# Radial motor nerve conduction study in posterior interosseous nerve syndrome and multifocal motor neuropathy

## TL;DR

This study helps distinguish between two nerve conditions by analyzing nerve conduction patterns, aiding in earlier diagnosis of multifocal motor neuropathy.

## Contribution

The study identifies conduction block as a key differentiator between posterior interosseous nerve syndrome and multifocal motor neuropathy.

## Key findings

- Conduction block above 60% was observed in 7 out of 12 MMN cases but absent in PIN syndrome.
- Axonal degeneration was present in all PIN syndrome patients but only 4 MMN patients.
- Conduction velocity and distal motor latencies showed no significant differences between the two groups.

## Abstract

Background
 Finger extension weakness could be a presentation of either posterior interosseous nerve (PIN) syndrome or multifocal motor neuropathy (MMN). However, there is a delay in the diagnosis of MMN in cases with a selective radial weakness, as they are frequently misdiagnosed as PIN.

Objective
 To analyze which variables in nerve conduction studies could aid in the early diagnosis of MMN.

Methods
 We reviewed charts of patients with diagnoses of MMN or PIN syndrome, from 2014 to 2022, in a single Brazilian reference center. Electrophysiological parameters included in the analysis were motor conduction velocity (CV), the presence and magnitude of conduction block (CB), distal motor latencies (DML), and the compound muscle action potential amplitude (CMAP) of the affected radial nerve.

Results
 A total of 44 radial nerves were included in the study. Axonal loss was associated with a diagnosis of PIN syndrome, while conduction block was associated with MMN (
p
 < 0.05). No patient with PIN had a CB over 60%, while 7 out of 12 radial CB blocks in patients with MMN were above that. Axonal degeneration was present in 4 MMN patients and in all patients with PIN syndrome. There was no difference in CV and DML between groups.

Conclusion
 The presence of CB or the absence of distal CMAP amplitude reduction should lead physicians to consider MMN, and a comprehensive nerve conduction study should be performed.