flow decreases that can be absent in sleep-related hypoventilation.
Comorbidity 
Sleep-related hypoventilation often occurs in a ssociation with a pulmonary disorder (e.g., in-
terstitial lung disease, COPD), with a neurom uscular or chest wall disorder (e.g., muscular
dystrophies, post-polio syndrome, cervical spinal cord in jury, obesity, kyphoscoliosis), or,