Computerized Dynamic Posturography (CDP): An Introduction

This video will explore what a Computerized Dynamic Posturography (CDP) assessment consists of, with a focus on the most important test of sensory integration: the Sensory Organization Test (SOT).

If you prefer reading, find the full transcript below.

Sensory and motor inputs

Maintaining balance relies on a combination of both sensory and motor inputs. The three sensory inputs that contribute to this process come from the visual, vestibular, and somatosensory systems. The information from these signals is integrated together in order to help determine an individual's body position.

And this links into the motor system by leading to decisions and choices regarding body movement to maintain stability and balance in a variety of scenarios. To assess an individual's balance function, it is important to investigate how well their sensory integration works and how good their motor control is. And this is where we can make use of computerized dynamic posturography.

What is computerized dynamic posturography (CDP)?

Computerized dynamic posturography (CDP) is an assessment of how well the patient integrates sensory information and how they make use of motor commands in complex environmental simulations that mimic real world situations.

Information needed before a CDP evaluation

If you were to ask a physical therapist about what information is needed before performing a CDP assessment, they will most likely suggest a set of tests of the patient's physical abilities, such as an assessment of their strength of their lower extremities, a sit to stand test, and evaluation of their gait.

However, if you were to approach an audiologist with the same question, they would most likely focus on the vestibular diagnostic test battery to:

• Measure the patient's VOR function
• Assess the peripheral vestibular organs
• Establish whether there is any central involvement

And in an ideal world, it would be great to have as much information as possible. Both the physical and audiological assessment would be highly informative. However, practically speaking, you don't actually have to have all or any of this information. You can still perform a CDP assessment and obtain a picture about someone's balance function without this information first.

And this is because the CDP assessment is not a diagnostic test. Its purpose is to provide a functional assessment of how the patient might be managing – or not – on a day-to-day basis. And there is some essential information that you do need to know before performing a CDP test.

1. Pre-existing physical or neurological conditions

As a bare minimum, any pre-existing physical or neurological conditions which could affect the assessment, or the rehabilitation process must be known about. It may be possible to account for these or it may confirm that other intervention is required first, before proceeding with the CDP assessment.

2. VOR deficit related to head movement

Any functional VOR deficit that relates to head movement will impact their rehabilitation. Tests such as dynamic visual acuity (DVA) and gaze stabilization testing (GST) can supply more information regarding this, and VOR exercises can be included in the rehabilitation program.

3. Ability to stand

Now, if the patient cannot stand on a hard surface unassisted for two minutes, then they simply won't be able to complete a CDP assessment. Other intervention or rehabilitation may be indicated in the first instance before proceeding to CDP.

What tests should you do?

The CDP/IVR™ offers tests to assess both the sensory system and the motor system. The most important sensory assessment test is called the sensory organization test, which we will be focusing on in more detail now.

What is the sensory organization test (SOT)?

The sensory organization test (SOT) is an assessment of the visual, vestibular, and somatosensory systems. The SOT provides an objective quantification of impairments in these systems and how the patient integrates the signals from the sensory system.

SOT conditions

There are six conditions within the SOT, and these are standardized conditions with normative data against which patient performance can be measured. Each of these conditions removes, reduces, or impairs one or more of the sensory inputs that the body uses to maintain its balance (Figure 1).

Organizing sensory information in daily life

The sensory and motor systems combine to contribute to our body's ability to maintain balance. The sensory information that is provided comes from visual, vestibular, and somatosensory cues. And in daily life, we encounter this information constantly and in a changing fashion.

Sometimes, this information is robust, straightforward, and reliable. For instance, in clear daylight, in a still environment, the visual cues that we receive are reliable and accurate. However, when we're in darker environments, when depth perception is affected, or there's a difference in object movement, these visual cues become less apparent or less accurate.

And the same applies for somatosensory cues. Walking on unstable surfaces provides us with unreliable sensory information.

What is sensory reweighting?

When sensory information is diminished or unreliable, then what the brain should do is re-weight the sensory information that it is receiving to prioritize the more relevant, accurate sensory information from an alternative source.

So for instance, when we're walking in the dark, our visual cues are diminished, and so somatosensory information about what is underfoot, and vestibular input, becomes more important. It gets reweighted and prioritized as more important than the visual cues.

However, what can happen is that the sensory reweighting process doesn't always work to the best of its ability. For instance, if there is a vestibular insult, the sensory reweighting process may try to prioritize the vestibular cues when visual cues aren't available or accurate. But these vestibular cues are in fact inaccurate themselves because of the vestibular problem, and so imbalance may occur.

Another example is when an individual has become over-reliant on visual information, and they find themselves in a visually disruptive environment. Here, the sensory reweighting process will try to suppress the visual cues and prioritize somatosensory cues. But if those visual cues cannot be suppressed effectively, then there will be a disruption to that person's balance.

SOT equilibrium score

In Figure 2, you can see that once the SOT has been performed, you are presented with an equilibrium score for each trial within each of the six conditions.

The SOT measures the amount of sway produced in each trial. The higher the score, the less sway was recorded. The grey bars refer to the normative data that is available. A green result means the patient's sway was less – and thus better – than the normative data. A red result means that their sway was greater – and thus worse – than the normative data.

Because the results are referenced to this normative data, it is possible to be better than this norm, which is why the results can exceed 100%.

Common patterns of abnormalities

It's very important to not just look at the individual condition scores produced in the equilibrium score graph, but to look at these conditions in relation to each other. By doing so, we can start to identify patterns and combinations of abnormalities.

The purpose of the SOT is to examine how the individual reacts to disturbances to the various different sensory inputs. To do this accurately, we must look at the overall picture rather than one individual condition in isolation. How they interact can tell us where the difficulties in sensory organization and reweighting are.

SOT sensory score

A set of sensory scores are also produced (Figure 3).

Motor strategy analysis

The SOT provides more than just an assessment of the amount of sway, however. The results also provide an insight into the motor strategy used by the individual in response to the conditions being tested. It becomes possible to identify whether the patient:

• Steps (which is known as a fall in the SOT)
• Is using an ankle strategy (which is what should be used)
• Is using a hip strategy (which is less preferable but may be effective in stopping them from falling)

By looking at the strategy analysis results, you can identify in which conditions which type of strategy was being used. This is also a powerful tool for analyzing progress and objectively showing improvement following therapeutic intervention.

In Figure 4, we can see that the patient used an ankle strategy for some conditions, and more of a hip strategy in others before therapy.

After therapy, an ankle strategy was being used in all conditions (Figure 5).

Center of gravity alignment

The SOT also measures the patient's center of gravity alignment, which can help to identify risk of falls. And once again, this is a useful method of quantifying progress and improvement following therapy.

Motor assessments

But the SOT does not stand in isolation. As already mentioned, it is important to complement the assessment of the sensory integration process with motor assessment tests. Computerized dynamic posturography supplies a range of motor assessments, including:

• Limits of stability
• The adaptation test
• The motor control test
• Rhythmic weight shifting

Each of these assesses the patient's motor assessment under different conditions. These motor control tests are beyond the scope of this video, however.