#12

Altered Vestibulo-Spinal Reflex (VSR)

  • Definition and Function: The VSR stabilizes the body by activating antigravity muscles through the spinal cord.
  • Clinical Manifestations:
    • When vestibular input is inconsistent or delayed, the outputs of the VSR can become either:
    • Hyperactive: This is displayed as rigid sway suppression.
    • Hypoactive: Resulting in excessive sway and loss of postural tone.
    • Patient Experience: Patients commonly report sensations of being "off balance, but not dizzy"; this is indicative of sensory mismatch attributed to musculoskeletal bias.

Degraded Vestibulo-Ocular Reflex (VOR) Calibration

  • Definition and Alterations: Head-neck alignment bias can disrupt eye-head coordination.
  • Clinical Implications:
    • VOR Gain Changes: may be reduced or variable, causing patients to experience visual blurring or heightened sensitivity to motion during head turns.
    • Advanced Chronic Cases: In these cases, visual dependency tends to increase, as vision becomes the primary source for maintaining balance control, which in turn raises the energy cost associated with postural control.

Skeletal-Level Effects on Vestibular Function

  • Regions and Effects:
    • Cervical Spine (CO-C2):
    • Forward head/extension posture alters otolith bias.
    • Implications on vertigo and balance perception.
    • Thoracic Spine:
    • Flattened kyphosis leads to reduced thoracic cage mobility, negatively affecting respiratory coupling.
    • Lumbopelvic Region:
    • Anterior tilt/lumbar extension bias shifts the center of mass (COM).
    • Lower Extremity:
    • Asymmetrical foot positioning can alter somatosensory input due to over-pronation or tibial rotation, impacting vestibular function.

Impact on Global Balance Strategies

  • Principles of Global Balance: The body employs three main postural correction strategies to maintain balance:

    1. Ankle Strategy:
    • Normal Function: Involves small corrections made via dorsiflexion and plantarflexion.
    • Chronic Adaptation: Results in impaired proprioceptive feedback, increased stiffness in the distal chain from underutilization of this strategy.
    1. Hip Strategy:
    • Normal Function: Utilizes medium corrections through trunk and hip torque.
    • Chronic Adaptation: Leads to underutilization, hip flexor facilitation, gluteal inhibition, resulting in inefficient torque generation, and increased shear through the spine.
    1. Stepping Strategy:
    • Normal Function: Employed to re-establish base of support (BOS) when the center of mass (COM) exceeds limits.
    • Chronic Adaptation: Results in delayed initiation due to slowed vestibulo-spinal timing, smaller corrective steps, or the condition termed "freezing".
    1. Suspensory Strategy:
    • Normal Function: Involves lowering the center of mass (COM) to increase stability.
    • Chronic Adaptation: Becomes a default due to fear and decreased adaptability, potentially resulting in a chronic crouched posture.