Goiter refers to an enlargement of the thyroid gland. The condition is estimated to affect 5% of the general population. While the definition of substernal goiter varies in the medical literature, goiters usually are considered substernal (also referred to as mediastinal, intrathoracic, or retrosternal) when more than 50% of the thyroid parenchyma is located below the sternal notch. Such tumors have been a focus of interest for surgeons for over 150 years. Klein is credited with being the first to successfully remove a mediastinal goiter in 1820, although the earliest surgical description of mediastinal thyroid extension dates back to Haller in 1749. Today, substernal goiters are treated by a number of different surgical specialists, including thoracic, general, and head and neck surgeons. Goiters account for as many as 10% to 15% of space-occupying mediastinal lesions and are the most common of the superior mediastinal masses.

Mediastinal goiters are classified as primary or secondary. Primary mediastinal goiters, also referred to as ectopic or aberrant goiters, do not possess any direct fibrous or parenchymal connections to the cervical portion of the gland. They are uncommon and represent fewer than 1% of all surgically excised goiters. Ectopic mediastinal thyroid tissue generally lies in proximity to the thymus owing to their shared embryological origins and to an intimate association with the thymothyroid ligament but also has been described in the pericardium and heart. Patients with ectopic thyroid tissue typically are clinically euthyroid, although hyperthyroidism has been described. The blood supply of these goiters originates from a mediastinal source, most commonly a branch from the internal mammary artery, the innominate artery, or the intrathoracic aorta itself. Other criteria used to define a primary mediastinal goiter include a normal or absent cervical thyroid gland, no history of prior thyroid surgery, and a lack of similar pathology in both the cervical and mediastinal portions of the thyroid. Confirmation of an ectopic thyroid gland can occur assuredly only at surgical resection if these criteria are met.¹

Secondary mediastinal goiters are a much more common clinical entity. As many as 5% to 15% of all goiters demonstrate some extension into the mediastinum. These goiters derive their blood supply from cervical branches of the superior and inferior thyroid arteries and therefore can be resected almost uniformly via a cervical collar incision. One exception to this rule, to be discussed later, is of special importance to the thoracic surgeon.

Substernal goiters are an important clinical entity for a number of reasons. Patients may eventually develop compressive or obstructive symptoms when the goiter, which is confined within the narrow thoracic inlet, begins to exert extrinsic compression on respiratory, esophageal, vascular, and/or neural structures. There is also a risk of malignant degeneration within the substernal goiter, reported to be as high as 15% to 20% in some published series.² In most situations, pathologic substernal goiter is an entity that is optimally managed surgically. Medical management in the form of thyroid suppression using exogenous thyroid hormone or radioactive iodine ablation can reduce the size of the gland by up to 20%, but these modalities are only temporizing. Delaying definitive surgical treatment of substernal thyroid goiter may allow for further growth and increase the technical difficulty of the operation and surgical morbidity. Most treating physicians express consensus supporting a surgical approach to the management of all substernal goiters.³

The thyroid gland is the first of the endocrine derivatives of the pharynx to develop and originates from the foramen cecum. It descends during the third week to reach its eventual position in the neck. It is postulated that ectopic thyroid tissue in the anterior mediastinum, pericardium, or heart originates from abnormal migration of thyroid tissue rudiments as the heart and great vessels develop in the chest, incorporating thyroid tissue into the mediastinum during embryologic unfolding.

Secondary substernal goiters tend to extend inferiorly from the neck as a result of anatomic factors facilitating downward growth into the mediastinum. The thyroid gland is limited superiorly by both the thyroid and cricoid cartilages, posteriorly by the prevertebral fascia and vertebral bodies, and anteriorly by the strap muscles and cervical fascia. Additional factors promote downward mediastinal growth of the goiter, including the negative intrathoracic pressure generated during respiration, gravity, and the downward traction that occurs during the act of swallowing. As thyroid tissue enlarges over time, it may become entrapped in the thorax and remain undiagnosed until compressive symptoms manifest.

Most substernal goiters extend anteriorly into the mediastinum (>85%), arising from the lower lobes of the thyroid or isthmus. They usually project anterolaterally to the trachea, lie anterior to the recurrent laryngeal nerves (RLNs), and tend to displace the great vessels laterally. Posterior goiters arise from the posterior aspects of the thyroid and descend posterior to the great vessels. Most posterior substernal thyroid goiters (including those arising from the left thyroid lobe) project to the right side of the thorax because the aortic arch and great vessels limit their leftward extension. Complex forms of substernal goiter are associated with more than one extension projecting into both the anterior and posterior mediastinum.

Substernal goiters are diagnosed most often in the fifth or sixth decade of life and are more common in women. Published reports describe a myriad of symptoms related to substernal goiters, including dyspnea, stridor, cough, hoarseness, dysphagia, superior vena cava syndrome, Pemberton sign which is evidence of venous engorgement of the face or neck when a patient raises his or her arms above the head, thyrotoxicosis, and Horner syndrome. However, 50% of patients are asymptomatic, with the mass found incidentally on routine physical examination coupled with either a chest x-ray (CXR) or CT scan performed for other indications. The most common symptoms attributable to substernal goiters are respiratory in nature (Table 157-1).⁴ Asymptomatic patients may demonstrate abnormal flow–volume loops on spirometry. In advanced cases, patients may have profound respiratory insufficiency. Many patients acknowledge exertional dyspnea on questioning (present in up to 60% of patients), and some have been treated for presumed asthma for years. A choking sensation with or without swallowing is also described commonly. Dysphagia may result from esophageal compression. Compression of neural structures can lead to hoarseness from transient vocal cord paralysis, permanent Horner syndrome when the cervical sympathetic chain is affected, or even less commonly phrenic nerve paralysis. If superior vena cava compression is present, patients can demonstrate Pemberton sign or even signs of superior vena cava syndrome. If Pemberton is suspected, the examiner should hold both the patient’s arms above his or her head for 1 minute and watch for distention of neck veins, facial plethora, difficulty swallowing, or worsening of respiratory status, including wheezing and stridor. Patients suffering from superior vena cava syndrome demonstrate these findings without provocative maneuvers. Compression of the carotid artery rarely can result in a transient ischemic attack.

Physical examination often identifies a cervical mass, although the lack of a cervical mass does not exclude the diagnosis of intrathoracic thyroid goiter. In published reports, up to 35% of patients lack a palpable cervical mass on examination. The presence of a substernal component of a cervical goiter is suggested when the caudal margin of the gland is undetectable on examination. Extending the patient’s neck may help to define the lower thyroid border. Displacement of the trachea may be evident if the gland is large or asymmetric. Dilated neck veins are an indication of significant blood vessel compression. Dysphonia should be evaluated and is present in up to 30% of patients. Biochemically, the incidence of hyperthyroidism varies widely in the literature from 5% to 50%. Thyrotoxicosis is seen in up to 10% of patients with a substernal goiter.

Preoperative Assessment

Radiographic assessment of a substernal goiter includes a CXR, which can demonstrate a mediastinal mass, superior mediastinal widening, tracheal deviation or compression, or all the above (Fig. 157-1). A CXR is negative in up to 30% of patients with a substernal goiter. A substernal thyroid is homogeneous, radiopaque, and smoothly contoured. Focal calcifications may be present. Past chest films may facilitate assessing the rate of growth over time.

Chest CT scans are used to define the full extent and anatomic relationships of the substernal thyroid to surrounding structures and to facilitate preoperative planning. A contrast-enhanced chest CT scan should fully characterize the extent of the thyroid gland, including continuous axial images from the neck into the mediastinum. Thyroid tissue exhibits early and prolonged enhancement after IV contrast administration. IV contrast is also helpful in defining the vascular anatomy and differentiating between blood vessels and lymphadenopathy. Nearly 90% of intrathoracic goiters have borders separated by fat planes from other mediastinal structures. If the CT scan demonstrates a posterior or complex intrathoracic goiter configuration, a careful dissection strategy should be formulated to avoid injury to the RLNs during resection.