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]