Brazil and Sweden models. Requires age and height.
Nerve Sural (Sensory)
Superficial fibular nerve (sensory)
Motor & F wave
PNP Z-Score Results
| Sweden index | -- |
| Brazil index | -- |
What is the polyneuropathy Z-score
Combines Z-scores of conduction parameters using Brazil or Sweden normative models. Elevated Z-scores across nerves quantify sensorimotor polyneuropathy severity.
The Combined Polyneuropathy Index (combined Z-score) is an advanced analytical tool that converts raw EMG/NCS findings into Z-scores and unifies them in a single index. It overcomes limitations of rigid normal values that often fail to account for individual variables such as age and height.
1. Logic Behind the Z-Score and Combined Index
In traditional neurophysiology, analyzing isolated parameters can yield false negatives in early stages, when the nerve is already affected but absolute values have not yet crossed the lower limit of normal. The Z-score addresses individualization: it calculates how many standard deviations the obtained value deviates from the expected mean, automatically correcting for age and height.
Z-score = (Observed value − Expected value) / Standard deviation
The combined index averages these Z-scores to capture the overall trend. If several parameters are subnormal, the index flags global abnormality, increasing sensitivity. If only one value is poor due to isolated technical error, the index dilutes that outlier, preserving specificity.
2. The 10 parameters used
Five parameters per lower limb, totaling 10 variables:
- Sensory amplitude of the sural nerve
- Sensory conduction velocity of the sural nerve
- Sensory amplitude of the superficial fibular nerve
- Motor conduction velocity of the fibular nerve
- Tibial F-wave latency
Natural logarithm (ln) for amplitudes
Sensory amplitudes are not Gaussian in the population. The calculator applies a logarithmic transform to amplitude values for correct Z-score statistics.
F-wave inversion
Because a pathological F wave has a higher value (prolonged latency) while other tests show lower values with disease, the F-wave Z-score is mathematically inverted to align with other index parameters.
3. Absent potentials (indeterminate parameters)
When polyneuropathy has already abolished a potential, the logarithm of zero is undefined. To avoid underestimating disease or calculation error, the calculator uses minimum clinically plausible residual values (e.g. absent amplitude → 0.1 µV; indeterminate velocity → 30 m/s).
If more than 3 parameters are fully indeterminate, the index loses refined statistical function and is considered indeterminate overall - because severe neuropathy is already clinically obvious.
4. Result Interpretation
Z between −2.0 and −2.5 - Borderline
Weigh strongly if the patient has suggestive symptoms (paresthesia, burning), indicating global functional nerve deviation.
Z below −2.5 - Abnormal
Suggests polyneuropathy even in patients who are still asymptomatic.
Z near 0 - Normal
Nerve conduction compatible with the expected mean for age and height.
Two models are calculated: Sweden (Solders 1993) and Brazil (Heise 2012). At least 3 parameters are recommended for reliable interpretation.
5. Advantages and limitations
Advantages
- Raises diagnostic sensitivity to ~80% in early stages vs. ~60% with isolated parameters, without sacrificing specificity.
- Yields a single number ideal for longitudinal follow-up - shows whether neuropathy is improving, stable, or progressing year to year.
Limitations
- Focal/multifocal neuropathies (e.g. MMN): healthy nerves may dilute the affected nerve, yielding a falsely normal index.
- Pure disorders (motor-only or sensory-only): the unaffected pathway pulls the index up, masking pathology.
- Confounding factors: not validated for extreme age/height, morbid obesity, or uncorrected limb cooling.
References:
· Heise CO, Machado FCN, de Amorim SC, Toledo SM. Combined nerve conduction index in diabetic polyneuropathy. Arq Neuropsiquiatr. 2012;70(5):353–358. doi: 10.1590/S0004-282X2012000500005
· Solders G, Andersson T, Borin Y, Brandt L, Persson A. Electroneurography index: a standardized method to assess peripheral nerve function in patients with polyneuropathy. Muscle Nerve. 1993;16:941–946.
Frequently asked questions
Which normative model: Brazil or Sweden?
Use the model aligned with your patient population and lab reference.
Does high Z-score always mean polyneuropathy?
It indicates statistical deviation; correlate with clinical pattern and other EMG findings.