Spirometry and Oximetry in neuromuscular diseases
Why perform spirometry and oximetry in neuromuscular diseases?
Neurological and neurodegenerative diseases are dysfunctions of the central or peripheral nervous system, which, based on the extent and focalization, may manifest themselves in different ways. Nevertheless, they are often severely crippling and sometimes fatal.
Among these is Multiple Sclerosis, a known neurodegenerative disease that affects the central nervous system (brain and spinal cord) with alteration of nerve pulse transmission, and Amyotrophic Lateral Sclerosis (ALS) that affects the cells that control muscle movement. Other diseases may affect the neuromuscular junction (Myasthenia gravis) or the muscles directly (such as muscular dystrophy, polymyositis and dermatomyositis)
In these diseases, involvement of the respiratory muscles causes progressively compromised respiratory function with serious disabilities and shorter survival.
Therefore, in neuromuscular diseases an instrumental pneumological assessment periodically needs to be performed, which includes spirometry and oximetry.
Spirometry in neuromuscular diseases
Among the spirometric parameters, measurement of FVC (Forced Vital Capacity) or SVC (Slow Vital Capacity) allows assessing the extent and following the decline over time of the restrictive defect typical of neuromuscular diseases due to involvement of the respiratory muscles.
The reduction in FVC (and/or SVC) in neuromuscular diseases is also an early indicator of the appearance of hypercapnia (increased carbon dioxide in the blood), further deteriorating the quality of life, as non-invasive assisted mechanical ventilation becomes necessary. When FVC is reduced to below 60%, sleep oximetry needs to be performed annually (see below) and the gases in the arterial blood analysed (blood gas analysis): if the values drop below 40%, much more frequent tests are required.
FVC reduction >20% measured in supine position compared to FVC measured in sitting position is very indicative of weakness of the diaphragm.
Another important spirometric parameter in neuromuscular diseases is PEF (Peak Expiratory Flow) which, related to the indices of expiratory muscle efficiency, may indicate involvement already in the initial stages of these diseases.
Moreover, in the more advanced stages of these diseases, PEF may reveal a compromised coughing mechanism which favours the onset of pneumonia.
In conclusion, these two spirometry parameters – FVC (and/or SVC) and PEF – may for all practical purposes be considered “biomarkers” of the respiratory complications of neuromuscular diseases (in particular, “ventilatory pump defect”).