# Tibial to ulnar nerve amplitude ratio as a marker of length-dependent neuropathy

## TL;DR

A new nerve conduction measure helps distinguish between acquired and inherited neuropathies with high accuracy.

## Contribution

The tibial to ulnar nerve amplitude ratio is introduced as a novel diagnostic marker for length-dependent neuropathy.

## Key findings

- The tibial to ulnar amplitude ratio is significantly higher in CIDP compared to CMT patients.
- The ratio has 95.5% sensitivity and 85.5% specificity for distinguishing CIDP from CMT.
- The marker performs well in CIDP-mimicking CMT cases with similar accuracy.

## Abstract

•Tibial to ulnar amplitude ratio separates acquired and inherited neuropathies.•The ratio is higher in chronic inflammatory demyelinating polyradiculoneuropathy.•Area under the curve 0.95 with 95.5% sensitivity and 85.5% specificity.

Tibial to ulnar amplitude ratio separates acquired and inherited neuropathies.

The ratio is higher in chronic inflammatory demyelinating polyradiculoneuropathy.

Area under the curve 0.95 with 95.5% sensitivity and 85.5% specificity.

To evaluate the utility of nerve conduction studies as a marker of length-dependent neuropathy.

We conducted a retrospective study of 44 chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) patients and 365 genetically confirmed Charcot-Marie-Tooth disease (CMT) patients, including those with PMP22 duplications or mutations in GJB1, MFN2, MPZ, and MME. Nerve conduction study parameters were compared, with subgroup analyses of CIDP-mimicking CMT (genetically confirmed CMT with a prior clinical diagnosis of CIDP) and gene-based classifications. Receiver operating characteristic (ROC) analysis assessed the sensitivity and specificity of these parameters.

The tibial to ulnar nerve distal compound muscle action potential (T/U CMAP) amplitude ratio was significantly higher in CIDP patients compared to those with genetically confirmed CMT, CIDP-mimicking CMT, and gene-based subgroups. This ratio yielded the highest area under the curve (AUC: 0.95) among all evaluated parameters, with a cutoff value of 0.385 demonstrating high diagnostic sensitivity (95.5%) and specificity (85.5%). In CIDP-mimicking CMT group, a similar sensitivity and specificity were observed.

The T/U CMAP amplitude ratio is a simple, robust electrophysiological index of length-dependent neuropathy.

This marker offers a reliable and accessible way to distinguish between acquired and inherited neuropathies, improving diagnostic accuracy and helping prioritize genetic testing.