How accurate is it? This is one of the most common questions I get asked about home sleep studies. In general, both patients as well as sleep doctors think that in-lab studies are much more accurate than home studies. To answer this question, we need to define what “accurate” means.
Suzy came to see me for her years of chronic fatigue and poor sleep quality. She was told she didn’t have obstructive sleep apnea based on an in-lab sleep study performed at a prestigious New York City–Ivy League medical school sleep lab. The report reveled that her AHI was 1, and that there was no evidence of obstructive sleep apnea. When I looked at her airway with a tiny flexible camera while she was flat on her back, I found that her epiglottis that was touching the back of her throat. The epiglottis is a cartilaginous hood-like structure on top of your voice-box. It’s thought to cover your airway when you swallow. Most likely, her epiglottis was flopping back with each inhalation, preventing her to take a full breath in while sleeping.
Not being satisfied with her formal in-lab study, I ordered one of my favorite home-based sleep test, called the WatchPAT. This test found that she stops breathing 19 times every hour.
Pros and cons of the standard in-lab sleep study
The in-lab sleep study hasn’t changed very much since it was creation in the 1950s. Standard measurements include brain waves (EEG), nasal and oral breathing, eye muscle (EMG) chin muscle (EMG), oxygen, heart rate (EKG), chest and abdominal movement, sleep position, and leg movements (EMG). Since the 1990s, a nasal pressure signal using nasal prongs were added to help detect more subtle breathing pauses.
As you might expect, sleeping wired up from head to toe (almost) in a foreign bed while someone is watching you might not be conducive to a good night’s sleep. Furthermore, it may not be possible to sleep in your preferred position, especially if you are a side or tummy sleeper. Some sleep labs tell you to sleep on your back.
However, the in-lab study is still the most comprehensive way of studying your sleep quality. Brain wave monitoring is crucial to pick up seizures and to detect sleep stages.
Pros and cons of the home sleep study
There are many variations of home tests. However, most home studies use a very limited number of sensors, which typically include a finger oxygen sensor, nasal pressure sensor and a chest strap to detect breathing. With few exceptions, most do not measure brain wave recordings or chin/leg muscle movements. So with this setup, you won’t be able to see sleep stages or limb movements.
For most patients with routine sleep apnea, a home test is more than adequate. In general, it correlates well with in-lab studies. In-lab studies were found to be slightly better in picking up breathing events in people with mild sleep apnea. However, if you have moderate to severe OSA, it’s a pretty good test.
One problem for both type of studies
I’ve also had many patients tell me that they couldn’t sleep at all. I can relate to this. During one of my past national sleep meetings, I tested a home sleep test which consisted of a finger oxygen probe, one chest strap and nasal prongs (like what’s used for oxygen in the hospital). Because of the prongs in my nose, I couldn’t breathe in through my nose, and I did not sleep well at all. Having a stuffy nose will cause your mouth to open, leading to more obstructed breathing. This artificially led to a diagnosis of mild sleep apnea, when an in-lab study I did previously showed no significant apneas (when I slept well).
What’s the controversy?
There are now numerous studies showing that home studies can be used as a first line test for most people with classic sleep apnea probability. Only if you have various other medical problems such as neurologic conditions, heart disease do you qualify for an in-lab study. Many insurance carriers require patients to start with home tests first. Children are still required to undergo in-lab testing. There’s really no controversy regarding which is better for sleep apnea as the sleep community has adapted to using both options as necessary.
The biggest debate seems which is better for detecting upper airway resistance syndrome. This is a condition where you stop breathing often, but the length of breathing pauses is not long enough to qualify as an event to be scored on the sleep study as an event that determines the final score that’s used the calculate the AHI (apnea hypopnea index). Apnea is any event where you have obstructed breathing for more than 90% and lasting 10 seconds or longer. Hypopnea is anything more than 30% obstructed breathing lasting more than 10 seconds, with either brain wave arousal, or your oxygen level dropping more than 4%. The total number of (apneas + hypopneas)/hour is the AHI. If your AHI is > 5 and you have certain symptoms or medical conditions, you have mild OSA. If you also all all other events that don’t meet the criteria for hypopnea but lasts for more than 10 seconds and leads to brain wave arousals, then it’s scored as a respiratory event related arousal (RERA). Adding RERAs to apneas and hypoponeas in the numerator will give you the Respiratory Disturbance Index, of the RDI.
Upper airway resistance syndrome
Upper airway resistance syndrome (UARS) has many definitions. The most common one is used as a waste basket diagnosis if you have severe sleep symptoms but don’t formally meet the criteria for sleep apnea. In the old days, a pressure catheter was placed in the esophagus to help detect UARS, but since the introduction of more sensitive of nasal pressure sensors, and more recently with drug induced sleep endoscopy, UARS can be diagnosed without invasive esophageal monitoring. In certain situations where the answer is not clear, esophageal pressure monitoring can be helpful.
Suzy, the patient mentioned in the beginning, had an AHI of 1, well below the level threshold needed for qualifying for treatment. A WatchPAT home study revealed an RDI of 19. In contrast to other home sleep tests, this model doesn’t use a nasal pressure reading or abdominal/chest sensors. Instead, it uses a finger probe (attached to a wrist watch) to measure your oxygen levels as well as what are called sympathetic arousals. Whenever you obstruct, the capillaries in your finger constrict due to the stress response created during your apnea. This is found to correlate highly with in-lab studies in numerous studies.
Because the WatchPAT doesn’t use traditional technology, the RDI is not really the same thing as the RDI on an in-lab sleep study. However, what’s more important is that it’s sensitive enough to detect Suzy’s obstructed breathing episodes with brain wave arousal that lead to finger artery constriction.
From my perspective, it really doesn’t matter which test you undergo. If you have any AHI > 5 and you have one or more the the signs or symptoms of untreated OSA (excessive daytime sleepiness, impaired cognition, mood disorders, insomnia, documented hypertension, ischemic heart disease or history of stroke), you’re eligible for treatment. It doesn’t matter if you have a 6 or 11. There are some practical implications of having mild, moderate or severe OSA, but that will be the topic of another blog post. For many of you, you just won’t have a choice. Your insurance policy mandates that you start off with a home study. If your sleep doctor is highly suspicious that you have sleep apnea despite a normal home sleep study, then he or she can request authorization for an in-lab study. Be patient. This process can be a bit involved or may take while for your sleep doctor’s office.
The biggest challenge is when you know you stop breathing a lot, but the AHI comes in less than 5. Your options include repeating the home test if you didn’t sleep that well, requesting authorization for an in-lab study, or try the WatchPAT test at home. In my experience, in-lab testing rarely comes back with numbers that are significantly different. What’s even more challenging is when both your home and in-lab studies come back completely “normal.” This will be the topic of a future blog post.
Suzy elected to undergo surgery for her floppy epiglottis, and now reports that her fatigue and daily headaches are much improved.
This is Part 2 in a series of articles describing the “Oxygen Illusion.”