Histological Investigation of the testicular nerves at the castration site in geldings

Abstract: Castration is one of the most common surgical procedures in equine medicine and associated with a number of postoperative complications such as swelling, infection, haemorrhage, peritonitis, funiculitis, omental herniation and eventration. But what happens with the testicular nerves after crushing and severing during the procedure? An injured nerve can respond with degeneration and/or regeneration. If the distal segment of the nerve is missing, such as after castration, the newly regenerating nerve fibres can be tangled in scar tissue, forming bundles of nerve fibres, creating a neuroma. In human medicine neuromas are well known a cause of pain sensations after different types of nerve injuries, for example in relation to amputations. Studies of animals have documented the existence of neuroma formation after somatic nerve injuries, and in a few cases signs of pain originating from such a neuroma have been described. Neuroma formation on the testicular nerves at the castration site in geldings has never been investigated before. Can neuroma develop as response to injury of the testicular nerves? If so could this cause pain in geldings? What could this pain lead to and which consequence could this have for the horse? As a first step to answer these questions, the aim of this pilot study was to investigate if neuromas can form at the castration site on the testicular nerves in geldings. Stumps of twelve spermatic cords were collected from six geldings (between 6-25 years of age and all of different breeds) admitted from the University Animal Hospital in Uppsala, Sweden for post mortem examination after euthanasia for unrelated reasons. Owners consent for examining the spermatic cords were obtained. The tissue samples were fixed in formalin and the spermatic cord was sectioned in a distal, mid and proximal segment. These where dehydrated and embedded in paraffin before sectioned in 4 μm longitudinal sections for histological investigation. The slides were stained with Haematoxylin and Eosin (HE) and Masson´s Trichrome (MT). After examination in light microscope twelve slides with presumptive neuroma formation were selected for further investigation. Expression of nerve specific enolase (NSE) for visualisation of nerve fibres and S-100 protein for detection of Schwann cells were examined in slides after immuno-histochemical labelling. The expression of nerve growth factor (NGF) by immuno-histochemical labelling was investigated as a possible pain marker. Neuromas were found in six spermatic cords belonging to four geldings. The neuromas consisted of an irregular and abnormal distribution of nerve fibres dispersed in and surrounded by connective tissue. Most of them were classified as delimited and containing Schwann cells. One neuroma was classified as dispersed, containing nerve fibres dispersed in a large area of connective tissue without Schwann cells. All neuromas expressed mild immunostaining of NGF, but no conclusions regarding clinical relevance from this could be drawn. This pilot study has proved that neuromas of the testicular nerves indeed may form at the castration site in geldings. Though, further studies are required to establish the extent of neuromas in this location. The influence of castration methods and the possible consequences for the individual horse need to be investigated to answer important questions on the possible welfare implications of this elective surgery. Possible clinical implications include documentation of presence of inguinal pain, unexplained hind limb lameness or movements, back pain or unexplained behavioural problems in geldings.