16.1 Introduction

In 1920, William Stuart Halsted wrote “the extirpation of the thyroid gland for goiter better typifies perhaps more than other operations, the supreme triumph of the surgeon’s art.” The extension of a goiter from its original cervical position into the mediastinum, known as substernal goiter, presents unique challenges for the thyroid surgeon. The anatomical distortion, restricted surgical access of the thoracic inlet, and unpredictable vascularity can make substernal goiter surgery challenging and technically demanding. This chapter reviews the definition, clinical presentation, and surgical technique of substernal goiter with an emphasis on the appropriate preparation for achieving optimal surgical outcomes.

16.2 Definition and Classification of a Substernal Goiter

The word goiter is derived from the Latin word gutter, meaning throat, and is an enlargement of the thyroid gland. Although it has been difficult to define what constitutes a goiter (some have defined it based on varying weights or lengths), the definition of a substernal goiter is even less clear. Multiple surgeons have described the substernal goiter in different ways. Early surgeons defined the substernal goiter by position. Kocher defined a substernal thyroid as a gland in which some portion remains permanently retrosternal, and Crile defined the substernal goiter as thyroid growth down to the aortic arch. ¹ Other positional definitions include a goiter with its lower position permanently remaining below the sternal notch with the neck in hyperextension or a goiter totally or partially located in the mediastinum that in operating position has its edge at least 3 cm below the sternal manubrium. ^{2 , 3 , 4} It has also been defined radiographically as thyroid growth to the level of the fourth thoracic vertebrae on X-ray examination. ⁵ Other definitions have focused on the technique required to remove the goiter, such as one that requires mediastinal exploration and dissection for removal. ⁶ In a review of the intrathoracic goiter definitions, the clinical definition found to be most relevant defined an intrathoracic goiter as a thyroid gland that has a portion which remains permanently retrosternal on neck examination without hyperextension. ⁷ This classification can be made on physical exam and was found to be as sensitive as the other definitions for determining the presence of compressive clinical features and development of postoperative complications.

In addition, several authors have offered various classification schemes for substernal goiter in order to objectively describe the degree of substernal extension. Lahey classified substernal goiters into two grades according to the relationship to the aortic arch: grade I includes those extending nearly to the arch of the aorta, and grade II includes those extending to the arch of the aorta or beyond. ⁸ Higgins described a classification scheme based on the percentage of goiter in the chest; goiters with > 50% in the neck are substernal, those with > 50% in the chest are partially intrathoracic, and those with > 80% in the chest are completely intrathoracic. ⁹ Similarly, Cohen and Cho graded substernal goiters according to the percentage of mediastinal or intrathoracic component of the goiter (grade 1: 0–25%, grade 2: 26–50%, grade 3: 51–75%, grade 4: > 75%). ¹⁰ A more practical classification scheme with associated anatomical correlates is useful for surgical planning and is demonstrated in Table 16.1. ¹¹

16.3 Pathogenesis of Substernal Goiter

The vast majority of substernal goiters derive from a caudal migration of cervical goiters. This downward migration of the cervical goiters into the thorax has been attributed to a combination of factors, such as the negative intrathoracic pressure generated during inspiration, repetitive forces of deglutition, the effect of gravity, and the large potential space of the mediastinum. The substernal components tend to migrate anterior to the trachea, esophagus, recurrent laryngeal nerve (RLN), and subclavian vessels in 85 to 90% of the cases. ^{12 , 13} They may extend evenly on both sides or asymmetrically. Although some series reported a higher incidence of substernal extension on the left side, others have reported the opposite phenomenon. ^{6 , 14 , 15 , 16}

In approximately 15% of substernal goiters, the goiter involves the posterior mediastinum. The posterior mediastinal goiter descends behind the trachea, great vessels, and RLN. The thyroid surgeon must be aware of this ventral position of the RLN so that it may be recognized before it is cut or stretched. The nerve may also be trapped between components of a posterior mediastinal goiter. The posterior mediastinal goiter may rest in a space bounded by the azygous vein inferiorly, vertebral column posteriorly, trachea and esophagus medially, and subclavian and innominate vessels anteriorly. ^{17 , 18} Posterior mediastinal goiters are more commonly found on the right side than the left, explained by the presence of the aortic arch and descending aorta, which obstructs the posterior descent on the left side. ¹⁹

Even rarer, approximately 1% of substernal goiters are isolated to the mediastinum without any connection to the normal cervical thyroid. Several interesting theories have been put forward to explain the pathogenesis of such goiters. Some isolated mediastinal goiters may form from embryological fragmentation with hyperdescent of thyroid anlagen associated with cardiac and great vessel descent. Another theory proposes that such goiters form as exophytic nodules from the thyroid inferior pole, and over time there is attenuation of the nodule-thyroid stalk. ¹¹ Finally, there is the “forgotten goiter,” which is a nodule representing a thyroid tissue fragment in the upper mediastinum from past goiter surgery. Recognition of the isolated mediastinal goiter is critical for presurgical planning because these goiters tend to derive their blood supply from the internal mammary and innominate arteries or directly from the intrathoracic aorta. Fig. 16.1 demonstrates a forgotten goiter recognized after hemithyroidectomy over 20 years prior.

16.4 Incidence and Prevalence

Multinodular goiter is common, affecting 4% of the US population and 10% of the British population. Iodine deficiency contributes to goiter formation, especially in noncoastal mountainous and lowland regions which are especially at risk for endemic goiter. The prevalence of substernal goiters is difficult to determine based on the various definitions and since the number of non-operative substernal goiters remains largely unknown. It is estimated to be present in 0.02% of the general population and 0.05% of females older than 40, based on screening radiography in Australia and the United States. ^{20 , 21} The incidence of substernal goiter significantly increases with age; 60% of substernal goiters occur in patients older than 60. ²⁰ In surgical series, the rates of substernal goiters in patients undergoing thyroidectomy have been estimated to be between 2.6 and 21%. ^{22 , 23 , 24}

16.5 Clinical Presentation and Physical Exam

Most substernal goiters arise in the setting of a preexisting cervical goiter, grow slowly, and are infrequently malignant. Most patients present in the fifth decade or later, and there is a female preponderance, with a female to male ratio of 3:1. ²⁵ A positive family history can be present in up to 30% of patients. ²⁶ Although some patients are asymptomatic, presenting after an incidental finding on radiographic studies, others present with a palpable neck mass or with respiratory symptoms varying from a simple irritative cough to hoarseness, inspiratory stridor, or frank shortness of breath. ^{2 , 13 , 27} Acute airway obstruction is an uncommon but life-threatening emergency for benign goiters and occurs almost exclusively in patients with substernal extension. ²⁸ Fig. 16.2 demonstrates a patient with acute airway obstruction. The exact reason for the sudden onset of airway obstruction is unclear, but this has been thought to be related to sudden enlargement of the goiter due to hemorrhage, cystic degeneration, or malignant change within the substernal component. ²⁹ In addition to respiratory distress, dysphagia and globus sensation are symptoms suggestive of local compression. Every patient should be questioned about signs of dyspnea, dysphagia, and dysphonia. When the neck vasculature is compressed, superior vena cava (SVC) syndrome or even cerebral edema may occur. ³⁰ True SVC syndrome is almost exclusively associated with substernal malignancy and should be carefully radiographically assessed preoperatively. Hematemesis secondary to esophageal varices, chylothorax secondary to thoracic obstruction, and transient ischemic attack through “thyroid steal syndrome” have all been reported as initial presentations of substernal goiters. ^{28 , 31}

16.6 Physical Examination

The physical exam of a patient presenting with a substernal goiter should focus on the airway. The larynx and trachea should be assessed for deviation from the midline, typically to the contralateral side of an asymmetrically enlarged cervical goiter. It is imperative to endoscopically examine the vocal cords in all patients with a substernal goiter. Vocal cord palsy without previous surgery is suggestive of the presence of invasive thyroid malignancy until proven otherwise. Occasionally, vocal cord paralysis may occur as a result of the mass effect of the large goiters, and bilateral vocal cord paralysis in a nonmalignant substernal goiter has been reported. ³² Patients should be asked to raise both arms above the neck to elicit the Pemberton’s sign (flushing of the face, dilation of the external jugular veins, and/or symptomatic airway compression). ^{4 , 30} Other clinical features, such as Horner’s syndrome, can occur occasionally. ²⁷ The size and consistency of the thyroid enlargement should be noted, although up to 30% of patients with substernal goiters may have no palpable cervical component. ^{6 , 13} The physical exam can underestimate the goiter, especially in patients with a short neck or morbid obesity. The World Health Organization grading system for goiters can be documented (Table 16.2). Finally, the cervical lymph nodes should be adequately assessed.

16.7 Preoperative Assessment

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