ObjectiveTo retrospectively compare the benefits (episode cessation) and risks of IV methylprednisolone (IV-MP) vs abstention during prolonged Kleine-Levin syndrome (KLS) episodes.MethodsA total of 26 patients with KLS received 1 g/d IV-MP for 3 days during 1 to 6 episodes each (totaling 43 IV-MP sessions). The change of episode duration with IV-MP (vs previous episode duration) was compared with the change duration between 2 consecutive episodes in 48 untreated patients matched for age, sex, age at KLS onset, number of episodes, and disease duration (more treated than untreated patients had long episodes).ResultsEleven patients (42.3%) had an episode that was at least 1 week shorter than the preceding one when they received IV-MP therapy, whereas shorter episodes were significantly less frequent (10.4%) in the untreated group. This benefit was more marked (65.5% responders, 12 fewer days in an episode vs 0 days in the untreated patients) when IV-MP was infused before the 10th day of the episode. Mild, transient adverse effects (insomnia, muscle pain, nervousness/restlessness, but no manic switching) were reported by 61.3% of patients. No specific responder profile was identified.ConclusionIn this open-labeled, naturalistic study, early IV-MP (following the protocol for multiple sclerosis relapses) had a good benefit/risk ratio during KLS episodes in patients with long episodes (with half of the patients having an early cessation of episodes).Classification of evidenceThis study provides Class IV evidence that for patients with long episodes of KLS, IV steroids decrease the duration of KLS episodes.