CPAP Is the First Choice of Physicians When Treating Obstructive Sleep Apnea, But CPAP Often is an Incorrect Therapy Option in Charcot Marie Tooth Disease
Sleep disorders in the general population are common. The American Sleep Association estimates that 50 – 70 million adults in the US have a sleep disorder, with 25 million having Obstructive Sleep Apnea (OSA). A much less common type of sleep apnea, called Central Sleep Apnea (CSA), affects less than 10% of people who experience Sleep Disordered Breathing (SDB). The go-to therapy for OSA and CSA is CPAP. CPAP is an acronym for Continuous Positive Airway Pressure. CPAP provides a constant supply of low-pressure air that keeps the airway open so that breathing does not stop. However, CPAP is contraindicated in Charcot Marie Tooth disease. Why is CPAP bad for a CMTer, and what are the options? To answer those questions, we must dig a little deeper.
Under Pressure
Sleep Disordered Breathing—a collection of sleep disorders characterized by irregular breathing while asleep, is common in the general population. Research suggests that CMTers might bear a predisposition to SDB. A study first published in 2007 established that especially CMT1A has a predisposition to OSA. Dziewas, et al. note in the study that, “pathophysiologically, one may assume that CMT1 related pharyngeal neuropathy increases the collapsibility of the upper airway which in turn leads to recurring obstructive respiratory events.” Treating SDB is straightforward in the general population. Basically, throw a bunch of air into the collapsing throat, and you are good to go. Neuromuscular disease, including CMT, however, presents additional challenges.
CMT experts and researchers hold that respiratory involvement outside of SDB conditions in CMT is rare. There is not much data depicting the prevalence of respiratory involvement in CMT. Anecdotal evidence disagrees with published literature. One must only look as far as social media CMTer communities to see the overwhelming number of personal stories about respiratory issues endured by CMTers, enduring often without treatment. This disparage between anecdote and expert consensus leaves CMT caused respiratory involvement a controversial topic. Nearly all CMTers who talk about experiencing respiratory impairment also talk about having SDB, be it OSA or CSA, with OSA being the most common. A good indicator of respiratory involvement as it pertains to the treatment of SDB in CMT is the CMTer’s response to the first line of therapy—CPAP.
In an oversimplified explanation of CPAP therapy, a machine outputs air at a specified pressure, through a hose connected to a mask. The unit of measure for this is centimeters of water pressure, or cmH2O. This pressure is exceptionally low. To give it some perspective, 1 psi is equal to 70.3 cmH2O, and maximum CPAP pressure is 25 cmH2O. Even at CPAP maximum pressure capability, the equivalent is only 0.35 psi. As a therapy for OSA, the continuous pressure output of CPAP forces the airway to stay open during inspiration (inhale) and expiration (exhale). The pressure remains the same for expiration as it is for inspiration. Even though CPAP pressure output is low, the continuous pressure can cause respiration difficulty in CMT, and especially during expiration. When difficulty is present, it is rooted in respiratory muscle weakness.
CMT does not cause disease in lung tissue. Rather, when there is respiratory involvement, the involvement is the result of the CMT disease process in the peripheral nerves that control the respiratory muscles. Just as CMT causes weakness in the muscles controlled by the motor nerves, CMT can also cause weakness in the muscles controlled by the somatic nerves that control breathing, i.e. the phrenic nerves and the intercostal nerves.
Respiration occurs because of muscle contractions. To inhale, the diaphragm contracts and moves downward. This downward motion creates a low-pressure area in the chest. Because pressure moves from a high-pressure to a low-pressure, and because the inside of the chest now has a lower pressure than what is outside the body, air rushes in to fill the lungs. To facilitate the lungs filling completely, the intercostal muscles of the rib cage contract, thereby expanding the chest. Once we have taken in as full of a breath as we can, the respiratory muscles contract in the opposite direction, forcing out the air from the lungs. This, of course, is an oversimplified explanation of respiration, but it is how the respiratory pump works.
When a CMTer has respiratory muscle weakness, it can be difficult to take in a full breath. Likewise, it can be difficult to exhale completely. When the respiratory muscles are weak because CMT has affected the nerves that control them, a CMTer might not have adequate muscle strength to get a full breath and/or to fully exhale. This creates a condition called hypoventilation. Barring any lung tissue disease that the CMTer might also have, oxygenation typically remains normal. However, an inability to fully exhale can lead to hypercapnia— an excessive level of CO2 in the body.
Every breath we take results in a gas exchange in the lungs. The lungs transfer oxygen into the blood stream during each inhale, and the lungs transfer carbon dioxide from the blood stream, expelling it with every exhale. This is an efficient process when everything is running as it should. When the respiratory muscles become weakened by CMT, a CMTer might not have the ability to take in a full breath. The most that a person can exhale with each breath is equal to the air volume that the person inhaled. If a CMTer cannot get a full enough breath to facilitate an adequate expiration volume of air that the lungs need for adequately filtering out CO2, CO2 will start to build up in the system. Adding to this dilemma, if a CMTer cannot then fully empty the lungs before taking the next breath, the CO2 left behind goes back into the bloodstream, thereby causing CO2 levels to rise. SDB leads to hypoventilation and the build-up of CO2 also.
Treat Me Right, Doc
Sleep Disordered Breathing, such as OSA and CSA, causes hypoventilation. Hypoventilation leads to excessive CO2 in the body. SDB causes hypoventilation by causing a disruption in regular breathing. SDB disrupts breathing via obstructing the airway (OSA), via a disruption in the communication channels between the brain and the respiratory system (CSA), or sometimes via a combination of both. To treat the conditions, physicians use CPAP. Typically, by keeping the airway open with CPAP, breathing during sleep is managed well. What happens when there is a neuromuscular component such as respiratory muscle weakness caused by CMT?
Obstructive Sleep Apnea is quite common in CMT. Research shows a predisposition to OSA in at least some types of CMT. CPAP is the first line of defense physicians deploy for treating Sleep Disordered Breathing, including OSA. However, according to Dr. Ashraf Elsayegh, MD, FCCP, Pulmonary/Critical Care, Cedars-Sinai Medical Center, CPAP is not recommended in neuromuscular disease. Despite its low-pressure, a CMTer can have difficulty exhaling against the continuous pressure of CPAP. This difficulty can fatigue the respiratory muscles. Respiratory muscle fatigue induced by CPAP can lead to hypoventilation; and of course, hypoventilation can lead to the build-up of CO2 in the body. CPAP induced hypoventilation is counterintuitive to the objective of CPAP therapy. Not only can CPAP not effectively treat SDB for a CMTer, CPAP can worsen the effects of SDB, at the hands of CPAP induced hypoventilation. If CPAP is out of the question, what are the options?
When a CMTer has a Sleep Disordered Breathing condition, CPAP is not the best option. When a CMTer also has respiratory muscle weakness, an SDB can be even more difficult to treat. An option that physicians sometime use is BiPap therapy. BiPap is the acronym for Bi-level Positive Airway Pressure. BiPap is like CPAP, but BiPap allows for a different and often lower expiration pressure. There are clear advantages for the CMTer with BiPap. However, the consensus is that Noninvasive Ventilation (NIV) is the best option for CMTers. What is NIV? Noninvasive ventilation sounds scary, sure, but it is just supercharged BiPap, but with a twist.
Noninvasive ventilation is a type of respiratory therapy that provides ventilation (respiratory) support, noninvasively. Conversely, invasive ventilation is mechanical ventilation support via intubation or via a surgical tracheotomy. NIV, much in the same way as CPAP, consists of a small tabletop machine that delivers air to a mask via a hose. Where NIV excels is the capability to provide volume support in addition to pressure therapy.
Continuous Positive Airway Pressure is a limited therapy. CPAP can successfully keep an obstructed airway open, but the rest of the respiratory system must function at peak. If there is any kind of deficiency, like even minimal respiratory muscle weakness that can be present in CMT, for example, CPAP often will not succeed in achieving therapy goals. BiPap therapy can provide an advantage over CPAP by allowing for a lower expiration pressure, but that is where BiPap reaches its therapeutic limit. Neither CPAP nor BiPap have the capability to treat the variability in respiration that a CMTer can have. NIV picks up where CPAP and BiPap leave off.
Exceeding the Limitations
Noninvasive ventilation exceeds the capabilities of CPAP and BiPap by having the ability to provide pressure support, to automatically adjust in real-time to the respiratory needs of the patient, and to provide volume support. NIV pressure therapy can function like BiPap insofar as NIV can use two different pressures—one for inspiration, and a lower one for expiration. However, NIV can automatically vary the delivered pressure according to respiratory demand, and NIV can make that adjustment in real-time. Where NIV really takes off is with volume support.
In addition to pressure support, noninvasive ventilation can provide volume support. As NIV pertains to CMT, the consensus is that NIV with volume support is the best option for a CMTer who is experiencing Sleep Disordered Breathing, and is the best option for a CMTer who is experiencing respiratory muscle weakness caused by their CMT. What is volume support?
Noninvasive ventilation provides volume support via a capability called Average Volume Assured Pressure Support, or AVAPS for short. NIV pressure support uses pressure to keep the airway open, and AVAPS uses volume to ensure that each breath is consistent and maximized. Like CPAP, the pressures are extremely low, and measured in cmH2O. The unit of measure for volume is cubic centimeters (cc) or milliliters (mL). Cubic centimeters and millimeters are interchangeable with one another. So, how does this work?
Average Volume Assured Pressure Support delivers a volume of air that is equal to the total lung volume of the patient, plus a little extra to allow for any leakage at the mask seal, and the volume is delivered within a specified duration of time, consistently with each breath. The lung volume, represented as Tidal Volume, tL, is determined during a test called a Pulmonary Function Test, or PFT. Although the pressure delivered by NIV might vary depending on respiratory demand, delivered volume will remain unchanged. I use NIV with AVAPS because I have respiratory muscle weakness caused by my CMT1A. I will use my machine set-up to explain the parameters.
Noninvasive ventilation uses many parameters to treat respiratory conditions. AVAPS adds an additional set of complex parameters. My NIV has been a game changer for me. I have respiratory muscle weakness caused by my CMT. I also have Obstructive Sleep Apnea, and I have Central Sleep apnea. My physician at the time started me on CPAP therapy. I had a tough time exhaling against the pressure of CPAP, and my symptoms of SDB worsened, especially the morning headaches. After a physician change, and with my measured CO2 high enough in the middle of the day to qualify as hypercapnia, the new physician switched me to NIV with AVAPS. The machine set-up is complex, but there are a handful of parameters that are straightforward.
My noninvasive ventilator is set-up to treat my Sleep Disordered Breathing conditions, and to also treat the neuromuscular respiratory weakness that my CMT causes. The pressure support half of my NIV treats the SDB conditions, and the AVAPS half treats the respiratory muscle weakness. Remember that I mentioned that NIV pressures can vary? My pressure parameters are set to have a minimum inspiratory pressure of 10 cmH2O and a maximum inspiratory pressure of 25 cmH2O. The pressure drops to 7 cmH2O for expiration. While expiration pressure does not vary, inspiration pressure will adjust up and down, in real-time, as the machine determines I need based on my actual breathing. My AVAPS parameters have my tidal volume set at 500 mL. This number is equal to my total lung volume, plus 150 mL to allow for any loss between the machine and my lungs. AVAPS delivers this 500 mL in 1.2 seconds with each breath. The 1.2 seconds is based on my actual PFT measured average inspiration time. AVAPS is set to deliver 12 breathes per minute. The breathes per minute is based on the average for my height, weight, and age. There are many other parameters used in my NIV, but these are the important ones.
Noninvasive ventilation pressure support provides a critical therapy for CMTers who are experiencing Sleep Disordered Breathing conditions. NIV with AVAPS provides an even more critical therapy for CMTers who experience even minimal respiratory muscle weakness. The volume support provided by AVAPS capabilities allows for the respiratory muscles to get a break by creating an opportunity for the muscles to not have to work as hard to facilitate breathing. The break that AVAPS can provide lessens the muscle workload, thereby lessening the opportunity for respiratory muscle fatigue. Why are these things important?
In Sleep We Trust
Successfully treating Sleep Disordered Breathing conditions is vital to overall health. Treating SDB can reduce the risk of developing heart disease such as congestive heart failure, coronary artery disease, and cardiac arrhythmias. Treating SDB can reduce the risk of stroke, diabetes, and obesity. Treating SDB can improve cognitive function, emotional health, and general mood. Successfully treating SDB conditions can even improve the neuropathy experienced by CMTers. Restorative sleep is critically important. SDB interrupts restorative sleep. The short-term and long-term health consequences can be significant. Lost sleep is forever gone, for there is no catching up on sleep. The longer it takes to successfully treat SDB conditions, the more severe the overall health problems can become.
There are several options available for treating Sleep Disordered Breathing in CMT. While the consensus is that NIV with AVAPS is the best option for CMTers, the therapeutic choice is up to the physician. It all comes down to what the individual patient needs. For some CMTers, CPAP is the perfect choice. For others, BiPap is the best option. Yet, for others, NIV with AVAPS is the needed therapy. What works for one, might not work for the other. If you are using a particular therapy but it is not achieving what you need it to, talk to your physician. There are options available. The sooner your condition becomes managed well, the better your overall health, and CMT condition, will be.
