Canalplasty

A canalplasty is performed to widen a narrowed (either congenitally or acquired) external auditory canal (EAC). The procedure is performed for a number of reasons. The most common reason for canalplasty is to enhance access for mastoid surgery or during a lateral graft tympanoplasty. Other primary reasons include removal of bony or soft tissue growths or scar tissue, or as part of surgery for aural atresia, which is addressed elsewhere. The motivation for surgery in these later cases may be chronic infection, trapped debris, or hearing loss due to an occluded EAC. The benefits of the procedure are improved drainage of the ear and potentially improved hearing as well.

Embryology

The external auditory canal (EAC) develops out of the first branchial groove or cleft. The groove deepens at 8 weeks to form the primary EAC that will develop into the adult cartilaginous EAC. A tubular structure called the meatal plate then forms at the medial aspect of this groove. This is a core of epithelial cells that typically canalizes around 21 weeks of development. The cells from the cord contribute to the bony EAC and the epithelial lining of the canal, including the lateral epithelium of the tympanic membrane.[1] Anomalies in this development can lead to canal aplasia, atresia, stenosis, or duplication.

Anatomy

In adulthood, the EAC is approximately 2.5 cm in length and composed of a lateral cartilaginous portion and a medial bony portion (see the image below). The medial bony structure of the external auditory canal primarily consists of the tympanic bone, a ringed lateral projection of the temporal bone. A superior notch in the tympanic bone is called the notch of Rivinus, which is located at the junction of tympanosquamous and tympanomastoid suture lines.

Sensory innervation to the EAC consists of the following:

• The auriculotemporal nerve (from the mandibular branch of the trigeminal nerve) provides sensory information from the anterior wall and roof

• The posterior wall and floor sensibility is carried in the nerve fibers of the auricular branch of vagus (Arnold nerve)

• The tympanic plexus offers some contributions

The arterial supply includes the posterior auricular artery, deep auricular branch of the maxillary artery, and superficial temporal artery.

Important anatomic relationships should be considered prior to performing a canalplasty. Just anterior to the bony portion of the EAC are the temporomandibular joint (TMJ) and the parotid gland. As this area is often the site of surgery, care is needed in the operating room to avoid penetration of the TMJ. In addition, posterior and inferior to the EAC is the mastoid portion of the temporal bone, in which the facial nerve courses, usually just lateral to the annulus in the posteroinferior quadrant of the tympanic membrane.[2]

For more information about the relevant anatomy, see Ear Anatomy.

Function

The primary purpose of the external auditory canal is conduction of acoustic energy to the tympanic membrane. The canal has a resonance frequency of 2-3 kHz, which may facilitate conductance of frequencies helpful for speech discrimination. The skin lining the EAC also allows clearance of debris and protection from pathogens. The outer third of the EAC is lined by skin containing hair follicles and cerumen-producing glands, which facilitate a slightly acidic environment. The frequent shedding and lateral migration of canal skin allows gradual clearance of debris.[3]

Although congenital lesions occluding of the EAC such as aural atresia or congenital canal stenosis are indications for canalplasty, acquired lesions are more commonly encountered. External auditory exostoses (EAE) and osteomas are hyperostotic growths of the external auditory canal (see image below). Increased prevalence has been noted in swimmers, scuba divers, and others exposed to cold water, and EAE is often found bilaterally.[4] The cause of this bony growth, however, is unknown. An additional acquired cause of EAC occlusion, medial canal fibrosis, is an inflammatory condition that leads to scar formation in the canal and that may result from chronic inflammation or prior otologic surgery.[5, 6]

The most common indication for surgery for acquired EAC occlusion is otitis externa refractory to medical management. Surgery can improve the success of topical treatment and improve access to prevent debris accumulation. Patient presenting symptoms include recurrent otitis externa, sensation of plugging, hearing loss, pain, and tinnitus.

Surgical indications include conductive hearing loss from impedance of air-conducted sound, chronic infection, or debris accumulation behind an obstruction. Additionally, canalplasty may be performed due to poor intraoperative visualization of the anterior sulcus of the external auditory canal when performing a tympanoplasty procedure.

Most surgeons prefer the patient not to be acutely infected with otitis externa at the time of operation and may therefore recommend ear canal debridement and topical medications prior to proceeding with surgery.

Facial nerve monitoring may be performed in some cases to assess proximity to the facial nerve. If facial nerve monitoring is used, discussion with the anesthesiology team should include limiting any use of long-acting paralytic agents, as this will inhibit accurate monitoring. Knowledge of anatomic relationships, particularly the location of the temporomandibular joint (TMJ), is essential for injury prevention.

Postoperative outcomes for stenosis of the external auditory canal are generally quite good.[7] As mentioned above, the rates of stenosis are low (4-10%). The use of ear plugs while swimming has been shown to decrease the risk of recurrence of exostoses in patients.[8]

Some surgeons may recommend their patients undergo treatment of external otitis media prior to surgery with topical antimicrobial drops that may include a steroid solution. Occasionally oral antimicrobial are required for severe infections.

Equipment

An otologic drill with tools including cutting and diamond burrs are required for many cases, as well as an operating microscope for improved visualization. A facial nerve monitoring unit may be desired if the surgeon feels the facial nerve may be at risk during the surgery.

Patient preparation

A single dose of antibiotics protective against skin flora is to be administered intravenously prior to skin incision.

Anesthesia

Canalplasty procedures are performed under general anesthesia. In cases in which facial nerve monitoring is required, long-acting paralytics are not recommended.

Local anesthesia is often also administered in the area around the planned incision, in the postauricular sulcus and in the external auditory canal. If feasible, injection of local anesthesia is performed in the cartilaginous, hair-bearing portion of the external auditory canal (EAC). Lidocaine mixed with diluted epinephrine (1:100,000) is typically used in order to decrease bleeding from the initial incisions

Positioning

After intubation, the patient is positioned supine on the operating room table with the head turned away from the surgeon. Typically, the bed is turned 180° away from the anesthesiology team to allow better maneuvering of the operating microscope.

Canalplasty is typically performed as an outpatient procedure. Patients are observed in the postanesthesia care unit until recovered from anesthesia before being discharged to home. To prevent re-stenosis, a stent may be fashioned and placed in the external auditory canal. Iodoform gauze may be used, with a cotton pack overlying the gauze that is replaced regularly by the patient at home.

Having the ear canal remain dry in the short-term postoperative period is usually advised; this allows healing and to prevent infection. Some surgeons suggest that patients may return to water exposure as early as one week postoperatively if wearing a silicone ear plug.[4] Antibiotic drops may be prescribed. Aside from local pain from surgery, discomfort during eating is common for up to 2 weeks postoperatively. A change in the shape of the ear after surgery may occur; however, this is usually due to swelling from the procedure and should resolve as swelling subsides.

Packing will be removed from the canal in approximately 2–3 weeks. Any nondissolving sutures will be removed in 7–10 days. Patients with canalplasty require frequent visits in the early postoperative period to ensure adequate healing of the wound. Long-term, EAC stenosis may recur in up to 10% of cases. If a skin graft was required during surgery, patients may require regular visits for debridement of the ear canal due to interruption of the natural epithelial migration of EAC skin.

Delayed healing

Postauricular approach is associated with prolonged healing in less than 5% of cases, tympanic membrane perforation in 0.2%, and a mild decrease in hearing at one frequency in less than 5% of patients.[9] The condition may recur in up to 10% of cases. Healing time of the canalplasty incisions can range from 6 weeks to 6 months. In a review of 100 cases, 90% of canalplasty incisions had healed by 2 months postsurgery.[4]

Facial nerve injury

The facial nerve is theoretically at risk during surgery at its tympanic segment. Injury to this nerve is one of the most feared complications of any otologic surgery. Approximately 25–30% of facial nerves have an aberrant course that can put the nerve at risk; however, risk to facial nerve injury in this procedure is low. A mild, transient facial weakness was noted in 2% of patients in one series.[4] This weakness resolved in both cases spontaneously.

Hearing loss

A tympanic membrane perforation can cause a conductive hearing loss. One series discovered an approximately 5% chance of greater than 15 dB hearing loss at 4000 Hz after canalplasty; however, the risk of a 15 dB change in pure tone average was less than 1%.

Change in taste

The nerve controlling taste to the side of the tongue on the side of surgery could be injured during the procedure.

Infection

Infection of the wound may occur and can manifest with erythema (redness of the skin), pain, or swelling in the area around the sutures. An oral antibiotic that covers skin flora may be required. Postoperative infection can also prolong the healing period.

Ear canal scarring

The rate of re-stenosis was reported as 4% and was primarily associated with use of a middle temporal flap to cover a bony defect.[9] Close postoperative monitoring is required to identify re-stenosis early.

No specific medical therapies have been shown to definitively improve exostoses or other causes of acquired external auditory canal (EAC) stenosis. Otitis externa may develop in the presence of acquired EAC stenosis, a condition requiring treatment with topical or systemic antimicrobials. For hearing loss associated with EAC occlusion, hearing aids may prove beneficial. In the setting of EAC obstructions, however, hearing aids may additionally cause inflammation that may exacerbate the condition and may be uncomfortable for the patient.[10, 11]

The goal of surgery is simply to establish an open external auditory canal, which may require addressing the cartilaginous or bony portions of the EAC. Preoperative planning should address which, if not both, are required.

Patients undergoing canalplasty who also have cosmetic deformities of the external ear should undergo any cosmetic procedures first, as a postauricular incision may affect blood supply to future planned repairs.

If acquired stenosis is the result of canal fibrosis, the lateral epithelial border of the TM may be included in the stenosis.

Several techniques for canalplasty have been advocated. For external auditory exostoses, osteotomes and otologic drills have both been used to remove excess bone.[12]  Typically, a postauricular approach is used with an endaural incision for exostoses and medial canal fibrosis. An endaural incision alone may be appropriate for an osteoma, as they often have a stalk that facilitates easier removal.

The planned incision areas in the postauricular sulcus and EAC are infiltrated with local anesthetic medication, typically 1% lidocaine with 1:100,000 parts epinephrine. The ear is then prepared and draped in a sterile manner.

The endaural incision is first made in the EAC as far medial as allowable given the constraints of the obstruction. A laterally-based vascular strip is developed in the EAC skin. Attention is then directed to the post-auricular area. A post-auricular incision is made approximately 7 mm behind the postauricular sulcus. The auricle is retracted laterally. The incision is then continued through the auricularis posterior muscle and down to temporalis fascia. Periosteum over the mastoid is incised and elevated anteriorly to the external auditory canal. The endaural incision is found from the postauricular approach, and the 2 incisions are joined. The EAC skin is carefully elevated off of the bony EAC and then retracted forward with the auricle.

In external auditory exostosis, the skin overlying the exostosis is elevated with a round knife and elevated toward the tympanic membrane. The exostosis is drilled down using cutting and diamond burrs in a lateral to medial direction. Curettes may be used to augment dissection of bony ledges. Typically, canalplasty for acquired EAC stenosis requires drilling of the anterior bony canal. When using the otologic drill, care must be taken to avoid contact with the ossicular chain, as this can contribute to hearing loss.

Additional care must be taken when drilling anteriorly to avoid penetration into the temporomandibular joint (TMJ). A technique for this portion of the procedure involves drilling away bone superior and inferior to the TMJ first, before carefully removing the buttress of bone overlying the joint.[4] If surgery is being performed for medial canal fibrosis, the scar can usually be dissected off of the tympanic membrane (TM), leaving an intact medial layer of the TM. Occasionally, however, a portion of the membrane must be resected and subsequently repaired.

If canalplasty is being performed alone, then after the exostosis is removed, the flap of ear canal skin is placed back in position. If necessary, releasing cuts may be made to allow the flap to rest properly. If insufficient skin is present to cover the defect, a split-thickness skin graft may be required. Out of convenience, the typical donor site is postauricular skin. Other donor sites, however, include the abdomen, leg, or forearm. A dermatome with a 2-inch blade is used with a thickness of 0.005-0.006 inches. Hemostasis is obtained at the donor site with pressure, topical epinephrine, or topical thrombin. The donor site can be dressed with an elastic bandage.

The postauricular incision is then closed in layers and the external auditory canal is packed with Gelfoam. A stent may be placed to assist with adherence of the graft to the EAC.

No specific laboratory studies are required for this procedure. Routine laboratory work may be recommended based on age and risk factors for undergoing general anesthesia.

In addition to the basic history and neurotologic physical examination including otoscopy, all patients should undergo a preoperative audiogram. This not only allows assessment of the degree of conductive hearing loss that may be present from external auditory canal occlusion but also provides information on any potential pre-existing sensorineural hearing loss.

Imaging Studies

Many situations requiring canalplasty require preoperative computed tomography (CT) imaging of the temporal bone without intravenous contrast. In situations in which the lesion is clearly identified on examination, such as a bony exostosis or benign soft tissue growth, CT imaging may not be necessary.

Staging

A rating scale for external auditory exostosis has evolved primarily for means of comparisons between studies. It is staged on scaled severity from 0-3 based on the percentage of canal obstruction, with 0 representing normal or no obstruction, 1 as less than 33%, 2 as 33-66%, and 3 as more than 66%.[9]