Respiratory Function in Multiple Sclerosis


Original Editor ­ Narayani Nagare Top Contributors - Ahmed M Diab, Narayani Nagare, Nicky Davis, Vidya Acharya and Alexandra Stead


Introduction

Multiple sclerosis (MS) is a chronic, autoimmune disorder of the central nervous system marked by inflammation, demyelination, and neurodegeneration. This process leads to disrupted nerve conduction, resulting in a range of neurological symptoms such as muscle weakness, vision disturbances, fatigue, cognitive impairment, and sensory abnormalities. MS follows distinct courses, commonly categorised as relapsing-remitting (RRMS), primary progressive (PPMS), and secondary progressive (SPMS).

Respiratory dysfunction in MS is a multifaceted issue arising from the disease's impact on the central nervous system (CNS). Demyelinating lesions are the pathophysiological hallmark of respiratory compromise, affecting one or more sites involved in the generation and/or transmission of nerve impulses to the respiratory muscles. Respiratory dysfunction can present as respiratory muscle weakness and impaired cough, bulbar muscle dysfunction, problems in breathing control, or respiratory failure, depending on the location and severity of demyelinating lesions.

Pathophysiology

Respiratory muscle performance may be independently compromised by medications, disease-related fatigue or nerve conduction block brought on elevated body temperature (Uhthoff phenomenon). Significant bulbar or limb paralysis is typically linked to respiratory failure which occurs in the later stages of MS.

Respiratory Muscle Weakness

Respiratory Muscle Weakness is common in Multiple Sclerosis. There is decreased strength of both inspiratory and expiratory muscles of respiration, with expiratory muscles typically weaker than inspiratory muscles. Respiratory weakness is more significant in bedridden or wheelchair-bound patients than in compared to ambulatory patients.

Diaphragmatic weakness usually occurs alongside weakness of other respiratory muscles and is diagnosed on the basis of orthopnoea, severe sleep dsiturbance, abdominal paradox and significant reduction in vital capacity in the supine position. Autopsy and MRI findings demonstrate that the cervical cord and medulla are often severely implicated in patients with advanced respiratory compromise.

There is generally no correlation between disease duration and respiratory muscle strength. However, patients with upper extremity weakness are more likely to have respiratory muscle weakness, particularly expiratory muscle weakness. Other clinical indicators that compromise the pulmonary index include weak cough, difficulty clearing secretions and reduced capacity to sustain phonation on a single exhalation. The predominance of expiratory muscle weakness in patients with upper arm involvement is explained by the pattern of paralysis progression; abdominal muscle involvement occurs before diaphragm and intercostal muscle involvement.

Standard spirometry may appear within normal limits in mild-to-moderate MS. Maximum inspiratory pressure (MIP) and maximum expiratory pressure (MEP) testing are more sensitive tools for detecting respiratory compromise at earlier disease stages.

Bulbar Muscle Dysfunction

Bulbar involvement in MS can impair swallowing, phonation and airway protection. Dysphagia in MS has a reported prevalence of approximately 45%. Swallowing disorders are associated with disease activity, number of relapses and respiratory function. The combination of impaired swallowing and weakened cough reflex increases the risk of aspiration, silent aspiration pneumonia and respiratory failure, particularly in later disease stages.

Breathing Control Dysfunction

Central lesions affecting the brainstem respiratory centres can result in abnormal breathing rhythms. These include:

• Loss of voluntary control (inability to voluntarily increase breath depth or pause breathing)
• Loss of automatic control (causing apnoea or respiratory arrest, particularly during sleep)
• Paroxysmal hyperventilation
• Apneustic breathing patterns
• Sleep-disordered breathing, including central and obstructive sleep apnoea, is more prevalent in MS than in the general population and can significantly contribute to fatigue and daytime somnolence.

Clinical Features and Associations

Fatigue and Respiratory Function

Fatigue is one of the most prevalent and disabling symptoms in MS, affecting up to 80% of patients. Respiratory muscle weakness and reduced pulmonary capacity contribute to fatigue, particularly during physical exertion. Decreased postural control and balance are associated with both respiratory capacity impairments and fatigue in persons with MS.

Respiratory Function, Balance and Postural Control

Reduced MIP, MEP, FVC and diaphragmatic excursion have been found to correlate with impaired balance scores (Berg Balance Scale, Mini-BESTest, TUG) and elevated fatigue ratings.

See also Respiration and Postural Stability

Assessment

A comprehensive respiratory assessment in MS should include:

Pulmonary Function Tests

• Spirometry: Forced Vital Capacity (FVC), FEV1, FEV1/FVC ratio Spirometry may be normal in mild-to-moderate MS; a normal result does not exclude clinically significant respiratory muscle weakness.

Respiratory Muscle Strength

• Maximal Inspiratory Pressure (MIP) is the most sensitive indicator of inspiratory muscle weakness.
• Maximal Expiratory Pressure (MEP) is important in expiratory muscle weakness as in MS.

Cough Efficacy

• Peak Cough Flow (PCF): patients with PCF<270 L/min are at significantly increased risk of respiratory complications, especially pneumonia.

Functional and Dyspnoea Assessment

• Borg Dyspnoea Scale
• Six-Minute Walk Test (6MWT)

Balance and Postural Assessment

• Berg Balance Scale
• Timed Up and Go Test (TUG)
• Trunk Impairment Scale

Physiotherapy Management

Inspiratory Muscle Training (IMT)

Typically performed using a threshold-loading device e.g. Threshold IMT, PowerBreathe. Training loads are set at 30-60% of MIP and progressed weekly. 10 weeks of training is recommended to achieve clinically meaningful improvements in MIP.

Expiratory Muscle Training (EMT)

EMT may be important particularly in MS because of predominance of expiratory muscle weakness in MS. EMT improves MEP and cough efficacy and may reduce the risk of aspiration pneumonia.

See also Respiratory Muscle Training

Breathing exercises have also been shown to reduce perceived fatigue.

Non-invasive ventilation (NIV)

Non-invasive ventilation e.g., BiPAP, CPAP may be necessary in cases of acute respiratory failure secondary to respiratory infection, neurogenic pulmonary oedema or perioperative management. NIV should also be considered for patients with significant nocturnal hypoventilation related to diaphragmatic or bulbar weakness. Initiation of NIV should involve the multidisciplinary team including respiratory medicine, neurology and physiotherapy.

Integrated Respiratory and Balance Rehabilitation

Combined exercise programmes addressing trunk stability, inspiratory muscle function and lower limb balance have shown promise.

Conclusion

Respiratory dysfunction in MS arises from demyelinating lesions that impair voluntary and autonomic breathing control. Expiratory muscle weakness tends to predominate. Respiratory impairment, fatigue and postural instability frequently co-occur in MS. Respiratory muscle training improves muscle strength and reduces fatigue.