# The deltoid muscle and the pattern of paresis in ALS

## TL;DR

This study shows that the deltoid muscle in ALS patients is affected early, similar to the biceps, based on corticospinal tract connections.

## Contribution

The study provides new evidence on the early involvement of the deltoid muscle in ALS, linked to corticospinal connectivity patterns.

## Key findings

- Deltoid muscle paresis in ALS resembles biceps involvement rather than triceps.
- The pattern aligns with corticospinal monosynaptic connectivity data.
- Early deltoid dysfunction contributes significantly to ALS disability.

## Abstract

There is neuroanatomical and clinical evidence that the corticospinal tract governs the patterns of pareses in sporadic ALS. These patterns are mirrored by phylogenetically young monosynaptic corticomotor neuronal connections. It is well known that, clinically, dysfunction of the deltoid muscle contributes considerably to the early disability of the ALS patient. In this study, we prospectively compared the degree of pareses of the deltoid muscle with the triceps and biceps brachii in N = 71 patients (426 muscles). We could show that the extent of involvement of the deltoid muscle early in the disease process resembles that of the biceps rather than the triceps brachii. This pattern is consistent with functional data of the corticospinal monosynaptic connectivity of all three muscles.