Implications of the neuroanatomy of the equine thoracolumbar vertebral column with regional anaesthesia and complications following desmotomy of the interspinous ligament

Abstract

Impinging/overriding dorsal spinous processes (DSPs) of the thoracolumbar vertebrae are a common cause of poor performance in horses. In the last five decades, numerous surgical treatments have been reported on, from transverse transection of the affected DSPs, and endoscopic resection of the affected DSPs, to transection of the interspinous ligament. Until recently, cosmetic outcomes have been reported as good to excellent in studies. However, a previously unreported complication of neurogenic atrophy of the contralateral epaxial muscle following desmotomy of the interspinous ligament has been recently reported. The authors hypothesised that this was because of a more lateral approach than previously described, resulting in the scissors being too far across midline and transecting a nerve in the region. Considering this finding, we have reviewed the literature on the neuroanatomy of the thoracolumbar region in the horse. Literature on the neuroanatomy of the horse is lacking when compared with that of humans and companion animals, with most of the work extrapolated from companion animals. Based on the current literature, we hypothesise that transection of an intermediate branch of the dorsal spinal nerve supplying the m. longissimus is potentially the cause of the post‐operative neurogenic atrophy. The lack of detailed knowledge of the neural anatomy of the equine back has resulted in the role of local anaesthesia in localising pain in the equine back being poorly understood. The wide variation in techniques used for localising back pain may explain why some horses suffering from poor performance or an abnormal gait because of back pain improve to local anaesthesia of the back while others do not. This review article highlights a lack of anatomical knowledge regarding the equine thoracolumbar region in the literature despite diagnostic local anaesthesia, medication, and surgery in this area being relatively common.

Erector spinae plane block as part of a multimodal analgesic approach in an anaesthetised horse undergoing dorsal spinous process ostectomy and desmotomy

“…Epidural spread was found in 20% of the horses (unilateral epidural spread occurred in one of the two injections for each horse). Epidural spread following ESP injections is inconsistent and difficult to predict and has been considered as one of the factors contributing towards clinical analgesia after performing the ESP block in both human and dogs 7,67–69 . However, due to the inconsistent and unpredictable epidural spread, one must consider that the final contribution towards analgesia will be uncertain for each patient.…”

Minimally invasive cranial ostectomy for the treatment of impinging dorsal spinous processes in 102 standing horses

“…The reported mechanism of action of the ISLD is to remove tension on afferent nociceptive receptors located at the level of the insertion in the interspinous ligament insertion to reduce pain by transection of the interspinous ligament, which can cause a mild enlargement in the space between impinging DSPs 2,7 . However, if extensive bone bridging has formed between the DSPs, then ISLD may be unsuccessful, and by positioning the Mayo scissors in a more obtuse angulation to facilitate transection, this can cause iatrogenic damage to the neighboring nerves 8,14 . This may lead to neurogenic atrophy of a small portion of the epaxial musculature and an associated cosmetic abnormality 8,14 .…”

“…2,7 However, if extensive bone bridging has formed between the DSPs, then ISLD may be unsuccessful, and by positioning the Mayo scissors in a more obtuse angulation to facilitate transection, this can cause iatrogenic damage to the neighboring nerves. 8,14 This may lead to neurogenic atrophy of a small portion of the epaxial musculature and an associated cosmetic abnormality. 8,14 In addition, by not removing the impinging and abnormal bone, recurrence of clinical signs could occur.…”

“…8,14 This may lead to neurogenic atrophy of a small portion of the epaxial musculature and an associated cosmetic abnormality. 8,14 In addition, by not removing the impinging and abnormal bone, recurrence of clinical signs could occur. 2 This was suggested to occur more frequently in horses that had a period of nonridden work and pasture turnout where core stability has been lost and highlights the importance of the rehabilitation in postoperative care.…”

Three-Dimensional Modeling and In Silico Kinematic Evaluation of Interspinous Ligament Desmotomy in Horses

“…The ISL occupies the space between the DSPs and interspinous ligament desmotomy (ISLD) has been shown to ameliorate back pain (Coomer et al, 2012) but its effects on increasing mobility of the spine are unknown. Several studies have shown a clinical improvement in horses suffering from poor performance post-ISLD (Prisk and García-López, 2019;Brown et al, 2020), however, others have mentioned long-term complications such as unilateral neurogenic atrophy of epaxial musculature (Derham et al, 2021). The main effects of ISLD appear to be due to reduced tension on the afferent nociceptive receptors located at the ligament insertion, which abolish the pain response (Coomer et al, 2012).…”