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The parotid gland is the largest of the paired major salivary glands, located in the parotid space. It secretes predominantly serous saliva via the parotid duct into the oral cavity to facilitate mastication and swallowing. During meals and mastication, the parotid gland is responsible for most of the saliva produced 14.
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The parotid gland is roughly triangular-shaped with a base superiorly and apex inferiorly. It is bounded superiorly by the zygomatic arch, posterosuperiorly by the external auditory canal and posteroinferiorly by the styloid process. The inferior projection of the gland is referred to as the "tail", which overlies the angle of the mandible.
The parotid gland is wrapped around the mandibular ramus, which subdivides the gland into superficial and deep lobes 15. The superficial and deep lobes are connected through an isthmus located behind the ramus of the mandible 13.
The facial nerve and its branches pass through the parotid gland, as does the external carotid artery and retromandibular vein. The external carotid artery forms its two terminal branches within the parotid gland: maxillary and superficial temporal artery 13.
The gland usually contains several intraparotid lymph nodes. These lymph nodes are the first station of lymphatic drainage of the skin of the pinna and periauricular skin.
A fibrous capsule surrounds the gland, formed by the superficial (investing) layer of the deep cervical fascia, creating the parotid space. Posteriorly, this fascia condenses to form the stylomandibular ligament.
Anteriorly, there is often an accessory parotid gland, which may be separate from the main gland.
The parotid gland typically becomes infiltrated or replaced by fat with age 6.
superior pole: anterior to external acoustic meatus, temporomandibular joint
lower pole: posterior to angle of mandible, anterior to the sternocleidomastoid and posterior belly of the digastric
lateral surface: subcutaneous tissue and skin
anterior surface: clasps the ramus of the mandible with the masseter on its outer surface and medial pterygoid on its inner surface inferiorly (separated by the stylomandibular ligament)
anterior border: formed by the lateral edge of the anterior surface where it meets the masseter
the parotid duct and five facial nerve branches emerge from this border
from the deeper part, the superficial temporal and maxillary arteries leave the gland
deep surface: indented by the mastoid process and its attached muscles (sternocleidomastoid and posterior belly of the digastric), styloid process and its attached muscles (stylohyoid, styloglossus, stylopharyngeus) and two ligaments (stylomandibular, stylohyoid)
the external carotid artery enters the gland through this surface
the styloid process separates the gland from the internal jugular vein and internal carotid artery
the temporozygomatic and cervicofacial branches of the facial nerve enter the gland between the mastoid and styloid processes 8
external carotid artery, and a specific branch of the artery, the transverse facial artery
the retromandibular vein drains the parotid gland and eventually into the external jugular vein
intraparotid nodes drain into the deep cervical chain
sympathetic: via plexus surrounding external carotid artery from the superior cervical ganglion
parasympathetic: otic ganglion via the auriculotemporal nerve
facial process: anterior extension of glandular tissue along the parotid duct continuous with the main gland
ectopic parotid tissue
parotid duct duplication 7
congenital agenesis: either unilateral and bilateral 10
Ultrasound is often the first diagnostic procedure to evaluate morphological and structural changes of the parotid gland. For small (<3 cm) and superficial lesions, the combination of ultrasound and cytology are often sufficient for a definitive diagnosis 2.
appears homogeneous with increased echogenicity compared to nearby muscle 1
intraparotid lymph nodes are normally seen (unlike the submandibular gland)
retromandibular vein and external carotid artery are also easily seen and by inference the facial nerve, which lies lateral to these vessels 1
limitations of ultrasound are:
difficulty visualizing deep lesions: the deep lobe is not able to be assessed as it is protected by the mandibular ramus
difficulty visualizing deep extension 3
CT and MRI provide useful additional diagnostic imaging if malignancy is suspected 4, with the sensitivity approaching 100% for detecting parotid neoplasms 5
the parotid duct and retromandibular vein are usually seen and approximate the plane separating the superficial and deep lobes 12
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