Histopathological assessment of intrinsic laryngeal musculature in horses with dynamic laryngeal collapse.

The study examines the possible causes of a condition called dynamic laryngeal collapse (DLC) in horses, which could be linked to weaknesses in the laryngeal muscles. The research did not find notable differences in muscle fibre size or type between horses with DLC and those without, indicating that neuromuscular issues may not be the primary cause of the condition.

Study Objective

• The main aim of the study was to find out the cause of bilateral dynamic laryngeal collapse (DLC), a condition that affects horses, and is suspected to be connected to laryngeal muscle weakness.
• The researchers conducted a histopathological examination of the cricoarytenoideus dorsalis, the cricothyroid (CT) and the cricoarytenoideus lateralis muscles in horses affected by DLC and compared these to unaffected horses.
• The hypothesis was that the DLC-affected horses would show signs of neurogenic atrophy in the CT or cricoarytenoideus dorsalis muscles or that the muscle fibre diameter would be reduced in DLC-affected horses, indicating an underlying weakness or paresis.

Methodology

• A case-control study design was used for the study, with five horses affected by DLC and eight unaffected horses as controls.
• After euthanization, muscle samples were collected for analysis. Attributes such as fibre type proportions, size and fibre type grouping were evaluated and compared between the two groups and the two sides (left/right) of each horse.

Results

• No statistically significant differences were observed in fibre type proportions, fibre size, and subjective assessment of fibre type grouping between horses with DLC and the controls.
• However, an objective assessment showed more large clusters of T1 fibres within the left cricoarytenoideus lateralis muscle in DLC-affected horses compared to controls, and within the right CT muscle of control horses compared to the DLC-cases.

Conclusions

• Findings did not support the notion that neuromuscular problems are the main cause of DLC, as there was no significant loss or atrophy in fibre type and size in the DLC-affected horses.
• The differences observed in objective assessments of fibre type grouping were inconsistent and did not conclusively point towards a neuromuscular cause of DLC. As such, other causes need to be explored as potential causes of DLC in horses.