Diaphragm reserve utilisation in motor neurone disease: a novel ultrasound-derived physiological index

Respiratory involvement is a major determinant of prognosis in motor neurone disease (MND; or amyotrophic lateral sclerosis), yet conventional static respiratory measures provide limited information about how much available diaphragm thickening capacity is recruited during quiet breathing. We evaluated diaphragm reserve utilisation (RU), an ultrasound-derived ratio of tidal diaphragm thickening relative to maximal diaphragm thickening, as a novel physiological descriptor in MND.

In a prospective observational study, 38 patients with MND and 20 healthy controls underwent right hemidiaphragm ultrasound to measure thickness at end-tidal expiration, peak tidal inspiration and peak maximal inspiration. Thickening fractions and RU were calculated. Between-group differences in diaphragm ultrasound indices were assessed. Secondary analyses examined relationships between RU, respiratory function and survival.

Tidal diaphragm thickening fraction was similar between patients with MND and controls, whereas maximal thickening fraction was lower in MND, resulting in higher RU in MND than controls. This pattern indicates that patients with MND maintained tidal diaphragm thickening while using a greater proportion of their available maximal thickening capacity. Higher RU was also associated with shorter survival, supporting its potential physiological and clinical relevance.

Diaphragm reserve utilisation describes the proportion of measured diaphragm thickening capacity recruited during tidal breathing. Patients with MND demonstrated preserved tidal thickening despite reduced maximal thickening capacity, resulting in higher RU than healthy controls. This supports RU as a novel physiological index of relative diaphragm thickening reserve utilisation. The relationship between RU and survival warrants validation in larger prospective cohorts.