Functional, immunohistochemical and morphometric analysis of peripheral nerves in horses with Recurrent Laryngeal Neuropathy

Abstract

Recurrent laryngeal neuropathy (RLN), a highly prevalent disease of tall horses, is one of the main causes of poor performance at exercise and during racing. The disorder is associated with degeneration and loss of large-diameter myelinated fibres within the recurrent laryngeal nerves, predominantly on the left side, with subsequent denervation atrophy and paresis of the intrinsic laryngeal muscles leading to reduced inspiratory airflow. The aetiopathogenesis remains unknown and it is still unclear whether this disorder is restricted to the recurrent laryngeal nerves or whether other long nerves are also affected. Further, it is unclear whether the disorder affects only motor axons or whether there is involvement of other axonal types. The work described in this thesis was aimed at classifying, in objective and blinded manner, whether RLN is a bilateral mono-neuropathy or a polyneuropathy, and examined other axon subtypes in horses with varying severities of RLN. A systemic investigation of multiple nerves was undertaken in RLN-affected horses, to characterise histologically the primary nature of the disease (axonopathy or demyelinating neuropathy) using blinded, objective and subjective criteria in the recurrent laryngeal nerves and to investigate the hypothesis that other long motor (phrenic, radial, peroneal) and sensory (palmar and plantar) nerves are also affected. Secondly, the axon composition of the recurrent laryngeal nerves was evaluated with a panel of immunohistochemical antibodies. Finally, conduction latencies to muscles in the distal limbs were evaluated using transcranial magnetic stimulation in horses with varying RLN severities. Results indicated that RLN is axonal in nature and limited to the left and right recurrent laryngeal nerves. It is characterised by loss of large diameter myelinated motor fibres within primarily the distal left recurrent laryngeal nerve with more proximal involvement in the most severely affected horses. Immunohistochemical investigation of distal recurrent laryngeal nerves revealed presence of motor, sympathetic and sensory axons; there was selective loss of motor fibres with preservation of other 4 axonal types in RLN. Conduction latencies, adjusted for body size, were not associated with RLN severity. RLN is best characterised as a pure motor, bilateral mononeuropathy. Selective loss of the largest diameter and longest motor axons in affected horses suggests a possible role for metabolic compromise in this disorder and is worthy of future investigation