MOVING PLATFORM POSTUROGRAPHY TESTING TO DETECT SYMPTOM EXAGGERATION AND MOVING PLATFORM POSTUROGRAPHY RESULTS IN PPPD

Timothy C. Hain, MD • Page last modified: April 4, 2021Return to testing index
cdp test Bertec CDP

Equitest dynamic posturography machine developed by Neurocom Inc. This device is the one we use in our clinic in Chicago.

Bertec CDP/IVR. This device is termed the "next generation" of dynamic posturography. (See Bertec web site). We do not know of any data about malingering with this device as of 2020

Moving platform posturography, also called computerized dynamic posturography (CDP), or posturography for short, is a method of quantifying balance (although the definition of balance can be tricky).

Symptom validity, in the context of this discussion,. means that symptoms are a true reflection of a physical illness.

CDP can be used to make inferences about symptom validity because it contains within it 6 similar procedures that vary in difficulty, and thus should be affected in a consistent way (i.e. get worse with more deficit), by disease. This assumption is somewhat simplistic, and likely is not always true, but is generally reasonable. In this context, It holds best in persons in whom there is no data supporting a deficit in sensory function -- i.e. people with good vision, good vestibular function, and good proprioceptive input.

General goals of a test to assess symptom validity.

According to Lockhart and Saytaya-Murti (2015), "symptom validity testing" using psychological tests depends on detecting:

  1. Responding at a below-chance level . This suggests deliberate selection of wrong answers.
  2. Abnormally poor performance on easy tests combined with chance-only performance on extremely difficult tests.
  3. Violation of "floor effect" design -- individuals with no quantifiable deficit performing worse than individuals with moderate/severe quantifiable deficits.

To summarize, these strategies involve looking for inconsistency between subject performance and expectations for other groups -- in #1, does the patient do worse than tossing a coin ? # 2 and #3 basically involve looking at the "floor" -- procedures that nearly anyone should be able to do. Of course, these are only a small subset of indicators of inconsistency. Missing here, is the commonly encountered situation where patients do better on harder tests, than they do on easier tests.

In CDP, There are 6 "sensory tests", which are arranged as follows:

Posturography conditions
Condition Vision Surface Visual Surround Difficulty
1 Eyes Open Stable Stable 1
2 Eyes Closed Stable Stable 2
3 Eyes Open Stable Sway-Referenced 3
4 Eyes Open Sway-Referenced Stable 4
5 Eyes Closed Sway-Referenced Stable 5
6 Eyes Open Sway-Referenced Sway-Referenced 6

One does not need to know exactly what is meant here by the CDP conditions, but importantly for assessment of symptom validity, the difficulty level rises with the subtest number, and also later tests are to some extent just combinations of earlier testing protocols, and if one cannot perform an earlier one (such as standing with eyes closed), it would be unreasonable to expect that they could do better on a later one (such as standing on a wiggly surface with eyes closed).

Malingering and symptom exaggeration.

Posturography sensory test profile suggestive of malingering. Scores on test 1 are unusually low (this is a floor effect). Scores on test 6, which is more difficult than test 5, are better than test 5. The "Cevette" algorithm applied to this profile suggested that it is an "aphysiologic" pattern.

 

There has been considerable evidence in the literature that posturography is helpful in detecting symptom exaggeration. We think that this indication for posturography needs to be pursued. In particular, how accurately can exaggeration be differentiated from imbalance due the large variety of organic balance disorders ?

Several studies (Cevette et al, 1995; , Gianoli et al, 2000; Goebel et al, 1997; Krempl 1998) suggest that CDP is useful in detection of malingering, or at least, imbalance that is not accompanied by objective sensory disturbances or other causes. On the other hand, Uimonen (1995) found that static posturography fared no better than clinical observation. We conclude from data available to date that CDP is useful in this context.

Not all patients who fail CDP symptom validity criteria are malingerers

We provide examples below of a person with a well documented organic disorders that was scored as "aphysiologic". Accordingly, it is clear to us that posturography is not 100% accurate. It certainly could be falsely negative. The example below show how it can be falsely positive. .

CDP in bilateral loss

This patient has complete bilateral vestibular loss, onset about 20 years prior to this test. Curiously, they had an "aphysiologic pattern" using the Cevette method of analyzing the conditions..

PPPD -- persistent postural perceptual dizziness.

Although it is not claimed that PPPD is a form of malingering or symptom exaggeration, it shares with these situations that there are symptoms lacking an underlying organic mechanism, and the technology used to detect symptom exaggeration might also apply here.

CDP in patients with PPPD
FIgure 1 from Sohsten et al
Figure 1 from Sohsten et al (2016) comparing CDP scores by subtest in patients with PPPD, recovered vestibular syndromes, and controls. Note the similarity between recovered and controls, and also the lack of error bars. There were less than 20 patients in each group.

 

Patients with PPPD score poorly on CDP compared to controls, but the pattern of their scoring is not specific. For example, there is no consistent visual preference pattern. (Sohsten, Bittar and Staab, 2016). This suggests both that CDP is not a good discriminator for psychogenic dizziness, or perhaps that PPPD is just a symptom collection that has no diagnostic pattern. The figure above is from a paper on the subject of CDP and PPPD. Note the lack of error bars as well as standard deviations. When authors leave out variability data, this often means that the data was very variable. This reduces the value of the report.

Bottom line regarding utility of CDP for symptom validity testing :

CDP is a useful tool to detect inconsistency, which often accompanies malingering of balance. It is not a good tool to differentiate PPPD from other types of dizziness.

We thank Neurocom Inc, for use of figures of their equipment to illustrate this page. (Neurocom was purchased by Natus in 2018, and no longer exists as a corporate entity).

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