The large thymus gland during childhood. Henry Gray (1918) Anatomy of the Human Body, Bartleby.com: Gray’s Anatomy, Plate 1178.
The thymus is the site for generation of mature thymic-derived lymphocytes (T cells) from lymphoid stem cells. During their development, T cells acquire antigen receptors in preparation for responding to antigenic challenges later in life. Once mature and functional, T cells leave the thymus and circulate in the blood and through secondary lymphoid tissues. The thymus develops at the eighth week of gestation and increases in size through fetal development and childhood. The process of thymic involution begins after puberty with size progressively decreasing over time.
Non-thymomatous Myasthenia Gravis
Myast henia gravis (MG) is an autoimmune condition caused by the formation of antibodies against acetylcholine receptors at neuromuscular junctions. Characteristic symptoms include weakness of skeletal muscles that increases with activity and improves with rest. The Osserman classification of myasthenia gravis (Table 6.1) is a commonly used tool to grade the severity of MG symptoms according to the distribution of affected muscles, which ranges from ocular muscles only in milder disease to generalized body weakness and even respiratory compromise during periods of crisis . Medical management is the primary therapy for MG and may include pyridostigmine bromide (an acetylcholinesterase inhibitor), immunosuppression with corticosteroids, nonsteroid immunosuppressants like cyclosporine, azathioprine and mycophenolate mofetil, immunoglobulin, and plasmapheresis in severe cases of myasthenic crisis. MG in children is rare, with an incidence of about 1.1 per million . It is more prevalent in black children and adolescents, with typical age of diagnosis around 10–12-year-olds (peri-pubertal) .
Osserman classification of myasthenia gravis.
Mild, generalized symptoms (including bulbar)
Moderate, generalized symptoms
Acute, fulminating symptoms
Alfred Blalock first described the benefits of thymectomy in adult patients with MG in 1939 , which has been confirmed by numerous adult studies. Thymectomy is currently the recommended therapy for all patients aged from puberty to 60 with generalized MG . The benefits of thymectomy in this population may include symptom reduction, decreased need for medication, and increased rates of medication-free remission for patients with MG. Enrolling a pediatric cohort for quality prospective trials is prohibitive due to the rare incidence of MG in children; many studies, therefore, tend to be small and retrospective in nature. Despite this limitation, there is sufficient evidence to suggest that thymectomy has a role in the pediatric MG population as well [7–11]. Current indications for thymectomy in patients with MG include failure of medical management or presence of a thymic mass. In a review of 479 pediatric patients who underwent thymectomy across 50 studies, 68 % saw symptomatic improvement, and 39 % were in medication-free remission at the time of follow-up .
The recommendations in children regarding timing of operation are complicated by the role of the thymus in development of the immune system. For that reason, surgical intervention is often delayed until the onset of puberty, if possible. However, for younger patients who are not responding to medical management and otherwise face-prolonged immunosuppression, thymectomy is an acceptable alternative. The literature demonstrates that children of all ages have undergone thymectomy safely and many enjoy symptom improvement without adverse outcome .
Traditionally, thymectomy has been performed via an open approach using a median sternotomy or cervical collar incision. Thoracoscopic thymectomy was first described in an adult population in 1995  and in a pediatric population in 2000 [14, 15]. As in adults, VATS thymectomy in pediatric patients does not cause any increased mortality or operative complication and does permit a comparably adequate resection. Importantly, thoracoscopy offers the advantages of improved cosmesis, reduced narcotic and oxygen requirements, decreased incidence of wound infection and pneumonia, and shorter hospital length of stay, all of which contribute to decreased overall cost [8, 12, 16–18]. This decrease in the morbidity of thymectomy may permit resection in patients at an earlier stage of disease, as there is some evidence to suggest that surgical intervention early in the disease course may result in superior symptom control and remission rates [9, 19, 20].
Both left- and right-sided thoracoscopic approaches have been described. Some advocate a right-sided approach because it allows (1) ready identification of the innominate using the vena cava as a landmark, (2) easier maneuverability of instruments in the larger right pleural cavity, and (3) greater ease for right-handed surgeons to dissect the thymus from inferior to superior [21, 22]. Others advocate a left-sided approach because it permits (1) avoidance of the vena cava and (2) easier dissection of the thymus around the left pericardiophrenic angle and aortopulmonary window with good visualization of the left phrenic nerve [23, 24]. Neither approach is clearly superior, and the laterality therefore should be selected based on patient factors and surgeon experience. Robotic-assisted thymectomy has also been described with early success and long-term outcomes pending [25–28]. A robotic approach may offer distinct advantages for minimally invasive thymectomy due to its 3D, high-definition vision system and its wristed instruments that bend and rotate allowing careful dissection in difficult access spaces such as the thoracic inlet and the neck .
A unilateral operative approach to thymectomy is widely accepted for its comparable safety and clinical outcomes [30–32]; however, care must be taken during dissection of the contralateral thymus. In particular, the contralateral phrenic nerve should be identified and dissected meticulously along with adjacent large vessels and vital structures that are at risk of injury. Further, when thymectomy is performed for MG, some advocate for the removal of all anterior pericardial fat, as this may occasionally contain ectopic thymic tissue . While this has the potential to be a challenging dissection, pericardial fat tends to be more prominent in older adults as compared to pediatric patients; children typically have little if any pericardial fat tissue to resect.
Other Indications for Thymectomy
Thymomas are neoplasms arising from thymic epithelial cells that are very rarely found in children. The incidence in children is very low; there are approximately 50 cases reported in the literature, with 62 % of cases reported in males and 7 (15 %) presenting with MG. Many patients were asymptomatic, though some experienced dysphagia or dyspnea due to mass effect and compression of nearby structures. All thymomas have the potential for malignant degeneration, and the current recommendation is complete surgical resection plus chemoradiation depending on staging . The International Thymic Malignancy Interest Group (ITMIG) has published recommendations to guide minimally invasive resection of thymic malignancies, namely, (1) a resection that includes the thymoma, thymus, and mediastinal fat; (2) dissection and visualization of the innominate vein and both phrenic nerves; (3) an access incision that is large enough to prevent specimen disruption, which should be removed in a bag and examined after removal for completeness of resection; and (4) conversion to open surgery if oncologic principles are compromised or violated .
Thymic cysts are thin-walled structures diagnosed histologically by the presence of thymic tissue in the cyst wall. They can be found anywhere along the anatomic course of embryologic descent of the thymus from the third pharyngeal pouch. On occasion thymic cysts may completely replace normal thymic tissue . Surgical resection is the primary treatment; depending on the location of the cyst (substernal versus cervical), this may necessitate a thoracoscopic- or robotic-assisted approach .
The procedure is performed under general anesthesia with endotracheal intubation. A double-lumen tube may be utilized to assist with ipsilateral lung collapse but is not necessary. The patient is positioned at 30–45° from horizontal using a roll under the back and lateral chest with the ipsilateral right arm raised above the head (Fig. 6.2). All pressure points should be padded appropriately and the endotracheal tube position confirmed after patient positioning is complete. The surgeon and assistant stand together on the side selected for the approach facing a monitor that is placed above the patient’s contralateral shoulder. Three 5-mm ports are placed inferior to the axilla: one in the fourth intercostal space of the anterior axillary line (camera), one in the second-third intercostal space of the midaxillary line (instrument), and one in the fifth-sixth intercostal space of the midaxillary line (instrument). Care should be taken to avoid violating mammary tissue (Fig. 6.3). Equipment may include a 30° telescope, tissue-grasping forceps, endoscopic scissors or hook cautery, LigaSure™ (Covidien, Mansfield, MA) or energy device, endoscopic clips, endokittner, and specimen retrieval bag.
Port positions for thymectomy. We start with three 5-mm ports, and the center port can be upsized to 10 mm or utility incision as needed to extract the specimen . Reprinted with permission from Springer.
Insufflation is achieved with CO2 to a pressure of 6–10 mmHg. Dissection begins at the inferior margin of the thymus, superior to the heart, and anterior to the phrenic nerve. Caution should be taken using energy devices in close proximity to the phrenic nerve, as heat spread and resulting injury may occur. Using a combination of blunt and sharp dissection parallel to the phrenic nerve and posterior to the thymic gland, the thymus is lifted away from the surface of the heart and great vessels. Anteriorly, the mediastinal pleura is incised, and the thymus is dissected free from its retrosternal attachments (Fig. 6.4). The dissection is carried inferiorly to superiorly, elevating the gland and ultimately exposing the vasculature. The thymic branches off the internal thoracic artery (thyrocervical trunk) originate cephalic and lateral to the gland, while the thymic vein branches drain posteriorly to the innominate vein. Blood vessels should be isolated and divided between clips or cauterized with a LigaSure™ energy device depending on surgeon preference. Next, the thymus is bluntly dissected from its contralateral pleural attachments taking care to avoid injury to the phrenic nerve. This allows the entire gland to be flipped superiorly to allow careful dissection off the innominate vein, freeing the thymic tissue from the great vessels. The remaining attachments in the cervical area, which may include tissue that joins the thymus and inferior thyroid, are separated; this may be assisted by inferior retraction of the superior horns of the thymus (Fig. 6.5).