Sleep-disordered breathing (SDB) is a common sleep disturbance in neuromuscular disease (NMD) affecting 36% to 53% of diagnosed adults (Arens R, et al.). Disturbances in sleep may serve as the earliest sign of muscle weakness in these patients, at times being detected before their underlying neuromuscular disease is diagnosed. This is of paramount importance to sleep medicine and pulmonary physicians who may be among the first specialists to evaluate these patients and can play a vital role in the recognition and diagnosis of neuromuscular disease. Herein, we will provide a guide to aid the reader in recognizing the early signs and symptoms of NMD as it pertains to sleep, as earlier diagnosis may lead to improved quality of life or possibly even survival, in some cases.
To begin, it is important to understand the pathophysiology of NMD and how it is altered during the sleep state. Sleep-related physiologic changes in healthy humans include reduction in upper airway muscle tone, blunting of chemoreceptors associated with pharyngeal dilator augmentation, and sleep stage-specific changes in skeletal muscle tone. In patients with NMD, these changes may not be adequately compensated for, leading to sleep-disordered breathing that can present as sleep apnea, hypoventilation, or hypoxia (Govindarajan R, et al.. Springer International Publishing AG, Springer Nature 2018).
Central respiratory control
The respiratory centers in the pons and medulla are generally spared from the primary effects of most NMD; however, over time, they may be affected secondarily. Similar to obesity hypoventilation syndrome (OHS), untreated chronic sleep-related hypoventilation from NMD can impair the sensitivity of respiratory chemoreceptors leading to worsening hypoventilation.
Upper airway resistance
Pharyngeal muscle tone is key to maintaining a patent airway during sleep. In some NMD, bulbar muscle weakness with pharyngeal dilator muscle hypotonia leads to increased upper airway resistance, especially during REM sleep, which can result in obstructive sleep apnea (OSA). In addition to weakness affecting the upper airway musculature, anatomical changes may also contribute to sleep-disordered breathing. In Pompe disease, for example, macroglossia and fibro-fatty replacement of tongue muscles may occur, leading to the development of OSA.
In NMD that affects the diaphragm, there is an increased reliance on the skeletal muscles of respiration to maintain adequate ventilation as the underlying disease progresses. Generally, weakness of the diaphragm will cause disturbances in REM sleep first as, during REM, ventilation predominately depends on the diaphragm and patients lose the assistance of their skeletal muscles. However, over time, the progressive weakening of the diaphragm will progress to involve NREM sleep as well, clinically manifesting with frank sleep apnea, hypoventilation, and, ultimately, chronic hypercapnic respiratory failure.
Inspiratory muscle weakness
As noted above, there are many other muscles used in inspiration in addition to the diaphragm. Other primary muscles include the intercostal and scalene muscles, and accessory muscles include the sternocleidomastoid, pectoralis, latissimus dorsi, erector spinae, and trapezius muscles. While sleep and breathing problems may begin early in the course of a neuromuscular disease, the complex restrictive lung disease pattern that we see in these patients may not develop until the respiratory muscles of the chest wall are involved. This restriction, which corresponds to lower lung volumes, leads to a fall in the caudal traction force of the airways which can lead to reduction in the pharyngeal airway cross section. Because these issues are worsened in the supine position, their pathophysiologic effects on respiration are most notable during sleep, putting patients at higher risk of OSA.