduce many complications associated with sleep-disordered breathing.