Substernal Goiter - Causes, Symptoms, Diagnosis, and Complications

Introduction:

Goiters may grow downwards into the thoracic cavity leading to a condition known as substernal or retrosternal goiter. The cause of substernal goiters is the same as that of cervical or simple goiters. These goiters remain asymptomatic for many years and are often incidentally diagnosed during a routine examination. The symptoms of substernal goiter do not greatly vary from cervical goiter, but they may become severe because of its location (within the chest cavity). In the case of substernal goiters, hypothyroidism, and hyperthyroidism can coexist and may have their respective signs and symptoms.

What Is a Goiter?

A goiter is a neck lump caused by thyroid gland enlargement. The thyroid is a tiny endocrine gland at the front of the neck that is vascular. It consists of two lobes on either side of the windpipe (trachea) beneath the voice box. The isthmus, a bridge of tissues, connects these lobes. The thyroid's principal purpose is to create hormones, referred to as thyroid hormones. The thyroid gland is normally not visible. However, with goiter, the gland enlarges and develops an easily noticeable lump. Iodine deficiency is responsible for over 90 percent of goiter occurrences worldwide. Most goiters are benign (non-cancerous), and only a few are cancerous.

What Is Substernal Goiter?

When the swelling or lump in the neck (goiter) extends into the thoracic cavity, it is known as a substernal goiter. The thoracic or chest cavity is a hollow area enclosed by the ribs, vertebral column, sternum (the t-shaped vertical bone that forms the anterior part of the chest wall), and diaphragm. A goiter is considered a substernal goiter when more than 50 percent of its total volume extends into the chest cavity. The goiter grows inferiorly and enters the thoracic cavity via the thoracic inlet. Substernal goiters result from the downward growth of the normal goiter (cervical goiter), with the primary blood supply remaining in the neck, mainly from the inferior thyroid artery. Substernal goiters may progress slowly with time and remain asymptomatic for a long time. The condition generally does not cause any pain unless the size of the goiters becomes extremely big and it starts compressing the adjoining structures, tissues, nerves, and vessels. Substernal goiters do not necessarily grow outwards, but they can develop inwards, towards the back portion of the neck, and down into the chest.

What Is the Location of Substernal Goiter?

• Substernal goiters primarily grow and descend into the antero-superior mediastinum (the area present between the lungs). This may lead to the displacement of the great vessels present near the capsule of the gland.

• Substernal goiters develop downwards on one or the other side of the windpipe. This leads to tracheoesophageal deviation to the opposite side of the growth.

• Substernal goiters that are centrally located may compress the windpipe (trachea) between the sternum and the spine, along with displacement of the food pipe (esophagus).

• In rare cases, substernal goiters can grow in the posterior portion of the mediastinum (area between lungs) and behind the windpipe. These goiters cause more severe complications.

What Are the Other Names of Substernal Goiter?

Substernal goiters are often also referred to as:

• Retrosternal goiter.

• Cervicothoracic goiter.

• Intrathoracic goiter.

• Endothoracic goiter.

• Mediastinal goiter.

How Rare Is Substernal Goiter?

• The exact rate of occurrence of the disease is unknown because the condition is rare, and not enough cases of substernal goiters have been reported and studied.

• However, one study suggested that the incidence rate of substernal goiters among patients with thyroid goiters is estimated to be between 5 to 15 percent.

• A recent surgical series suggest that around 7 percent of the patients who underwent thyroid surgery had substernal goiters.

• The common age of onset is after 50 years.

• Substernal goiters are four times more common in women as compared to men.

What Causes Substernal Goiter?

The primary cause of substernal goiter is the same as that of goiters in general. Goiters can occur due to iodine deficiency in the diet, which is more prevalent in developing countries and people with low socioeconomic status. Goiters can also occur through the consumption of certain substances known as goitrogens. These goitrogens block the production of thyroid hormones leading to the development of a goiter in the gland. These goitrogens can be found in vegetables like cabbage, turnip, kale, etc. Goiters can also develop due to the following causes:

• Long-standing multinodular goiters are the most common cause of substernal goiters. These multinodular goiters grow over several years and into the thoracic cavity.

• Congenital hypothyroidism is a congenital disability in which the baby can not produce thyroid hormone.

• Pituitary diseases can cause over-secretion of the thyroid-stimulating hormone from the pituitary gland, which can cause goiters.

• Fluid-filled lumps, often called nodules, develop on both sides of the thyroid gland resulting in overall enlargement.

• Inflammation of the thyroid gland, known as thyroiditis, can also lead to the development of goiters.

When these cervical goiters grow downwards into the thoracic cavity through the thoracic inlet, it causes the development of substernal goiters. In extremely rare cases, ectopic thyroid tissues in the chest can cause new substernal goiter formation in the thoracic cavity.

What Are the Symptoms of Substernal Goiter?

An individual suffering from substernal goiter may remain asymptomatic for a long time. Around 15 to 50 percent of the patients are asymptomatic. When the goiter grows extensively large, it can cause multiple compressive symptoms. In addition, the location of substernal goiter can lead to compressive symptoms. The common symptoms of substernal goiters include:

• Difficulty in breathing after exertion.

• Horseness of voice (due to compression of the recurrent laryngeal nerve).

• Whistling sound while breathing (stridor).

• Cough and wheezing.

• Discomfort due to the presence of thyroid mass.

• Chocking sensation.

• Difficulty in swallowing due to compression of the esophagus (windpipe).

• The jugular vein becomes more prominent.

• Presence of Pemberton’s signs- During physical examinations, the patients are asked to raise both of their hands, so the middle part of the arm touches the lateral side of the head, and they are asked to hold this position for a minute. In the case of substernal goiters, there will be congestion of the face, facial flushing, stridor, bulging of the neck veins, dyspnea, etc. A positive Pemberton sign suggests thoracic inlet compression.

How Are Substernal Goiters Diagnosed?

Substernal goiters remain asymptomatic for a long time. Therefore, incidental finding of substernal goiter during routine examinations like x-ray or chest scans is a common incident. In addition, the diagnosis of substernal goiter is made in the following ways:

• Medical History: A detailed medical history, family history, and patient physical examination should be made.

• Blood Examinations: The levels of thyroid-stimulating hormone (TSH), free T3 (triiodothyronine), and free T4 (thyroxine) levels are measured to check the thyroid functioning of the gland.

• Thyroid Ultrasound: Thyroid ultrasonography is done to assess only the part of the goiter that is present outside the thoracic cavity, which is the cervical component of the goiter. A thyroid ultrasound cannot assess the part of the goiter inside the thoracic cavity because the ultrasonic waves cannot penetrate bones and do not flow well through the air in the lungs.

• CT Scan (Computed Tomography) or MRI (Magnetic Resonance Imaging): These are the preferred methods of imaging tests used to diagnose substernal goiters. CT scans and MRIs accurately assess the size, structure, extent, and location of the substernal goiter in the thoracic cavity. These imaging tests also evaluate the effect of a substernal goiter on the surrounding structures, tissues, vessels, and nerves. The compression of the windpipe and food pipe can also be easily determined.

• Barium Esophagography: Upper endoscopy is done in case of dysphagia due to substernal goiter. This test may help demonstrate extensive deviation and compression of the food pipe due to the goiter.

How Are Substernal Goiters Treated?

The treatment of substernal goiter includes:

• Treatment of hypothyroidism or hyperthyroidism with the help of hormone replacement therapy and anti-thyroid drugs, respectively.

• In case of asymptomatic substernal goiter and absence of any compressive symptoms, there may be no requirement for surgical treatment, and only follow-up and monitoring of the goiter is recommended. A close watch should be kept to determine any changes in the size of the goiter, thyroid functioning, and diameter of the windpipe. In this approach, the patient is also asked to avoid iodine in their diet to avoid the development of iodine-induced hyperthyroidism.

• The absolute treatment of symptomatic substernal goiter is surgery. When windpipe narrowing exceeds 35 percent, surgical intervention is recommended. Complete surgical removal of the thyroid gland or total thyroidectomy is preferred. A cervical approach is used in most cases, but in rare cases, an extra cervical approach is also used. An extra cervical approach may involve partial sternotomy or thoracotomy (these are surgical processes to gain access to the organs of the chest or the heart and its surrounding vessels).

• After complete thyroidectomy, the patient is kept on thyroid replacement therapy lifelong. A dose of Levothyroxine 1.6 mcg/ kg (micrograms per kilogram) body weight is recommended.

What Are the Possible Complications of Substernal Goiter?

The possible complications of substernal goiter are:

• Compression of the food pipe, windpipe, veins, vessels, and adjoining structures in the thoracic cavity.

• A substernal goiter hemorrhage can result in rapid thyroid tissue enlargement and airway constriction. An upper respiratory tract infection can exacerbate upper airway obstruction symptoms.

• Thyroid cancer.

• Paralysis of the phrenic nerve.

• Horner syndrome (due to compression of cervical nerves).

• Blockade of the jugular vein with blood clots (thrombosis).

• Superior vena cava syndrome (group of disorders occurring due to slowing down of the blood flow through the superior vena cava).

Conclusion:

The symptoms of substernal goiters appear gradually, and the diagnosis is usually incidental. Substernal goiters can be hidden in the chest cavity for a long time. Therefore, they can grow and become very large before being noticed. While the symptoms develop slowly and gradually, more severe symptoms like airway obstruction can develop abruptly. Substernal goiters are generally noncancerous, but in very rare cases, they can be associated with the development of thyroid cancers.