Computerized Dynamic Posturography (CDP) Sensory Test Scoring: SOM sensory analysis

Timothy C. Hain, MD • Page last modified: July 15, 2022Return to testing indexReturn to Sensory analysis page

In these pages we are taking a deep dive into the method of scoring the Neurocom CDP sensory tests.

Typical Sensory Analysis

On the above graphic, a rather typical sensory analysis is shown for an older dizzy patient. The "SOM" score is to the far left.

Our take on the "SOM" score is that it can be safely ignored.

Processed CDP scores
Score Formula Compared Apparent Intent
Som 2/1 (V+F)/(V+E+F) Take out eyes Feet

The "Som" score is computed from the ratio of the CDP scores for test 2 (eyes closed) divided by test 1 (eyes open). One would expect that in normal persons, test 1 (where eyes, ears and feet are presumably engaged), would have a higher CDP score than test 2 (where the eyes are closed), and thus the ratio should be a little < 100% in normal people, and a lot lower than 100% in persons who have no ankle proprioception. For example, people with peripheral neuropathy who can't feel things in their feet.

As is the situation for other sensory analysis tests, the "SOM" score actually is not based on manipulation of somatosensory input at all, but rather is the result of taking away vision. It is thus a highly indirect measure, that at best, might sometimes correlate with loss of somatosensory input.

If vestibular function was poor, then one would also expect a reduction in the "SOM" score, because loss of vestibular function would make the person more dependent on vision and proprioception. Thus the "SOM" score should also be sensitive to vestibular impairment.

SOM scores with respect to age in a broad group of dizzy patients.

CDP SOM scores in dizzy patients
Som vs age
The graph above shows mean SOM scores as a boxplot from almost 2000 dizzy patients tested at Chicago Dizziness and Hearing. Age is associated with only a small reduction in SOM scores. Graph made with "R". The small error bars show that SOM score doesn't vary much in dizzy patients, but there are some outliers (possibly patients with poor somatosensation or vestibular function). Of course, these measurements are not from "normal" subjects, but rather a mixture of many subjects with individual conditions. So some of the variability is likely due to subject composition.

 

Discussion of the SOM score in CDP

The SOM score is an insensitive measure of the contribution of proprioception, not actually implemented by altering proprioception at all, but rather just by eliminating vision.

As the new Bertec version of the Neurocom CDP system mainly varies in visual sway referencing, which is not used to compute the SOM score, one would expect similar results from the newer device. This score may be meaningless (see next).

The literature has no papers about the utility of the "SOM" score, or lack thereof. It is difficult to envision a clinical use for the Equitest SOM score, at least in situations where one has already ascertained the subject's proprioceptive function through a physical exam.

 

References (for all posturography pages)