Boundaries of the Superior Mediastinum
Contents of the Superior Mediastinum
Great vessels arising from the aortic arch:
Superior vena cava and its branches:
The thymus is the most obvious structure of the superior mediastinum and sits atop the innominate artery. The gland may extend anteriorly against the posterior surface of the sternum.
The trachea bifurcates at the carina and is located just posterior to the aorta.
The cervical esophagus is located just behind the pharynx.
The thoracic duct runs to the left of the cervical esophagus and enters the junction of the left internal jugular and subclavian veins.
Since the superior vena cava traverses the superior mediastinum, it can be obstructed from any number of causes. The superior vena cava syndrome is a medical emergency and is most often due to a malignant process in the chest. While lung cancer is the most common cause, other causes include aortic aneurysms, mediastinal fibrosis, pericarditis, thrombosis as a result of venous catheters and infections like histoplasmosis. Superior vena cava syndrome is often seen in middle-aged individuals. Once diagnosed, the condition needs to be treated. Otherwise, it can lead to brain and upper airway edema. Radiotherapy is often used as the initial treatment when the diagnosis is uncertain. In the past surgery was done to reconstruct the superior vena cava but today percutaneous angioplasty and stenting offer a better option. With stenting, symptomatic relief is obtained within 24 to 48 hours.
Arterial and venous structures found in the superior mediastinum include the following:
Both the vagus and phrenic nerves descend into the mediastinum. Both the proximal trachea and esophagus descend into the superior mediastinum. They may be exposed via a left cervical neck incision.
The left recurrent laryngeal nerve originates from the left vagus and traverses the aortic arch in between the left common carotid artery and left subclavian artery. The nerve then passes underneath the aortic arch, just adjacent to the ligmentum arteriosum. From here the nerve descends into the tracheoesophageal groove.
The sympathetic trunk crosses bilaterally next to the vertebral bodies.
The thymus is a small triangular gland containing lymphoid tissue. In the infant, the thymus is quite large and can occupy most of the space behind the manubrium. It is a primary lymphatic organ responsible for the development of cellular immunity. The thymus reaches its maximum just after puberty and then gradually atrophies. In the adult, only fat cells and small fibrous strands remain. In about a quarter of individuals who develop myasthenia gravis, the thymus may be enlarged (thymoma). The thymus gland is accessed via the median sternotomy and removed in toto. Great care should be taken during excision of the thymoma as the phrenic nerve runs laterally and can easily be damaged. The only way to determine if the thymoma is benign or malignant is by visual inspection of its location and invasion of adjacent structures. Malignant lesions tend to be invasive and adherent to nearby structures.
Sometimes a thyroid goiter may extend into the superior mediastinum; the mass may be large and located just behind the manubrium. To excise the sternal goiter, the surgeon will either make a cervical neck incision and bluntly dissect the lesion with fingers or partially open the manubrium.
In infants, a cystic hygroma or a cystic lymphangioma can also be found in the superior mediastinum. This multiloculated lesion often develops in the posterior triangle of the neck and then descends into the superior mediastinum. This may be removed surgically or with the use of sclerosing agents. Recurrence is not uncommon.
Mediastinal nodes can be enlarged around the trachea in patients with lung cancer, sarcoidosis, lymphoma, or tuberculosis. If the nodes are on the right side of the trachea (paratracheal), then they can be biopsied with a mediastinoscope. Left-sided paratracheal nodes can be biopsied safely with a mediastinoscopy and need a left anterior thoracotomy.
Aneurysms of the great vessels are not uncommon in the superior mediastinum. Surgeons usually need to perform a median sternotomy incision to get access to these aneurysms. Access to the proximal left subclavian artery usually requires a very high left thoracotomy.
The mediastinum is also a common location for extragonadal lesions like germ cell tumors. About 10% of germ cell tumors occur in the superior mediastinum. These lesions may be benign or malignant. The benign lesions include teratomas or dermoid cysts. Malignant germ cell tumors of the superior mediastinum include nonseminomas and seminomas. These lesions often present with chest heaviness, cough, chest pain or dyspnea. The diagnosis is aided by measurement of alpha-fetoprotein and human chorionic gonadotrophin. CT scan is vital in determining the location. Treatment of germ cell tumors depends on the type. Benign lesions are surgically removed, whereas malignant lesions are responsive to chemotherapy and or radiation.
Lymphomas are also known to occur in the superior mediastinum. These tumors are seen in young adults in between the second to fourth decade of life. While the majority of mediastinal lymphomas are B cells and have an indolent course, about 5% will be very aggressive lesions. The majority of patients with lymphoma of the superior mediastinum present with vague chest pain, dysphagia, dyspnea, night sweats, hoarseness or weight loss. The symptoms typically depend on the size of the lesion. Imaging procedures are essential in determining the size and location of these lesions. A biopsy- usually a mediastinal lymph node biopsy is required to confirm the diagnosis. The biopsy specimens have to be stained and immunotyped to determine the type of lymphoma. The majority of patients with mediastinal lymphomas are managed with chemotherapy and/or radiation.