Computerized Dynamic Posturography (CDP) Sensory Test Scoring

Timothy C. Hain, MD • Page last modified: March 4, 2021Return to testing index

Example of SOT test

There are 6 "sensory tests", numbered 1-6, which are arranged as follows:

Posturography conditions
Condition Vision Surface Visual Surround Intended Sensory Input
1 Eyes Open Stable Stable

Vestibular+Eye+feet

2 Eyes Closed Stable Stable Vestibular+Feet
3 Eyes Open Stable Sway-Referenced Vestibular+Feet, distorted eyes
4 Eyes Open Sway-Referenced Stable Vestibular+Eyes, distorted feet
5 Eyes Closed Sway-Referenced Stable Vestibular+distorted feet
6 Eyes Open Sway-Referenced Sway-Referenced Vestibular+distorted feet+distorted eyes

Scores for each trial of each subtest are computed from the angular difference between the patient's calculated maximum anterior to posterior displacements to the theoretical maximum displacement, and this is expressed as an inverse percentage between 0 and 100. In other words, 100 is no sway at all, and 0 is 12.5 deg of sway. This produces a vector of 6 sensory (equilibrium) scores, which are actually (1-sway). I It also produces a vector of sensory conditions that becomes more challenging as the condition # increases, and usually results in a pattern where patients score the highest on test 1, and lowest on test 6.

The meaning of sway-referencing:

Three subtests, 4,5 and 6, are "sway referenced" with respect to the ankles, meaning that pressure is used to control the pitch angle of the platform with the goal of keeping ankle angle constant. This is not really technically possible, but it does distort ankle angle input and makes the ankles less reliable sources of orientation information.

Two tests, 3 and 6, are sway referenced with respect to vision, meaning that pressure of the feet is used to attempt to keep visual angle constant in spite of pressure differential across the lateral axis of the ankles, as measured by the pressure sensors on the device. Again, it is not really possible to do this accurately with this limited device (Di Fabio et al, 1998), but it makes vision less reliable and the entire test more difficult. The newer version of CDP, from Bertec, uses a different method for visual sway referencing, and for this reason, normal values for the older Neurocom device are not accurate for the Bertec device. Trueblood et al (2018) reported that "notable differences in SOT condition 4 and ADT. SOT composite and conditions 4-6" and furthermore said "The differences are likely related to the virtual surround having a more compelling effect on balance even in conditions with a stationary surround"

Further processing of scores.

These 6 sensory scores are then processed in an attempt to provide something clinically useful. The sensory-analysis page provides the formulae, and the intended purpose of the scores.

References (for all CDP pages)