The disorder
Causes & mechanism
Both sides fail together, so there is no asymmetry to spin the room — instead the two systems that normally stabilise gaze and posture quietly drop out. Then comes the search for a cause.
Why both sides failing looks the way it does
The balance organ does two jobs: it keeps your eyes still when your head moves, and it helps you stay upright. When only one side is damaged you feel spinning, because the two sides no longer match. When both sides fail equally there is no mismatch, so there is little spinning — but your eyes can no longer hold a steady picture during movement (vision blurs), and balance fails when it is dark or the ground is uneven.
A unilateral lesion produces vertigo and nystagmus from the static asymmetry between the two sides. In bilateral vestibulopathy the loss is broadly symmetric, so there is no spontaneous nystagmus and little rotational vertigo. What remains are two functional deficits: a failed VOR — measurable as low VOR gain and seen clinically as oscillopsia — and loss of the vestibular contribution to postural control, hence imbalance that decompensates in the dark.1
The VOR fails across the frequency range, which is why testing spans frequencies (caloric at ~0.003 Hz, rotational chair at mid frequencies, vHIT at the high, functional end). Central compensation — so effective after unilateral loss — has little to work with when both labyrinths are gone, so substitution by vision and proprioception, and saccadic strategies, carry the recovery.3 The vestibular contribution to hippocampal spatial processing also explains the spatial-memory and navigational difficulty many patients describe.
The causes
A cause is found in roughly three-quarters of patients; about a quarter remain idiopathic after a full work-up.2 The most important preventable cause is aminoglycoside ototoxicity — notably gentamicin, which is selectively vestibulotoxic, frequently spares hearing, and can appear even after apparently safe serum levels. Other causes include bilateral Ménière’s disease, autoimmune inner-ear disease, meningitis (a classic cause in children), genetic and degenerative disease — above all CANVAS — and neoplastic or surgical loss (bilateral schwannomas in NF2; bilateral nerve section, the original Dandy’s syndrome).
Precipitating events
Illustrative distribution of causes — about a quarter remain idiopathic after a full work-up; ototoxicity is the most important preventable cause.
Where to read the underlying anatomy
The labyrinth, the vestibular nerve and the central pathways are covered in Anatomy and Physiology. The tests that quantify the deficit each have their own chapter — vHIT, rotational chair and the caloric component of VNG.
Key points
- Symmetric bilateral loss → no static asymmetry, so little spinning vertigo or spontaneous nystagmus.
- The deficits that remain are a failed VOR (oscillopsia, low gain) and loss of vestibular postural control (imbalance worse in the dark).
- Central compensation has little to work with; substitution by vision and proprioception drives recovery.
- Aminoglycoside ototoxicity is the key preventable cause; ~25% are idiopathic; consider CANVAS, bilateral Ménière’s, autoimmune and genetic causes.