The Neurophysiological Index (NI) integrates CMAP, distal latency, and F-wave frequency as a lower motor neuron loss biomarker.
NFI = (CMAP amplitude × F-wave frequency) / Distal motor latency
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What is the Neurophysiological Index (NI)
The NI = (CMAP/DML) × F-wave frequency (%). Reduced values reflect lower motor neuron loss in ALS.
The Neurophysiological Index (NFI) - also called NPI (Neurophysiologic Index) - is a quantitative electrophysiological biomarker originally described by Mamede de Carvalho in 2000 to assess and monitor lower motor neuron dysfunction. It provides an objective measure of motor neuron disease progression, with particular relevance to amyotrophic lateral sclerosis (ALS). This calculator automatically processes entered parameters and generates the index, supporting longitudinal follow-up.
1. Why the Index Is Needed (Pathophysiology)
Isolated CMAP amplitude assessment has a major early limitation: it falls significantly only after motor neuron loss reaches a critical threshold (usually 50–80%). Before that, collateral reinnervation - surviving axons sprouting to orphan muscle fibers - masks neuronal loss, keeping CMAP apparently normal.
The Neurophysiological Index overcomes this by combining different aspects of lower motor neuron pathology in one number, becoming more sensitive to early change.
2. Formula Used
The index can be calculated from routine standard EMG/NCS data, without additional tests:
NFI = (CMAP amplitude × F-wave frequency) / Distal motor latency
- CMAP amplitude (mV): reflects viable muscle fibers and motor units.
- F-wave frequency / persistence (%): reflects anterior horn motoneuron pool excitability. In ALS it falls markedly due to hypoexcitability and α-motoneuron loss.
- Distal motor latency (ms): reflects terminal motor nerve conduction to the muscle.
Nerves used: preferably ulnar (ADM) as gold standard; median (APB) equally accepted. For lower limbs, deep fibular (EDB) or tibial (AH). Normal ulnar values are typically > 5 - but temporal trend matters more than any single value.
3. Advantages and Clinical Use
Disease progression follow-up
A simple, highly useful tool for clinical follow-up, allowing quantitative visualization of disease progression rate over months.
Research use (biomarker)
Meets characteristics of an ideal clinical-trial biomarker - objective measure of response to new therapies.
Practicality and accessibility
Unlike MUNE/MUNIX (complex and time-consuming), NFI is simple to perform from routine EMG/NCS data without extra specialized training.
4. Limitations
Despite excellence for limb-onset ALS, the Neurophysiological Index has technical limitations in bulbar-onset or bulbar-predominant ALS, where limb nerves may be relatively preserved early in the disease.
References:
· de Carvalho M, Swash M. Nerve conduction studies in amyotrophic lateral sclerosis. Muscle Nerve. 2000;23(3):344–352.
· Menon P, Kiernan MC, Vucic S. Cortical hyperexcitability precedes lower motor neuron dysfunction in ALS. Clin Neurophysiol. 2015;126(4):803–809.
· Vucic S et al. ALS diagnostic index (ALSDI): integrating peripheral NI with TMS-derived cortical parameters. Brain. 2021;144(10):2967–2973.
Frequently asked questions
What is NI used for in ALS?
Quantifies lower motor neuron dysfunction combining distal amplitude/latency and F-wave excitability.
How to calculate NI?
NI = (CMAP amplitude / distal motor latency) × F-wave frequency as percentage.