First branchial cleft cyst

First branchial cleft cysts are a type of branchial cleft anomaly. They are uncommon and represent only ~7% of all branchial cleft cysts.

Epidemiology

First branchial cleft cysts are usually diagnosed in middle-aged women ^3-4. 

Clinical presentation

Their presentation can be in the form of ³

• asymptomatic, e.g. incidental finding on imaging

• a palpable lump or inflammatory mass in the parotid region

• spontaneous fluid draining from a pit-like depression on the skin, which may be mucus or pus depending on the presence of associated infection

• facial nerve palsy ⁴

Pathology

First branchial cleft cysts develop as a result of the incomplete fusion of the cleft between the first and second branchial arches (see branchial apparatus). There may be a sinus with drainage to the external ear or skin. They typically occur within or close to the parotid gland or external auditory canal.

Subtypes

They can be divided into three types based on location:

• type I: inferoposteromedial to the pinna

• type II: between the angle of the mandible and the external auditory canal

• type III: periparotid

They can also be classified based on histology:

• type I

  • purely ectodermal

  • extremely rare

  • appear histologically as cysts lined by squamous epithelium

  • presents as a cystic mass or fistula posterior to the pinna 

  • usually located superior to the main trunk of the facial nerve and ends in a cul-de-sac on or near a bony plate at the level of the mesotympanum

• type II

  • contain ectodermal and mesodermal elements

  • comparatively common

  • represent a duplication of both membranous and cartilaginous portions of the external auditory canal

  • contain skin as well as adnexal structures and cartilage

  • may be associated with the parotid gland

  • often associated with fistulae in the concha or external auditory canal +/- fistulous openings in the neck

  • incorporates some portion of the first and second arch as well as the cleft

Radiographic features

They are typically well-defined cystic masses located superficial to, within, or deep to the parotid gland. If they form a sinus, the sinus tract can be identified draining into the external auditory canal or even extending to the hyoid bone.

Ultrasound

Sharply demarcated thin-walled cyst with variable echogenicity: anechoic (most common) to different degrees of heterogeneity depending on internal debris. 

When infected, sonographic features of parotitis may be present. 

CT

Sharply circumscribed, fluid density, and thin-walled mass. The wall thickness and enhancement are variable and tend to increase with recurrent infections ⁴.

MRI

• T1: variable signal dependent on protein content

  • high protein content: high signal

  • low protein content: low signal

• T2: high signal

• T1 C+ (Gd): no enhancement in uncomplicated lesions

Differential diagnosis

• parotid sialocele

• ​necrotic lymph node(s)

• Warthin tumor