The Combined Sensory Index (CSI) (Robinson) integrates transcarpal sensory latency differences for suspected carpal tunnel syndrome.
CSI = Ringdiff + Thumbdiff + Palmdiff (ms) - normal < 0.9
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What is the Combined Sensory Index (CSI) and how to interpret it
Robinson's CSI sums absolute SNAP latency differences between nerves vs the median. High values suggest diffuse sensory polyneuropathy; isolated median involvement favors CTS.
Reference values
| CSI (ms) | Interpretation |
|---|---|
| Low / normal | No diffuse sensory polyneuropathy |
| Elevated | Probable sensory polyneuropathy |
| Isolated median | Favors CTS vs PNP |
The Combined Sensory Index (CSI), also known as the Robinson index, is a quantitative electrophysiological tool developed to maximize sensitivity and specificity in diagnosing carpal tunnel syndrome (CTS), especially in mild and incipient forms. It is considered a sensitizing neurophysiological test for focal median neuropathy at the wrist. This calculator automatically sums latency differences from three distinct comparative studies performed on the same hand, yielding a single value that defines the degree of focal median slowing at the wrist.
Why Was the Index Created? (False-Positive Mathematics)
In neurophysiology practice, if the clinician runs multiple independent tests and requires only one to be abnormal to confirm diagnosis, the additive false-positive rate rises sharply. For example, with six tests each at 2.5% individual error, the chance of a false positive across the full study can reach 15%.
The Robinson index addresses this statistical problem. Instead of analyzing each test in isolation, it sums the results into a single criterion. This dilutes random technical error, reduces the effect of a casual outlier, and amplifies small pathological differences, raising sensitivity without sacrificing specificity.
How the Index Is Calculated
The formula uses the sum of peak latency differences (ms) from three comparative techniques (antidromic or orthodromic, per laboratory routine):
4th digit difference - Ringdiff
Median-D4 minus Ulnar-D4, stimulated at the wrist at 14 cm. Normal difference: ≤ 0.4 ms.
1st digit difference - Thumbdiff
Median-D1 minus Radial-D1 (superficial sensory branch), stimulated at the wrist at 10 cm. Normal difference: ≤ 0.5 ms.
Transcarpal difference - Palmdiff
Median palm–wrist minus Ulnar palm–wrist (usually mixed nerve), stimulated in the palm at 8 cm. Normal difference: ≤ 0.3 ms.
CSI = Ringdiff + Thumbdiff + Palmdiff
Note: if one test is already clearly abnormal and conclusive, full CSI may be omitted. It excels in borderline cases.
Result Interpretation
CSI ≥ 0.9 ms - Abnormal
Most widely adopted lower abnormality threshold. 83–84% sensitivity and 95% specificity for CTS diagnosis.
CSI ≥ 1.1 ms - High specificity
Raising the cutoff to 1.1 ms slightly lowers sensitivity (~82%) but specificity approaches 100%, virtually eliminating false positives.
CSI < 0.9 ms - Normal
No focal transcarpal slowing. Symptomatic clinical CTS becomes very unlikely.
Technical Advantages and Prognostic Value
Because all comparisons are between nerves in the same topography, the final calculation is highly resistant to temperature variation and limb cooling - a common source of error in electrodiagnostic studies.
CSI is among the few electrophysiological tools that can predict success of carpal tunnel release surgery. Patients with CSI between 2.5 ms and 4.6 ms have the best postoperative pain and paresthesia resolution.
- Values well below this range may indicate subclinical cases where immediate surgery may not yield perceptible benefit.
- Values above this range often indicate late disease with severe axonal degeneration, reducing the chance of full sensory recovery even after ligament release.
References:
· Robinson LR et al. Combined sensory nerve conduction index in CTS. Arch Phys Med Rehabil. 1998;79(3):258–261.
· Uncini A et al. Combined sensory index: early detection of CTS. Electroencephalogr Clin Neurophysiol. 1993;89(2):131–135.
Frequently asked questions
When is CSI positive for carpal tunnel syndrome?
In isolated CTS, only the median nerve is abnormal; CSI stays low. Elevated CSI favors polyneuropathy.
How to calculate Robinson's combined sensory index?
Sum absolute SNAP latency differences between sural, ulnar, radial and median nerves in the same limb.