Respiratory Muscle Assessment in Acute Guillain-Barré Syndrome

The neurology of acutely failing respiratory mechanics

Forces involved in breathing-which effectively pull in air-are the diaphragmatic, intercostal, spine, and neck muscles. Equally important is the bulbar musculature maintaining the architecture of a patent airway conduit and abdominal wall and internal intercostal muscles providing cough. Acute injury along a neural trajectory from brainstem to muscle will impair the coordinated interaction between these muscle groups. Acutely failing respiratory mechanics can be caused by central and peripheral lesions. In central lesions, the key lesion is in the nucleus ambiguus innervating the dilator muscles of the soft palate, pharynx, and larynx, but abnormal respiratory mechanics rarely coincide with abnormalities of the respiratory pattern generator. In peripheral lesions, diaphragmatic weakness is a main element, but in many neuromuscular disorders mechanical upper airway obstruction from oropharyngeal weakness contributes equally to an increased respiratory load. The neurology of breathing involves changes in respiratory drive, rhythm, mechanics, and dynamics. This review focuses on the fundamentals of abnormal respiratory mechanics in acute neurologic conditions, bedside judgment, interpretation of additional laboratory tests, and initial stabilization, with practical solutions provided. Many of these respiratory signs are relevant to neurologists, who in acute situations may see these patients first. Ann Neurol 2017;81:485-494.

Optimal method for assessment of respiratory muscle strength in neuromuscular disorders using sniff nasal inspiratory pressure (SNIP)

Background

The ability to accurately determine respiratory muscle strength is vitally important in patients with neuromuscular disorders (NMD). Sniff nasal inspiratory pressure (SNIP), a test of inspiratory muscle strength, is easier to perform for many NMD patients than the more commonly used determination of maximum inspiratory pressure measured at the mouth (MIP). However, due to an inconsistent approach in the literature, the optimal technique to perform the SNIP maneuver is unclear. Therefore, we systematically evaluated the impact of performing the maneuver with nostril contralateral to the pressure-sensing probe open (SNIP_OP) versus closed (SNIP_CL), on determination of inspiratory muscle strength in NMD patients as well as control subjects with normal respiratory muscle function.

Methods

NMD patients (n = 52) and control subjects without respiratory dysfunction (n = 52) were studied. SNIP_OP, SNIP_CL, and MIP were measured during the same session and compared using ANOVA. Agreement and bias were assessed with intraclass correlation coefficients (ICC) and Bland-Altman plots.

Results

Mean MIP values were 58.2 and 94.0 cmH2O in NMD and control subjects, respectively (p<0.001). SNIP_CL was greater than SNIP_OP in NMD (51.9 ±31.0 vs. 36.9 ±25.4 cmH₂O; p<0.001) as well as in controls (89.2 ±28.1 vs. 69.2 ±29.2 cmH₂O; p<0.001). In both populations, the ICC between MIP and SNIP_CL (NMD = 0.78, controls = 0.35) was higher than for MIP and SNIP_OP (NMD = 0.53, controls = 0.06). In addition, SNIP_CL was more often able to exclude inspiratory muscle weakness than SNIP_OP.

Conclusions

SNIP_CL values are systematically higher than SNIP_OP in both normal subjects and NMD patients. Therefore, SNIP_CL is a useful complementary test for ruling out inspiratory muscle weakness in individuals with low MIP values.