Parotid Region

INTRODUCTION

– The parotid region lies on the posterolateral aspect of the face and contains the parotid gland, facial nerve, and associated vessels.

Boundaries of Parotid Region

• Anteriorly: It is bounded by the ramus of mandible, masseter, and medial pterygoid.
• Posteriorly: It is bounded by the mastoid process and sternocleidomastoid.
• Superiorly: It is bounded by the external acoustic meatus and posterior part of the temporomandibular joint.
• Medially: It is bounded by the styloid process and its attached muscles.

Salivary Glands

• The three pairs of major salivary glands are the parotid, submandibular, and sublingual glands.
• Numerous minor salivary glands are distributed throughout the oral mucosa, particularly in the tongue, lips, cheeks, and palate.
• The salivary glands collectively produce approximately 0.5–1.5 litres of saliva daily.

PAROTID GLANDS

• The parotid glands are the largest salivary glands and produce predominantly serous (watery) secretion.
• Weight: Each gland weighs approximately 25 g.
• Appearance: It is a lobulated, yellowish-brown gland.

Location

– The parotid gland occupies the pyramidal retromandibular fossa, also called the parotid bed.

Boundaries of the parotid bed

• Anteriorly: Ramus of mandible, masseter, and medial pterygoid.
• Posteriorly: Mastoid process, sternocleidomastoid, and posterior belly of digastric.
• Superiorly: External acoustic meatus and temporomandibular joint.
• Medially: Styloid process and its attached muscles.

Extent of Parotid Gland

• Anteriorly: It extends over the masseter muscle.
• Posteriorly: It overlaps the sternocleidomastoid.
• Superiorly: It extends towards the external acoustic meatus.
• Inferiorly: It extends below the angle of mandible towards the carotid triangle.
• Medially: Its deep part extends towards the styloid apparatus.

Accessory parotid gland

• It is a separate mass of parotid tissue situated over the masseter, usually between the zygomatic arch and parotid duct.
• Its secretion drains into the main parotid duct.

Capsule or Coverings of Parotid Glands

The parotid gland has two coverings:

1. True capsule: It is formed by condensation of the fibrous connective tissue of the gland.
2. False capsule (parotid sheath): It is derived from the investing layer of deep cervical fascia and consists of two laminae.

Superficial lamina

• It is thick, tough, and firmly attached to the gland.
• Superiorly, it attaches to the inferior border of the zygomatic arch.
• Anteriorly, it blends with the fascia covering masseter to form the parotidomasseteric fascia.

Deep lamina

• It is thin and attaches to the tympanic plate and styloid process.
• Its thickened portion forms the stylomandibular ligament, extending from the styloid process to the angle of mandible.
• The stylomandibular ligament separates the parotid gland from the submandibular gland.

External Features

• The parotid gland resembles an inverted pyramid with its apex directed downward.
• It presents:
  • One apex, directed inferiorly.
  • Four surfaces: Superior (base), superficial, anteromedial, and posteromedial.
  • Three borders: Anterior, posterior, and medial.

Relations of Parotid Gland

Apex

• The apex is directed downward and overlaps the posterior belly of digastric.
• Structures emerging through the apex:
  1. Cervical branch of facial nerve.
  2. Anterior and posterior divisions of retromandibular vein.

Superior surface (base)

• It is concave and related to the external acoustic meatus and posterior part of the temporomandibular joint.
• Structures emerging through the superior surface:
  1. Superficial temporal vessels.
  2. Auriculotemporal nerve.

Superficial surface

• It is the largest surface and faces laterally.
• It is covered by the following structures:
  1. Skin.
  2. Superficial fascia containing the great auricular nerve.
  3. Platysma.
  4. Superficial lamina of parotid fascia.
• Note: The superficial parotid (preauricular) lymph nodes lie in the superficial fascia.

Anteromedial surface

• It is deeply grooved by the ramus of mandible.
• Its relations include:
  1. Masseter.
  2. Posterior border of ramus of mandible.
  3. Medial pterygoid.
  4. Temporomandibular joint.
  5. Emerging branches of facial nerve.
  6. Maxillary artery.

Posteromedial surface

• It is irregular and related to the mastoid and styloid processes and their attached muscles.
• Superficial relations: Mastoid process, sternocleidomastoid, and posterior belly of digastric.
• Deep relations: Styloid process and its attached muscles.
• Structures deep to the styloid apparatus: Internal carotid artery, internal jugular vein, and cranial nerves IX, X, XI, and XII.
• Structures entering through this surface:
  1. Facial nerve.
  2. External carotid artery.

Anterior border

• It separates the superficial and anteromedial surfaces.
• Structures emerging from above downward:
  1. Temporal branch of facial nerve.
  2. Zygomatic branch of facial nerve.
  3. Transverse facial vessels.
  4. Upper buccal branch of facial nerve.
  5. Parotid duct.
  6. Lower buccal branch of facial nerve.
  7. Marginal mandibular branch of facial nerve.

Posterior border

• It separates the superficial and posteromedial surfaces and overlaps sternocleidomastoid.
• The posterior auricular branch of the facial nerve emerges near its deep aspect.

Medial border (pharyngeal border)

– It separates the anteromedial and posteromedial surfaces and is related to the pharyngeal wall.

Structures within Parotid Gland

– The three principal structures within the gland are arranged from superficial to deep as follows:

1. Facial nerve and its branches.
2. Retromandibular vein.
3. External carotid artery.

Facial Nerve

• The facial nerve emerges through the stylomastoid foramen and passes downward and forward for approximately 1 cm before entering the posteromedial surface of the gland.
• Within the gland, it forms the parotid plexus and usually divides into two principal trunks:
  • Temporofacial trunk: It predominantly gives temporal and zygomatic branches.
  • Cervicofacial trunk: It predominantly gives buccal, marginal mandibular, and cervical branches.
• Pes anserinus: The five terminal branches spread out within the gland in a pattern resembling a goose’s foot, called pes anserinus.
• Note: The facial nerve passes through the parotid gland but does not provide its secretomotor supply.

Retromandibular Vein

• It is formed by the union of the superficial temporal and maxillary veins within the parotid gland.
• It descends through the gland and divides into anterior and posterior divisions.
• Anterior division: It joins the facial vein to form the common facial vein, which drains into the internal jugular vein.
• Posterior division: It joins the posterior auricular vein to form the external jugular vein.

External Carotid Artery

• It enters the lower part of the posteromedial surface of the parotid gland.
• It ascends within the gland and divides near the neck of mandible into:
  1. Superficial temporal artery.
  2. Maxillary artery.
• The superficial temporal artery gives rise to the transverse facial artery within the gland.

Parotid (Stenson’s) duct

• The parotid duct is a thick-walled tube, approximately 5 cm long, that carries saliva from the parotid gland to the oral cavity.
• Extent: It extends from the anterior border of the parotid gland to the oral vestibule, where it opens on the parotid papilla opposite the upper second molar tooth.

Course

• The duct emerges from the anterior border of the gland and runs forwards over the masseter, between the upper and lower buccal branches of the facial nerve.
• At the anterior border of masseter, it turns medially and passes through:
  • Buccal fat pad.
  • Buccopharyngeal fascia.
  • Buccinator muscle.
  • Oral mucosa.
• It runs obliquely beneath the mucosa before opening into the oral vestibule.
• Functional significance: The oblique course and compression by buccinator help prevent air from entering the duct during blowing.

Blood Supply and Lymphatic Drainage

Blood supply

• Arterial supply: The gland receives branches from the external carotid artery, particularly the superficial temporal and posterior auricular arteries.
• Venous drainage: Blood drains mainly into the retromandibular vein and subsequently into the external jugular venous system.

Lymphatic drainage

• Lymph from the parotid gland drains into the superficial and deep parotid lymph nodes.
• These nodes subsequently drain into the deep cervical lymph nodes.

Nerve supply

• Parasympathetic (secretomotor): Preganglionic fibres arise from the inferior salivatory nucleus and relay in the otic ganglion. Postganglionic fibres reach the gland through the auriculotemporal nerve.
• Sympathetic: Fibres arise from upper thoracic spinal segments, relay in the superior cervical ganglion, and reach the gland through periarterial plexuses.
• Sensory: The auriculotemporal nerve supplies the gland, while the great auricular nerve (C2–C3) provides sensation to the parotid fascia and overlying skin.

Development

• The parotid gland develops from the ectoderm lining the primitive stomodeum.
• An epithelial furrow develops near the angle of the primitive mouth and extends posterolaterally between the maxillary and mandibular prominences.
• The epithelial cord canalises to form the parotid duct.
• Its terminal portion undergoes repeated branching and canalisation to develop the ductules and secretory acini.
• The parotid duct eventually opens into the oral vestibule.

SECRETOMOTOR PATHWAY OF PAROTID GLAND

Parasympathetic Pathway

Preganglionic fibres

• They arise from the inferior salivatory nucleus in the medulla oblongata.
• They travel through the glossopharyngeal nerve and its tympanic branch to reach the tympanic plexus in the middle ear.
• Fibres continue through the lesser petrosal nerve, which passes through the middle cranial fossa and usually exits through the foramen ovale.
• They synapse in the otic ganglion.

Postganglionic fibres

– They arise from the otic ganglion and reach the parotid gland through the auriculotemporal nerve.

Sympathetic Pathway

Preganglionic fibres

• They originate mainly from the lateral horn of the T1 spinal cord segment.
• They enter the sympathetic trunk and ascend to the superior cervical sympathetic ganglion, where they synapse.

Postganglionic fibres

– They form plexuses around the carotid arteries and reach the parotid gland mainly along branches of the external carotid artery.

Functional significance

• Parasympathetic stimulation: It produces abundant watery saliva.
• Sympathetic stimulation: It promotes a relatively protein-rich, more viscous secretion and regulates glandular blood flow.

FREY’S SYNDROME

• Synonyms: Auriculotemporal syndrome, Frey–Baillarger syndrome, and gustatory sweating.
• Definition: It is a condition characterised by sweating and flushing over the parotid region during eating.
• Cause: It commonly develops following parotid surgery or trauma involving parasympathetic fibres supplying the gland.

Anatomical Basis

• Injury to the auriculotemporal nerve damages parasympathetic secretomotor fibres supplying the parotid gland.
• During regeneration, some parasympathetic fibres grow towards sweat glands and cutaneous blood vessels instead of salivary tissue.
• Consequently, stimulation during eating produces sweating and flushing over the affected parotid region rather than normal salivary secretion from those fibres.

Clinical Integration