Anatomy and Anaesthesia of the Equine External Ear Canal

Abstract

Anaesthesia of the external ear canal (external acoustic meatus) is usually performed by blocking both the great and internal auricular nerves by regional infiltration. However, exact landmarks for blocking the internal auricular nerve to accomplish effective anaesthesia have not been described yet. In this study, detailed anatomical dissection of the equine external ear canal and its nerve supply was carried out on fifteen cadaver heads. Tissue samples of the dissected nerves were taken from two cadaver heads processed and were evaluated microscopically. Prior to the dissection, the region of interest was evaluated ultrasonographically, and injection of a local anaesthetic was simulated with an injection of methylene blue on ten cadaver heads. The tympanic membranes of three cadaver heads were obtained by microdissection and processed for microscopic evaluation. The entrance point of the internal auricular nerve, which is a branch of the facial nerve, into the ear canal is formed by the styloid process of the auricular cartilage. Using ultrasound, the styloid process presented as a thin hyperechoic line 2.17-2.97 cm deep, based on the skin surface. Landmarks for performing a complete and reliable anaesthesia of the external ear canal were established, and the simulated anaesthesia with methylene blue injection was evaluated as successful in all ten cases. Additionally, the histological composition of the equine tympanic membrane is described and illustrated.

Resection of a ceruminous adenocarcinoma in a horse by a modified vertical ear canal ablation

“…Based on equine cadaver studies, the external ear canal measures approximately 9.2 cm in total length (Sommerauer et al . ), with the osseous component being 2.52–2.65 cm (Sargent et al . ) and there appears to be little difference in size between breeds.…”

“…If the medial wall is extensively diseased, it would not be possible to perform this modified procedure. Based on equine cadaver studies, the external ear canal measures approximately 9.2 cm in total length (Sommerauer et al 2012), with the osseous component being 2.52-2.65 cm (Sargent et al 2006) and there appears to be little difference in size between breeds. Therefore, almost half of the vertical canal was resected in this case, which may be the limit for the procedure to be successful.…”

Temporohyoid osteoarthropathy in the horse

“…Effective blocks of the equine ear may be performed either by instilling local anaesthetic into the external ear canal, as described by Bras et al (2014), or by perineural anaesthesia of the internal and great auricular nerves (McCoy et al 2007;Sommerauer et al 2012). In the report by Bras et al (2014) clinical signs of headshaking were significantly improved after infusion of 2% lidocaine into the ear canals.…”

Histological Study of the External, Middle and Inner Ear of Horses

“…Sommerauer et al. () established the nerve block anaesthesia of the great and inner auricular nerves to gain access to the equine acoustic organ in standing sedated horses. In addition, the endoscopic examination of the equine external ear canal (EEEC) and tympanic membrane (TM) was performed following a standardized protocol (Sommerauer et al., ; Blanke et al., ).…”