How Does Your Dog Hear? Decision Making in Ear Canal Surgery
World Small Animal Veterinary Association World Congress Proceedings, 2008
D.J. Brockman, BVSc, CVR, CSAO, DACVS, DECVS, FHEA, MRCVS
Department of Small Animal Medicine & Surgery, Royal Veterinary College
North Mymms, Hatfield, Herts, UK

External Ear Canal Anatomy

The external ear canal is a funnel-shaped cartilaginous tube which extends from the external auditory meatus to the tympanic membrane. It comprises a vertical canal and a horizontal canal. The terminal portion of the horizontal canal is formed by a bony projection of the petrous temporal bone.

Click on the figure to see a larger view.


 

Surgical Procedures When, How and Why

Lateral Wall Resection

Lateral wall resection (LWR) increases drainage and improves ventilation of the ear canal. It also facilitates placement of topical medications in the external ear canal.

It is indicated:

 For patients with otitis externa where there is minimal hyperplasia of the epithelium of the external auditory meatus

 For removal of small mass lesions of the lateral aspect of the vertical canal

 To gain access to the lower portions of the canal

It should not be performed: on patients with obstruction or stenosis of the horizontal canal, alone to manage dogs with concurrent otitis media or severe epithelial hyperplasia. Dogs with an underlying disease, e.g., hypothyroidism, or primary seborrhoea, may respond poorly. This procedure is not curative and long term medical management of the underlying otitis externa may be required.

Surgery

The patient is placed in lateral recumbency and two parallel incisions are made over the vertical canal, from the ventral aspect of the opening of the external ear canal. These extend for one and a half times the depth of the vertical canal. The incisions are joined ventrally and the skin flap thus created is dissected free of the underlying tissue and reflected dorsally.

The exposed vertical canal is dissected free of the overlying muscle tissue and the parotid gland is reflected ventrally. Two parallel incisions are made in the vertical canal and the cartilage flap is reflected ventrally. The majority of the cartilage is resected, leaving the distal one third. This is sutured to the skin edges below the opening to the horizontal canal to act as a drainage board to prevent the skin from becoming excoriated. The skin edges are the apposed to the remaining vertical canal to close the wound.

Complications

Wound contamination is difficult to avoid and some degree of minor wound swelling, exudation or dehiscence is not uncommon, but rarely requires any further attention.

Despite its widespread use, it is apparent that this technique does not provide a solution to all cases of chronic ear disease.

The reasons for failure of LWR include:

1.  Poor surgical technique e.g., failure to drain the horizontal canal properly

2.  Poor patient selection, i.e., chronic irreversible changes in the remaining ear canal

3.  Failure to control the underlying ear disease

4.  Unremitting otitis media

Vertical Canal Ablation

Vertical canal ablation (VCA) is indicated in the management of cases of otitis externa in patients where the disease is confined to the vertical canal and who have a normal horizontal canal. In fact, this is an uncommon situation, and most patients will have chronic changes throughout the external ear canal and a total ear canal ablation is a more suitable alternative. Neoplasia or trauma to the vertical canal may be amenable to management with this technique. Since the tympanic membrane and bulla are left intact, VCA will interfere less with hearing post-operatively than total ear canal ablation.

Surgical Technique

A vertical incision is made over the vertical canal, extending to below the junction of the horizontal and vertical canal as for the LWR. A circular incision is made around the opening to the external auditory meatus and the entire vertical canal is dissected free of surrounding tissue. The vertical canal is transected 1-2 cm dorsal to the horizontal canal and two parallel incisions are made in the cranial and caudal aspects of the remaining vertical canal. These flaps are reflected dorsally and ventrally to create a dorsal and ventral drainage board as described for the LWR, and are sutured to the skin. The dead space is obliterated and the wound closed.

Alternatively, this technique may be performed using a pull-through technique. A circular incision is made around the opening to the external auditory meatus and an incision is made at the junction of the horizontal and vertical canals. The vertical canal is then dissected free of surrounding tissue and pulled through the lower incision. This technique is technically more demanding and apart from a claim for improved cosmesis, there is little to recommend it.

Total Ear Canal Ablation/Lateral Bulla Osteotomy

This is indicated in:

1.  Chronic proliferative changes in the ear canal beyond the vertical canal

2.  Complete ear canal stenosis

3.  Continuing otitis externa following LWR or VCA

4.  Unremitting middle ear disease

5.  Neoplastic disease of the ear canal or tympanic bulla.

6.  Severe trauma to the external ear canal

7.  Para-aural abscessation

Many animals requiring this surgery will have undergone prolonged treatment previously and many will have had unsuccessful surgical intervention.

Surgical Technique

A circular incision is made around the opening of the external ear canal and a vertical incision is made over the vertical canal. The entire ear canal is dissected free of surrounding tissue to the level of the osseous external auditory prominence (OEAP) at the lateral aspect of the tympanic bulla. The external ear canal is amputated at the level of the OEAP, taking care to avoid the facial nerve, which exits the skull via the stylomastoid foramen, caudoventral to the OEAP.

The integument which lines the OEAP and tympanic bulla is removed with a curette. In the vast majority of animals, this requires a lateral bulla osteotomy (LBO). Rongeurs are used to remove the lateral aspect of the tympanic bulla, ventral to the OEAP. The tympanic cavity is irrigated and curettage proceeds until there is no integument or debris in the middle ear.

The soft tissues are closed over the bulla, taking care to obliterate the dead space. A Penrose drain may be placed, extending from the bulla to exit ventral to the skin wound. The skin is closed in a T or inverted L shape.

Complications

1.  Facial nerve injury. This may be temporary, due to stretching (neuropraxia) or permanent, due to transection (paralysis).

2.  Wound dehiscence and infection

3.  Haemorrhage from the retroglenoid vein

4.  Vestibular signs (inner ear disease)

5.  Hypoglossal nerve dysfunction

6.  Chronic sinus tracts and para-aural abscessation

7.  Deafness

The prognosis is good if a meticulous surgical technique is employed with long-term success rates of 90-95% reported. However, the complication rate may be high with poor surgical technique or if integument is left behind in the middle ear.

Myringotomy

If the tympanic membrane is intact but discoloured or bulging outwards, a myringotomy or incision of the tympanic membrane is performed. The aims of myringotomy are:

 To obtain samples for culture

 To drain the middle ear

 To relieve pain and pressure associated with otitis media

 To allow lavage and instillation of medication

After the external ear canal is cleaned, the otoscope is introduced and the tympanic membrane perforated caudal to the malleus. The contents of the middle ear may be sampled after introducing a 20 gauge spinal needle through the tympanic membrane and aspirating. The middle ear is then lavaged with sterile saline by repeatedly instilling and aspirating sterile saline until the fluid recovered is clear. Long term antibiotic therapy, for 3 to 6 weeks, is required, the choice of antibiotic being dependent on culture results. Common pathogens isolated from the middle ear include Staphylococcus spp., Streptococcus spp., Pseudomonas spp., E. coli, and Proteus mirabilis.

The disadvantages of myringotomy as the sole method of treatment include:

 Poor exposure of the tympanic cavity

 Poor post-operative drainage

 Exposure of the middle ear to the external ear canal, which may itself be infected

 Damage to the structures of the middle ear

In addition, otitis media which is not secondary to otitis externa is relatively uncommon and this procedure is uncommonly indicated. Surgical drainage is indicated if this therapy proves ineffective, or if a neoplastic lesion, inflammatory mass or foreign body is found.

Ventral Bulla Osteotomy

This technique allows access to the tympanic cavity. Ventral bulla osteotomy (VBO) gives better access to the tympanic bulla than LBO, more consistent drainage and allows both bullae to be explored without repositioning the patient. In the patient with middle ear disease in conjunction with otitis externa, access to the tympanic bulla is best gained by a LBO following a TECA. Although this gives a more restricted exposure, it avoids repositioning the patient. VBO is indicated in the management of middle ear neoplasia and nasopharyngeal polyps.

Surgical Technique

The patient is placed in dorsal recumbency. The bulla may be palpated immediately caudal and medial to the vertical ramus of the mandible. An 8-10 cm paramedian incision is made just medial to the mandibular salivary gland and centred midway between the angular process of the mandible cranially and the wings of the atlas caudally. The incision is continued through the platysma muscle. The digastricus muscle and linguofacial vein are retracted medially and the styloglossus and hyoglossus muscles and the hypoglossal nerve are retracted laterally to expose the bulla.

A Steinman pin or small drill may be used to perform the osteotomy, which is then enlarged with small rongeurs. The contents of the bulla may then be examined. The feline bulla is divided by a thin bony septum into two compartments; a small craniolateral compartment and a larger caudomedial compartment. The bulla may be lavaged and, if necessary, a drain placed.

General Considerations for Surgery of the Ear Canal

Surgical Asepsis

It is difficult, if not impossible, to ensure that the surgical site is aseptically prepared. Rather than perform repeated lavage of the external ear canal, it is probably better to accept that aural surgery is a contaminated procedure. Hence, perioperative antibiotics are indicated. The requirement for post-operative therapy depend on the nature of the disease and the surgical procedure and may be determined once the degree of contamination has been assessed.

Analgesia

With the exception of minor surgery of the pinna, most aural surgery is extremely painful. Particular attention should be paid to achieving adequate analgesia, both from a humanitarian aspect and also to prevent self-trauma.

Wound Management

Aural surgery is generally considered contaminated. Measures should be taken to prevent self-trauma. Bandaging the ears is difficult and incorporation of the ears into a bandage which passes round the neck may lead to asphyxiation. Prevention of self-trauma with an Elizabethan collar is recommended. The wound should be inspected frequently, and cleaned as required, particularly if a Penrose drain has been placed.

Nerve Damage

Damage to the facial nerve may result in a loss of the blink response and a loss of parasympathetic innervation to the lacrimal glands. The eye should be kept lubricated with artificial tears or a suitable ophthalmic lubricant.

Curettage of the tympanic bulla may result in Horner's syndrome, particularly in the cat. This is generally transient and may improve with a short course of corticosteroids.

Vestibular signs, either as a result of extension of the disease from the middle ear or, rarely, from over-zealous curettage of the bulla, may cause the patient to become disorientated.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

D.J. Brockman, BVSc, CVR, CSAO, DACVS, DECVS, FHEA, MRCVS
Department of Small Animal Medicine & Surgery
Royal Veterinary College
Hatfield, Hertfordshire, UK


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