Longitudinal Improvement in Respiratory Function Following Plasma Exchange in Patients with Severe Myasthenia Gravis.

Background: There are no data on the effect size and timing of plasma exchange (PLEX) in patients with myasthenic crisis (MC).

Methods: We retrospectively analyzed measurements of forced vital capacity (FVC) and negative inspiratory force (NIF) in the days before and after PLEX (administered every other day) in patients with MC admitted to a tertiary hospital over 4 years. For multiple measurements in one day, the average value was used. The day immediately before the first treatment with PLEX was considered baseline. Using time as a continuous or categorical variable in mixed-effects multiple linear regressions, we estimated predicted values for these tests.

Results: Twenty-two patients (mean age 67.3 years, 51.9% male patients) with 27 MC episodes and 508 measurements (234 FVC and 274 NIF; from 5 days before to 20 days after PLEX) were included. Presence of antibodies was detected in 70.4%. Intubation and mechanical ventilation occurred in 36.6% of patients. The mean number of PLEX was 5.1 (range 3-11). NIF values decreased before the first PLEX but increased after by on average 1 cm H₂O/day (95% confidence interval [CI] 0.68-1.32, p < 0.001). FVC fluctuated before the first PLEX but then increased by on average 51.2 mL/day (95% CI 35.8-66.1, p < 0.001). The maximum increase in NIF occurred during the day of the first PLEX (9.2 cm H₂O, 95% CI 3.3-15.1, p = 0.002) and rather slowed after day 10. FVC increase compared to baseline became significant the second day after the first PLEX (287 mL, 95% CI 7.5-567.6, p = 0.04) and continued overall to increase (with fluctuations) up to day 17.

Conclusions: Significant increases in bedside respiratory measurements are observed as soon as the first PLEX day but with more variability on FVC than NIF, which may either reflect more FVC technique inconsistencies or more consistent effect of the treatment on NIF.