How to Perform Effective Blocks of the Equine Ear

1. Introduction

Because of the sensitivity of many horses regarding handling and manipulation of the ears, procedures involving these structures may become quite complicated and frustrating for the veterinarian [1]. This is especially true for painful procedures, such as biopsies or laceration repairs that need minimal movement by the patient. Local infiltration of an anesthetic agent around the procedure site can be useful but may be difficult to perform even with adequate sedation. Regional infiltration around the base of the ear can also be performed [2], but can be time consuming and needs several injections. Directly blocking the nerves that innervate the pinna at the level of the base of the ear is a technique that can reduce patient stress and the amount of time needed to perform a procedure.

2. Materials and Methods

Equipment

A 1-in, 22-gauge needle, 5/8-in 25-gauge needle, 2% lidocaine, and a syringe are needed.

Restraint

Both the internal auricular nerve block and the great auricular nerve block are well tolerated by the sedated horse. Additional restraint is generally not necessary.

Anatomy

The internal auricular nerve (Fig. 1, A and C) branches off of the facial nerve (cranial nerve VII) after the facial nerve passes through the stylomastoid foramen. It runs parallel to the rostral auricular artery and vein as it courses superficial to the parotid gland before passing through the auricular cartilage. This nerve provides sensation to the inner auditory meatus and the inner surface of the auricular cartilage [2-7].

The great auricular nerve (Fig. 1, A and B) arises from the second cervical nerve (C2) and travels dorsally along the neck to the caudal aspect of the ear, where it branches just ventral to the auricular cartilage. The branches innervate the cervicoauricularis superficialis muscle and the caudal aspect of the auricular cartilage [3-7].

Figure 1. Gross dissection of the internal and great auricular nerves. (A) Arrows mark the relative positions of the nerves. The asterisk marks the approximate location of the "notch" palpated externally. (B) View of the great auricular nerve on the caudal aspect of the pinna (arrow). (C) Isolation of the internal auricular nerve as it enters the ear (Cr, cranial; Cd, caudal).

Technique: Internal Auricular Nerve Block

Palpate the lateral aspect of the base of the auricular cartilage. A small "notch" or "divot" should be identified. This is the opening in the auricular cartilage through which the internal auricular nerve passes. Direct a 25-gauge needle into the "notch" perpendicular to the skin surface and deposit 2 ml of 2% lidocaine. The block takes ≈5 min to become fully effective.

Technique: Great Auricular Nerve Block

Palpate the caudal aspect of the pinna at its base. The great auricular nerve runs rostral-caudal and will "slip" beneath the fingers when palpated. Using a 22-gauge needle, deposit 3 ml of 2% lidocaine subcutaneously over the nerve. The block takes ≈5 min to become fully effective.

3. Results

We have successfully used these blocks (in conjunction with sedation) in a number of horses in which punch biopsies of the inner surface of the pinna were collected. Compared with unblocked sedated patients, horses that had both the great and internal auricular nerves blocked were more tolerant of the procedure. The addition of these local blocks resulted in less head shaking and other avoidance behaviors. The effectiveness of these blocks can be tested by tapping (with a finger, pen, etc.) the external and/or internal surface of the pinna. After IV sedation, touching an ear generally resulted in both ear movement and head shaking. In the population of horses that received the local blocks, these behaviors were not observed once the local blocks had taken effect. We have successfully substituted 2% mepivacaine for the lidocaine when a longer duration of effect was desired.

4. Discussion

Blocking the great auricular and internal auricular nerves is an easy, effective method to desensitize the pinna for short procedures. The procedure is well tolerated in sedated horses and has been used successfully in a population of horses from which punch biopsies were taken from the ear. Patience and knowledge of relevant anatomy is important, because there is a good deal of variety between horses in the exact location of the internal auricular nerve, and the appropriate "notch" may be more difficult to palpate in some patients than in others. No complications have been noted with the use of this block, and they should be rare as long as proper aseptic technique is observed. The most likely complication would be local irritation from the lidocaine.