Pulmonary complications are a significant cause of morbidity and mortality in patients with multiple sclerosis (MS). Symptoms are insidious, and respiratory dysfunction may present as late as 9 years after the onset of neurologic symptoms.¹ The pattern, severity, and morbidity of respiratory failure is related to the size and location of the demyelinating plaques within the central nervous system. The respiratory dysfunction is related to the degree of neurologic involvement and related respiratory muscle weakness. This is manifested as loss of ventilation control, sleep apnea, difficulty swallowing, and an ineffective cough. Current and future treatments for MS may also result in a decreased immune response to infection. The respiratory failure treatment is often supportive, with both invasive and noninvasive mechanical ventilation.

Pulmonary function tests may not be the most sensitive marker of respiratory muscle dysfunction in MS.² Common measurements used to measure respiratory function in patients with MS include vital capacity (VC),
maximal inspiratory pressure (MIP), maximal expiratory pressure (MEP), and maximal voluntary ventilation (MVV).

Reduction of the VC suggests significant diaphragmatic weakness. Patients often exhibit correlative signs such as paradoxical abdominal breathing. The MIP, MEP, and MVV, in contrast, are more sensitive indicators of respiratory muscle weakness.³ Similar to other neuromuscular diseases, the decline in MIPmax and MEPmax precede changes in lung volumes. Patients with moderately severe MS on average exhibit a 40% decline in MIP and a 60% decline in MEP.⁸ MVV measures the respiratory muscle endurance. This decreases in parallel to the decrease in MEP.⁴

Studies have demonstrated a larger decrease in expiratory muscle strength than in inspiratory muscle strength. Expiratory muscle weakness can occur without inspiratory muscle weakness in patients with mild disease.⁵ This pattern can be explained by the progression of paresis in patients with MS. In MS, muscular impairment generally progresses from lower to upper extremities. The abdominal muscles become impaired earlier than the muscles of the diaphragm and intercostal muscles. This results in impaired expiration before inspiration.⁶ This can be clinically manifested as an ineffective cough and an inability to clear secretions.

Respiratory muscle weakness occurs both in patients who are ambulatory and wheelchair bound. However, patients who are ambulatory with minimal upper extremity involvement have a much smaller decline in MEP, VC, and MVV and essentially no decline in MIP when compared with patients who are wheelchair bound with significant upper extremity impairment⁴ (Figure 13.1). Patients with a mild degree of neurologic impairment rarely show any abnormalities in pulmonary function testing with normal (VC), forced expiratory volume in 1 s (FEV1), total lung capacity (TLC), and residual volume (RV). In contrast, patients with more advanced disease may show lung volume loss with a decreased VC and a preserved FEV1/FVC ratio. This pattern suggests restrictive lung disease; however, the TLC tends to be preserved in MS⁵ (Figure 13.2). This is attributable to submaximal inspiratory and expiratory efforts that are expected in patients with whole body muscle weakness, which results in an increase of the residual volume (RV). The rise in the RV correlates with the decline in the MEP.

Although MIPs and MEPs provide for a more useful tool than usual pulmonary function tests, they can be difficult to administer in patients with facial and bulbar muscle weakness.⁷ Clinical indices may be the best predictor of expiratory muscle weakness.⁵ Activities such as talking, coughing, and upper extremity strength serve as reliable markers of muscle strength. Talking requires complex coordination of upper and lower respiratory muscles, and coughing requires muscle contraction against a closed glottis. Smeltzer et al developed a pulmonary index score that appeared to be the best predictor of expiratory muscle weakness. The score comprised four parameters: patient’s report of his or her ability to handle

secretions/mucus, patient’s report of a weakened cough, examiner’s evaluation of a patient’s cough when asked to voluntarily cough as forcefully as possible, and the patient’s ability to count on a single exhalation. Using stepwise regression analysis it was concluded that the index score, upper extremity weakness, and MVV accounted for 60% of the variance in maximal expiratory muscle strength.⁵