Occasional review Ethical and clinical issues in the use of home non-invasive mechanical ventilation for the palliation of breathlessness in motor neurone disease

Motor neurone disease (MND) is a devastating and progressive neurological disorder in which degeneration of motor neurones results in weakness and wasting of the dependent muscles. The prevalence of MND is approximately five per 100 000 and approximately 30% of patients have primary bulbar symptoms. Half of the patients die within 36 months of experiencing the first symptom. Presentation with ventilatory failure is recognised but uncommon, occurring in less than 5% of cases; however, abnormalities of respiratory muscle function are frequently detectable. 5 As the condition progresses respiratory muscle strength diminishes ; indeed, the rate of change in respiratory function is the only measurable parameter which predicts survival. When the load placed on the respiratory muscle pump exceeds the capacity of the respiratory muscles, then the patient is at risk of ventilatory failure. Initially abnormalities may be present only in sleep or on exercise but, frequently, established ventilatory failure ensues. Ventilatory failure is usually manifest by dyspnoea which may be worse on lying flat (if diaphragm weakness is prominent) or sitting upright (if expiratory muscle weakness predominates). More commonly there is generalised weakness and then there are no clear cut postural symptoms. Direct questioning may elicit symptoms suggestive of disordered sleep architecture—for example, daytime somnolence, diYculty concentrating, or “respiratory” nightmares—or carbon dioxide retention—for example, morning headache. Examination may show paradoxical abdominal motion during respiration. This indicates substantial diaphragm weakness combined with preservation of enough upper thoracic musculature to generate suYcient negative intrathoracic pressure to cause inward abdominal motion. In more advanced disease paradoxical motion may be diYcult to elicit and the patient may simply appear tachypnoeic without useful rib cage movement. In established MND patients may also complain of choking symptoms; most commonly this represents swallowing diYculties. However, if unrelated to food, this symptom may be a manifestation of respiratory muscle weakness but occasionally endoscopy will show the cause to be abnormal vocal cord movement. In patients with MND in whom respiratory muscle weakness is suspected, our practice is to confirm (or refute) the diagnosis using appropriate tests and to seek evidence of ventilatory failure from measurement of daytime blood gas tensions and overnight transcutaneous oxygen saturation and carbon dioxide tensions. Ideally all symptomatic patients should be considered for polysomnography, but this investigation is especially useful for patients who deny sleep related symptoms but have proven severe weakness and those with sleep related symptoms in whom strength tests fail to demonstrate severe respiratory muscle weakness. If the diagnosis of ventilatory failure due to respiratory muscle weakness is secure, the patient, his or her carers, and the clinician need to consider what treatments they wish to pursue.