Substernal Thyroid Goiter 

Updated: Dec 11, 2015
Author: Steven K Dankle, MD; Chief Editor: Arlen D Meyers, MD, MBA 

Overview

Background

The frequency with which large goiters are encountered in the United States has progressively declined over the past several decades. This decrease is largely due to an increase in dietary iodine, particularly iodized salt. Despite the declining frequency of goiter diagnoses in the United States, substernal goiter remains a significant consideration in the differential diagnosis of mediastinal masses, particularly those located in the anterior mediastinum. Substernal goiter is generally defined as a thyroid mass that has 50% or more of its volume located below the thoracic inlet.

See the image below.

Patient with a goiter. Prominent side-view outline Patient with a goiter. Prominent side-view outline.

See 21 Hidden Clues to Diagnosing Nutritional Deficiencies, a Critical Images slideshow, to help identify clues to conditions associated with malnutrition.

Problem

Substernal goiters can remain asymptomatic for many years. Consequently, incidental discovery of these goiters is not uncommon, for example, on routine chest radiography. When symptoms arise, they are usually related to compression and compromise of adjacent structures from continued growth of the goiter within the bony confines of the thoracic cavity.[1] Symptoms of substernal goiter typically develop slowly and insidiously over long periods. Because such goiters can remain hidden within the thoracic cavity, they can occasionally become massive before discovery.

Conversely, while symptoms of substernal goiter can often develop slowly over many years, serious symptoms, such as airway compromise, can develop precipitously with little or no advance warning. Malignancy usually must be ruled out when substernal goiters are discovered in asymptomatic individuals, although fine-needle aspiration is often difficult or inadvisable in this location. For these reasons, many authors have advocated surgical removal of all substernal goiters, even when these goiters are asymptomatic.

Epidemiology

The incidence of substernal goiter is difficult to assess but reportedly ranges from 2.6-21% of patients undergoing thyroidectomy. The wide range in reported incidence is largely due to variation in the definition of substernal goiter. Substernal goiters represent up to 7% of mediastinal tumors.[2]

Etiology

Most (85-95%) substernal thyroid masses represent benign goiter. Historically, goiters have generally occurred because of iodine deficiency, although this is now observed primarily in developing nations. Goiters can also be caused by ingestion of goitrogens (ie, substances that block formation of thyroid hormone), which can be found in turnip, cabbage, and kale, or they can occur as a familial form.

Malignancy occurs in 5-15% of substernal goiters.

Pathophysiology

Substernal goiters are largely considered to result from the descent of a cervical goiter with the primary blood supply remaining in the neck, primarily from the inferior thyroid artery. Some theories postulate that small cervical thyroid nodules descend beneath the pretracheal fascia, possibly aided by the negative intrathoracic pressures that normally occur during respiration and swallowing. As the nodule gradually enlarges, it eventually becomes trapped below the thoracic inlet, where it continues to enlarge within the confines of the thoracic cavity, commonly hidden from view.

Substernal goiters originating from truly ectopic thyroid tissue located in the mediastinum are controversial. Arguments have been advanced that goiters, which are isolated within the mediastinum, may actually represent extensions of cervical goiters that have become sequestered (remaining connected only by fascial attachments), rather than arising truly de novo in the chest.

As substernal goiters enlarge within the bony thoracic cavity, vascular and visceral structures may slowly become compressed and compromised. Goiters often enlarge posteromedially to the great vessels and have the potential of compressing the vascular structures against the bone of the sternum or thoracic cage. Such compression can lead to superior vena cava syndrome.[3] Airway obstruction can occur because of compression or displacement of the trachea. Furthermore, over long periods, such compression can result in tracheomalacia. Dysphagia may originate because of compression of the esophagus. Dysphonia can occur because of compressive effects on the trachea or compromise of the recurrent laryngeal nerve.

Presentation

Substernal goiters may be discovered incidentally in asymptomatic patients. For example, routine chest radiography may reveal a mediastinal mass or tracheal deviation.

The most common symptoms of substernal goiter result from compression of the trachea and/or esophagus and include dyspnea, choking sensation, cough, and dysphagia. Progressive hoarseness and superior vena cava syndrome are less common symptoms.

Hyperthyroidism may be observed in cervical or substernal goiters. Thyrotoxicity in goiters may be due to an autonomously functioning nodule or may be precipitated by ingestion of iodides found in certain expectorants or in radiographic contrast media.

Indications

Most authors consider the mere presence of a substernal goiter an indication for surgical removal. The rationale for this recommendation is primarily related to the risk of serious complications that can occur because of compression of visceral or vascular structures within the bony thoracic cavity, the most significant of which is airway compromise. Recognize that airway compromise, while generally occurring slowly and progressively, can occur precipitously, thus resulting in the need for emergent airway management.

Additional arguments in favor of thyroidectomy for even small substernal goiters include the relative ineffectiveness of suppressive therapies and concerns that delaying surgery until symptoms develop may render the operation more technically difficult.

Finally, malignancy has been reported in 5-15% of substernal goiters, and fine-needle aspiration for cytology may be dangerous or impossible. However, reasonable arguments could be made that close observation may be appropriate for a small substernal goiter observed in an asymptomatic patient of very advanced age without radiographic evidence of visceral or vascular compromise.

Relevant Anatomy

Substernal goiters most commonly descend into the anterosuperior mediastinum. Recognize that the great vessels may be near the capsule of the gland or displaced anteriorly or inferiorly, although the predominant vascular supply is cervical. Typically, substernal goiters descend to one side of the trachea or the other, causing tracheoesophageal deviation to the contralateral side. Centrally located substernal goiters may compress the trachea between the sternum and spine, usually displacing the esophagus. When goiters are located in the anterosuperior mediastinum, the recurrent laryngeal nerve is not typically displaced from the tracheoesophageal groove.

Occasionally, substernal goiters may extend into the posterior mediastinum behind the trachea or even into the posterior thoracic cavity. These goiters are typically associated with greater anatomic considerations and complexity. Such goiters may extend between the trachea and esophagus, significantly displacing these structures or greatly distorting the normal relationship of the recurrent laryngeal nerve with the trachea. Goiters located in the posterior mediastinum or posterior thoracic cavity may also be complicated by a relatively normal location of the great vessels, making anterior exposure more difficult. Finally, note that goiters extending into the posterior thoracic cavity are extrapleural and behind the lung, which also makes anterior exposure more difficult.

Contraindications

Thyroidectomy for substernal goiter is contraindicated in patients who are inappropriate candidates for surgery in general (eg, patients who have conditions that carry significant or more serious comorbidity). Many practitioners consider the known presence of anaplastic carcinoma of the thyroid a relative contraindication to surgery for substernal goiters because treatment of this condition often is futile, and the likelihood of malignant invasion of critical structures (including the great vessels) may be high. Finally, while toxic substernal goiters may ultimately require surgery for cure, hyperthyroidism requires preoperative medical management to avoid thyroid storm intraoperatively.

 

Workup

Laboratory Studies

All patients require thyroid function studies to assess for hyperthyroidism.

Consider a preoperative serum calcium study.

Imaging Studies

CT scanning or MRI

CT scanning and MRI generally are the most useful and important diagnostic and preoperative imaging studies because they allow for a fairly detailed assessment of the relevant anatomy, including surrounding tissue planes. CT scanning with iodinated contrast media should generally be avoided to preclude triggering of thyrotoxicity. However, if CT scanning with contrast is performed, it should follow thyroid scanning because nuclear imaging is not possible for several weeks after this iodine load.

Chest radiography

These images often are quite useful because they can reveal the presence of tracheal deviation or compression.

Chest radiography sometimes provides the first evidence of a mediastinal mass.

Barium esophagraphy

Barium esophagraphy is often obtained in the evaluation of dysphagia because it may demonstrate extrinsic compression or deviation. Barium esophagraphy is often not particularly helpful in the preoperative assessment of known substernal goiter.

Nuclear thyroid imaging and sonography

Nuclear thyroid imaging may demonstrate thyroid activity in the mediastinum, but the absence of uptake in the mediastinum does not exclude a diagnosis of substernal goiter.

Sonography may demonstrate the presence of a mediastinal mass, but it is not as helpful as chest radiography or CT scanning.

Neither nuclear imaging nor sonography is necessary in the preoperative assessment of known substernal goiter.

Diagnostic Procedures

Fine-needle aspiration of goiters for cytologic analysis may be helpful when a significant cervical component exists; however, they often are not recommended for substernal goiters because they may be dangerous or impossible to obtain.[4]

 

Treatment

Medical Therapy

Suppressive therapy generally is ineffective in the management of substernal goiters, but consider this therapy when a contraindication for surgical intervention exists. Radioactive iodine therapy may be useful in the treatment of hyperthyroidism associated with goiters but usually is unhelpful in the management of obstructive symptoms.

Surgical Therapy

Surgical intervention is the treatment of choice for substernal goiter.[2, 5] In the case of benign disease, the extent of surgery is determined by the removal of the entire substernal component of a goiter as well as any other thyroid tissue responsible for obstructive symptomatology. If this goal of surgery can be met, while limiting surgery to removal of a single lobe and the isthmus, the risk of complication is reduced because injury of the contralateral recurrent laryngeal nerves and parathyroid glands is avoided.[6] In the case of malignant disease, total thyroidectomy is generally recommended, particularly in tumors larger than 2 cm in which postoperative radioactive iodine treatment may be used.

Preoperative Details

Preoperative management for substernal thyroidectomy focuses on (1) avoidance of thyroid storm, (2) detailed comprehension of the patient's anatomic considerations, and (3) preparation for airway difficulties during anesthesia. Accordingly, all patients undergoing thyroid surgery should have preoperative thyroid function studies as well as appropriate medical management of any thyrotoxicity. Although passing an endotracheal tube through and past an area of extrinsic tracheal compression caused by benign substernal goiters rarely is problematic, distortion of the normal position of the larynx may make peroral intubation more difficult.

In addition, patients who have difficult upper airway anatomy and tracheal compression from substernal goiter or those who become symptomatic with neck extension may pose significant hazards during induction of anesthesia. Therefore, an appropriate preoperative discussion with the anesthesiologist regarding airway management issues is recommended. Additionally, the anesthesiologist should have access to and expertise in fiberoptic intubation.

CT scanning currently is the most useful tool in preoperative assessment of patient anatomy. In most cases, substernal goiters can be removed successfully through a transcervical approach, although transthoracic approaches are occasionally required. Certain radiographic findings may alert the surgeon to the possible need for such an approach. For example, posterior mediastinal and posterior intrathoracic goiters may be difficult or impossible to remove through a cervical incision. Occasionally, removal of massive centrally located goiters that are wedged between the sternum and spine presents difficulties. These anatomic characteristics or substernal goiters occurring in patients who have had prior thyroid surgery (in which fibrosis can make dissection more difficult) may necessitate the assistance of a surgeon experienced in sternotomy or thoracotomy.[7, 8]

Intraoperative Details

Many substernal goiters that are not easily detected by palpation of a patient in a sitting patient can become detectable in a supine patient whose neck is properly extended.

After standard preparation and draping (taking into consideration the possible need for sternotomy or thoracotomy), commence the procedure using a low collar incision followed by elevation of subplatysmal flaps superiorly above the thyroid notch and inferiorly to the clavicle. Generally, transection of the strap muscles is unnecessary. Therefore, initially divide the strap muscles in the midline and retract them laterally to expose the cervical portion of the thyroid gland.

Carefully examine the gland by palpation, using cautious blunt finger dissection around the lobe extending into the mediastinum. Some authors advise ligation of the middle thyroid vein and inferior thyroid artery in the neck before attempting to deliver the goiter from the mediastinum. This author prefers to work first in the mediastinum, using gentle finger dissection and working circumferentially around the substernal extent, carefully separating fibrous attachments. At this point, using both hands sometimes helps to dissect and carefully lift the goiter from the mediastinum into the neck.

Whether the substernal goiter can be delivered into the neck through the collar incision can usually be determined at this early juncture in the procedure. If such determination cannot be made, consider alternative exposures. In most cases in which the goiter cannot be removed through a neck incision, a ministernotomy or partial upper sternotomy provides excellent exposure with little or no additional morbidity. Alternative approaches include partial resection of the clavicular head or thoracotomy. While many authors have correctly pointed out that the vast majority of substernal goiters can be successfully removed through a neck incision,[9] the ability to pursue transthoracic approaches when circumstances warrant nevertheless is important.

Generally, identification of the parathyroid glands or the recurrent laryngeal nerve before delivering the goiter from the mediastinum is not possible. Once the goiter is elevated from the mediastinum, complete the balance of the operative procedure as with any standard thyroidectomy. Identify and ligate the middle thyroid vein, followed by identification and preservation of the recurrent laryngeal nerve and parathyroid glands. Next, ligate the superior pole vessels and trace the recurrent laryngeal nerve to its entry into the larynx below the cricothyroideus muscle. Then, identify and ligate terminal branches of the inferior thyroid artery, after which the Berry ligament can be sharply divided with the isthmus elevated off the underlying trachea.

In most cases, lobectomy and isthmusectomy are the appropriate extent of surgery. At this point, assess the trachea for any residual displacement or distortion. If significant tracheomalacia with residual narrowing or kinking of the lumen exists following goiter removal, the tracheal airway may be improved by using traction sutures placed around cartilaginous rings. The sutures then are secured to strap muscles or are brought out through the skin tied over a button. Suction drains are commonly employed because of the significant dead space often created by removal of large goiters.

Postoperative Details

Monitor the serum calcium level. In certain cases of massive multinodular goiter, particularly with significant cervical and substernal involvement, identification of the parathyroid glands may be difficult. Even when dissection is limited to a single tracheoesophageal groove, typically transient hypocalcemia may ensue.

Complications

Removal of substernal goiters carries a very low risk of operative morbidity or mortality. In the hands of experienced surgeons, such removal can be considered safe. Potential complications include vocal cord paralysis, hypoparathyroidism, bleeding, mediastinal infection, tracheal or esophageal perforation, or pneumothorax. Removal of substernal goiters that require extensive dissection or total thyroidectomy may be associated with a somewhat greater risk of morbidity when compared with thyroidectomy in general. However, many authors have reported overall incidence of complication in removal of substernal goiters as similar to that for standard thyroidectomy.