Assessing Inspiratory Muscle Strength for Early Detection of Respiratory Failure in Motor Neuron Disease: Should We Use MIP, SNIP, or Both?

ABSTRACT

BACKGROUND: Motor neuron disease (MND) invariably impacts on inspiratory muscle strength leading to respiratory failure. Regular assessment of sniff nasal inspiratory pressure (SNIP) and/or maximal mouth inspiratory pressure (MIP) contributes to early detection of a requirement for ventilatory support. OBJECTIVES: The aim of this study was to compare the feasibility, agreement, and performance of both tests in MND. METHODS: Patients with MND followed by a multidisciplinary consultation were prospectively included. Pulmonary follow-up included forced expiratory volumes, vital capacity (VC) seated and supine, MIP, SNIP, pulse oximetry, and daytime arterial blood gases. RESULTS: A total of 61 patients were included. SNIP and MIP could not be performed in 14 (21%) subjects; 74% of the subjects showed a decrease in MIP or SNIP at inclusion versus 31% for VC. Correlation between MIP and SNIP (Pearson's rho 0.68, p < 0.001) was moderate, with a non-significant bias in favor of SNIP (3.6 cm H2O) and wide limits of agreement (-34 to 41 cm H2O). Results were similar in "bulbar" versus "non-bulbar" patients. At different proposed cut-off values for identifying patients at risk of respiratory failure, the agreement between MIP and SNIP (64-79%) and kappa values (0.29-0.53) was moderate. CONCLUSIONS: MIP and SNIP were equally feasible. There was no significant bias in favor of either test, but a considerable disparity in results between tests, suggesting that use of both tests is warranted to screen for early detection of patients at risk of respiratory failure and avoid over diagnoses. SNIP, MIP, and VC all follow a relatively linear downhill course with a steeper slope for "bulbar" versus "non-bulbar" patients.