Computerized Dynamic Posturography (CDP)
OVERVIEW
The human postural control system is uniquely designed to maintain the center of gravity (COG) over a small base of support (BOS) by integrating sensory inputs (vision, somatosensory, vestibular) and coordinating motor output (Nashner, 2021). The vestibular system is necessary for postural control when standing on an unstable or moving support surface and in visually complex or darkened environments. The visual system aids in balance control with an unstable/moving surface and a stable visual scene: the somatosensory system is the primary sensory input with a stable support surface.
Limits of Stability
The vestibular system also contributes to a limits of stability (LOS) map or the maximal amount of body movement (i.e., COG sway) that a person maintain without needing to alter their BOS (Horak, 2006; Nashner, 2021). The LOS is often depicted as a cone with an estimated range of movement in all directions (anterior, posterior, medial, and lateral) a person can comfortably make without falling. Vestibular dysfunction and the aging process may disrupt the internal stability map (LOS), which impacts the appropriate selection of balance correction strategies (i.e., ankle, hip, stepping) to maintain balance, resulting in an increased risk of falls (Horak, 2006).
Balance Correction Strategies
Volitional and automatic balance control responses stabilize the body to prevent a fall during activities of daily living, but not all balance corrections require the same balance strategy. Individuals may utilize an ankle strategy when on a firm, flat surface, and quickly change to a hip strategy when on a complaint or narrow BOS (Shepard & Telian, 1996). Both of these strategies keep the feet in place, but when this is not possible, a stepping strategy is used to alter the BOS to control balance (Horak, 2006). Repeated exposure to destabilizing environments corrects for inappropriate responses through adaptive learning (Shepard & Telian, 1996). This learning process begins in infancy and is modified with advanced activities and challenging balance scenarios.
CLINICAL CONSIDERATIONS FOR COMPUTERIZED DYNAMIC POSTUROGRAPHY (CDP)
CDP quantifies balance function during challenging tasks and brief, unexpected movements (Nashner & Shepard, 2021). COG is estimated through the use of a force measurement system (Shepard and Telian 1996), and patient demographics (i.e., height and weight; Nashner & Shepard, 2021). Specifically, the patient stands on forceplates that sense horizontal shear force and vertical force movements (Nashner & Shepard, 2021), and pressure exerted on these sensors estimate COG sway.
The primary use of CDP is to further examine the sensory contributors (somatosensory, visual, and vestibular) and motor control output to maintain balance. CDP is ideal for monitoring the functional capabilities of the patient with imbalance and falling concerns and to evaluate the status of compensation in patients with vestibular dysfunction (Shepard & Telian, 1996). The NeuroCom EquiTest System was considered the gold standard CDP device that utilizes a movable support system and visual surround, both of which moves in anterior/posterior directions according to the patient’s body sway (i.e., sway-referenced). However, advances in technology (Bertec CDP with Immersive Virtual Reality, Bertec Inc.) provide an interactive visual display (i.e., virtual reality) as opposed to a sway-referenced movable surround. The full-field visual surround is projected within a stationary dome (Figure 9–1). Recent work by Trueblood, Rivera, Lopez, Bently, and Wubenhorst (2018) compared the new Bertec CDP with the standard NeuroCom CDP. Overall, the systems produce comparable results in healthy controls, with no significant differences when evaluating vestibular contributions to maintain balance. Both systems utilize the same CDP subtests: sensory organization test, motor control test, and adaptation test.
• Sensory Organization Test (SOT) measures an individual’s functional ability to coordinate vision, vestibular, and somatosensory systems to maintain balance via six increasingly challenging conditions.
Figure 9–1. Schematic of a patient positioned within full-field visual immersive posturography. Reprinted with permission of Bertec Corporation, Columbus, OH, Copyright 2020.
• Motor Control Test (MCT) assesses the function of the long-loop motor pathways involved in maintaining balance after an unexpected movement of the support surface.
• Adaptation Test (ADT) evaluates the adaptive postural response to maintain stability with unexpected repeat movements of the support surface.
Patient Preparation
The following steps are recommended for optimal patient setup, as outlined by Nashner and Shepard (2021):
1. Consistent with other aspects of vestibular testing, the patient should avoid alcohol and medications that may impact balance control upwards of 48 hours before testing.
• Note: The patient should continue to take all life-sustaining medications.
2. The patient should wear loose-fitting pants and remove all footwear. Booties may be worn for infection control purposes.
3. The patient is secured in a safety harness, making sure that weight on the harness is transferred to the lower trunk.
4. The patient steps onto the right and left forceplates. The patient’s feet are aligned with the medial malleoli (medial ankle bones) set over the wide, horizontal line, and the lateral calcaneus (outer ankle bones) are adjusted according to patient’s height (small, medium, or large).
5. The safety harness straps are connected to the overhead bar allowing for unrestricted movement to measure COG sway (i.e., not too tight).
6. Finally, the patient is instructed to maintain upright stance without touching the walls or harness and keep their eyes open until told otherwise. Patients should be given instruction before each task; however, no feedback is generally given during testing.
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