The Storage Velocity Mechanism: A Guide for Vestibular Therapists

In an effort to better understand the Storage Velocity Mechanism (SVM) and its clinical significance—something that isn’t talked about much in courses—I decided to dive deeper into what it is, why it matters, and what I should know as a vestibular therapist. Here’s what I found.

What is the Storage Velocity Mechanism?

As we know, the semicircular canals detect angular acceleration. However, with motion at a steady speed the cupula in the semicircular canal returns to its resting position within 7 seconds. Despite this, we can maintain visual stability during sustained rotations, thanks to the Storage Velocity Mechanism (SVM).

The SVM essentially integrates and "stores" velocity information within the vestibular nuclei in the brainstem, extending the response time up to an additional 20 seconds. This prolonged signal allows for smoother adaptation to sustained head movements, maintaining gaze stability, balance, and spatial awareness.

Examples of the SVM in Action

1. Post-Rotational Nystagmus

One classic example of the SVM in action is post-rotational nystagmus. After spinning and stopping abruptly, the neural signals stored by the SVM continue to generate eye movements (nystagmus) and a sense of motion. A fun example of this can be seen with Broom races,  where participants spin rapidly around a broomstick (activating the SVM), then attempt to run. The lingering signals cause poor balance, unsteady movements, and a lot of laughs, along with some potential abrasions and bruises.

2. The Graveyard Spiral in Aviation

In aviation, the Graveyard Spiral demonstrates the dangers of SVM decay. Pilots flying without visual references (e.g., in clouds) rely on vestibular inputs to detect rotation. Over time, the SVM’s extended signal decays, leading pilots to incorrectly perceive they are flying straight while they are actually banking. This results in progressively tighter turns and eventual loss of control.

This example highlights how critical the SVM is for spatial orientation and the importance of visual/instrument cues to avoid relying solely on the vestibular system.

What Impairs the SVM?

The SVM can be impaired by:

• Brainstem Lesions: Disrupt the vestibular nuclei’s ability to integrate signals.
• Neurological Disorders: Conditions like multiple sclerosis impair central vestibular processing.
• Vestibular Dysfunction: Peripheral vestibular deficits weaken the input signals.
• Medications: Sedatives or vestibular-suppressing drugs can dampen neural processing.

Symptoms of SVM Impairment

When the SVM is impaired, patients may present with:

• Dizziness or motion sensitivity.
• Visual instability during head movements.
• Loss of balance.
• Spatial disorientation.

Assessing the SVM Clinically

The SVM can be formally evaluated through Rotational Chair Testing, but these are usually not accessible. There is an alternative, however, that you may want to consider being the Post-Rotational Nystagmus Assessment:

Steps for Post-Rotational Nystagmus Assessment:

1. Preparation:
   
   • Seat the patient securely in a position aligned with the axis of rotation.
   • Use VNG or Frenzel goggles to monitor nystagmus.
2. Induction:
   
   • Rotate the patient at ~120°/sec for 60 seconds.
   • Instruct the patient to keep their eyes open during rotation.
3. Observation:
   
   • Stop rotation abruptly and observe nystagmus duration, amplitude, and direction.

Interpretation:

• Short-Lived Nystagmus (<10 seconds): Suggests vestibular hypofunction.
• Prolonged Nystagmus (>30 seconds): Suggests central dysfunction.
• Asymmetric Nystagmus: Suggests unilateral vestibular loss.

Follow-up tests (e.g., calorics, head impulse, VEMPs, imaging) can help confirm findings.

Treatment Strategies for SVM Impairment

When SVM function is impaired, the following strategies that most of us are familiar with already may help:

1. Vestibular Adaptation Exercises:
   
   • VOR x1 and x2 exercises strengthen vestibulo-ocular pathways.
2. Balance and Habituation Training:
   
   • Gradual exposure to rotational movements improves central integration.
3. Education:
   
   • Train patients to rely more on visual and proprioceptive cues for stability.
4. Address Underlying Causes:
   
   • Treat the peripheral vestibular deficits or central neurological issues.

Final Thoughts

The Storage Velocity Mechanism is a critical, though under-discussed, component of vestibular function. It compensates for the natural decay of semicircular canal input, ensuring gaze stability, balance, and spatial awareness during sustained head movements. It is important to understand its function and relevance.

With investigating this topic it was clear that SVM has been studied significantly with living in space and dealing with Mal de Débarquement Syndrome. With that being said, maybe the reason it is not in the forefront of courses that we take is simply due to the fact that it is impacted by vestibular conditions we are assessing anyways and there are no unique treatment strategy to address it when it is impaired.

Sources:

1. Baloh RW, Honrubia V. Clinical neurophysiology of the vestibular system. Contemp Neurol Ser. 1979;18:1-21. PMID: 378525.
2. Hain T. Neurophysiology of Vestibular Compensation - 2023.
3. Lackner JR & DiZio P. Velocity storage: its multiple roles.Journal of Neurophysiologuy 2020. 123: 3, 1206-1215.
4. Raphan T, Matsuo V, Cohen B. Velocity storage in the vestibulo-ocular reflex arc (VOR). Exp Brain Res. 1979 Apr 2;35(2):229-48. doi: 10.1007/BF00236613. PMID: 108122.