Lungs themselves are not affected by paralysis, but the muscles of the chest, abdomen and diaphragm, can be. As the various breathing muscles contract, they allow the lungs to expand, which changes the pressure inside the chest so that air rushes into the lungs. This is the process of inhaling – which requires muscle strength. As those muscles relax, the air flows back out of the lungs.
If paralysis occurs in C3 or higher, the phrenic nerve is no longer stimulated and therefore the diaphragm does not function. This means mechanical assistance – usually a ventilator – will be required to facilitate breathing. When the injury is between C3 to C5 (the diaphragm is functional), respiratory insufficiency still occurs: The intercostals and other chest wall muscles do not provide the integrated expansion of the upper chest wall as the diaphragm descends during inspiration.
People with paralysis at the mid-thoracic level and higher may have trouble taking a deep breath and exhaling forcefully. Because they may not have use of abdominal or intercostal muscles, these people also lose the ability to force a strong cough. This can lead to lung congestion and respiratory infections.
Source: The Christopher & Dana Reeve Foundation
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