Which diagnostic tests are recommended to confirm a diagnosis of MND, and how should I interpret their results?

Recommended diagnostic tests to confirm motor neuron disease (MND) include clinical assessment and exclusion of other causes, with referral to a neurologist for specialist evaluation. The diagnosis is primarily clinical, based on progressive muscular weakness, muscle wasting, fasciculations, and bulbar symptoms such as speech or swallowing difficulties. There is no single definitive test for MND, so diagnosis relies on a combination of clinical features and investigations to exclude mimics.

Respiratory function tests are important as part of the initial assessment or soon after diagnosis to establish baseline respiratory status. These include pulse oximetry (SpO2) at rest on room air, forced vital capacity (FVC) or vital capacity (VC), sniff nasal inspiratory pressure (SNIP), and maximal inspiratory pressure (MIP). If pulse oximetry is not possible locally, referral to a respiratory ventilation service is recommended. Interpretation of these tests helps assess respiratory muscle involvement and guide management.

Arterial or capillary blood gas analysis is indicated if SpO2 is ≤94% (or ≤92% in those with lung disease) to assess for hypoventilation and hypercapnia. If PaCO2 is >6 kPa, urgent referral to a respiratory ventilation service is required.

Cognitive and behavioural assessments should be considered if there are concerns about frontotemporal dementia or other cognitive changes associated with MND. Formal assessment may be needed in line with dementia guidelines.

Interpretation of results: Clinical features consistent with progressive upper and lower motor neuron signs support the diagnosis. Respiratory tests showing declining function indicate disease progression and need for respiratory support. Blood gases indicating hypercapnia signal respiratory failure risk. Cognitive assessments identify associated frontotemporal dementia, which affects management.

Referral to a neurologist with expertise in MND is essential for diagnosis confirmation and management planning.

In summary, diagnosis is clinical supported by respiratory and cognitive assessments, with exclusion of other causes, and specialist neurological evaluation is key.

All recommendations are based on NICE guideline NG42 on Motor neurone disease: assessment and management ¹.