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A rare cause of respiratory failure

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Q1: How would you interpret the arterial blood gases and the lung function tests?

Arterial blood gases show compensated type 2 respiratory failure. Lung function tests show predominantly restrictive lung disease with greater than predicted transfer factor. These two tests indicate that the main problem is that of chest wall or neuromuscular weakness rather than alveolar level problem.

Q2: What is the differential diagnosis?

The differential diagnosis is:

  • Neuromuscular pathology, for example, motor neurone disease, Eaton-Lambert syndrome, myasthenia gravis.

  • Diaphragmatic paralysis.

  • Kyphoscoliosis.

Q3: What do the chest and cervical radiographs show? (see p 208)

The chest radiograph is normal with no evidence of diaphragmatic paralysis. Cervical radiography shows no evidence of kyphoscoliosis.

Q4: What further investigations might be helpful?

Nerve conduction studies and Tensilon test (edrophonium chloride).

Q5: What would be your initial management of this patient?

Controlled oxygen therapy and non-invasive positive pressure ventilation or full ventilation. Despite treatment with controlled oxygen therapy, intravenous doxapram, steroids, nebulisers and antibiotics, the patient continued to deteriorate, and required endotracheal intubation and invasive ventilation. Non-invasive ventilation was not tried, as it is not currently available in our hospital.

Once he was stable a tracheostomy was performed to aid “weaning”. The Tensilon test was normal but electromyography showed scattered denervation and clear neurogenic weakness in all muscles sampled. There was no evidence of an underlying myopathy but changes indicating lower motor neurone weakness with partial denervation in some muscles were present. This was consistent with the diagnosis of motor neurone disease (amyotrophic lateral sclerosis). It proved extremely difficult to wean him off the ventilator. As the motor neurone disease was previously undiagnosed, the question of life support had not been discussed, and yet he had been precipitated into complete dependency on a ventilator. As he had normal cognitive function and was able to communicate, his diagnosis and prognosis were discussed with him and his family in a frank and compassionate manner. The patient and his family were given ample time to consider their options. They decided that they would like him to be considered for home non-invasive positive pressure ventilation. Unfortunately, while still being ventilated, he developed a pneumothorax (treated with intercostal chest drain) followed by an overwhelming nosocomial pneumonia and died. The rapidity of the clinical progression suggested fairly aggressive disease.

Q6: What is the long term respiratory management of this condition?

Specific respiratory treatment involves protection against aspiration and non-invasive ventilation. Aspiration can be prevented by mechanical means, for example, raising the head, regular swallowing assessment, use of anticholinergic drugs to reduce oral secretions, chest physiotherapy, early insertion of a gastrostomy tube, and in some specific cases, a tracheostomy.1 As the expertise of portable non-invasive home ventilation increases (either applied through use of a nose or facemask or via tracheostomy), it should be offered to selected patients (see discussion), as this has been shown to prolong survival.2

Discussion

Motor neurone disease (amyotrophic lateral sclerosis) is a progressive disease of adults resulting from variable degeneration of the upper motor and lower motor neurones. Worldwide annual incidence rates for classical amyotrophic lateral sclerosis ranges from 0.4 and 1.8/100 000 population.3 A point prevalence study of patients in the counties of South Glamorgan, Mid Glamorgan, and Gwent gave a point prevalence of 2.73/100 000 population. In the same study, 94% of the patients were living at home, of whom 65% had some degree of mobility impairment and 20% were wheelchair bound. Swallowing speed was reduced in 67% of patients and 8% had a percutaneous endoscopy gastrostomy feeding tube. Only one patient (2%) was receiving home non-invasive ventilation.1 In patients with amyotrophic lateral sclerosis, significant bulbar and respiratory weakness occurs in about 50%,1 and in few cases respiratory failure requiring mechanical ventilation is the presenting complaint.4 Respiratory compromise is due to impairment of any or all of the three muscle groups, which are essential for normal ventilation: the muscles of inspiration, muscles of expiration, and the muscles that control the upper airway.2 5 The manifestations of amyotrophic lateral sclerosis on respiratory muscles can be additive and progress in a vicious cycle to respiratory failure. Aspiration and loss of cough causes atelectasis and decreased compliance, decreased inspiratory muscle strength causing further atelectasis, and infection or aspiration pneumonia, increasing respiratory demand. Hypoxaemia and acidosis also impair respiratory muscle function by direct effect on the diaphragm.6 The usual relentless course of amyotrophic lateral sclerosis leads to death in 50% of patients within three years from the onset, invariably from respiratory failure. The three major predictors of survival in classical amyotrophic lateral sclerosis are age of onset, clinical variant of disease (predominantly bulbar involvement), and pulmonary function.1

As portable non-invasive positive pressure ventilation becomes more readily available it should be considered for selected patients, as this has been shown to prolong survival. These forms of non-invasive positive pressure ventilation have the added advantage of stabilising the oropharyngeal airway and prevent upper airway obstruction during sleep. Sherman et al have shown that in patients with amyotrophic lateral sclerosis and respiratory failure, the institution of non-invasive mechanical ventilation significantly prolonged survival in the treatment group (80.4 days v 19.25, p<0.01).2

Long term assisted ventilation in a disease that ultimately produces paralysis in the majority of patients causes significant ethical, legal, and psychological concerns. It is imperative that discussion with patients and family members about the pros and cons of ventilatory care be frank, and take place early in the course of the disease, so as to help the patient make an informed decision. This will enable patients to make advance directives (living will) or have an appropriate legal surrogate in cases when the patient's condition deteriorates.

Using home ventilation requires cooperation from patients, carers (who need to be trained), and the multidisciplinary input from doctors and other health care workers. Not only does this involve much time and energy from family and health care workers, but it also has a large cost implication.7 Due to the progressive nature of this incurable disease, the decision to utilise these modalities must be made with realistic consideration of the patient's quality of life.

Final diagnosis

Respiratory failure secondary to motor neurone disease (amyotrophic lateral sclerosis).

References

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