Parotid gland

The parotid gland is the largest of the salivary glands and secretes saliva via the parotid duct into the oral cavity to facilitate mastication and swallowing. It is located in the parotid space. 

Gross anatomy

The parotid gland is wrapped around the mandibular ramus and extends to a position anterior and inferior to the ear within the parotid space. It has superficial and deep lobes.

The facial nerve and its branches pass through the parotid gland, as does the external carotid artery (ECA) and retromandibular vein. The ECA forms its two terminal branches within the parotid gland: maxillary and superficial temporal artery. There is moderate fatty infiltration or fatty replacement of the parotid glands with age ⁶.

The gland usually contains several intraparotid lymph nodes. These are typically situated in two locations within the gland: pre-trial (pre-auricular) and in the apex of the superficial lobe. These lymph nodes are the first station of lymphatic drainage of the skin of the pinna and peri-auricular skin.

There is a fibrous capsule surrounding the gland, formed by the split layers of the investing layer of the deep cervical fascia. Posteriorly this fascia condenses forming the stylomandibular ligament. The inferior projection of the parotid is often referred to as the "tail" which overlies the angle of the mandible. Anteriorly there is often an accessory parotid gland which may be separate to the main gland.

Relations

• superior pole: external acoustic meatus, temporomandibular joint
• lower pole: behind the angle of the mandible, anterior to the sternocleidomastoid and posterior belly of the digastric
• lateral surface: subcutaneous tissue
• 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 5 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

Blood supply

• arterial: ECA and a specific branch of the artery, the transverse facial artery
• venous drainage: plexus of veins into the internal jugular vein

Lymphatic drainage

Intra-parotid nodes drain into the deep cervical chain. 

Innervation

• sensory: auriculotemporal nerve, greater auricular nerve
• parasympathetic: via auriculotemporal nerve
• sympathetic: via plexus surrounding external carotid artery from superior cervical ganglion. 

Variant anatomy

• accessory parotid gland
• facial process: anterior extension of glandular tissue along the parotid duct continuous with the main gland
• ectopic parotid tissue
• parotid duct duplication ⁷

Radiographic appearance

Ultrasoundis often the first diagnostic procedure to evaluate morphological and structural changes of the parotid gland; for small (<3 cm) and superficial lesions, ultrasound and cytology are often sufficient for a definitive diagnosis ².

Ultrasound

• 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 visualising deep lesions: the deep lobe is not able to be assessed as it is protected by the mandibular ramus
  • difficulty visualising deep extension ³

CT/MRI

• CT and MRI provide useful additional diagnostic imaging if a malignancy is suspected ⁴
• the sensitivity of CT/MRI approaches 100% for detecting parotid neoplasms ⁵

Related pathology

• parotid enlargement
  • salivary gland tumours
  • cystic parotid lesions
• parotitis
• sialolithiasis