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 otolaryngology head and neck surgeons. Goiters account for as many as 10–15% 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 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 5–15% 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. The 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 eventually will 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. Also, there is a risk of malignant degeneration within the substernal goiter, reported to be as high as 15–20% 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 in nature. Delaying definitive surgical treatment of substernal thyroid goiter will increase the technical difficulty of the operation and surgical morbidity. Most treating physicians express consensus regarding a surgical approach to the management of all substernal goiters.³

ANATOMY AND PHYSIOLOGY

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 and 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.

GENERAL PRINCIPLES

Substernal goiters are diagnosed most often in the fifth or sixth decade of life and are more common in women. Published reports describe myriad symptoms related to substernal goiters, including dyspnea, stridor, cough, hoarseness, dysphagia, superior vena cava syndrome, Pemberton's sign, thyrotoxicosis, and Horner's syndrome. However, 50% of patients are asymptomatic, with the mass found incidentally on routine physical examination coupled with either a chest x-ray or CT scan performed for other indications. The most common symptoms attributable to substernal goiters are respiratory in nature⁴ (Table 133-1). 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 by patients. Dysphagia may result from compression of the esophagus. Compression of neural structures can lead to hoarseness owing to vocal cord paralysis that may be transient, permanent Horner's 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's sign or even signs of superior vena cava syndrome. Pemberton's sign is evidence of venous engorgement of the face or neck when a patient raises his or her arms above the head. If 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 chest x-ray, which can demonstrate mediastinal mass, superior mediastinal widening, tracheal deviation or compression, or all the above (Fig. 133-1). It 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 afford the opportunity to assess the rate of growth of the goiter 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 material administration, and IV contrast material is helpful to define the vascular anatomy and differentiate 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 recurrent laryngeal nerves during resection.

While ultrasound is useful for defining anatomy in the neck, it cannot define substernal or posterior thyroid extensions. MRI adds little to preoperative assessment and planning over high-resolution CT scanning. Fine-needle aspiration of thyroid nodules is not necessary for preoperative planning. Malignancy may be missed, and the information does not change the operative approach. The decision to operate is not based on cytopathology.

Evidence of preoperative vocal cord palsy/dysfunction or extracapsular malignancy noted preoperatively in the neck generally mandates preoperative collaboration with an otolaryngologist-head and neck surgeon. In these situations, it is of utmost importance to preserve the function of the remaining RLNs to avoid the need for tracheostomy postoperatively. Even in patients who present with severe airway compromise and stridor preoperatively, tracheostomy generally is avoided because it does not alleviate the lower tracheal compression that is present, is often technically challenging, and raises the risk of definitive operation. In most of these patients, endotracheal intubation is successful. A preoperative serum thyroid-stimulating hormone measurement should be obtained. If hyperthyroidism is present, medical management with antithyroid medications and beta blockade, as needed, should be undertaken before elective resection.

Pulmonary functiong testing, including spirometry and flow-volume loops, is useful as part of the preoperative evaluation. This testing is not done, however, in patients with severe airway compromise. The presence of abnormalities in flow-volume loops is corroborative evidence of the need for resection in patients with substernal thyroid goiters because abnormalities are indicative of, at least, moderate compromise of airway anatomy by the substernal goiter. Flow-volume loops will demonstrate derangement of the inspiratory phase, suggesting intrathoracic obstruction, or a combined defect, suggesting a fixed obstruction at the thoracic inlet. Postoperative reversal of pulmonary function testing is often not immediate. A prolonged recovery is seen in those with the most severe preoperative airway obstruction.

Surgeons should discuss these patients with the anesthesiologist in advance of surgery, and an adequate anesthetic plan should be formulated before the operation. There is often a significant amount of anxiety and a variable level of comfort of anesthesiologists in regard to airway management of these patients. Close communication between the attending surgeon and the anesthesiologist will lessen the chances for an adverse airway issue to occur on induction of anesthesia.

Indications for Resection

The presence of a substernal goiter (with or without associated symptoms) is an indication for removal. Since mediastinal goiters eventually will cause respiratory problems, resection is indicated if the patient is medically suitable to undergo an operation. These tumors cannot be managed medically with thyroid replacement or radioactive iodine suppression, nor can their growth be followed easily over time or translated readily into degree of anatomic physiologic derangement. Surgery is also indicated in cases of unsuccessful medical therapy (i.e., progressive enlargement of cervical thyroid goiters), thyrotoxicosis in goiters with substernal extension, and to relieve symptoms or effects of extrinsic compression on vital structures, including the trachea, esophagus, nerves, and vessels. Furthermore, surgery will prevent potential airway obstruction and treat overt (abnormal fine-needle aspiration biopsy) or subclinical thyroid malignancy, reported to be as high as 20%.

Substernal thyroid goiters become increasingly difficult to extirpate if resection is delayed until compressive symptoms arise. There is also a small risk of spontaneous or traumatic hemorrhage into a substernal goiter resulting in acute or complete airway compromise, which could be fatal owing to the fixed space of the thoracic inlet and superior mediastinum. Therefore, all patients with a substernal goiter, unless medically unfit for any procedure, should be offered resection and a chance for cure.

SURGICAL TECHNIQUE

The options for surgical approach to a substernal goiter include collar incision, median sternotomy, and thoracotomy (Fig. 133-2). Most reviews confirm that most substernal goiters (>90%) can be removed via a collar incision exclusively.^5–8 Exceptions that may necessitate median sternotomy (partial) or thoracotomy (posterolateral or anterior) include a primary retrosternal goiter, atypical anatomy, dense adhesions from prior neck or thyroid surgery, inability to deliver the gland into the neck, overt or extracapsular extension of malignancy in the mediastinum, recurrent intrathoracic goiter, goiters that extend to the tracheal carina or pose significant life-threatening compression of mediastinal structures, and patients who develop significant mediastinal bleeding intraoperatively⁶ (Table 133-2). In certain circumstances, a thoracoscopic approach to an ectopic thyroid mass may be considered if deemed accessible and safe by the operating surgeon, and the details of this approach have been reported previously.¹

A total or near-total thyroidectomy is performed. Prophylactic antibiotics are administered intravenously. The surgeon should be present in the OR on induction of anesthesia in the event of complete airway obstruction. Patients with significant airway compression or deviation should undergo awake, fiberoptic intubation with variable sedation. Tracheostomy is rarely needed.⁹

The patient is positioned supinely with the arms tucked at the sides. A transverse shoulder roll is positioned behind the upper chest to extend the neck. Alternatively, an inflatable bladder is positioned behind the upper chest, which can be used to extend (when inflated) or flex (when deflated) the neck during various portions of the operation (Fig. 133-3). The anterior neck, chest, and upper abdomen are prepped in their entirety and draped using Ioban to keep towels in appropriate position. Access for complete median sternotomy, if necessary, is achieved. The reverse Trendelenburg position is used to diminish venous congestion.

A collar (Kocher) incision approximately 2 cm above the sternal notch is used for initial exploration of thyroid goiter (Fig. 133-4). The platysma is divided using electrocautery, and skin/soft tissue flaps deep to the durable platysma are developed superiorly and inferiorly to the level of the hyoid and sternal notch, respectively. Exposure extends to the sternocleidomastoid muscles bilaterally. The strap muscles are exposed for their length and divided in their midline to expose the thyroid capsule. Frequently, the initial position of the neck in extension elevates the substernal goiter from an intrathoracic position to a predominantly extrathoracic position, allowing complete extirpation of the thyroid gland through a collar incision alone, without the need for an additional incision, counterincision, or sternotomy.

Once access to the thyroid has been completed and exposure optimized, mobilization of the gland is facilitated by elevating the intrathoracic portion of the thyroid and by using blunt finger dissection circumferentially around the gland immediately on the capsule of the thyroid inferior to the middle thyroid vessels. Blunt dissection on the posterior aspect of the manubrium is performed. Some advocate the use of several large sutures (1 or 0 PROLENE noncutting sutures in a figure-of-eight pattern) for traction as the mediastinal portion of the gland is mobilized, whereas others describe the use of a sterile spoon to aid in goiter delivery into the neck (Fig. 133-5). If additional exposure is needed, the strap muscles or medial portions of the sternocleidomastoid muscles can be divided transversely.

Ligation of the superior and middle thyroid vessels is next accomplished as close to the thyroid capsule as possible. This procedure should be begun on the smaller or most straightforward side of the goiter. Once the superior and middle vessels are ligated, the lateral thyroid is bluntly and sharply medialized by the development of areolar planes along the sides of the goiter. Care should be taken to identify and preserve, if possible, the superior parathyroid glands because the inferior glands may be more difficult to identify. The recurrent laryngeal nerve is identified and preserved. Only careful retraction is used during this portion of the procedure to avoid injury.

The inferior thyroid vessels are ligated close to the thyroid capsule, and the inferior parathyroid glands are identified, if possible. It is not uncommon for parathyroid glands to be removed inadvertently or devascularized during the mobilization of large substernal goiters, especially the bilateral inferior glands. If parathyroid glands are identified and appear to have been injured, incidentally removed, or devascularized, autotransplantation of the minced gland into the sternocleidomastoid mass should be performed. A small frozen section of the presumed parathyroid tissue is obtained before autotransplantation to ensure that parathyroid gland and not lymph node or cancer is being autotransplanted. Once the goiter has been mobilized to the midline, sharp dissection is used to release its attachment to the trachea to avoid inadvertent tracheotomy or airway fire (Fig. 133-6).

There will be times when an intrathoracic extension of a thyroid goiter cannot be delivered safely into the neck via a collar incision. This may not be obvious during preoperative planning or before an attempt at resection is made. A partial (manubrial split) or full sternotomy or thoracotomy may be necessary. Incision choice is a clinical decision dictated by the anatomy of the remaining mediastinal component and should be evident from preoperative scanning coupled with the intraoperative findings and limitations. Primary ectopic thyroid goiters recurring after previous thyroid surgery tend to parasitize mediastinal blood vessels, making cervical resection more hazardous. If a thoracoscopic approach is undertaken, resection is accomplished with isolation and ligation of the arterial supply, safe dissection, and identification of the phrenic and/or vagus nerves. The tumor should be removed en bloc and should only be cautiously grasped directly with instruments to ensure complete resection.

Hemostasis is achieved. Divided muscles are reapproximated. Soft suction drains sometimes are placed at the discretion of the surgeon at the conclusion of the procedure. The incision is closed anatomically in layers. Steri-Strips and a sterile nonconstricting dressing are applied.

POSTOPERATIVE COMPLICATIONS

Mortality should occur in fewer than 1% of patients with appropriate preoperative planning, management of the airway, and good surgical technique. Length of stay for an uncomplicated procedure is overnight, and patients can be discharged uneventfully with calcium or calcitriol supplementation as dictated by laboratory values and/or clinical symptoms of hypocalcemia. If a thoracotomy or sternotomy is required, length of stay is increased, although morbidity is not affected.

Occasionally, postoperative hemorrhage has been reported, necessitating a return to the OR for control and hematoma evacuation. The major complications after thyroidectomy for substernal goiter involve injury to the trachea, parathyroid glands, or recurrent laryngeal nerves. The length of stay for patients with marked intrathoracic extension is increased, as is the need for prolonged intubation. Historically, tracheomalacia was reported as a significant problem in patients with long-standing tracheal compression, although this appears to be a rare phenomenon (0.001–1.5%). When present, tracheomalacia may result in an extended period of postoperative intubation. One study reported that 10% of patients required extended intubation for airway issues postoperatively, with all patients eventually being extubated by postoperative day 10. However, these patients required multiple attempts at intubation preoperatively, making their need for prolonged postoperative intubation harder to simply attribute to tracheomalacia, as some authors have done. Postoperative airway complications are more likely to occur in older patients with large goiters (>200 g) who demonstrate significant tracheal compression on preoperative imaging. The need for tracheostomy is rare, and it should be performed only in cases of tracheal infiltration by an undifferentiated cancer or when it can be expected that there is bilateral vocal cord paresis postoperatively. This typically occurs when a patient has a known preoperative unilateral vocal cord palsy from prior thyroid surgery and intraoperative dissection has placed the remaining RLNs at risk.

Transient and permanent hypoparathyroidism has been noted to occur postoperatively in fewer than 10% and 2% of cases, respectively. This is the most frequent complication after total thyroidectomy and is more likely to occur when the goiter is extensive, displacing normal anatomic landmarks and requiring marked dissection for gland resection. There is a higher risk for hypoparathyroidism after resection of intrathoracic goiter compared with removal of a cervical goiter alone because the lower parathyroid glands tend to be at increased risk. These glands are situated along the thyrothymic ligament and can be devascularized more readily or even not identified at all. This complication can be minimized by careful and meticulous dissection on the thyroid capsule and performing autotransplantation when the viability of the parathyroid gland appears marginal. The postoperative use of oral and IV calcium as needed, as well as calcitriol, can avoid prolonged hospitalization for this complication. Difficult cases of hypoparathyroidism should be managed with the help of an endocrinologist, who will taper these medications appropriately. In most cases, transient hypoparathyroidism from parathyroid stunning will resolve by 6 months postoperatively.

Transient unilateral vocal cord paralysis has been reported to occur after substernal goiter resection in up to 4% of patients and, if suspected postoperatively, warrants consultation with otolaryngology. Permanent RLN paralysis should occur in fewer than 2% of patients with careful surgical technique and dissection. This is more likely to occur in patients with large or extensively intrathoracic goiters or those with extracapsular extension of malignancy.

SUMMARY

Substernal goiters typically are benign neoplasms that can be removed via a cervical collar incision. Patients present with a wide variety of symptoms, ranging in severity from asymptomatic to stridorous requiring emergent intubation. Symptoms experienced by patients are a function of the external compression of vital structures such as the trachea, esophagus, superior vena cava, or nerves such as the RLNs, phrenic, or cervical sympathetic chain. All patients with substernal goiter should be evaluated for resection if they are medically fit to undergo a surgical procedure because there is no effective medical treatment, and delay will only increase the morbidity and difficulty of the operation. Preoperative anatomic characterization of the tumor by CT scan is required for optimal surgical planning. Fine-needle aspiration of the lesion is not warranted because many tumors possess small foci of cancer that likely would be missed. Certain characteristics and intraoperative findings related to the tumor may necessitate sternotomy or thoracotomy for safe resection. For example, patients with tumors found to have large posterior extensions that are difficult to visualize, who are being reoperated for a thyroid malignancy, who have a primary ectopic tumor, or who are known preoperatively to have extracapsular extension of their malignancy are more likely to need a thoracic incision to remove the tumor safely. RLN palsy and hypoparathyroidism are complications that may ensue and may warrant close collaboration with otolaryngology or endocrinology to optimize their quality of life postoperatively.

EDITOR'S COMMENT

Although most large thyroid glands can be removed through a cervical incision, thyroid glands with their dominant girth within the thorax will likely require at least partial sternotomy. Also, a feature of thyroid tissue is that it avidly retains intravenous contrast; an imaging characteristic that can be useful for both diagnostic identification and surgical planning.

–SJM

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