Exploring the pathophysiology of Mal de Debarquement

Abstract

Mal de debarquement syndrome (MdDS) is a disorder of perceived motion most often occurring following a cruise. We report findings from a patient with MdDS who underwent a transcranial magnetic stimulation protocol to quantify cortical excitability. These data were compared to those from 40 controls. The patient with MdDS exhibited high levels of intracortical facilitation. These findings will provide insight on the pathophysiology of MdDS, and will help guide future work on this rare condition. Mal de debarquement syndrome (MdDS) occurs when habituation to passive background movement becomes resistant to readaptation to stable conditions and results in a phantom perception of self-motion. MdDS typically disappears quickly after return to stable ground; however, in rare instances the symptoms may persist for months to years (persistent MdDS). While the etiology of persistent MdDS is poorly understood, it does not appear to originate from vestibular dysfunction as vestibular tests are normal and the symptoms do not respond to vestibular therapy [2]. Rather, MdDS appears to be a disorder of neuroplasticity and sensory rearrangement [2]. To our knowledge, there have been no studies examining the neurophysiologic characteristics of the disorder. In this case study we report findings from a patient with persistent MdDS who underwent an extensive transcranial magnetic stimulation (TMS) protocol to quantify motor cortical excitability. Specifically, we quantified the duration of the corticospinal silent period using single-pulse TMS. The silent period is observed when a subject performs a slight contraction and a single TMS stimulus is applied to the motor cortex (Fig. 1a). Additionally, we quantified intracortical facilitation (ICF) and short-interval intracortical inhibition (SICI) using paired-pulse TMS. Paired-pulse TMS combines a conditioning stimulus (CP) with a test stimulus (TP) at different interstimulus intervals, and the relative change in motor evoked potential (MEP) size is expressed relative to an unconditioned test pulse. At a short interstimulus interval of 3 ms, the CP inhibits the MEP in comparison to the TP only (SICI), whereas at a longer interstimulus intervals of 15 ms it facilitates the MEP (ICF) (Fig. 1b). The patient was a 63-year-old female (160.2 cm, 77.3 kg) who complained of sensations of unsteadiness and a feeling of the floor moving under her feet after disembarking from a 7-day cruise *3.5 years ago (3/2007). She reported having to regularly reach out to steady herself against stationary objects, and that MdDS had greatly impacted her quality of life (subjective scoring of six out of ten on a visual analog scale for the impact of MdDS on her quality of life). Several other hallmark features of MdDS were present including mild cognitive slowing and transient improvement in symptoms with re-exposure to passive movement [2]. Physical examination was unremarkable. Diagnostic testing including sural sensory and peroneal motor conduction studies, needle EMG, and MRI of the brain were all normal. Additionally, the patient had unremarkable findings on Dix-Hallpike testing and the caloric reflex test. The patient did not have migraine or other primary headache as defined by the International B. C. Clark (&) Department of Biomedical Sciences, Ohio Musculoskeletal and Neurological Institute (OMNI), Ohio University, 211 Irvine Hall, Athens, OH 45701, USA e-mail: clarkb2@ohio.edu