Branchial Cleft Cyst

Description

Branchial cleft cysts are benign congenital lesions resulting from an incomplete obliteration of the branchial clefts during embryological development. They are a common cause of lateral neck masses in both children and adults, with no significant gender predilection.

Pathogenesis

During the 4th to 6th week of embryological development, the primitive pharynx gives rise to the branchial apparatus, which comprises six pairs of branchial arches, clefts, and pouches. Normally, the second to fourth clefts form the cervical sinus of His, which is eventually obliterated. Failure of this process leads to the formation of branchial cleft anomalies, including cysts, sinuses, and fistulas.

Subtypes

Branchial cleft cysts are classified based on their location and presumed embryological origin:

• First branchial cleft cysts: Located near the external auditory canal or parotid gland. They may present with ear symptoms due to proximity to the facial nerve.
• Second branchial cleft cysts: Located along the anterior border of the sternocleidomastoid muscle. These are the most common type (accounting for 95% of cases) and usually present as a painless mass.
• Third branchial cleft cysts: Located lower in the neck, posterior to the sternocleidomastoid muscle, and can extend into the lower neck or chest.
• Fourth Branchial Cleft Cysts: Located in the lower neck and may extend into the mediastinum. Rare.

Epidemiology, Risk Factors & Associations

• The majority of branchial cleft cysts are diagnosed in young adults, although they can be found at any age (no significant gender predilection).
• They are associated with certain syndromes, including Branchio-oto-renal syndrome and EEC syndrome (ectrodactyly, ectodermal dysplasia, and cleft lip/palate).

Clinical Features

• Typically present as a painless, smooth, fluctuant mass in the neck.
• May increase in size during episodes of upper respiratory tract infection due to secondary infection of the cyst.
• First branchial cleft cysts can lead to facial nerve palsy or conductive hearing loss.

Complications

• Secondary infection can lead to abscess formation, cellulitis, and fistula formation.
• Rarely, malignant transformation into squamous cell carcinoma has been reported.

Pathological Features

Histopathology

• Macroscopic: Cysts are typically well-circumscribed and filled with clear or turbid fluid.
• Microscopic: Cyst lining usually demonstrates squamous or columnar epithelium. Wall contains lymphoid tissue with germinal centres.

Radiological Features

General Features

• Characteristically demonstrates a well-defined, thin-walled cystic mass in the neck.
• The location and relation to surrounding structures can help differentiate between first and second branchial cleft cysts.
• May have rim enhancement post-contrast administration.
• 2nd branchial cysts are classically located at the anteromedial border of the sternocleidomastoid muscle, lateral to the carotid space and at the posterior margin of the submandibular gland.
  • It displaces:
    • Sternocleidomastoid muscle posteriorly or posterolaterally
    • Carotid space vessels medially or posteromedial
    • Submandibular gland anteriorly
  • Beak sign may be seen on axial CT or MR images – represents curved rim of tissue or ‘beak’ pointing medially between the internal and external carotid arteries (pathognominic sign)

CT

• Non-contrast: Hypodense well-circumscribed lesion in the expected location.
• Contrast-enhanced: Cyst wall may show rim enhancement.

MRI

• T1WI: Hypointense relative to muscle.
• T2WI: Hyperintense.
• T1 C+ (Gadolinium): Rim enhancement can be seen after contrast administration.
• DWI/ADC: Restricted diffusion is not typically observed.

US

• B-mode:
  • Sharply marginated, round to ovoid with thin walls
  • Centrally anechoic posterior acoustic shadowing. Occasionally fine, indistinct, internal echoes representing debris.
  • Displaces surrounding tissue. Compressible.

Diagnosis

The diagnosis of a branchial cleft cyst typically involves clinical examination, imaging studies, and may be confirmed with fine needle aspiration or excisional biopsy.

Differential Diagnosis

• Lymphadenopathy: Can be differentiated by the presence of illness, tender and hard nodes.
  • Metastatic nodes from head-and-neck malignancy, especially papillary carcinoma of the thyroid, are the most common types of nodal metastases presenting as cystic masses in the neck. On US, central cystic area with thick irregular walls or an eccentric solid component with increased peripheral or intralesional vascularity. Punctate calcification favours papillary thyroid carcinoma.
• Thyroglossal duct cyst: Typically located in the midline and moves with swallowing or tongue protrusion.
• Dermoid cyst: May also be midline but often have a more superficial location and can have characteristic dermoid features on imaging. On ultrasound, they appear as complex cystic masses with internal echoes due to sebaceous material and hair. CT imaging reveals well-circumscribed cystic lesions with fat attenuation and possible calcifications. On MRI, variable signal intensities can be observed depending on the cyst content, including high signal on T1-weighted images due to sebaceous material, confirmed on fat-suppressed sequences.
• Parotid gland tumours (for first branchial cleft cysts): Parotid gland tumours may demonstrate intrinsic vascularity on Doppler ultrasound and typically enhance on CT or MRI.

Management

The treatment of choice for branchial cleft cysts is surgical excision to confirm the diagnosis, relieve symptoms, and prevent complications.