الفهرس 
Anatomy of the MediastinumOnly 14 pages are availabe for public view
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Abstract
Primary mediastinal tumors and cysts are relatively rare in the pediatric population. Many of which are malignant and life-threatening. In fact, the majority of mediastinal masses in children are malignant. Mediastinal masses in children present in a variety of ways. Tracheal compression most commonly occurs with benign lesions in infants. Fever and constitutional symptoms frequent older children with malignancies. Over one-half of children with mediastinal masses are symptomatic. Malignant lesions have a greater chance of being symptomatic than benign lesions. The most common symptoms in children are dyspnea, cough and fever and malaise. Most mediastinal masses, regardless of whether they are malignant or benign, require surgical intervention either for diagnosis or for definitive treatment. The basic principles involved in the treatment of the mediastinal masses include protection of the airway from compression, accurate diagnosis, and complete resection with minimal morbidity when indicated.
The postero-anterior and lateral chest radiograph is the first test to be obtained that ninety percent of mediastinal masses can be seen. Computerized tomography of the chest is most often indicated as the second imaging test. CT scan establishes the site of the mediastinal mass, the consistency of the mass, and the relationship to adjacent structures. Spiral CT scan is better than conventional CT scan to evaluate enteric and bronchogenic masses. Magnetic resonance imaging (MRI) may have some advantages over CT scan in defining the relationship of tumors to the spine and possible invasion into adjacent structures. Myelography, barium esophagrams, and bronchography should rarely be necessary in this age of radiological imaging; however, barium esophagram may be useful for suspected enterogenic cysts or other suspected esophageal lesions. Once a mediastinal mass is diagnosed, complete excision is indicated. Therefore, incisional biopsy is rarely employed. The exception to this is in the case of suspected lymphoma or nonseminomatous germ cell tumors. In these cases, limited incisional or fine needle biopsy is indicated. CT guided fine needle biopsy is often the preferred approach to diagnosis. Mediastinoscopy is rarely indicated in the pediatric patient.
Most tumors within the anterior compartment are of the thymic, lymphatic, or germ cell origin. Occasionally, especially in children, tumors of vascular origin are found in the anterior superior compartment. Lymphoma is the third most common malignancy in children, but is the most common malignancy of the mediastinum in children, representing approximately one- half of all mediastinal malignancies. Lymphomas may involve all compartments in the mediastinum. In children, lymphoma occurs most frequently in the anterior and middle compartments. It should be said that there is very little role for surgical resection of lymphomas. Many children with larger lymphomas may present with airway compression and symptoms. If these patients require a surgical procedure, preoperative radiation and chemotherapy may be indicated to decrease this tumor bulk. In very rare situations in which a child presents with so bulky a tumor that airway compression is life-threatening, surgical debulking may be indicated. Non-Hodgkin’s lymphoma represents approximately twothirds of mediastinal lymphomas and Hodgkin’s lymphoma represents about one-third of the cases in children. Non- Hodgkin’s lymphoma is usually synonymous with lymphoblastic lymphoma in children, although non-lymphoblastic (large cell) may occur rarely. Children with non-Hodgkin’s lymphoma often present with symptoms related to local compression such as cough, stridor, respiratory distress, superior vena cava obstruction. Generalized symptoms are often present and include fever, night sweats and weight loss. Pleural effusions and cervical adenopathy are commonly present and provide a convenient opportunity for diagnosis. Bone marrow involvement occurs in over one-third of patients. Hodgkin’s lymphoma presents in adolescents and young adults. Children with Hodgkin’s lymphoma are more likely to experience systemic symptoms of weight loss, night sweats, and malaise and less likely to have pleural effusions. In most children, the diagnosis can be made by lymph node biopsy. Nodular sclerosing is the most common cell type.
Germ cell tumors may be both benign or malignant neoplasms, believed to originate from primordial germ cells that fail to complete the migration from the urogenital ridge and come to rest in the mediastinum. These tumors are classified as teratomas and teratocarcinomas, seminomas, embryonal cell carcinomas, choriocarcinomas, and endodermal cell tumors. While these tumors are identical cell types to those that occur in the gonads, they are not considered to be metastases from the gonadal origin. In children, the mediastinum is the fourth most common site for teratomas. Mature teratomas are those with mature tissue elements. Immature teratomas are those with embryonic mesenchyme, immature neuroectoderm and/or blastema. Occasionally, malignant germ cell elements may occur within mature teratomas, but this is very uncommon. The majority of benign mediastinal teratomas occur in the anterior mediastinum, however some occur in the posterior mediastinum. Most mature teratomas are large well-circumscribed masses that contain tissue derived from three germinal cell layers. Most contain mature ectodermal elements such as skin, pilo-sebaceous units, and mature neuroglial tissue. Cystic elements are often lined by various epithelia including respiratory and mucinous. Other tissues that may be present include bone, cartilage, smooth muscle, fat, and endocrine tissue, including pancreatic and thyroid tissue. Clinically, patients present as infants or as young adults. In infants, the tracheal cartilages are less rigid and may be compressed by a large tumor resulting in stridor, tachypnea, and severe respiratory distress, especially while lying supine. Surgical resection is indicated not only to relieve symptoms, but also to rule out malignant elements. Resection of anterior mediastinal teratomas is best achieved by a median sternotomy while those rare posterior mediastinal teratomas may be excised by thoracotomy.
Vascular anomalies are reported to account for between 3% and 6% of mediastinal masses in children. Broadly speaking, these include hemangiomas, hemangiosarcomas, and cystic hygromas. Hemangiomas are extremely rare in the mediastinum with a reported incidence of 0.05 to 1.7%. When they do occur in the mediastinum, their location is varied. They tend to be well-circumscribed, resulting in few if any symptoms for example chest pain, cough, dyspnea, stridor, or wheeze. Spontaneous hemothorax can also occur, though far more infrequently. The diagnostic workup should include chest radiograph, bronchoscopy and CT or MRI. Recent studies have suggested that MRI with angiography is most helpful. Since many of these lesions are known to regress spontaneously, a conservative approach with serial follow-up and imaging should be employed in those who are asymptomatic. Involution of hemangiomas can be facilitated by corticosteroids, interferon, and arterial ligation of feeding vessels, laser therapy, and cryotherapy. Surgical resection is indicated if there are life-threatening symptoms, namely tracheal compression, and a median sternotomy approach is advised. Angiosarcoma is a malignant vascular tumor. They have not been reported to occur in the mediastinum, except when they originate in the heart, pericardium or great vessels. Hemangiosarcomas are a distinct group of vascular tumors that have a histologic appearance between hemangiomas and angiosarcomas, and share intermediate behavior between the two. Treatment for hemangiosarcomas is wide local excision, including regional lymph node dissection. Excision of metastases and local recurrences is indicated. Additionally, radiation and chemotherapy may also be used in the treatment of metastases or recurrences. The majority of vascular anomalies involving the mediastinum are actually lymphatic malformations. These are commonly referred to as cystic hygromas or lymphangiomas. Isolated lymphatic malformations of the mediastinum are rare. In fact, most cystic hygromas in the mediastinum represent an extension of cervical lesions. Axillary and chest wall lesions may also extend into the mediastinum. Cystic hygroma typically involves the anterior mediastinum. It is often discovered at birth and usually asymptomatic. Infection, hemorrhage, or continued growth result in symptoms. The diagnostic modality of choice is ultrasound or CT scan. Surgical intervention is only necessary when patients are symptomatic. Excision is accomplished through a cervicomediastinal approach.
Thymic lesions can be separated into thymic cysts, thymic hyperplasia, and thymic tumors. Thymic cysts are uncommon lesions that may present as either mediastinal or neck masses in children. Mediastinal thymic cysts are generally asymptomatic and found incidentally. Their origin remains unclear. True thymic cysts are thin-walled, usually unilocular cysts that have thymic tissue in the cyst wall. They are uniformly benign in children. The cyst can be located either entirely in the mediastinum or the neck, but frequently includes both sites. Most are asymptomatic, but hemorrhage into the cyst cavity may produce tracheal compression and associated respiratory distress. When a thymic cyst is suspected, trans-thoracic ultrasound, CT scan, or FNA may help confirm the diagnosis. Thymic cysts should be treated by surgical excision to prevent complications of local expansion. Thymic hyperplasia can be divided into two types: lymphoid or follicular hyperplasia and true hyperplasia, often termed hypertrophy of the gland. In lymphoid hyperplasia, the gland may be normal or abnormal in size, weight, or both. This type of hyperplasia is frequently associated with autoimmune diseases. True hyperplasia, itself, can be further divided into two categories: thymic hyperplasia with massive enlargement and thymic rebound after treatment of some tumors and other systemic states. Thymic hyperplasia with massive enlargement is frequently referred to as idiopathic thymic hyperplasia. This is a very rare finding usually asymptomatic sometimes gives respiratory complaints, including pulmonary infection, respiratory distress or cough. Surgical excision is curative and advocated only when symptomatic. Thymic rebound is a poorly understood phenomenon. It is traditionally seen in those children who have undergone treatment for Hodgkin’s disease as well as for other malignant conditions. It is self-limited and resolves on its own. No treatment is necessary for the hyperplasia of thymic rebound. While thymomas are the second most common mediastinal mass in adults, they are relatively rare in the pediatric population. They are neoplasms derived from the epithelial and lymphoid components of the thymus. Thymomas are a varied spectrum of primary thymic tumors with bland cytologic features and no overt histologic evidence of malignancy. Thymomas are histologically classified either by the predominance of epithelial or lymphocytic cells or by the morphologic resemblance to cortical or medullary epithelium. Presentation of thymoma ranges from asymptomatic discovery on chest radiograph to chest pain, shortness of breath, superior vena cava syndrome, and pleural and pericardial effusions. CT scan is the diagnostic gold standard. For all thymomas, surgical excision is the mainstay of therapy. Complete resection, with contiguous structures, if necessary, should be the goal. For malignant lesions, postoperative chemotherapy and radiation have been used with limited success. Thymomas are generally slow-growing tumors with low malignant potential. Their pattern of behavior is varied.They can be found as a well-encapsulated focal lesion amenable to local excision or as one which demonstrates evidence of local invasion and metastatic spread with eventual death.
Benign congenital cystic lesions have been referred to by various names, including bronchogenic cyst, esophageal duplication cyst, enteric cyst, and neuroenteric cyst. They are all believed to constitute congenital abnormalities of division of the embryonic primitive foregut. Bronchogenic cysts are the most common of the bronchopulmonary malformations encountered in children. These lesions are usually unilocular, are lined by ciliated columnar epithelium and contain fragments of cartilage and smooth muscle within the wall. Bronchogenic cysts usually demonstrate no patent communication to the airway but are almost always closely attached to the major airways or to the esophagus by dense fibrous tissue. Enteric cysts of the mediastinum contain gastric mucosa and may or may not communicate with the gastrointestinal tract below the diaphragm. These cysts may be associated with vertebral anomalies, but there exist no intervertebral communication. Neuroenteric cysts contain both endodermal and ectodermal, or neurogenic, elements. Characteristically, a neuroenteric cyst is connected by a stalk to the meninges and spinal cord. They are associated with spinal deformities, including congenital scoliosis, hemivertebrae, and spina bifida. In children these cysts are usually symptomatic and sometimes life-threatening. Symptoms include persistent cough, progressive dyspnea, wheeze, stridor, and cyanosis. Additionally, gastroenteric cysts might be associated with pain or melena from peptic complications secondary to the activity of the gastric tissue. Plain chest x-ray is the single most useful diagnostic tool in establishing the diagnosis of these foregut cysts. It demonstrates the cyst in the mediastinum displacing or compressing the trachea, bronchi or esophagus. Useful adjuncts include CT scan, bronchoscopy, and echocardiography. Treatment of foregut cysts is always complete surgical excision, traditionally through a thoracotomy.
Neurogenic tumors are usually located in the posterior mediastinum and originate from the sympathetic ganglia (ganglioma, ganglioneuroblastoma, and neuroblastoma), the intercostals nerves (neurofibroma, neurilemoma, and neurosarcoma), and the paraganglia cells (paraganglioma). These show a larg percentage of malignancy in children. Ganglioneuroma, ganglioneuroblastoma, and neuroblastoma represent a continuum of maturation and differentiation. Ganglioneuromas, composed of ganglion cells and nerve fibers, are benign tumors originating from the sympathetic chain. These are the most common neurogenic tumors in children. Traditionally, they present at an early age and as an incidental finding on routine chest radiography. Rarely, they cause symptoms by pressure on the nerve root or spinal cord. MRI is useful in defining the relationship of the neurogenic tumors to adjacent structures like the spinal cord, aorta and esophagus. Surgical resection provides definitive cure. Ganglioneuroblastomas exhibit an intermediate degree of differentiation between ganglioneuromas and neuroblastoma. The ganglioneuroblastoma is malignant, possesses a recognizable capsule and usually generates symptoms. They are composed of both mature and immature ganglion cells. Neuroblastoma is a highly malignant tumor that occurs in young children and infants. Most are found in children younger than 3 years of age. These tumors account for over 50% of the neurogenic tumors in the mediastinum, and the intrathoracic lesions account for approximately 20% of all neuroblastomas in children. Grossly, it is a smooth mass, Histologically, these tumors consist of small, round, immature cells organized in a rosette pattern which are highly malignant that frequently have metastasized before diagnosis Presenting symptoms are variable and include cough, dyspnea, chest pain, Horner’s syndrome, paraplegia, fever, and lassitude. Prognosis is variable and highly dependent on stage of disease. For all patients with ganglioneuroblastomas, 5-year survival is 88%. An overall 5-year survival rate of 22% is reported for neuroblastomas. Neurolemomas are benign tumors arising from perineural Schwann cells. These tumors are well circumscribed and have a defined capsule. Surgical excision results in cure.