How does an SCI affect the respiratory system?

Author: Sharon Jang | Reviewer: Tova Plashkes | Published: 24 September 2020 | Updated: 7 December 2021

When someone is unable to breathe or cough by themselves, their function, independence, and health are affected. These respiratory complications arise in 36-83% of individuals after SCI. This is due to disruption of the breathing nerves after an injury, and to secondary complications of an SCI, such as spasticity. After an SCI, breathing muscles may be partially or completely affected, depending on the completeness of the injury. However, breathing ability may improve over time.

Many muscles are required for breathing. The diagram above shows the main muscles of breathing, and the sections of the spinal cord that innervate them.²

The chances of experiencing respiratory complications depend on a variety of factors, including:

• The level of injury (dictates which muscles are spared)
• The completeness of an injury
• Timing of a tracheostomy*
• The cause of injury
• Age*

* Conflicting evidence

Refer to our page on Evidence Rating for more information on conflicting evidence.

After injury, the amount of air that can be inhaled and exhaled are significantly reduced for people with

cervical

and higher

thoracic

(neck and upper back) level injuries. More specifically, the amount of air that can fill the lungs (known as the

total lung capacity

) is reduced to 60-80% of normal values. Additionally, the amount of air that can be exhaled after the biggest breath in (known as the

vital capacity

) is reduced to 50-80% of normal values. This contributes to inefficient breathing that may be tiresome and difficult. In addition, a lower vital capacity can impact voice volume, making it difficult to speak at louder volumes.

An SCI can affect the lung itself. The main change is a reduction in lung compliance, or the lung’s ability to stretch and expand. As a result, the lung does not spring back “closed” after being open. In addition, the compliance of the rib cage (chest wall) may also decrease, causing the chest to become rigid in individuals with tetraplegia. Reduced compliance results in a decreased ability to take a deep breath independently or with the help of a breathing bag or ventilator.

Coughing is important to keep the airway and lungs clear from mucus. This is because a build up of mucus can collapse the lungs, and mucus in the airways can result in infection. In order to perform a cough, one needs to inhale deeply then have a forceful exhale while a structure called the

glottis

closes the entrance to the

windpipe

. The intercostal muscles and the abdominal muscles assist with the ability to increase the force of exhaled air. As these muscles are innervated by nerves in the chest region, individuals with spinal cord injuries may have an impaired coughing function. Cough function may be completely absent in some individuals, while others may have limited or ineffective coughing abilities.

Soon after injury, it is common for individuals with high-level SCI to produce a lot of mucus in their lungs and have smaller airways deep in the lung. The lungs are also very irritable to stimuli like too much suctioning of mucus, or smoking.  This may be due to the increased influence of the parasympathetic nervous system after SCI. In people with acute tetraplegia, it has been reported that an excess of up to 1 liter of mucus is produced each day. In combination with an inability to cough, this excess production of mucus can result in a buildup of fluid in the lungs and airway.

Although swallowing is important for eating, it is also important for clearing the throat to prevent food, drink, stomach contents, or saliva from entering the lungs (also known as aspiration). After SCI, the risk of aspiration increases as:

• • Your ability to cough may be limited by medical conditions and weakness due to your injury
  • Surgical procedures on the spine may compress your throat
  • You may feel less alert due to sedative medications
  • Some medications you may be on can lead to dry mouth
  • Your sensation may be impaired, which prevents you from feeling food or liquid in the spaces at the back of your throat

The lack of effective swallowing can cause mucus to collect in your airway. Over time, the stagnant mucus can encourage the growth of bacteria, which may travel down to your lungs and potentially result in pneumonia.