Dynamic SVV
Centrifugal stimulation, off-vertical axis rotation, and unilateral galvanic paradigms unmask the SVV asymmetries that static testing has compensated away.
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Static SVV tests the otolith system at zero frequency — a single steady measurement, the patient sitting still. Dynamic SVV tests it at higher frequencies and accelerations, where central compensation works less well.
The clinical reason to bother is simple. A peripheral vestibular lesion produces a large static SVV tilt that the brain compensates over weeks. By three months, the static SVV may be back to normal even though the lesion is still there. Dynamic paradigms unmask that residual asymmetry.
Centrifugation is the most direct approach. The patient sits on a motorised chair displaced four centimetres off the rotation axis. Constant-velocity rotation produces a centripetal acceleration along the inter-aural axis.
That acceleration stimulates one utricle selectively, depending on which side the patient is displaced toward. The patient sets the SVV during steady rotation.
An asymmetric response — settings tilted more on one side than the other by more than five degrees — indicates utricular asymmetry, even when static SVV is normal.
Off-vertical-axis rotation, or OVAR, tilts the rotation axis ten to thirty degrees from vertical. The otoliths experience a continuously changing gravitational vector. Healthy subjects track it smoothly. Utricular asymmetry shows up as an asymmetric modulation of perceived vertical across the rotation cycle.
Galvanic vestibular stimulation passes a small DC current across the mastoids. It produces a perceived tilt toward the anode and a measurable SVV shift. It is the only dynamic paradigm that does not need to move the patient.
Where does dynamic SVV earn its place? Three main settings: compensated unilateral vestibulopathy where you suspect residual asymmetry; vestibular schwannoma surveillance; and pre-operative baselines before intratympanic gentamicin.
Dynamic SVV does not help in acute presentations, where static SVV is already tilted. It does not help in bilateral failure, where there is no asymmetry to detect. And it does not help in functional dizziness, where vestibular testing must be normal by definition.
Why static SVV is not the whole story
Static SVV asks the patient to set a luminous line to vertical while sitting upright in a dark, silent environment. The vestibular asymmetry produced by a peripheral lesion is a tonic signal — and the brain compensates tonic signals well[2]. Within weeks of an acute vestibular lesion, the static SVV may normalise even though caloric responses, head impulse gain, and dynamic visual acuity all remain abnormal.
Dynamic SVV paradigms probe the otolith system at higher frequencies and accelerations. Central compensation works less well at these frequencies. A chronic asymmetry that is invisible to static SVV will reveal itself when the patient is rotated, displaced, or electrically stimulated.
Centrifugal SVV (unilateral centrifugation)
The patient sits on a centrifuge displaced 4 cm to one side of the rotation axis. During constant-velocity rotation, the centripetal acceleration (typically 1 g) acts along the inter-aural axis and selectively stimulates the laterally-displaced utricle. The patient is asked to set the SVV during steady rotation.
- Healthy adults set SVV within ±5° of the gravito-inertial vector under these conditions.
- A unilateral utricular lesion produces an asymmetric response: SVV is more tilted when the patient is displaced so that the lesioned utricle is stimulated.
- Differences between left-stimulus and right-stimulus settings of more than 5° indicate utricular asymmetry, even when the static SVV is normal[2].
Centrifugation requires a motorised vestibular chair with a translation track and a luminous-line projection inside an otherwise dark cabin. Outside of dedicated vestibular laboratories, this paradigm is rarely available.
Off-vertical axis rotation (OVAR)
The whole rotation axis is tilted 10–30° from vertical and the patient is rotated at constant velocity. The otolith system experiences a continuously changing gravitational vector. Healthy subjects perceive themselves as on a cone; their SVV settings track the dynamic gravito-inertial vector smoothly. Patients with utricular asymmetry show an asymmetric modulation of perceived vertical — the SVV trace is flattened on one side of the rotation and exaggerated on the other.
OVAR is sensitive to subtle utricular asymmetries that escape both static and centrifugal SVV. It is also the most demanding paradigm — the equipment is rare and the patient must tolerate sustained off-vertical rotation.
Galvanic vestibular stimulation (GVS)
A small DC current (1–3 mA) applied across the mastoids stimulates vestibular afferents bilaterally and produces a perceived tilt and a measurable SVV shift toward the anode. Asymmetric responses to bilateral stimulation indicate central or peripheral asymmetry; unilateral stimulation tests one side at a time.
GVS is the only dynamic SVV paradigm that does not require motion of the patient. It is available in research settings but has not yet entered routine clinical practice.
When to consider dynamic SVV
| Clinical question | Static SVV | Dynamic SVV |
|---|---|---|
| Acute UVL: is there utricular involvement? | Diagnostic | Not needed |
| Chronic, compensated UVL: is there residual asymmetry? | Often normal | Unmasks the asymmetry |
| Pre-operative vestibular schwannoma baseline | Often normal | Quantifies residual function |
| Bilateral vestibulopathy | Normal (no asymmetry to detect) | Normal — wrong test |
| Acute Wallenberg / central lesion | Grossly tilted | Adds little |
| Suspected functional dizziness (PPPD) | Normal — supportive | Normal — supportive |
The main clinical role for dynamic SVV is in chronic or compensatedperipheral lesions, particularly when the question is whether a residual asymmetry persists. For acute presentations, static SVV is sufficient.
Pitfalls
- Motion sickness. Centrifugation and OVAR provoke nausea. Test in short blocks with rest periods.
- Motor adaptation. Repeated trials in one direction can produce a short-term adaptation that biases subsequent measurements. Randomise stimulus order.
- Visual frame leakage. Any stray light from chair instrumentation contaminates the SVV setting. Verify true darkness before each block.
- Head position errors. Even a 5° head tilt during rotation changes the gravito-inertial vector geometry; head restraint is essential.