Mediastinal Masses

Timothy M. Fernandes

INTRODUCTION

The mediastinum is located in the central thorax and is bound by the thoracic inlet apically, the diaphragm caudally, and the sternum and spinal column in the anteriorposterior direction. The space may be divided into three anatomic compartments, knowledge of their normal contents helps narrow the differential diagnosis of masses that may arise in them. The anterior compartment is the area between the sternum and the anterior pericardium. Important structures contained in the anterior compartment include the trachea, ascending aorta, thymus gland, and numerous lymph nodes. The middle compartment, sometimes referred to as the visceral compartment, contains the pericardium and its contents along with the distal trachea, carina, main bronchi and their associated lymph nodes. The posterior compartment extends from the posterior pericardium to the vertebral column and the paravertebral gutters. This compartment includes the descending aorta, the esophagus, the sympathetic ganglion, and peripheral nerves.

CLINICAL PRESENTATION

Masses that arise in the mediastinum are extremely variable in the symptoms, or lack of symptoms, that are found at presentation. In adults, approximately half of all mediastinal masses are asymptomatic and are found incidentally on radiographic examination of the thorax. In children, however, the presentation may be subtle, with complaints such as cough or vague pain. The contents of the normal mediastinum may suffer specific effects when they are compressed by enlarging masses, which may give clues to trigger further investigation. In the anterior compartment, a mass may grow to compress the airways, causing recurrent pneumonia, wheezing, hemoptysis, or an intrathoracic obstruction seen on pulmonary function testing. Masses in the middle compartment also may cause airway compression. The recurrent laryngeal nerve is found in the middle compartment. Its usual course is to wrap under the left main bronchus; its disruption produces hoarseness and paralysis of the left vocal cord. The phrenic nerve arises from the cervical roots of the spinal cord and travels through the mediastinum to innervate the diaphragm; its disruption may lead to an elevated hemidiaphragm. The posterior mediastinum also contains the sympathetic chain ganglia which, if severed, can lead to Horner syndrome with the classic triad of ptosis (drooping of the upper eyelid), miosis (small pupils), and anhidrosis (lack of sweat production). Horner syndrome frequently is seen with masses that occur near the thoracic inlet and these may compress the superior vena cava leading to facial swelling and plethora (the superior vena cava syndrome).

Mediastinal masses (both malignant and benign) that originate from structures found within the mediastinum can cause associated symptoms, paraneoplastic syndromes, or other systemic diseases. Lymphomas in the mediastinum may present with “B symptoms” which include fevers, night sweats, and weight loss. Thymomas are found in the anterior mediastinum and are closely associated with myasthenia gravis, which may manifest as diplopia, muscle weakness, or easy fatigability. Other rare problems associated with thymomas include red cell aplastic anemia (5% of cases) and hypogammaglobinemia (5%–10% of cases). Parathyroid adenomas can present with hyperparathyroidism resulting in hypercalcemia and its associated features. Paraganglionomas may manifest clinically the features of pheochromocytoma, including intermittent flushing, headaches, and sweating. Intrathoracic goiters may occasionally be associated with thyrotoxicosis.

DIAGNOSIS

The evaluation of a mediastinal mass is directed by the differential diagnosis which, in part, is dictated by the anatomic location and the age of the patient. Routine chest radiographs should be carefully evaluated for abnormalities in the mediastinum. In the anteroposterior (AP) or posteroanterior (PA) views, subtle densities along the paratracheal stripe may hint at the presence of a mass but widening of the mediastinum is a more characteristic sign. A normal mediastinum should be less than 8 cm in transverse diameter in the PA view. The lateral film is useful in defining the compartments of the mediastinum. The retrosternal clear space, retrotracheal space, and the aortopulmonary window should all be carefully examined. Comparison with prior radiographs may aid in diagnosis of subtle abnormalities.

Chest computed tomography (CT) with intravenous contrast can be the first-line diagnostic imaging modality for mediastinal masses. Chest CT offers the ability to determine the exact location of a mass in relation to vessels or other structures of the thorax. The tissue attenuation also may suggest the etiology of the mass. Magnetic resonance imaging (MRI) is seldom needed to identify mediastinal masses themselves, although it may be useful for neurogenic masses and enteric foregut duplication cysts. In addition, MRI can provide information about vascular involvement, invasion of tissue planes, and can distinguish between cystic and solid masses. Other imaging modalities may be considered based on the suspected etiology of the mass. Radionucliotide scanning can be helpful for certain diagnoses. Thyroid ¹²³I scans (for thyroid masses and goiters), metaiodobenzylgnanidine (MIBG) scans (for suspected pheochromocytomas and paraganglionomas), and sestambi scans (for masses of parathyroid origin) all have utility based on clinical suspicion. Whole body position emission tomography-CT (PET-CT) is useful to rule out metastatic disease, especially for lymphomas, that may be easy to reach bronchoscopically for definitive tissue diagnosis. Laboratory testing may be helpful depending on the clinical context of the presentation. Germ cell tumors (GCT) should be suspected in a young male with an anterior mediastinal mass. Elevation of α-fetoprotein (AFP) and β-human chorionic gonadotropin (β-hCG) levels is found in nonseminomatous GCT. Other laboratory testing may be helpful, depending on the suspected etiology. Thyroid function tests may help diagnose a functioning goiter or intrathoracic thyroid. Serum calcium, phosphate, and parathyroid hormone levels are useful in parathyroid adenomas. In patients with paravertebral masses, paraganglionoma and pheochromocytoma may be suspected. These tumors commonly produce norepinepherine or epinephrine; serum free metanepherines are less sensitive than a 24-hour urine collection for metanepherines, urinary homovanillic acid, and vanillylmandelic acid in diagnosing these tumors.

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