Index
It is the largest salivary gland located on each side of the head in front of the ears.
The parotid gland (or simply parotid, Latin: parotid gland, parotid gland) is a paired organ. It has two parts: the deep portion found topographically in the retromandibular fossa and the superficial portion of the parotid gland that lies within the superficial tissue on the lateral side of the face, in front of and below the ear.
The parotid gland is located in the region around the ramus of the mandible. It is situated in the masseter muscle, it extends upwards to the zygomatic arch, downwards to the mandibular angle, and in the posterior part, it reaches the external acoustic meatus.
The parotid fascia covers the parotid gland. The parotid gland secretes saliva through a tube that opens into the oral cavity, called the parotid duct. The parotid duct is five to six centimeters long and crosses the cheek, piercing the buccinator muscle.
The parotid gland duct opens into the oral vestibule at the second upper molar tooth level.
Structure and function
The parotid gland can be palpated anterior and inferior to each ear on the lateral surface of the cheek. The bad aspect is posterior to the level of the angle of the mandible, and the only element is adjacent to the zygomatic arch.
Each parotid gland comprises a superficial lobe and a deep lobe divided by the posterior facial vein and the facial nerve. Between the lobes of the gland, there is fatty tissue to accommodate the movement of the jaw. The superficial lobe lies lateral to the facial nerve and overlaps the lateral surface of the masseter muscle.
The deep lobe lies medial to the facial nerve and is located between the mastoid process of the temporal bone and the mandibular ramus. Around each gland is a fascial capsule called the carotid sheath.
The parotid duct (Stensen’s duct), which provides an outlet for the gland’s secretions, passes through the anterior border of the gland in the superficial lobe, turns medially at the anterior wall of the masseter, penetrates the buccinator, and then enters to the lateral oral cavity up to the maxillary second molar.
As one of the three large paired salivary glands, the parotid is a serous gland that produces saliva to moisten the mouth, helping to chew, swallow, phone, and digest. Saliva also works to prevent tooth decay.
Parotid tissue comprises serous acinar cells and may contain accessory glandular tissue consisting of mucinous acinar cells. Each serous acinus is surrounded by myoepithelial cells that are bordered by a layer of the basement membrane.
Myoepithelial cells contract to help expel secretions from the acini. Saliva is first produced in the acinar lumen and then altered into a mixture of electrolytes and macromolecules as it is actively transported through the ducts.
Saliva is hypotonic when it reaches the mouth, but salivary flow rates can influence the composition of the electrolyte. In addition to electrolytes, saliva also contains mucin and digestive enzymes. The most important enzyme is amylase, which is essential in the initial digestion of carbohydrates.
Under certain pathological conditions such as dehydration, the salivary flow may decrease, the gland may swell, and infection may occur. This painful condition is called sialadenitis.
Embryology
The salivary glands begin to develop between weeks 6 and 7 of gestation. They start as epithelial buds in the oral cavity and then spread into the underlying mesenchyme.
Blood and lymphatic supply
Blood supply
The external carotid artery (ACE) is the arterial supply to the parotid. The ECA bifurcates into the superficial temporal artery (STA) and the maxillary artery (MA) medial to the parotid gland.
The STA branches into the transverse facial artery, which runs anteriorly between the zygoma and the parotid duct and supplies the parotid duct, the parotid gland, and the masseter muscle. AFTER EXITING THE MEDIAL PAROTID, the MA supplies the infratemporal fossa and the pterygopalatine fossa.
The retromandibular vein is formed from the superior and superficial temporal veins and provides a venous flow to the parotid. It goes deep into the facial nerve and may have variable anatomy before joining the external jugular vein.
Lymphatic
There are many lymph nodes in and around the parotid glands. The parotid is the only salivary gland with two nodal layers that drain into the superficial and deep cervical lymphatic systems. Most nodes are found within the superficial lobe between the gland and the parotid capsule.
The parotid gland and the external auditory canal drain to the superficial and deep nodes. Superficial nodes also drain the pinna, scalp, eyelids, and lacrimal glands. Deep nodes drain the middle ear, nasopharynx, and soft palate.
Cancers of the skin of the face and scalp, as well as other head and neck cancers, can cause pathologically enlarged lymph nodes in the parotid gland, which may indicate the regional spread of the carcinoma.
Nerves
The parasympathetic innervation of the parotid comes from the glossopharyngeal nerve (CN IX) and causes the secretion of saliva. The parasympathetic fibers of CN IX originate in the inferior salivary nucleus in the medulla and travel through the jugular foramen to the inferior ganglion.
A small branch of CN IX, called the tympanic or Jacobsen nerve, forms the tympanic plexus within the middle ear.
These preganglionic fibers become the lesser petrosal nerve and travel through the middle cranial fossa before exiting through the foramen ovale; They synapse in the otic ganglion postganglionic parasympathetic fibers join the auriculotemporal nerve to innervate the parotid gland to secrete saliva.
The neurotransmitters acetylcholine (ACh) and norepinephrine (NE) act within the parotid. ACh binds muscarinic receptors to stimulate acinar activity and ductal transport.
It also utilizes second messenger activity by producing inositol triphosphate, which increases the concentration of calcium within cells to increase the secretion of salivary volume.
Norepinephrine transmits impulses from the sympathetic nervous system through postganglionic sympathetic fibers to the salivary glands. NE binds to beta-adrenergic receptors, resulting in the activation of the second messenger system, adenylate cyclase, which leads to the cAMP formation and the phosphorylation of various proteins and activation of enzymes.
The sympathetic outlet thickens the saliva.
The facial nerve passes through the parotid gland, providing motor supply to the muscles of facial expression but does not supply innervation to the gland itself.
Surgical Considerations
Because the facial nerve runs through the glandular substance of the parotid gland, an accurate understanding of the anatomy is crucial when performing parotidectomies. Any surgery for a parotid tumor must identify the facial nerve.
Identification of the facial nerve trunk (TNF) and its branches is essential to avoid nerve damage.
Some commonly used bone and soft tissue landmarks to identify TNF include the cartilaginous tragal pointer, tympanomastoid suture, posterior digastric abdomen, styloid process, and retromandibular vein.
If an obstructive stone (sialolithiasis) causes refractory inflammation despite medical treatment, a sial endoscopy may be done for relief. A sial endoscopy uses an endoscope to visualize the rock to aid management.
The neoplastic disease can affect the parotid gland. The most common primary parotid tumor is the pleomorphic adenoma. Metastatic disease can also affect the parotid through lymph node involvement. Treatment of parotid neoplastic disease generally includes surgical resection.
Clinical significance
Sialadenitis
Sialadenitis is due to inflammation of the salivary gland caused by obstruction and infection by bacteria, viruses, or stones.
Signs and symptoms include pain, swelling of the gland, and fever.
The most common microorganisms that cause sialadenitis are staphylococcal bacteria and the mumps virus.
Treatment includes antibiotics for bacterial infections, oral hydration, warm compresses, and sialagogues. For cases of refractory disease, surgical treatment (i.e., to drain the abscess) may be indicated.
Sialolithiasis
Sialolithiasis is a stone lodged in a salivary duct. It is the most common cause of obstructive salivary gland disease and is responsible for half of all major salivary gland disorders.
Signs and symptoms include pain and swelling in the affected duct, particularly during and after eating.
Ultrasound is the first step in the diagnosis. Computed tomography, MRI, and MRI sialography can be used in patients with a high suspicion of ductal obstruction but a negative or inconclusive ultrasound study.
The goal of treatment is to increase saliva flow through the canal with oral hydration or sialogogues. Surgical removal of the stone is necessary for chronic sialolithiasis that has failed conservative treatment.
