Surgical Anatomy
The ear is divided into three sections: the external, the middle and the inner ear. The external and middle portions of the ear are of primary importance to the veterinary surgeon, but knowledge of the proximity of the inner ear is critical to avoid potential iatrogenic complications.
The external ear consists of the pinna, the vertical canal, and the horizontal canal. The pinna varies dramatically between different breeds of dog and has numerous muscular attachments to facilitate movement. The pinna is covered in skin on both sides. The skin is fixed firmly to the perichondrium on the concave side with more movement possible on the convex side. The auricular cartilage is the larger cartilaginous portion of the external ear, and this constitutes the vertical ear canal. The stiff tragus forms the most dorsolateral aspect of the vertical canal. There is a distinct junction or 'corner' at the termination of the vertical canal at the distal aspect of the horizontal canal. The horizontal ear canal is formed by the annular cartilage, which is connected to the skull and the auricular cartilage by small ligaments. The tympanic membrane separates the external ear from the middle ear and is made up of tense (pars tensa) and flaccid (pars flaccida) portions. The pars tensa is translucent, and a portion of the manubrium (one of the auditory ossicles) can be seen through its dorsal aspect.
The facial nerve provides motor innervation to the external ear and a branch of the carotid artery (the great auricular artery) provides the main blood supply. The retroglenoid vein runs rostral to the tympanic bulla and the internal carotid artery lies medial to the bulla; knowledge of the anatomical positioning of these vessels is vital before attempting surgical procedures such as bulla osteotomy.
The middle ear consists of the tympanic cavity, which is predominantly air-filled but also contains the ossicles, the auditory tube, and several neural structures (branches of CN IX and autonomic fibres). In the cat, the tympanic cavity is divided into two compartments (rostrolateral and ventromedial) by a bony septum, and the sympathetic nerve fibres are more exposed on the promontory of this septum.
The facial nerve exits the skull via the stylomastoid foramen and runs ventral to the horizontal canal and tympanic bulla.
The inner ear is completely contained within the temporal bone of the skull and is divided into the vestibule, cochlea, and semicircular canals. These structures are innervated by the vestibular and cochlear nerves.
Clinical Signs
Most dogs and cats presenting with ear disease will have characteristic clinical signs of head shaking, scratching, and rubbing. Animals may also present with erythema of the pinna and visible component of the vertical ear canal that can progress to an odorous aural discharge, aural pain, and occasionally head tilt, facial nerve paralysis, and Horner's syndrome.
Up to 80% of dogs with severe otitis externa will have concurrent skin disorders, so a full dermatologic examination is indicated in all patients. Treatment of these skin conditions can aid in the medical and surgical management of ear disease.
Diagnostic Techniques
Otoscopy is the most common procedure used to diagnose ear disease. Material for cytological analysis and bacterial culture should be obtained from the deeper part of the horizontal ear canal, and a fine-needle aspirate should be taken from any aural mass lesions identified.
Radiography can be used to evaluate the external ear canals and tympanic bullae.
General anesthesia and symmetrical anatomic radiographic positioning are essential. Best images of the tympanic bullae are obtained with the use of multiple views including lateral, dorsoventral, and open-mouthed oblique positioning.
Computed tomography (CT) provides a superior image of the ear canals and tympanic bullae, allowing complete assessment of the entire ear and the presence or absence of any bony changes to the bones of the skull. Magnetic resonance imaging (MRI) can also be used, but due to the signal void that can result from gas in the tympanic bullae, MRI probably provides most benefit in cases with suspected inner ear disease.
Perioperative Considerations
A thorough knowledge of the complex anatomy of the ear as briefly outlined earlier is an important consideration before tackling ear surgery. The facial nerve is prone to damage and can be difficult to identify in cases with chronic inflammatory changes to the external auditory meatus. The sympathetic nerve plexus in the middle ear can be easily damaged, and this appears to be more common in cats due to its more prominent location. The most commonly performed external/middle ear surgery, the total ear canal ablation and lateral bulla osteotomy (TECA/LBO) will result in hearing loss, although this may not be easily recognized, as many animals will have substantial hearing loss from the disease process already.
Ear surgery is regarded as painful, and consideration should be given to making an analgesia plan before surgery. Preoperative application of a fentanyl patch the day prior to surgery is recommended. However, as many animals are only admitted on the day of surgery, then I use a peri- and postoperative fentanyl continuous-rate infusion (CRI) often combined with a 'splash block' of bupivacaine (up to 2 mg/kg) in the surgical site prior to closure. A morphine, lidocaine, and ketamine (MLK) CRI could also be considered, and as dogs can appear dysphoric following ear surgery, tranquilizers such as acepromazine (0.025–0.05 mg/kg) can be useful.
Antibiotic therapy is almost always indicated before, during, and following ear surgery, as the primary reason for considering ear surgery is infection. A combination of topical and systemic antibiotics based on a bacterial culture and sensitivity is recommended. Some Gram-negative species (e.g., Pseudomonas, Enterococcus) are notoriously resistant in ear infections, and often resolution preoperatively will not be possible. In many cases with chronic ear infection, I will perform surgery in the face of infection in an attempt to remove the nidus and provide an opportunity for systemic antibiotics to be more effective.
As most patients with ear disease are generally healthy, routine anesthesia protocols are adequate. The hair is clipped from the dorsal midline of the skull to the ventral border of the mandible and as far cranially as the lateral canthus of the eye. Preparation of the skin of the external ear canal should avoid the use of chlorhexidine or iodine, as these are ototoxic and the tympanic membrane is often ruptured.
Instrumentation
Electrocautery, preferably bipolar, and suction are required for surgery on the ear canal. These procedures are performed around inflamed and infected tissue where frequent and profuse hemorrhage can be encountered. The surgical site and remaining tympanic bullae must be carefully flushed and debris removed to reduce postoperative infection development. In addition, high-quality surgical lights are required to visualize the deep aspects of the bulla. Use of a head lamp and magnification loupes are recommended when a total ear canal ablation/lateral bulla osteotomy (TECA/LBO) is performed. The bulla osteotomy requires the use of several pairs of differing sized rongeurs that are sharp, or a neurosurgical bone burr for large dogs with a thick, bony meatus. Self-retaining retractors such as Gelpi and Lone Star are of huge benefit.
Specific Procedures
Surgery of the Pinna
Aural hematoma is a common occurrence in dogs and may be due to trauma from a direct injury or repeated head-shaking. The resultant rupture of the branches of the great auricular artery within the cartilage causes blood to accumulate on the concave surface of the pinna. If the hematoma is not resolved by repeated needle drainage and head bandaging, then surgical drainage with an S-shaped incision to remove the blood and fibrin followed by placement of full-thickness, vertically oriented mattress sutures with or without stents should be performed. Bandaging of the ear and head following drainage is important to provide compression and reduce further trauma.
Lacerations of the pinna can occur with fights or head trauma. Full-thickness defects generally require cleaning, debridement, and primary repair to avoid deformation of the pinna. Postoperative bandaging is again important.
Neoplastic conditions of the pinna can occur, most commonly, squamous cell carcinoma in cats; resection with wide surgical margins should be performed. The skin of the convex surface should be sutured to the skin of the concave surface for closure, avoiding placing sutures in the auricular cartilage.
Surgery of the External Ear Canal
Lateral Ear Canal Resection
Resection of the lateral wall of the vertical ear canal (often referred to as the Zepp's procedure) was first described in 1931 as a surgical treatment for ear disease in dogs. Despite the technique being performed in many dogs, it is apparent that clinical signs continue in over 50% of cases. The procedure is contraindicated in animals with stenosis or obstruction of the horizontal ear canal. The procedure involves making two parallel incisions ventrally from the tragus to a point ventral to the horizontal ear canal. The incisions are joined and the skin undermined to expose the cartilage. The cartilage is then incised in parallel incisions that do not converge to provide a wide and lengthy 'drain board' of cartilage that is folded ventrally and sutured to the skin. The excess skin and cartilage are transected, leaving direct exposure of the horizontal ear canal with a drain board providing a stable orifice and restricting hair growth.
Vertical Ear Canal Resection
Removal of the entire vertical ear canal can be performed when the disease process involves only this portion of the ear. This is very unlikely in dogs or cats with otitis externa but may be evident with neoplastic conditions. The procedure is similar to that described for the lateral ear canal resection, except 'drain boards' are created dorsally and ventrally and the remaining cartilage removed.
Total Ear Canal Ablation and Lateral Bulla Osteotomy (TECA/LBO)
TECA/LBO is a salvage procedure and has the potential for serious complications. It is absolutely contraindicated to perform a TECA without an LBO, as recurrent deep middle ear infection will develop in inadequately drained and curetted tympanic bullae.
With the patient in lateral recumbency, a T-shaped incision is made over the vertical ear canal. The subcutaneous tissues are dissected to expose the lateral wall of the vertical ear canal. Key considerations during the procedure are to keep the dissection along the plane of the cartilage. Retraction is aided by self-retaining retractors. The canal is cleaned down to the attachment at the osseous external auditory meatus. The facial nerve is identified caudoventral to the canal at this level. The canal is sharply amputated from the bony auditory meatus. Frequently, purulent or granulomatous material is evident in the meatus; this is removed by suction and curettage to expose the entire opening. Rongeurs or a bone burr is then used to create a keyhole-shaped osteotomy in the lateral wall of the bulla to allow curettage of the tympanic cavity. Care is taken to avoid damage to the close vascular structures rostrally and ventrally. Aggressive curettage of the dorsal aspect of the bulla is not recommended to avoid damage to the structures of the inner ear. The bulla is lavaged numerous times prior to closure of the deep and subcutaneous soft tissues and skin. Drains are not placed.
Surgery of the Middle Ear
Ventral Bulla Osteotomy
Using the ventral approach to provide curettage or drainage of the tympanic bulla is most commonly indicated in cats with inflammatory polyps, although the technique is also used rarely in dogs with isolated middle ear disease.
Inflammatory polyps are benign growths that originate in the tympanic bulla or auditory tube and result in clinical signs related to their extension either ventrally into the nasopharynx (dyspnea, nasal discharge, sneezing) or laterally into the external ear canal (head tilt, aural discharge/mass, head-shaking). They can occur in any aged cat but are most commonly seen in cats younger than 2 years of age. The underlying cause remains unknown, and diagnosis is made from otoscopic or nasopharyngeal visualization of a mass. Radiographs of the skull may demonstrate a soft-tissue opacity in the tympanic bulla(e), whereas CT or MRI imaging will provide a superior evaluation of the external ear canals, tympanic bullae, and the nasopharynx.
Surgical treatment of feline inflammatory polyps via ventral bulla osteotomy has demonstrated the lowest recurrence rates (< 2%) and is strongly recommended particularly when imaging demonstrates the presence of a soft-tissue mass within the bulla.
With the cat in dorsal recumbency, the ventral aspect of the tympanic bulla is palpable caudal and medial to the ramus of the mandible. A paramedian skin incision is made and the underlying tissues dissected avoiding the linguofacial vein, lingual artery and hypoglossal nerve to expose the curved ventral aspect of the bulla. A small hole is made in the bone using a bone drill or sharp Steinmann pin, taking care to avoid damage to the deeper structures. The hole is enlarged using rongeurs, and the ventromedial compartment of the tympanic cavity is lavaged and suctioned. Another hole is made through the septum into the dorsolateral compartment. Samples of tissue should be obtained for histopathology as well as bacterial culture and sensitivity. The exposed bulla compartments should be gently curetted and flushed prior to routine closure of the soft tissues and skin.
Ventral bulla osteotomy in dogs follows a similar technique to cats, except the ventral aspect of the bulla is less palpable through the skin, and dogs do not have two compartments to their tympanic bulla.
Prognosis and Complications
TECA/LBO is reported to be a successful procedure with resolution of often debilitating clinical signs in up to 92% of cases. The primary complications with TECA/LBO relate to facial nerve damage and recurrence of infection in the tympanic bulla. Facial nerve paralysis is reported in up to 58% of cases, although most surgeons would expect the complication rate to be < 10%. Clinical signs of facial nerve paralysis include absence of the palpebral reflex and drooping of the ipsilateral ear or lip. This complication should resolve in 90% of cases, but short-term treatment with artificial tears will be necessary. If permanent facial nerve damage occurs, this does not appear to cause long-term corneal dryness, as the nictitating membrane and globe retraction act to keep the eye lubricated. Postoperative infection is reported in approximately 12% of cases following TECA/LBO, which seems high but this number includes infections that range from superficial pinnal infection only to the long-term development of a draining sinus. The more superficial infections are generally resolved by wound cleansing and antibiotics therapy. The presence of a draining sinus is likely to indicate the redevelopment of otitis media. Dogs that develop a draining sinus following TECA/LBO should have advanced imaging (CT/MRI) of the head performed to evaluate the bone for signs of osteomyelitis. Surgical reexploration may be indicated.
Cats have a higher reported rate of neurologic complications following ear surgery, with 56% reported to have facial nerve paralysis and 42% reported to have Horner's syndrome (ptosis, miosis, enophthalmos, and nictitating membrane prolapse) after TECA/LBO. Greater than half of these neurologic complications resolved over weeks to months.
Horner's syndrome is the most common complication (up to 83%) following ventral bulla osteotomy in cats. This is thought to be due to damage to the exposed and sensitive autonomic nerve plexus in the tympanic bulla during curettage. Fortunately, the clinical signs resolve in almost all cats over days to weeks postoperatively.