MIDDLE-POSTERIOR AND PARAVERTEBRAL MEDIASTINUM
The middle-posterior mediastinum is the compartment located between the anterior and the paravertebral compartments. The paravertebral compartment is located posterior to an imaginary line drawn just posterior to the anterior margins of the thoracic vertebral bodies. Approximately 10% to 15% of mediastinal tumors occur in the middle-posterior mediastinum. The most common abnormalities are caused by lymph node enlargement from lymphoma, granulomatous disease caused by tuberculosis, fungal infection, or non-infectious conditions of sarcoidosis or silicosis. Metastatic lymphadenopathy may be caused by cancers of the lung, kidney, breast, or gastrointestinal tract. Rare causes of lymphadenopathy are Castleman disease and amyloidosis. Symptoms may be absent or related to the underlying systemic disease process, such as fever and night sweats caused by lymphoma or an infectious process. Some patients may complain of dysphagia caused by compression of the esophagus or vague chest discomfort.
Congenital foregut cysts are a common cause of middle mediastinal lesions. These include bronchogenic cysts, esophageal duplication cysts, and (uncommonly) neurenteric cysts. Bronchogenic cysts are most common and are thought to be caused by an abnormal budding of the foregut during development. Most are located paratracheally or subcarinally. These cysts are lined by respiratory epithelium (pseudostratiﬁed, columnar, ciliated). Enteric cysts (esophageal duplication and neurenteric) arise from the dorsal foregut and are usually located in the middle-posterior mediastinum. Enteric cysts are lined by squamous or enteric epithelium. The cyst walls have smooth muscle layers with a myenteric plexus. Esophageal duplication cysts usually adhere to the esophagus. Neurenteric cysts may be associated with the esophagus or cervical or upper thoracic vertebral abnormalities with an attachment or extension into the spine. Most enteric cysts are diagnosed during childhood. Radiographically, these cysts are well-circumscribed, spherical masses. On computed tomography (CT) scan, they are unilocular, homogeneous, and nonenhancing.
Asymptomatic cysts may be observed, but their chance or rate of enlargement is uncertain. Large cysts may compress the airways and lead to pneumonia or dysphagia with esophageal compression. The treatment of choice for patients with symptomatic cysts is surgical resection. Unresected cysts rarely transform into malignant lesions.
Esophageal disorders such as achalasia, benign tumors, diverticula, and carcinoma are common causes of middle-posterior mediastinal lesions. Hiatal hernia is very common and presents as a mass in the inferior middle-posterior mediastinum, usually seen as a retro- cardiac mass on routine chest radiography. Barium swallow studies and endoscopy are generally diagnostic. Vascular lesions may present as a mass in the middle-posterior mediastinum and should always be considered before attempting biopsy. Thoracic aortic aneurysm is the most common of these, but pulmonary artery aneurysm and mediastinal hemangiomas are occasionally encountered. Contrast-enhanced CT chest examinations are generally diagnostic for a vascular aneurysm and likely demonstrate the vascular nature of hemangiomas.
Tumors of the paravertebral compartment are generally caused by neurogenic neoplasms. Neurogenic tumors account for 20% of adult and 40% of pediatric mediastinal tumors. The large majority of these tumors in adults are benign, but 50% of the neurogenic tumors in children are malignant. Schwannomas (also called neurilemmomas) and neuroﬁbromas are the most common neurogenic neoplasms. More than 90% are benign, and a small percentage are multiple. They are slow growing and arise from a spinal nerve root. Neuroﬁbromas often occur in individuals with von Recklinghausen disease (neuroﬁbromatosis). They may have multiple tumors, and malignant transformation is more common with this disease. These tumors are most commonly asymptomatic and detected accidentally. Occasionally, they may result in pain that leads to discovery of the tumor. The malignant form of these neurogenic tumors is classiﬁed as a malignant peripheral nerve sheath neoplasm. Radiographically, schwannomas and neuroﬁbromas are well-marginated, spherical, or lobular paravertebral masses. They are usually small and span one to two vertebrae but can grow to a large size. They may cause erosion of the rib or vertebral body, and 10% grow through and enlarge the neuroforamina and expand on either end to give a “dumbbell” shape. For this reason, magnetic resonance imaging of the spine is indicated before surgical resection is attempted. Surgery is the treatment of choice.
Ganglioneuromas are benign neoplasms of the sympathetic ganglia that typically occur in older children or young adults. They may be asymptomatic or symptomatic because of local tumor effects. They are well-demarcated, oblong paravertebral masses that usually span three to ﬁve vertebrae. Ganglioneuroblastomas and neuroblastomas are malignant sympathetic ganglia neoplasms that occur in young children.
Pheochromocytomas or paraganglionomas are rare neoplasms of paraganglionic tissue and rarely occur in the mediastinum, usually adjacent to the aorta or pulmonary arteries, but they may present in the posterior mediastinum. A lateral thoracic meningocele is uncommon and may be multiple. Radiographic studies demonstrate the typical cystic structures in the paravertebral foramina location. Asymptomatic lesions do not require treatment.