Function of the ramus communicans of the medial and lateral palmar nerves of the horse

Reasons for performing study: The role of the communicating branch between the medial and lateral palmar nerves of horses (i.e. the ramus communicans) in conveying sensory impulses proximally should be determined to avoid errors in interpreting diagnostic anaesthesia of the palmar nerves. Hypothesis: Sensory nerve fibres in the ramus communicans of horses pass proximally from the lateral palmar nerve to merge with the medial palmar nerve, but not vice versa. Objective: To determine the direction of sensory impulses through the ramus communicans between lateral and medial palmar nerves. Methods: Pain in a thoracic foot was created with set‐screw pressure applied to either the medial or lateral aspect of the sole of each forelimb of 6 horses. The palmar nerve on the side of the sole in which pain was created was anaesthetised proximal to the ramus communicans with local anaesthetic. Lameness was evaluated objectively by using a wireless, inertial, sensor‐based, motion analysis system (Lameness Locator). Lameness was also evaluated subjectively by using a graded scoring system. Local anaesthetic was then administered adjacent to the ramus communicans to determine the effect of anaesthesia of the ramus communicans on residual lameness. Results: When pain originated from the medial or the lateral aspect of the sole, anaesthesia of the ipsilateral palmar nerve proximal to the ramus communicans did not entirely resolve lameness. Anaesthesia of the ramus communicans further attenuated or resolved lameness. Conclusions: Sensory fibres pass in both directions in the ramus communicans to connect the medial and lateral palmar nerves. Potential relevance: When administering a low palmar nerve block, both palmar nerves should be anaesthetised distal to the ramus communicans to avoid leaving nondesensitised sensory nerve fibres passing through this neural connection. Alternatively, local anaesthetic could also be deposited adjacent to the ramus communicans when anaesthetising the palmar nerves.