Parotid gland

The parotid glands are the largest of the major salivary glands. They are paired, predominantly serous salivary glands situated in the parotid region of the face, below and anterior to the external acoustic meatus, between the ramus of mandible and masseter anteriorly, while sternocleidomastoid muscle and mastoid process posteriorly. The parotid gland is often associated with an accessory parotid gland lying over the masseter muscle.

Lobes of parotid gland

The parotid gland can be divided into superficial and deep lobes in reference to the location of the facial nerve that traverses through the gland:

1. Superficial lobe

The superficial lobe of the parotid gland is the larger and more externally visible portion of the gland located lateral to the facial nerve. It lies between the skin and the ramus of the mandible and forms most of the palpable bulk of the parotid region. Because of its superficial position, swellings or tumours of this lobe usually present as visible facial swelling anterior and inferior to the ear. It is also the part most commonly involved in parotid surgery.

2. Deep lobe

The deep lobe of the parotid gland is the smaller portion situated medial to the facial nerve and behind the ramus of the mandible. It extends into the parapharyngeal region and is closely related to the lateral wall of the pharynx. Lesions of the deep lobe may not produce obvious external swelling but can present as a bulge within the oropharynx or tonsillar region. Due to its deep location and proximity to important neurovascular structures, surgery involving the deep lobe is more complex.

Surfaces and Relations

1. Superficial surface is related to:

• Skin

• Superficial fascia

• Great auricular nerve

• Parotid fascia

2. Anteromedial surface is related to:

• Ramus of mandible

• Masseter

• Medial pterygoid

• Temporomandibular joint (TMJ)

3. Posteromedial surface is related to:

• Mastoid process

• Sternocleidomastoid

• Posterior belly of digastric

• Styloid apparatus

• Carotid sheath structures

Parotid fascia and capsule

The parotid gland is enclosed within a parotid fascia (parotid sheath) and a parotid capsule. Both are closely related but are not exactly the same.

The parotid fascia (parotid sheath) is the outer fascial covering of the gland derived mainly from the investing layer of the deep cervical fascia. It encloses the parotid gland externally and is particularly thick and tough.

The parotid capsule refers to the thin connective tissue capsule that is directly adherent to the gland substance itself. It is considered the true capsule of the parotid gland.

The tough parotid fascia restricts gland expansion during inflammation, which is why conditions such as mumps or acute parotitis are particularly painful.

Structures Within the Parotid Gland

From superficial to deep, these include:

Facial Nerve in the Gland

After exiting the stylomastoid foramen, the facial nerve enters the parotid gland and divides into terminal branches: Temporal, zygomatic, buccal, marginal mandibular, and cervical.

These branches form the parotid plexus. Please note that even though the facial nerve traverses the gland but does not provide secretomotor innervation to it.

Parotid Duct (Stensen Duct)

The parotid duct emerges from anterior border of gland, runs horizontally across masseter (between upper and lower branches of facial nerve), turns medially at the anterior border of the muscle. It then pierces the buccal fat pad and buccinator muscle to open into oral vestibule opposite the upper second molar tooth.

-------Clinical correlate:

Duct obstruction may cause painful gland swelling.

Blood Supply

Arterial supply is derived mainly from:

• External carotid artery

• Superficial temporal artery

• Maxillary artery

• Transverse facial artery

Venous Drainage occurs via the retromandibular vein into external and internal jugular veins.

Lymphatic drainage occurs to parotid lymph nodes and upper deep cervical lymph nodes.

Innervation

1. Sensory Supply

Sensory innervation of the parotid gland mainly travels through the auriculotemporal nerve, a branch of the mandibular division of the trigeminal nerve (V3). These sensory fibres carry pain, temperature, and other general sensations from the parotid gland, capsule, and overlying fascia and transmit them back to the trigeminal sensory nuclei in the brainstem.

Additional sensory supply to the overlying skin and fascia is provided by the great auricular nerve (C2, C3) from the cervical plexus.

-------Clinical correlate:

Pain from inflammation of the parotid gland (e.g., parotitis or mumps) is transmitted mainly via the auriculotemporal nerve, which is why pain may be referred to the temporal region, external ear, or temporomandibular joint area.

2. Parasympathetic (secretomotor) Supply

Pathway:

• Inferior salivatory nucleus > Glossopharyngeal nerve (CN IX) > Tympanic branch > Tympanic plexus > Lesser petrosal nerve > Otic ganglion (synapse) > Auriculotemporal nerve → parotid gland

3. Sympathetic Supply

• From vasomotor postganglionic sympathetic fibres that reach the parotid gland via the plexuses around the external carotid artery and its branches.

Clinical correlates

• Mumps (viral parotitis): causes painful parotid swelling, fever, and pain worsened during eating due to increased salivation.

• Frey syndrome (auriculotemporal syndrome): occurs after parotid surgery or trauma due to abnormal regeneration of parasympathetic fibres, leading to gustatory sweating and facial flushing during meals.

• Parotid tumours: most salivary gland tumours arise in the parotid gland; facial nerve involvement may suggest malignancy.

• Sialolithiasis: formation of salivary duct stones causing obstruction and gland swelling; less common in the parotid gland than the submandibular gland.

• Facial nerve injury: may occur during parotidectomy, trauma, or tumour invasion and results in ipsilateral weakness of the muscles of facial expression.

• Sialography: radiographic contrast study used to assess the parotid duct system for obstruction, strictures, and branching abnormalities.