Online Quiz

CASE 6

History :
4 year female child presented with complaints of neck swelling.

Question:

Your diagnosis / differentials.

Please see the answer below.

ANSWER

FINDINGS:


































  • T1 & T2 bright lesion in the lower aspect of left side of neck noted causing mass effect and shift of trachea to right side.
  • MRA - source & TOF images show - The lesion causes splaying of left carotid and brachiocephalic trunk.

Diagnosis: Cervical thymic cyst

TEACHING POINTS:
Most paediatric neck masses are inflammatory and resolve with conservative therapy. In clinically atypical lesions, imaging studies can provide useful clues regarding the nature and prognosis of the mass. Magnetic resonance (MR) imaging is very useful in the evaluation of paediatric neck masses. Cystic components bright on T1-weighted and T2-weighted images and location are suggestive of cervical thymic cyst. Lesional hyperintensity on T1-weighted images can also reflect high cyst protein content or cholesterol granulomas.

DISCUSSION /GENERAL CONSIDERATIONS:

EMBRYOLOGY:
The thymic gland is embryologically derived from ventral sacculation of the 3rd pharyngeal pouch during the 6th week of development. Formed paired thymic buds on each side begin to migrate caudally to form a thymopharyngeal duct. Migration continues and the duct becomes separated from the pharynx. Cellular proliferation gives rise to paired solid masses by the 8th week, which descend into the mediastinum where they fuse and form the bilobed thymus. During the 3rd fetal month, cellular differentiation and continuing proliferation separate cortex from medulla.

Thymic cyst can be:

  • Congenital: contains thymic tissue in wall, often unilocular.
  • Acquired: often multi locular. secondary to thoracotomy, following chemotherapy or radiotherapy for mediastinal malignancy, inflammatory.
  • Can be broadly divided into: cervical thymic cyst, mediastinal thymic cyst

The cervical thymic lesions include the following categories distinguished by anatomic location and the nature of the thymic gland tissue.

Accessory cervical thymus :
Solid cervical thymic tissue is sequestered from the main gland, along the normal descent path, with or without parathyroid. Previous terms include aberrant, ectopic, undescended, persistent, or accessory thymus.

Cervical thymic cyst:
Sequestered cystic cervical thymus is found along a normal path of descent, with or without parathyroid glands. It is a cystic version of accessory cervical thymus and may have fibrous band or a solid thymic cord connection to the pharynx or mediastinum.

Undescended cervical thymus :
This occurs when a solid lobe of thymus fails to descend entirely, with or without a parathyroid complex. It differs from accessory cervical thymus in that only half of the normally blobbed thymus is present in the mediastinum, conceivably it may also become cystic.

Persistent thymopharyngeal duct cyst:
This is the same as undescended cervical thymus; however, the thymic duct is cystic. The thymus is solid, with or without parathyroid complex, and probably represents undescended thymus. A variant would be the cervical cystic duct leading to the mediastinal thymus.

Persistent thymic cord :
This is the cervical prolongation of a solid thymic cord which is continuous with the mediastinal thymus. The cystic variant may overlap with the histology and clinical appearance of the cervical thymic cyst if a true connection to mediastinal thymus cannot be documented.

Cervical extension of mediastinal thymus :
This appears as low midline solid thymus at the thoracic inlet due to incomplete mediastinal descent. It may transiently present with increased intrathoracic pressure.

Ectopic thymus :
This is the rare, solid thymic tissue in abnormal locations, eg., in the pharynx, trachea, or base of skull. Ectopic cervical thymic cyst has not been reported. The increasing number of cervical thymic cysts reported in the last few years probably reflects greater awareness of this condition among pathologists. It is also possible that in the past, many cases of thymic cyst had been missed and diagnosed as brachial cleft cyst because of inadequate sampling of the specimen. The frequent atrophic condition of the thymic remnants may require sampling from various portions of specimen before a diagnosis of thymic cyst could be rendered. Clinically, in most cases, cervical thymic lesions present as a unilateral asymptomatic neck mass, commonly on the left side of the neck and 75% of patients present before 20 years of age.

CLINICAL FEATURES:

Enlarging asymptomatic neck mass is the most common complaint.

IMAGING FINDINGS:

In children, normal thymic tissue is homogeneousand slightly more intense than muscle on T1-weighted MR images and slightly less intenseor isointense relative to fat on T2- weighted images. The MR appearance ofsolid cervical thymic tissue parallels that of mediastinalthymus. Cystic componentsof aberrant cervical thymus were bright on T1-weighted images. Mediastinalthymic cysts are usually hypointense on T1- weighted images and bright on T2-weighted images. If hemorrhagic, the cyst contents can behyperintense on T1-weighted images because of the T1 shortening effect of methemoglobin. Lesional hyperintensity on T1-weighted images can also reflect high cyst protein content or cholesterol granulomas. Bright signal intensity has been noted in middle ear and petrous apex, cholesterol granulomas on T1-weighted MR images.

TREATMENT:

Cervical Thymic Cysts are uncommon lesions causing neck swelling and are often misdiagnosed preoperatively. Surgical excision and histological examination of the specimen usually makes the diagnosis. The presenceof mediastinal thymic tissue can be confirmedpreoperatively in children, avoiding the potentialrisk of iatrogenically compromised immunity.

DIFFERENTIAL DIAGNOSIS:

The differential diagnosis of paediatric neck swelling include

  • Branchial cyst
  • Necrotic or suppurative lymphadenopathy
  • Thyroglossal duct cysts
  • Cystic hygroma
  • Thyroid and parathyroid gland lesions
  • Lymphoma
  • Simple or complicated (hemorrhagic or infected) third branchial cleft cysts
  • Lymphangiomas
  • Venous malformations
  • Epidermoid and dermoid cysts

REFERENCES:

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