40. Surgery of the Parotid Gland
1. Introduction: Historically tumours of the parotid gland came to be treated by plastic surgeons because as they emerged from their general surgical background they continued to treat various abnormalities of the face. The planning of incisions (to avoid or hide scars), the meticulous closure of these incisions, the intricate dissection that was needed to avoid injury to the facial nerve, the repair of the nerve (sometimes with nerve grafts) (see chapter 11) under a microscope (Fig. 1-4) and the need to do flaps when skin had to be sacrificed (recently with free microvascular flaps) (Fig. 32-46) has meant that the plastic surgeon will remain involved in the surgery of the parotid gland. This chapter deals with not so much the diseases of the parotid gland but concentrates on the surgical craft needed for these diseases.
2. Essential anatomy: That the parotid gland has two lobes, deep and superficial and they are joined by an isthmus has been the standard teaching for generations (Fig. 6, top left, see also Fig. 8 and 21). Of these, the deeper lobe is smaller and occupies the space behind the posterior border of the vertical ramus of the mandible and the rest of the gland is subcutaneous (Fig. 5). This anatomical arrangement might be convenient to understand but it must be remembered that embryologically this arrangement comes about as the salivary gland proper (ectoderm), the mandible (mesoderm), the facial nerve (neuroectoderm) and the oral mucosa (entoderm) into which the parotid duct opens (Fig. 7) develop from a mass of cells which differentiate and migrate to take their positions as the embryo expands in various directions (please see chapter 13, Flaps are a Vascular Network and chapter 29, Craniofacial Clefts). On this background as the mandible grows itself in between the two lobes of the parotid gland though a single isthmus between the two lobes is the most frequently encountered arrangement, multiple connections are not infrequent and the division of the facial nerve within the gland into two main branches one below and one above the isthmus may not be seen in each case.
It is for this reason that is to save the facial nerve that the nerve must be traced from behind forwards as it emerges from the stylomastoid foramen and enters the parotid gland (Fig. 8).
The nerve is surrounded by a leash of veins called the neuro-venous plexus of Patey which must be followed with fine dissection, preferably under a microscope, to surgically divide the isthmus. In inflammatory diseases of the gland with formation of an abscess, it is best to approach the gland from behind near the mastoid process, and to enter it by incising the capsule of the gland, a part of the deep cervical fascia attached to the mastoid process, and then the suppurating area is reached by blunt dissection so as to avoid injury to the facial nerve (Fig. 9), which may inadvertently occur if the abscess is incised in the front.
In lesions of the deep lobe of the parotid, usually in the form of neoplasms, the swelling will appear in the para-tonsillar area of the soft palate within the mouth or the lateral pharyngeal wall because it is restrained on the outside by the mandible. An abscess of the deep lobe of the parotid gland may also appear as a swelling adjacent to the soft palate, in the para-tonsillar area and is best drained under proper general anaesthesia with intra-tracheal intubation from the inside (Fig. 10, 11). See also Fig. 26-31 for tumours of the deep part of the parotid gland.
3. The advent of fine needle aspiration biopsy (FNAC) and its reliability, ultrasound examination together with CT scans and magnetic resonance imaging and occasionally a retrograde sialogram has meant that the nature of the tumour and its spread is now known to the surgeon ahead of surgery. Surgery therefore can be planned and can be divided into various categories as below:
a. A benign tumour restricted to the lower pole for e.g. a Warthin’s tumour (a lympho-epithelioma) or a pleomorphic adenoma can be approached via a pre-auricular incision by going around the ear lobule and then lifting a skin flap as in the face lift operation, or with an extension in the sub-mandibular region parallel to the lower border of the mandible. This gives enough exposure to excise the whole of the lower pole. Here the danger of damaging the facial nerve is minimal though this form of surgery is best done under magnification and a nerve stimulator is always handy in all forms of surgery on the parotid gland (Fig. 12-16).
b. A benign tumour partly occupying the superficial lobe (mostly pleomorphic adenomas) in an area in front and below the ear invariably produces a pseudo capsule around it. A similar incision as in Fig. 12 followed by raising an anterior skin flap exposes the tumour which can be easily removed by dissection (sometimes even by enucleation) leaving the pseudo-capsule behind. Here too the facial nerve is not likely to be damaged (Fig. 17-19).
c. A benign tumour which occupies almost the whole of the parotid gland however must undergo what is called a superficial paratoidectomy with a more extensive incision (see Fig. 20) and involves identifying the facial nerve as it emerges from the stylo-mastoid foramen. This lies between the tympanic and the mastoid bone which are both parts of the temporal bone (see Fig. 21 (E,F,C)). First the deep cervical fascia attached to the mastoid bone (see Fig. 21 (C)) is incised. A blunt dissection is then employed to remove or separate loose areolar tissue. The posterior belly of the digastric muscle is now visible (Fig. 21 (B)) and is perhaps the most important landmark in locating the nerve here at some depth. As the dissection is deepened with a mosquito forceps in the upper part of the field, the external visible part of the auditory canal comes in to view and is formed in its depth by the tympanic bone (Fig. 21 (E,F). It is at this wedge between the tympanic bone and the mastoid that the nerve emerges. It is sometimes advisable and also safe to chip away at the mastoid bone (Fig. 21 (C)) to reach the nerve. The area is somewhat vascular because of the leash of veins surrounding the nerve (see para 2) and if a fine pointed cautery is used to stop oozing, it acts like a nerve stimulator and causes twitching in the face which must be kept fully exposed when the patient is draped for surgery. Once the trunk is identified it is followed anteriorly and somewhat superiorly, preferably by blunt dissection, as it enters the parotid gland. From here onwards the tumourous gland is retracted away from the nerve with hooks or a cats’ paw from the lower pole to the upper convex border (Fig. 22). The dissection here can be sharp and very close to the gland till the whole of the tumour and the gland is lifted up to its duct. As the dissection progresses the single or multiple connections to the deeper lobe are cut and cauterized. In superficial parotidectomy the duct needs to be clamped, cut and sacrificed as it passes over the masseter. The deep part of the gland drains in to the superficial lobe via a single or multiple connections and the literature is silent on what happens to the deep lobe of the parotid gland when it is deprived of its drainage system. It probably atrophies. In malignant tumours the duct is a part of the peri-glandular tissue and lymph nodes which are excised en masse. The wound can be closed in two layers with good dermal approximation to avoid bad scars (Fig. 23,24).
4. Solitary benign tumours of the deeper, smaller lobe of the parotid gland are comparatively rare and usually make their appearance in the throat around the para-tonsillar area or in the soft palate. FNAC will reveal the diagnosis but tumours of the small salivary tissue in this area can be ruled out ideally by a CT-scan. This will usually show a tumour of the deep lobe to be dumbbell shaped because it enters through the pharyngeal fascia by way of a small window and then expands after the resistance of the fascia is overcome. Here too the facial nerve needs to be identified up to its course into the parotid gland but then onwards the dissection is deep to the nerve. The submandibular incision extended up to the tragus is usually employed and the dissection begins at the posterior edge of the sub-mandibular salivary gland with which the deep lobe of the parotid gland is in close proximity and in the same plane. A bi-manual maneuver in which a finger is placed over the tumour’s intra-oral extension helps to deliver the gland within the operative field (Fig. 26-31). The bed on which this dissection comes to be performed is fairly vascular and several veins need to be identified and clamped to avoid bleeding. The internal jugular vein also lies in this area at some depth. More frequently a tumour occupies both the superficial and deep lobe and when the tumour is benign, a total paratoidectomy is performed after identifying and securing the facial nerve. The surgeon must be familiar with the anatomy of this area (Fig. 25).
5. Malignant tumours of the parotid of either lobe will need a total paratoidectomy with or without excision of the draining lymph nodes depending upon the method of staging usually decided by the oncologist. The incisions are more extensive and if the facial nerve is to be saved is also the Oncologist’s discretion. If the facial nerve is injured or cut, or deliberately excised, its reconstruction is usually done by a plastic surgeon under a microscope (see para 1, Fig. 1-4).
6. In the event that skin is sacrificed the plastic or maxillofacial surgeon can undertake distant flaps (Fig. 35-46) or free microvascular flaps (see Fig. 32-34). In tumours that have invaded the mandible which gets excised the bone is usually reconstructed by a free microvascular fibular graft.
The above case photograph courtesy: Nisheet Agni, Rajiv Borle, Monograph Salivary Gland Pathologies, Jaypee 2013.
This chapter draws heavily from an excellent Monograph by Nisheet Agni, associate professor and Rajiv Borle, professor and head, dept. of Oral and maxillofacial surgery, Sharad Pawar Dental College, Datta Meghe Institute of Medical sciences (deemed university, Nagpur, Maharashtra, India) published by Jaypee 2013 and their debt is acknowledged. The rest of the drawings are done by Ravin Thatte, the compiler of these short notes by tracing from Gray’s anatomy (recent edition).