Can transcranial magnetic stimulation be used to evaluate patients with narcolepsy?

Narcolepsy is a rare, chronic sleep disorder characterized by excessive daytime sleepiness, cataplexy and other manifestations of dissociated rapid eye movement in sleep. We assessed the utility of transcranial magnetic stimulation (TMS) as an objective tool to elucidate the cortical excitability changes and also to analyze its role in assessing the treatment efficacy in narcolepsy. Eight patients with narcolepsy under our regular follow-up from 2000 to 2009 at our Sleep disorder clinic were chosen. All of them underwent polysomnography, multiple sleep latency tests and TMS. Resting motor threshold (RMT), cortical silent period (CSP) and central motor conduction time (CMCT) were assessed using TMS in both drug-naïve and post-treatment states. Eight controls were also subjected to all the three investigations. Appropriate statistical methods were used. The mean RMT (%) pre-treatment was higher in narcolepsy patients than that in controls, and it normalized following treatment. CSP and CMCT were unaffected in narcolepsy patients as compared to controls. This study shows that the cortical excitability is significantly low in narcolepsy patients. This motor cortex hypoexcitability becomes normal with the institution of treatment, pari passu with the control of symptoms. In future, TMS may be considered as an effective tool for documenting the treatment efficacy in patients with narcolepsy.