Parotidectomy

EMBRYOLOGY

The epithelial lining of the primitive mouth gives rise to solid buds that develop into the parotid glands. The surrounding capsule is derived from the adjacent mesenchymal tissue. The basal motor plate of the myelencephalon contains the dorsal nucleus of the vagus and the inferior salivatory nucleus, which innervate the parotid gland via the glossopharyngeal nerve.

ANATOMY

The parotid gland is the largest of the paired salivary glands. It is a wedge-shaped gland located in a compact area that is bounded by the following bony structures: the ramus of the mandible anteriorly, the mastoid process posteriorly, and the styloid process medially. Skin and the deep investing cervical fascia cover the superficial surface of the gland. The posteromedial surface of the gland lies on the mastoid process, the sternocleidomastoid, and the posterior belly of digastric muscle. The anteromedial surface extends over the masseter muscle laterally and the medial pterygoid muscle medially. The deep surface of the gland is intimately related to the styloid process and its attached muscles. Emerging from the anterior border of the parotid gland is the parotid duct (Stenson’s duct), which passes over the masseter muscle and then dives immediately to pierce the buccal mucosa and opens opposite the upper second molar tooth. The structures that lie within the parotid gland from the medial to the lateral aspect are (1) the external carotid artery, (2) the retromandibular vein, (3) the facial nerve, and (4) the lymph nodes lying immediately beneath the parotid fascia. The facial nerve, which is the nerve of the second pharyngeal arch, emerges from the stylomastoid foramen and enters the parotid gland, where it divides into five branches that supply the muscles of facial expression. These five branches are the temporal, zygomatic, buccal, marginal mandibu-lar, and cervical.

The arterial supply of the parotid gland is derived from the external carotid artery. The venous drainage flows into the retromandibular vein. The sympathetic supply to the parotid gland is from the superior cervical ganglion. The parasympathetic fibers from the inferior salivary nucleus of the glossopharyngeal nerve reach the parotid gland via the otic ganglion and auriculotemporal nerve.

PREOPERATIVE PREPARATION

Apart from basic investigations, the imaging studies of the parotid gland, particularly the computed tomography scan, must be reviewed, although parotidectomy is usually based on clinical suspicion and the results of fine-needle aspiration cytology. Preoperatively the function of the facial nerve must be confirmed to be intact by physical examination. Informed consent, informing the patient about the risks of damage to the facial nerve, loss of sensation over the earlobe, and the potential for gustatory sweating over the parotidectomy area, must be obtained.

Operative Procedure

POSITION

The patient undergoes general anesthesia with endotracheal intubation, but anesthesiologists must refrain from using paralyzing agents because these make the use of the nerve stimulator ineffective. The patient is placed in a supine position with a shoulder roll to extend the neck. The head is turned away from the affected side and elevated to a 45-degree position to reduce venous congestion. The operative area is prepped and draped. The face is left exposed by placing a transparent dressing to allow facial movements to be observed during facial nerve stimulation.

INCISION

A preauricular incision is made starting at the upper end of the ear and passing vertically downward and then gently curving away from the angle of the mandible. The incision is continued anteriorly approximately two fingers-breadth below the inferior border of the mandible to avoid injury to the marginal mandibular branch of the facial nerve. To avoid capillary oozing, the skin can be infiltrated with 1% lidocaine with epinephrine.

Figure : A, The incision used to perform parotidectomy. B, The cartilaginous part of the external auditory meatus is identified in relation to the mastoid process. The facial nerve is found at the depth of the posterior belly of the digastric muscle coursing forward toward the parotid gland. SCM, sternocleidomastoid.

PROCEDURE

The anterior flap is created in the subplatysmal plane close to the parotid fascia. Mobilization of the flap should not proceed beyond the anterior border of the parotid gland to avoid injury to the terminal branches of the facial nerve, because they lie in very superficial positions. The posterior flap is elevated to expose the sternocleidomastoid muscle, the mastoid process, and the cartilage of the external auditory canal. To maintain exposure, the skin flaps can be sutured with 2-0 silk sutures to the drapes, and the earlobe should also be retracted superiorly using a 2-0 silk suture.

Posteriorly the lower border of the parotid gland is dissected off the sternocleidomastoid muscle. The following two structures are encountered: (1) the external jugular vein, which is divided and ligated with 2-0 silk sutures, and (2) the anterior branch of the greater auricular nerve that runs over the parotid gland, which is sacrificed. After these structures have been dealt with, the parotid gland is elevated until the anterior border of sternocleidomastoid muscle is identified. This dissection is continued cephalad toward the mastoid process. Deep to the sternocleidomastoid, the posterior belly of the digastric muscle is exposed. Using a fine Halsted mosquito clamp, the operator develops a plane of dissection between the gland and the cartilaginous part of the external auditory meatus until the bony part is reached. This dissection is continued superiorly as far as the zygomatic process of the temporal bone. Once the posterior dissection is complete, the parotid gland can be retracted forward and outward to begin identification of the facial nerve.

First, the V-shaped sulcus between the mastoid process and the cartilaginous part of the external auditory meatus is felt. Next, the tympanoparotid fascia will be encountered; this extends between the tympanomastoid fissure and the parotid gland. This fascia is elevated and divided with a scalpel. Approximately 1 cm anterior to the apex of this V-shaped sulcus, the facial nerve should be sought at the depth of the posterior belly of the digastric muscle. At this point the facial nerve is identified coursing forward toward the parotid gland and is less deeply placed, thus simplifying its exposure. A small artery, always encountered just superficial to the trunk of the facial nerve, requires ligation because it can bleed vigorously. Damage to the nerve is prevented if meticulous blunt dissection is directed in the axis of the nerve.

After the trunk of the seventh nerve has been isolated, dissection proceeds forward in the plane of its branches. If dissection is carried only along the branches, several tunnels will be created but the bulk of the superficial lobe will remain unelevated. Instead, we prefer to create a plane just superficial to the branches. While keeping these branches under view, fine blunt hemostats are passed through the parotid gland at a right angle to the axis of the branch. The tissue is divided with bipolar forceps to avoid heat injury to the facial nerve branches. Use of bipolar electrocautery also avoids the troublesome bleeding that occurs when the parotid parenchyma is divided. Any vessel greater than 2 mm in diameter is ligated in continuity with 3-0 silk sutures and divided. If a dry field is maintained, the nerve is less likely to be inadvertently damaged. The gland is thus sequentially elevated off all the branches of the facial nerve. At the anterior border of the superficial lobe, the parotid duct (Stensen’s duct) is identified, divided, and ligated. The superficial lobe is removed, and any persistent bleeding is controlled with bipolar electrocautery or fine ligatures.

If the deep lobe of the parotid gland also needs to be removed, several vascular structures first need to be secured. At the lower border of the deep portion of the parotid gland, the posterior facial vein is isolated, divided, and ligated with 2-0 silk sutures. Next, the posterior facial vein is separated from the marginal mandibular nerve before it is divided and ligated with 2-0 silk sutures. The lower border of the gland is elevated; deep to the posterior belly of the digastric muscle, the external carotid artery is divided and ligated. Posteriorly, the superficial temporal vessels are divided. At the anterior border of the gland, the internal maxillary and transverse facial vessels are divided. The individual branches of the facial nerve are dissected free from the parotid gland and retracted gently with either nerve hooks or vessel loops. The deep lobe is removed from the space between the divisions of the facial nerve or from below the lowermost division.

CLOSURE

A small 7-mm Jackson-Pratt drain is placed through a separate stab incision. The platysma is approximated with 3-0 absorbable sutures. The skin is closed with subcuticular 4-0 absorbable sutures. The closure can be reinforced with Steri-Strips.