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Disorders that cause respiratory failure

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CNS

Sleep

Medication

OHS

Neuromuscular

disorders

Figure 1. Schematic illustration of the eect of reduced capacity due to NMD/CWD on the load/capacity ratio of the respiratory muscle pump.

–

Neural

respiratory

drive

Respiratory

muscles

Breathlessness

Hypercapnic respiratory

failure

Obesity

LoadCapacity

further (e.g. on exertion) or drive is diminished (e.g. at sleep onset or during REM sleep atonia), then a more marked imbalance in the load/capacity ratio will lead not only to symptoms, but also the onset of hypoventilation.

In chronic conditions, this occurs typically ﬁrst during REM sleep, due to physiological REM-sleep atonia of the skeletal muscles. This leads to reduced ventilation, particularly if the diaphragm is also aected by the disease. With further progression and a more signiﬁcant imbalance of the load/capacity ratio, hypoventilation will also start to occur in non-REM sleep. Finally, this progresses to sustained hypoventilation during sleep and when awake, where full activation of skeletal respiratory muscles with increased neural respiratory drive, even during wakefulness, is no longer sucient to maintain appropriate ventilation (table 1).

Aetiology

CNS/brainstem

Neurological conditions involving the CNS/brainstem can cause hypoventilation by inhibiting central control of respiration and eerent output. This process involves interpretation of aerent signals to breathe from central and peripheral chemoreceptors, and proportionate eerent signalling to the respiratory muscles. Congenital central hypoventilation syndrome (Ondine’s curse) presents in early childhood and causes autonomic hypoventilation during sleep. Late-onset central hypoventilation syndrome presents slightly later, with associated endocrinopathy and/or hypothalamic dysfunction. Injury to the brainstem (such as aer a stroke or motor vehicle accident) or medications such as opioids, benzodiazepines and neuropathic agents can also inhibit central control of breathing and cause hypoventilation and respiratory failure.

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Disorders that cause respiratory failure

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Table 1. Examples of NMDs/CWDs associated with hypercapnic respiratory failure

Rapid

progression

CNS/brainstem Acute stroke Central hypoventilation

Motor neurone ALS/MND Spinal bulbar muscular

Neuromuscular junction

Peripheral nerves Muscles Rhabdomyolysis

Chest wall Trauma

Guillain–Barré

syndrome

Critical illness

polyneuropathy/

myopathy

Tumour

Pneumothorax/

haemothorax

Variable progression Slow or stable

syndromes (Ondine’s

curse)

Medications (e.g.

opioids, neuropathic

agents)

atrophy

Myasthenia gravis

Lambert–Eaton

myasthenic syndrome

Charcot–Marie–Tooth

disease

DMD

Myotonic dystrophy

Mitochondrial

myopathies

Danon disease

Limb girdle muscular

dystrophy

Myoﬁbrillar myopathy

Obesity

Ankylosing spondylitis

Scar tissue

progression

Post-stroke

Spinal muscular

atrophy

Polyneuropathies

Facioscapulohumeral

muscular dystrophy

Post-polio syndrome

Becker muscular

dystrophy

Inclusion body myositis

McArdle disease

Kyphoscoliosis

Thoracoplasty

Motor neurones

These disorders include amyotrophic lateral sclerosis (ALS)/motor neurone disease (MND), spinal and/or bulbar muscular atrophy, poliomyelitis or post-polio syndrome. ALS/MND is a progressive neurodegenerative condition aecting the pyramidal tract and anterior horn cells, which is ultimately fatal, in many cases due to progressive respiratory muscle failure. The rate of progression can vary between individuals. Progressive respiratory muscle weakness leads to initial nocturnal hypoventilation followed by sustained daytime hypoventilation that may require ventilatory support. Other types of SDB, such as OSA, are also common in these patients. Associated symptoms may include orthopnoea, sleep fragmentation and sleepiness, diculty managing respiratory or oral secretions, or insomnia due to mood-related issues.

Spinal muscular atrophy is an autosomal recessive condition caused by mutations or deletions in the SMN1 gene leading to progressive muscle weakness. With childhood onset, spinal muscular atrophy is associated with tetraplegia and respiratory failure. Later onset is associated with milder disease. Respiratory failure due to muscle weakness in childhood is oen complicated by scoliosis due to the weak postural muscles. Spinal and bulbar muscular atrophy (Kennedy disease) is primarily associated with bulbar and distal limb weakness, but can be associated with hypoventilation and OSA.

Acute poliomyelitis is now rare in industrialised countries, but can be associated with rapidly progressive muscular palsy, atrophy and weakness. In post-polio syndrome,

265ERS Handbook: Respiratory Sleep Medicine

Disorders that cause respiratory failure

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hypoventilation can develop gradually over time, even aer decades, with muscle atrophy leading to both weakness and chest wall restriction.

Neuromuscular junction

Diseases of the neuromuscular junction can be congenital or acquired, and are characterised by progressive muscle weakness and fatiguability. The most common example is myasthenia gravis, an autoimmune condition causing weakness of the skeletal muscles through acetylcholine receptor antibodies. Lambert–Eaton syndrome is a similar variant caused by anti-voltage-gated calcium channel receptor antibodies, usually via an autoimmune or paraneoplastic process. Respiratory function is aected by bulbar or respiratory muscle involvement. Younger patients with a shorter disease course may experience OSA, although hypoventilation is less likely. Patients who are older may have comorbidities such as obesity, which increases the load on the respiratory mechanics in the setting of reduced capacity, precipitating respiratory failure.

Peripheral neuropathies

Congenital or acquired peripheral neuropathies can cause hypercapnic respiratory failure due to phrenic nerve and respiratory muscle involvement. Charcot–Marie– Tooth disease, also known as hereditary sensory motor neuropathy, is a congenital demyelinating polyneuropathy that can cause OSA by way of pharyngeal neuropathy, or hypoventilation with diaphragmatic involvement.

Guillain–Barré syndrome is an acquired inﬂammatory demyelinating polyneuropathy that can rapidly progress to phrenic nerve and diaphragmatic involvement, causing acute and chronic hypercapnic respiratory failure. However, with ventilatory support, Guillain–Barré syndrome may resolve over time.

Other secondary acquired polyneuropathies, such as diabetic neuropathy, cause SDB and hypoventilation less commonly, for example by impairing chemosensitivity to carbon dioxide (CO2). Diaphragmatic weakness caused by trauma, such as surgery or injury, compression of the phrenic nerve, or neuralgic amyotrophy can also lead to respiratory failure; this is more common in bilateral rather than unilateral diaphragmatic palsy.

Myotonic and muscular dystrophies

Myotonic dystrophy (type 1 and 2) is an autosomal dominant condition causing myotonia and muscle weakness, primarily aecting facial (facies myotonicus) and distal muscles. Type 2 myotonic dystrophy is typically less severe than type 1. Respiratory failure may result from involvement of the diaphragm, as well as laryngeal and pharyngeal muscles, causing hypoventilation and OSA. Oropharyngeal muscle involvement impairs swallowing and increases the risk of aspiration, which can precipitate acute respiratory failure. Excessive sleepiness is a common symptom in these patients, caused by SDB, as well as centrally via neurodegeneration of serotoninergic neurones and hypothalamic dysfunction.

Muscular dystrophies are conditions characterised by degeneration of muscle ﬁbres, which frequently present in childhood and are congenital or of metabolic origin. The most common of these is Duchenne muscular dystrophy (DMD), an X-linked recessive disorder. Respiratory failure presents due to oropharyngeal muscle weakness, nocturnal hypoventilation, with obstructive and central apnoeas, and eventually daytime hypercapnic respiratory failure. Patients with DMD may develop extrathoracic

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Disorders that cause respiratory failure

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lung restriction due to chest wall deformities; an increased load on the already impaired capacity. Acid maltase deﬁciency myopathy can cause rapid and signiﬁcant diaphragmatic weakness and respiratory failure.

Chest wall disorders

Chest wall disorders can result in hypoventilation due to a permanent increased load on the respiratory mechanics with reduced compliance of the chest wall and lungs, leading to increased work of breathing. This is most noticeable in patients with signiﬁcant kyphoscoliosis, while minor chest wall deformities (e.g. minor pectus excavatum/carinatum) may not typically impact chest wall compliance to the point that patients develop respiratory failure.

Obesity

Obesity increases the load on the respiratory muscles, with signiﬁcantly increased intra-abdominal pressures raising the diaphragm, lowering lung volumes and reducing lung and chest wall compliance. OHS may further develop through resistance of leptin (a respiratory stimulant protein), and reduced chemoreceptor sensitivity to a sustained nocturnal increase in CO2. This ‘blunted respiratory drive’ contributes to sustained daytime hypoventilation. Patients are treated with nocturnal PAP or NIV, and with weight loss in the long term.

Summary

An imbalance of the load/capacity ratio of the respiratory muscle pump can lead to symptoms such as breathlessness and acute and chronic hypercapnic respiratory failure. Neuromuscular and chest wall disorders most commonly aect the capacity side of the respiratory muscles, but may also have an impact on central motor eerent output. Hypercapnic respiratory failure can be treated by providing mechanical ventilation, in the acute setting in critical care and in the chronic situation with home mechanical ventilation (noninvasively or via tracheostoma). Many patients with NMDs/CWDs beneﬁt from the input of the wider multidisciplinary team (e.g. physiotherapist, specialist nurse) and nonrespiratory specialties (e.g. neurology, ENT).

Further reading

• American Academy of Sleep Medicine (2014). International Classiﬁcation of Sleep Disorders.

3rd Edn. Darien, American Academy of Sleep Medicine.

• Boentert M (2021). Neuromuscular disoders. In: Bassetti C, et al., eds. European Sleep

Research Society Sleep Medicine Textbook. 2nd Edn. Bonn, European Sleep Research Society; pp. 991–1000.

• Curtis A, et al. (2020). The value of a post-polio syndrome self-management programme.

J Thorac Dis; 12: Suppl. 2, S153–S162.

• Falcoz P-E, et al. (2013). Chest wall disorders. In: Palange P, et al., eds. ERS Handbook of

Respiratory Medicine. 2nd Edn. Sheeld, European Respiratory Society; pp. 448–450.

• Jolley CJ, et al. (2009). Neural respiratory drive in healthy subjects and in COPD. Eur Respir J;

33: 289–297.

267ERS Handbook: Respiratory Sleep Medicine

Disorders that cause respiratory failure

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• Masa JF, et al. (2019). Obesity hypoventilation syndrome. Eur Respir Rev; 28: 180097.

• Patout M, et al. (2020). Long-term survival following initiation of home noninvasive ventilation:

a European study. Thorax; 75: 965–973.

• Polkey MI, et al. (2019). Neuromuscular disorders and the diaphragm. In: Palange P, et al.,

eds. ERS Handbook of Respiratory Medicine. 3rd Edn. Sheeld, European Respiratory Society; pp. 443–447.

• Shah NM, et al. (2020). The adult multidisciplinary respiratory neuromuscular clinic. Breathe;

16: 200121.

• Steier J, et al. (2009). Neural respiratory drive in obesity. Thorax; 64: 719–725.

• Steier J, et al. (2014). Observational study of the eect of obesity on lung volumes. Thorax; 69:

752–759.

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ERS Handbook: Respiratory Sleep Medicine

Assessment of respiratory

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muscle weakness

Neeraj M. Shah, Georgios Kaltsakas and Joerg Steier

Assessment of respiratory muscle strength is an important element of the evaluation of patients with unexplained breathlessness and hypercapnic respiratory failure. Comprehensive assessment can oen reveal the speciﬁc pathology, particularly when it is not detected in standard respiratory investigations (e.g. pulmonary function testing).

Here, we provide a comprehensive and stepwise approach to the assessment of respiratory muscle strength, beginning with clinical examination, followed by noninvasive bedside assessment and concluding with advanced tests of respiratory muscle strength, which can provide speciﬁc insight into the function of individual muscle groups. Isolated test results of respiratory muscle strength are dependent on multiple factors, including patient motivation and accurate measurement, and tend to lead to an overdiagnosis of weakness. The combination of multiple tests of respiratory muscle strength can be a useful approach to a patient with suspected respiratory muscle weakness and can help identify inspiratory, expiratory or diaphragmatic muscle involvement (table 1).

Clinical assessment

Patients with respiratory muscle weakness may present with breathlessness that does not have an obvious cause. Clinical assessment can reveal preceding symptoms of broader skeletal muscle weakness, such as diculty when tying shoelaces, when

Key points

• Weakness or paralysis of the respiratory muscles, particularly the diaphragm, leads to symptoms and hypercapnic respiratory failure.

• Respiratory muscle strength should be investigated in patients with unexplained breathlessness.

• In most cases, respiratory muscle weakness can be diagnosed using noninvasive investigations.

• More advanced, invasive methods of respiratory muscle testing are available where noninvasive testing may not deliver conclusive results, for the diagnosis of (hemi)diaphragmatic paralysis, or in research settings.

• Assessing respiratory muscle strength by combining results from multiple tests can improve diagnostic accuracy.

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Assessment of respiratory muscle weakness

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Table 1. Tests of respiratory muscle strength and the information they can provide

Test Possible information provided

Noninvasive tests

Spirometry (supine versus erect) Restrictive lung volumes; reduction in VC in the

supine position

Imax

Emax

SNIP Diaphragmatic strength Cough peak expiratory ﬂow Expiratory muscle function Chest radiograph Hemidiaphragm paralysis or weakness Diaphragm ultrasound Diaphragm dysfunction Overnight oximetry/respiratory PG SDB/hypoventilation

Invasive tests

Arterial blood gas analysis Hypercapnic respiratory failure associated with

oes

gas

Phrenic nerve stimulation Diaphragm strength and phrenic nerve function Phrenic nerve conduction Aetiology of neuromuscular (diaphragm)

Respiratory muscle EMG Respiratory muscle activation/innervation

PG: polygraphy.

General inspiratory muscle strength General expiratory muscle strength

inspiratory muscle weakness General inspiratory strength Diaphragmatic strength Expiratory muscle strength

dysfunction

lying supine, with immersion in water, when climbing stairs or when standing from a chair. The patient may have falsely attributed these symptoms to advancing age or lack of physical ﬁtness. By the time they attend a respiratory clinic, they may be breathless on exertion progressing to breathless at rest; they may report orthopnoea or they might be experiencing recurrent lower respiratory tract infections with

weight loss (due to muscle wasting, poor oral intake or swallowing diculties, for example) and impaired cognitive function or memory, which could be explained by sleep disturbance and the development of hypercapnic respiratory failure. Inspection during the physical examination may reveal the use of the accessory muscles of respiration, a paradoxical breathing pattern, atrophic or compensatory hypertrophic muscle groups and tachypnoea. Patients might complain of breathlessness with certain postures, when squeezing the abdomen or when bending forwards. Any unexplained clinical features warrant further investigation; early referral to a centre of expertise with access to advanced investigations should also be considered.

Noninvasive tests

Spirometry is oen the ﬁrst investigation applied in patients with breathlessness. Respiratory muscle weakness is associated with reduced lung volume (restrictive picture), speciﬁcally with reduction in the vital capacity (VC). It is important to note that reduced VC is a sign of signiﬁcantly impaired inspiratory muscle function; a normal VC should not exclude the possibility of respiratory muscle weakness. In patients with diaphragmatic weakness, VC reduces further when lying down, so it can be useful to compare upright and supine VC; a reduction in FVC of >15% in the supine posture is suggestive of (hemi)diaphragm weakness or paralysis, and a reduction of >30% is pathognomonic and suggestive of (bilateral) diaphragmatic weakness or paralysis.

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Assessment of respiratory muscle weakness

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Chest radiography may identify a raised hemidiaphragm. However, it is less appropriate for the accurate diagnosis of weakness or residual function. Diaphragm ultrasound may also be useful for imaging diaphragmatic motion and for assessing muscle thickness. Emerging evidence indicates that MRI can be used to identify early diaphragm weakness.

SDB, particularly nocturnal hypoventilation, can sometimes be the ﬁrst identiﬁed clinical manifestation of inspiratory muscle weakness. Overnight oximetry and respiratory sleep studies may therefore be useful tools in the systematic assessment of a patient with unexplained breathlessness.

Respiratory muscle strength can also be assessed by measuring force and pressure. The least invasive way of assessing general inspiratory muscle strength is by testing the maximal inspiratory pressure (P

), which involves a maximal inspiratory eort

Imax

against a closed shutter. Similarly, general expiratory muscle strength is assessed by testing the maximal expiratory pressure (P

). These tests can be dicult for the

Emax

patient to coordinate and require reliable patient eort. A more diaphragm-speciﬁc assessment of inspiratory muscle strength is to test the

sni nasal inspiratory pressure (SNIP), which simply requires the patient to take maximal snis. An additional means of assessing expiratory muscle strength is to test cough peak expiratory ﬂow, which is intuitively easier for the patient than the

test. This can be supplemented by invasive measurement of cough gastric

Emax

pressure (P

gas

By convention, these tests are repeated three to ﬁve times (10 times for SNIP) in order to achieve the maximal and repeatable measurement. Based on the principle that a patient cannot generate a false high pressure, the highest pressure recorded is accepted as the true reading, i.e. if a patient generates a pressure within the normal range, then they have normal respiratory muscle strength. Low pressures, however, may be falsely low for a number of reasons, such as eort, seal and method.

It is important to note that all volitional tests of respiratory muscle strength require intact cortical function. In patients with impaired cortical function, or where the patient is unable to generate a maximal eort, test results are likely to be inaccurate.

Invasive tests

Advanced tests of respiratory muscle strength can be dicult to access. However, they do have a useful role when anatomical features, such as glottic dysfunction or nasal obstruction, cause underestimation of intrathoracic pressure when using nasal or mouth measurements. They are also used in research studies where accurate measures of respiratory muscle strength are essential.

Established volitional and invasive tests involve the measurement of oesophageal pressure (P

) and transdiaphragmatic pressure (Pdi) (the dierence between the

oes

pressure in the abdominal (P of Pdi is performed by introducing catheter(s) that contain either balloons or pressure transducers through the nasopharynx, accurately positioning them so that the P taken ∼10 cm above the gastric cardia, and the gastric balloon sits within the stomach (i.e. rostral and caudal to the diaphragm). Pdi can be measured during dierent manoeuvres (e.g. when spontaneously breathing, or during maximal snis or P to assess the work of breathing and the inspiratory muscle strength. With continuous monitoring and when expressed as a product of pressure and time, Pdi can be used to measure the impact of therapies such as NIV in the awake and asleep state.

) and the thoracic (P

gas

) compartments). Measurement

oes

Imax

)

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Non-volitional assessment involves neural stimulation, either using an electrical current or magnetic stimulation. It can be performed transcranially, cervically or, most commonly, via the phrenic nerves (e.g. unilaterally or bilaterally on the neck). It produces a short twitch of the diaphragm (twitch Pdi), which tests the function of the neuromuscular units (i.e. the nerves and the diaphragm). It is a useful measure of diaphragmatic function that is independent of patient eort and can be used in the intensive care environment (e.g. in ventilated patients).

Twitch Pdi is a speciﬁc measure of diaphragmatic strength. Phrenic nerve conduction studies can also be used to establish phrenic nerve latency and compound muscle action potential (CMAP), which can further identify the aetiopathophysiology of the cause of diaphragmatic weakness. Parallel measurement of the EMG of the respective muscle groups can complement the assessment of their respective function.

Lastly, twitching the 10th thoracic segment while simultaneously measuring gastric pressure is a non-volitional test of expiratory muscle strength that is less wellvalidated due to submaximal stimulation.