parotid space

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The parotid space is one of the deep compartments of the head and neck and, as the name suggests, is mostly filled with the parotid gland. It is the most lateral major suprahyoid neck space.

Gross anatomy

The parotid space is a roughly pyramidal space, the broad elongated base facing laterally, formed by the superficial layer of the deep cervical fascia overlying the superficial lobe of the parotid gland, and its apex pointing medially. It is traversed by the external carotid artery, retromandibular vein and facial nerve.

Relations

lateral to the parapharyngeal space
medial to superficial space and subcutaneous tissue
anterior to the carotid space
- posterior belly of the digastric muscle forms a variable portion of the posteromedial border of the parotid space and at times this muscular band helps to differentiate a deep-lobe parotid space lesion to one arising in the carotid space
posterior to the masticator space

Boundaries

The parotid space is circumscribed by the superficial layer of the deep cervical fascia :

superior margin: external auditory canal; apex of the mastoid process
inferior margin: inferior mandibular margin (although the parotid tail can extend further inferiorly below the angle of the mandible)
anterior margin: masticator space

Radiographic appearance

Ultrasound

with the parotid gland filling around two-thirds of the parotid space, US findings will reflect those of the parotid gland
intraparotid lymph nodes (may number up into the twenties) are also evident as rounded/bean-shaped hypoechoic structures with an echogenic central fatty hilum within the hyperechoic parotid gland
- most frequently located in the preauricular region and they provide lymphatic drainage of the external ear and lateral scalp
external carotid artery and retromandibular vein will be found posteriorly
- retromandibular vein crosses the superficial lobe of the parotid gland and continues longitudinally until it reaches the inferior margin of the parotid gland to join with the external jugular vein
- external carotid artery travels in the same route, however, it is a larger structure and is found in a deeper plane to the retromandibular vein
facial nerve is not visualized on US, but is inferred to be located lateral to the retromandibular vein

CT

lower attenuation in comparison to muscle due to the combination of fat and glandular tissue
allows for good visualization of deep lesions as well as reliable assessment of size, location, margins, extracapsular extension, involvement of adjacent structures, and areas of necrosis, hemorrhage, calcification of cysts
CT is preferred for tender, recurrent parotid masses that are likely to be inflammatory whereas MRI is better for assessment of a painless parotid lump

MRI

T1: higher signal than muscle
T2: lower signal than muscle
MRI is preferred for imaging the parotid space in children
- neonates and young children have limited amounts of fat within the parotid gland which decreases the amount of natural contrast available on CT, which increases the difficulty to perceive a mass lesion and define its margins.
- MRI is significantly less affected by the relative lack of native fat

Related pathology

Any mass originating from the parotid space will be centered within the parotid gland.

A large mass arising within the deep lobe of the parotid will medially displace fat in the parapharyngeal space and cause posteromedial displacement of the posterior belly of the digastric muscle and carotid space. There is also often associated widening of the stylomandibular notch .