Hypothesis: The Vestibular and Cerebellar Basis of the Mal de Debarquement Syndrome

Abstract

The Mal de Debarquement syndrome (MdDS) generally follows sea voyages, but it can occur after turbulent flights or spontaneously. The primary features are objective or perceived continuous rocking, swaying, and/or bobbing at 0.2 Hz after sea voyages or 0.3 Hz after flights. The oscillations can continue for months or years and are immensely disturbing. Associated symptoms appear to be secondary to the incessant sensation of movement. We previously suggested that the illness can be attributed to maladaptation of the velocity storage integrator in the vestibular system, but the actual neural mechanisms driving the MdDS are unknown. Here, based on experiments in subhuman primates, we propose a series of postulates through which the MdDS is generated: (1) The MdDS is produced in the velocity storage integrator by activation of vestibular-only (VO) neurons on either side of the brainstem that are oscillating back and forth at 0.2 or 0.3 Hz. (2) The groups of VO neurons are driven by signals that originate in Purkinje cells in the cerebellar nodulus. (3) Prolonged exposure to roll, either on the sea or in the air, conditions the roll-related neurons in the nodulus. (4) The prolonged exposure causes a shift of the pitch orientation vector from its original position aligned with gravity to a position tilted in roll. (5) Successful treatment involves exposure to a full-field optokinetic stimulus rotating around the spatial vertical countering the direction of the vestibular imbalance. This is done while rolling the head at the frequency of the perceived rocking, swaying, or bobbing. We also note experiments that could be used to verify these postulates, as well as considering potential flaws in the logic. Important unanswered questions: (1) Why does the MdDS predominantly affect women? (2) What aspect of roll causes the prolongation of the tilted orientation vector, and why is it so prolonged in some individuals? (3) What produces the increase in symptoms of some patients when returning home after treatment, and how can this be avoided? We also posit that the same mechanisms underlie the less troublesome and shorter duration Mal de Debarquement.

Keywords: baclofen; bobbing; gravity; nodulus; orientation vector; rocking; swaying; vestibular-only neurons.

Figure 1

Frequencies of rocking (A) and swaying (B) in Mal de Debarquement Syndrome patients (2). The frequencies were determined on a Nintendo Wii board. The rocking frequencies were tightly centered around a maximum at 0.2 Hz, more for the rocking than the swaying. When there was no actual rocking or swaying, the perceived frequencies were determined with the elbow stabilized on a board, and the patient moved the forearm at the frequency of the perceived movement.

Figure 2

(A) Site of recording of the neuron of the nodulus shown in (B). (B) Climbing Fiber-driven Purkinje cell activity from the site shown in (B). The Purkinje cell fired three to five times. Each time, the animal was rolled into the left side-down position. The oscillation in roll is shown by the bottom trace. The oscillation amplitude is shown by the bar on the right, and the time base by the lowest trace. This figure is reprinted with permission. For further details, see the article by Barmack and Shojaku (70).