The syndrome

Pathophysiology

MdDS is not a disease of the labyrinth but of adaptation. The brain learns the rhythm of the boat and then cannot unlearn it.

Maladaptive adaptation

On a boat, the brain adapts its vestibular and visual processing to the rhythmic, rolling motion so that you can function. On returning to stable ground, that adaptation should rapidly reverse. In MdDS it does not: the adapted internal model persists, and the patient continues to feel the rocking that is no longer there.1 This is the maladaptive-adaptation model, and it explains the two most peculiar features — that the trigger is motion, and that motion relieves it.

Entrained velocity storage and the VOR

A leading mechanistic account centres on velocity storage — the brainstem–cerebellar integrator that prolongs vestibular signals — becoming entrained to the roll motion of the vessel. The vestibulo-ocular reflex adapts in the roll plane during exposure and fails to re-set afterwards. This model is more than theoretical: it is the rationale for the readaptation protocol, in which rolling the head while viewing moving stripes re-tunes the maladapted reflex and relieves symptoms.2

A central, not peripheral, disorder

Peripheral vestibular function is normal in MdDS; the disorder is central. Functional and metabolic imaging has shown altered activity and connectivity in vestibular, visual and limbic networks — including the entorhinal cortex and amygdala — consistent with a disorder of central sensory integration and its emotional weighting rather than end-organ damage.3

The female predominance

MdDS overwhelmingly affects women in midlife. The reasons are not settled, but a hormonal contribution is suspected given the demographic skew and reports of symptom fluctuation with hormonal changes; anxiety and migraine are also over-represented, particularly in the spontaneous-onset form.4

Key points

  • MdDS is a disorder of maladaptive central adaptation, not labyrinthine disease.
  • Velocity storage and the roll-plane VOR become entrained to the vessel and fail to re-set.
  • That model underpins the VOR readaptation treatment.
  • Imaging shows central vestibular–visual–limbic network changes; peripheral function is normal.
  • Strong female, midlife predominance; hormonal, anxiety and migraine associations are described.