|Of all the pathologies affecting the parotid gland, few can be treated without surgery parotidectomy forms a preliminary step in certain major procedures on the head and neck such as temporal bone resections and is needed as an approach to areas such as the Infratemporal fossa. The proximity of the great vessels of the head and neck especially the internal carotid artery & the intimate relationship of the facial nerve to the gland demand a sound knowledge of the surgical anatomy to Facilitate surgery of this gland. Work in the last few decades has improved the standards of surgical treatment particularly for the most frequent conditions where parotidectomy is undertaken; tumors and chronic parotitis. The latter is fortunately relatively uncommon. The histopathology of the tumours remained for a longtime a histopathology of the tumours remained for a longtime a subject of discussion, but recently a classification of tumours approved by health organization has been developed.
The investing layer of the deep fascia neck splits to enclose the gland from behind and below and its therefore continuous with the fascia over the sternomastoid and the fascia of the neck . The fascia is closely adherent to the gland. Various septae in the substance it from the styoid apparatus and thickens between the styoid process and the angle of the mandible to attain a ligamentous character called the stylomandibular ligament . The rest of this fascia is know as the stylomandibular membrane through which portions of the gland may herniated .The fascia of the parotid is attached superiorly to the zygomatic arch.
The parotid duct leaves the gland at its anterior border close to its superior edge and runs over the masseter for a short distance before piercing the buccinator to enter the oral cavity. Its course approximates roughly to a line drawn from the tragus to the midpoint of a line between the ala of the nose and the angle of the mouth. the duct sweeps around the anterior border of the masseter pierces the buccopharyngeal fascia and the buccinator muscle, and enters the oral cavity through the buccal mucosa at the level of the second upper molar tooth.
The parotid gland is roughly triangular in shape with its base superiorly. Its lateral surface is superficial and visible as the skin flaps are raised at surgery . Traditionally the gland has been described as having a superficial and a deep lobe, the latter comprising that part which sweeps around the posterior border of the mandible to lie deep to the mandibular ramus and partly against the medial surface of the medial pterygoid muscle.
The apex of the triangle is known as the lower pole of the parotid. It is note worthy that there is no upper pole. The upper end of the gland is concave and abuts against the external auditory meatus.
The relations of the parotid gland are probably the most important part of its surgical anatomy. Branches of the facial nerve emerge from its anterior border and proceed superficial to the fascia of the masseter muscle.
The temporal branch is the first. It is referred to as the frontal nerve in American texts. It comes out from the superior border and proceeds superficial to the zygomatic bone, but deep to the superficial fascia, and runs towards the temple and supplies the frontalis part of the occipitofrontalis muscle, the auricularis anterior, the auricularis superior and upper part of the orbicularis oculi muscles. The zygomatic branch is by far the most vital branch of the facial nerve. It is usually comprised of several branches, one running superficial to the zygoma to supply the orbicularis oculi and is responsible for the closure of the eye. Another branch runs along the lower border of the zygomatic bone under cover of the. zygomatic muscles and supplies the muscles of the nose and muscles between the eye and mouth.
The buccal branch runs parallel to and about I cm under the parotid duct and supplies the muscles of the cheek. They have a variable origin, either from the upper division of the nerve, or more often from the lower division and occasionally from the main nerve at its bifurcation.
The mandibular branch is also known as the marginal nerve because of its relationship to the lower margin of the mandible. It is an important branch and supplies the depressor angluli oris. Its relationship deep to the investing layer of the deep cervical fascia and superficial to the facial vein at the lower margin of the mandible is a reliable landmark in its dissection both in the surgery of the parotid gland as well as in surgery of the neck.
The cervical branch emerges from the lower division of the facial nerve and is seen in the neck deep to the . deep fascia emerging from the lower pole of the gland running forwards and downwards. At its origin within the lower role It is superficial to the posterior facial vein also termed the retromandibular vein.
Close to the auricle are the superficial temporal artery and vein. These vessels are accompanied by the auricular. temporal nerve Which is a branch of the mandibular nerve and supplies the auricle and the external acoustic meatus.
The external carotid artery gives off the posterior auricular artery as it emerges above the cranial border of the posterior belly of the digastric muscle. The artery runs through the gland and in so doing grooves it. At the level of the neck of the mandible the artery divides into its terminal branches, the superficial temporal artery running vertically upwards almost as a continuation of the maintrunk while the maxillary artery travels at right angles and medially.
The facial nerve emerges from the styloid foramen medial to the mastoid process and medial to the parotid gland. It runs lateral to the styloid process and passes forwards and upwards giving off two branches namely the posterior auricular nerve and the nerve to posterior belly of digastric and the stylohyoid muscles. The nerve enters the posteromedial surface of the gland and becoming more superficial, runs forwards and finally divides into its two main trunks from which the five terminal branches emerge. It is important to realise that at the point of its exit from the stylomastoid foramen and until it enters the gland the nerve is deep to all parotid tissue. Not only the main trunk of the nerve but also the branches, especially the zygomatic division, continue to become more superficial as they proceed forwards. The realisation of this anatomical fact will prevent unwanted injury to the nerve and its branches The nerve divides the gland into a superficial and a deep lobe. Some texts mention an isthmus, the exact location of which is cranial to the main trunk of the nerve. This feature is however not referred to in many texts on anatomy.
The size of the parotid gland is variable. Lymph nodes are found in its substance which drain the preauricular, temporal, infraorbital and buccal areas.
The greater auricular nerve is the source of innervation for the gland. It originates from the cervical plexus at about the midpoint along the posterior border of the sternomastoid muscle and runs obliquely across superficial to the muscle, to the lower pole of the gland where it divides into several branches. The nerve serves as a useful source for grafting purposes. The para- sympathetic supply to the gland comes from the glosso- pharyngeal nerve. These fibres are relayed to the auricul-otemporal nerve via. the otic ganglion.
WHO classification of salivary gland tumours
The next step is the identification of the surgical landmarks for the dissection of the facial nerve. These landmarks are the posterior belly of the digastric muscle and the pointer or the medial inferior end of the cartilaginous external auditory meatus. The main trunk of the facial nerve lies approximately one cm in front of the pointer and the same distance above the superior ,border of the posterior belly of the digastric muscle deep all parotid tissue.
It is easiest to begin with identification of the posterior belly of the digastric. The fascia over the sternomastoid muscle is opened exposing the parotid gland beyond the sternomastoid as far as the posterior border of the mandible. The visceral layer of the deep fascia of the neck between the angle of the mandible and the sternomastoid is now opened with dissecting scissors. As the fascia is slit open in a vertical direction the posterior belly of the digastric is recognised just under the angle of the mandible. The sternomastoid muscle is retracted laterally and the digastric is followed to the mastoid process. The posterior auricular vessels run close to the muscle and need to be coagulated or tied.
Dissection is now begun between the parotid gland and the cartilaginous ear canal. Scissor dissection followed by blunt dissection with a small rounded ball of gauze helps both minimize trauma as well as provide haemostasis. This dissection needs to carried out for about 3 cm until the antero-inferior end is visualized. This end points to the direction of the nerve. All parotid tissue between the mastoid process and the pointer is now carefully dissected with scissors and divided. There are a pair of tiny vessels at this stage passing to and from the parotid towards the ear canal. They are ligated or coagulated. Care should be exercised in using diathermy and the use of bipolar diathermy although not essential, is advisable, at least for the beginner. The parotid gland is gently retracted with a pair of right angled retractors and further dissected medially with gauze. This brings the maintrunk of the nerve into sight (Fig. 1 ). The nerve is recognised by its ivory colour. It must be realised that the space at this point of dissection is very limited and flanked by the mastoid, the ear canal and the parotid gland. Every drop of blood that appears anywhere in the surgical area trickles down this narrow area of dissection and obstructs the view. It is essential therefore to maintain perfect haemostasis. Irrigation of the wound with saline may be needed to clear out any clots of blood and improve vision. This also helps in clearing any blood that may be sticking on to the nerve and thus assist in its identification. Unless absolutely necessary , it is wise not to use suction in the operating field at this stage and should this be necessary it should be used against a gauze.
With the gland now retracted forwards, parotid tissue overlying the nerve is dissected free from the superficial surface of the nerve, picked up with a pair of fine non- toothed forceps and cut with the scissors. The scissors are again introduced between the glandular tissue and the nerve and the same process repeated. As more and more glandular tissue is freed, it is held by a pair of Ellis forceps or retracted away until the bifurcation of the nerve is recognised This technique is easy while dissecting normal parotid tissue, but is difficult if a hard tumour mass or a soft cystic lesion is pressing on the nerve.
The position of the facial nerve may be altered if a tumour originates in the deep lobe pushing the nerve laterally or it may be exceptionally deep if a superficially located tumour in the preauricular area pushes it medially. The styloid process so often described in the anatomical texts lies deep to the nerve. The presence of a thin fibrous membrane makes it difficult to see and it is better to identify it by palpation instead of dissecting it.
The branches of the facial nerve are now dissected in the same fashion. A tumour that is adherent to the nerve or its branches or involving the deeper lobe or the masseter may make dissection difficult. A good policy in these circumstance is to identify a peripheral branch and follow it cranially in order to approach the tumour from a different aspect. Some advocate tieing the posterior facial (retromandibular) vein, cutting the greater auricular nerve and lifting up the lower pole for dissection. This step should be delayed until later, since tying the vein produces venous stasis and the oozing makes dissection difficult. Once the nerve branches have been dissected free, however, and the tumour mobilised the vein can be ligated at the lower pole and divided. The superficial lobe is lifted up along with the tumour, dissected free from the cervico-facial division of the facial nerve, the buccal, zygomatic and the temporal nerves and the vein then religated to allow removal of the lobe. Throughout the procedure, the nerve is handled as little as possible. It is better to hold some tissue next to it than to handle the nerve itself. Temporary and partial palsy after surgery disappears after a period that varies from a few days to a couple of weeks. The preauricular part of the lobe above the tragus and below the zygoma may have to be removed separately. It harbours the superficial temporal vessels and the auriculo temporal nerve. Approximately 2 to 5.% of pleomorphic adenomas become malignant.
Table l-Commonly encountered tumours of the parotid gland in order of frequency.
The next most common benign tumour is an adeno-lymphoma. It originates most commonly at the lower pole and is essentially benign. Table 1 shows a list of benign and malignant tumours of the parotid in order of their frequency. Malignant tumours demand a total parotidectomy.
Total parotidectomy technique
Dumbbell and para pharyngeal tumours
The surgical approach for their removal should in principle be from the external route. A superficial parotidectomy is carried out and the nerve branches are mobilised. The stylomandibular ligament is divided and if the styloid process is ossified it is removed. This creates space behind the ascending ramus of the mandible which is retracted upwards and forwards with a large single hook. The parapharyngeal tumour can then be dissected with blunt dissection. A combined external and transoral approach has been reported.
All wounds after tumour surgery are irrigated thoroughly with a view to preventing iatrogenic implantation of tumour cells. A closed drainage system is used which is loosely tacked onto the sternomastoid muscle at some distance from the nerve. Often when a parapharyngeal or a deep lobe tumour is large its removal may cause stretching of the nerve. Although such a nerve is intact, it may take as long as 6-10 months for function to recover completely. When tumours arise close to the exit of the facial nerve a mastoidectomy and exposure of the nerve in the fallopian canal may be necessary occasionally a large malignant tumour may demand partial temporal bone resection.
Total parotidectomy for chronic recurrent parotitis is indicated when the frequent episodes make patients life uncomfortable, It is not In common to see this condition at a very young age. Patients have been treated several times with antibiotics and analgesics before they present to the surgeon, The indication and its consequences should be thoroughly discussed with the patient. Often patients are keen to have surgery because they are tired of the painful episodes.
Parotidectomy in chronic parotitis should not be performed during an acute episode. Often the condition has been precipitated by sialography performed elsewhere. This diagnostic procedure does not add much to the clinical evaluation which is more important. Because of recurrent inflammations of the gland the facial nerve is closely adherent to the glandular tissue and needs extremely careful dissection. This procedure should therefore preferably by performed only by experienced surgeons.
In recent years infections with Mycobacterium Avum have frequently been seen. They affect children usually between 5 to 12 years of age and typically involve the lymphnodes in the parotid gland. Surgical removal is the treatment of choice in the form of a superficial parotidectomy. Needless to say, such surgery should again be performed by someone who is used to operating in this area , the same being true for recurrent tumours.
Head & Neck Surgical oncologist,
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