Ectopic thymus is a condition where

thymus The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or ''T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. ...

tissue is found in an abnormal location. It usually does not cause symptoms, but may leads to a mass in the neck that may compress the

trachea The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...

and the

esophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...

. It is thought to be the result of either a failure of descent or a failure of

involution Involution may refer to: * Involute, a construction in the differential geometry of curves * '' Agricultural Involution: The Processes of Ecological Change in Indonesia'', a 1963 study of intensification of production through increased labour inpu ...

of normal thymus tissue. It may be diagnosed with radiology, such as an

ultrasound Ultrasound is sound waves with frequency, frequencies higher than the upper audible limit of human hearing range, hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hea ...

magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...

. If it causes illness, surgery can be used to remove it. Recurrence after surgery is very unlikely.

Signs and symptoms

Ectopic thymus most often does not cause symptoms. It is most frequently discovered as a soft mass or swelling in the neck of infants and children. However, when symptoms do occur they are most commonly due to compression of nearby structures such as the

and

. This can lead to

hoarse voice A hoarse voice, also known as dysphonia or hoarseness, is when the voice involuntarily sounds breathy, raspy, or strained, or is softer in volume or lower in pitch. A hoarse voice, can be associated with a feeling of unease or scratchiness in the ...

stridor Stridor (Latin for "creaking or grating noise") is a high-pitched extra-thoracic breath sound resulting from turbulent air flow in the larynx or lower in the bronchial tree. It is different from a stertor which is a noise originating in the pha ...

difficulty breathing Shortness of breath (SOB), also medically known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathe well enough. The American Thoracic Society defines it as "a subjective experience of breathing disc ...

and/or

difficulty swallowing Dysphagia is difficulty in swallowing. Although classified under " symptoms and signs" in ICD-10, in some contexts it is classified as a condition in its own right. It may be a sensation that suggests difficulty in the passage of solids or liqu ...

. Pain is uncommon.

Cause

During embryological development, the thymus is formed from the third and fourth pharyngeal pouches. It descends along a pathway from the mandible to its final resting place of the

mediastinum The mediastinum (from ) is the central compartment of the thoracic cavity. Surrounded by loose connective tissue, it is an undelineated region that contains a group of structures within the thorax, namely the heart and its vessels, the esophagu ...

. When the thymus tissue fails to descend appropriately or fails to involute, thymus tissue remains in various locations along this pathway. Locations that solid thymus tissue has been reported include near the

thyroid The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...

(most common), within the thyroid, the base of the skull, and within the

pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...

or trachea.

Diagnosis

Ultrasound Ultrasound is sound waves with frequency, frequencies higher than the upper audible limit of human hearing range, hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hea ...

is the recommended diagnostic modality used to diagnose cervical ectopic thymus. The thymus has a unique appearance on ultrasound, which allows for specific diagnosis. Ectopic thymus appears

hypoechoic Echogenicity (misspelled sometimes as echogenecity) or echogeneity is the ability to bounce an echo, e.g. return the signal in ultrasound examinations. In other words, echogenicity is higher when the surface bouncing the sound echo reflects increa ...

, with characteristic linear echogenic foci.

Magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...

may be utilized as well to better characterize and identify the location of the ectopic thymus. On MRI, ectopic cervical thymus appears as a homogeneous mass which is isointense to muscle on T1-weighted scans and hyperintense on T2-weighted scans. Biopsy or

histological Histology, also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...

examination upon resection can also be used to make a definitive diagnosis. Sometimes, ectopic thymus is found incidentally during neck surgery.

Differential diagnosis

An appropriate differential diagnosis depends upon location of the ectopic thymus. For cervical ectopic thymus, the differential diagnosis should include additional causes of neck masses. This includes common causes of neck masses in children, including: *

thyroglossal duct cyst A thyroglossal cyst is a fibrous cyst that forms from a persistent thyroglossal duct. Thyroglossal cysts can be defined as an irregular neck mass or a lump which develops from cells and tissues left over after the formation of the thyroid gland d ...

. * branchial cleft cyst. *

dermoid cyst A dermoid cyst is a teratoma of a cystic nature that contains an array of developmentally mature, solid tissues. It frequently consists of skin, hair follicles, and sweat glands, while other commonly found components include clumps of long hair, ...

. * inflammatory

lymphadenitis Lymphadenopathy or adenopathy is a disease of the lymph nodes, in which they are abnormal in size or consistency. Lymphadenopathy of an inflammatory type (the most common type) is lymphadenitis, producing swollen or enlarged lymph nodes. In cli ...

. * salivary gland

infection An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...

. * sternocleidomastoid tumor of infancy. *

benign tumor A benign tumor is a mass of cells (tumor) that does not invade neighboring tissue or metastasize (spread throughout the body). Compared to malignant (cancerous) tumors, benign tumors generally have a slower growth rate. Benign tumors have re ...

. Rare causes of neck masses in children include: *

thyroid cancer Thyroid cancer is cancer that develops from the tissues of the thyroid gland. It is a disease in which cells grow abnormally and have the potential to spread to other parts of the body. Symptoms can include swelling or a lump in the neck. C ...

. *

lymphoma Lymphoma is a group of blood and lymph tumors that develop from lymphocytes (a type of white blood cell). In current usage the name usually refers to just the cancerous versions rather than all such tumours. Signs and symptoms may include enlar ...

. *

rhabdomyosarcoma Rhabdomyosarcoma (RMS) is a highly aggressive form of cancer that develops from mesenchymal cells that have failed to fully differentiate into myocytes of skeletal muscle. Cells of the tumor are identified as rhabdomyoblasts. There are four subt ...

. *

thyroid nodule Thyroid nodules are nodules (raised areas of tissue or fluid) which commonly arise within an otherwise normal thyroid gland. They may be hyperplastic or tumorous, but only a small percentage of thyroid tumors are malignant. Small, asymptomatic n ...

Treatment

If the patient is asymptomatic and the mass is identified based upon radiologic findings,

biopsy A biopsy is a medical test commonly performed by a surgeon, interventional radiologist, or an interventional cardiologist. The process involves extraction of sample cells or tissues for examination to determine the presence or extent of a diseas ...

and/or resection may be avoided. Surgical removal of the mass is the definitive treatment for ectopic thymus tissue that is causing symptoms. It has been reported that the ectopic thymus tissue can transform into cancerous tissue. However, due to most diagnosed ectopic thymus tissue being resected due to this concern, the natural progression is not well explored. The data supporting malignant transformation is limited, and ectopic thymus tissue that is not causing problems can likely be left to involute. Given the thymus's role in the body's adaptive immune system, it should be confirmed that the patient has a mediastinal thymus prior to surgery in order to prevent the potential for future

immunodeficiencies Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...

Prognosis

Following surgical removal of the ectopic thymus, there have been no reported recurrences.

Epidemiology

Because ectopic thymus usually does not cause symptoms, it is hard to determine prevalence. Ectopic thymus is rarely reported in the literature. The prevalence of ectopic thymus reportedly ranges from 1 to 90%. This variation in prevalence is largely dependent upon the method of investigation used and how extensive the workup is. With most ectopic thymus tissue being

asymptomatic In medicine, any disease is classified asymptomatic if a patient tests as carrier for a disease or infection but experiences no symptoms. Whenever a medical condition fails to show noticeable symptoms after a diagnosis it might be considered asy ...

, it is likely the prevalence is higher than typically reported.

References

{{Congenital lymphatic organ disorders Immune system disorders