Extended Versus Standard Thymectomy for Myasthenia Gravis


Myasthenia subtype

Thymectomy indicated

Quality of evidence

Recommendation

AchR thymoma

Yes

Moderate

Weak

AchR early onset generalized

Yes

Moderate

Weak

AchR early onset ocular

No

Low

Not recommended

AchR late onset

No

Low

Not recommended

MUSK

No

Low

Not recommended

LRP4

Unknown
  
seronegative

Unknown
  

AchR acetylcholine receptor, MUSK muscle-specific kinase, LRP4 lipoprotein receptor-related protein 4





Optimal Surgical Approach to the Thymus


The thymus originates from the third and fourth pharyngeal pouches which move towards the midline and subsequently descend into the mediastinum. The thymus has a typical H-shape with two cervical poles which are less developed, and two mediastinal poles which are broadly extending along the pericardium inferiorly as far as the anterior cardiophrenic recesses. A highly variable anatomy has been described which has profound surgical implications when discussing thymectomy and the most appropriate surgical approach [10]. Precise anatomical and pathological studies have demonstrated that additional ectopic thymic tissue may be discovered in 32–98 % of patients when an extended resection has been performed [10, 11]. Non-encapsulated lobules of thymus and microscopic foci of thymus may be present in the pretracheal and anterior mediastinal fat from the level of the thyroid to the diaphragm and bilaterally from beyond each phrenic nerve. Microscopic foci of thymus have also been found in subcarinal fat and in the aortopulmonary window.

Many different approaches exist to perform a thymectomy and these can be subdivided into open procedures, minimally invasive and combined interventions (Table 52.2). The different steps of these procedures have been described in detail [1214]. The specific indication discussed within a multidisciplinary setting, the experience of the thoracic surgeon and the patient’s preference will determine the ultimate choice. In case of myasthenia a “maximal” thymectomy is advised removing as much thymic tissue as possible with surrounding mediastinal fat from the cervical region to the diaphragm extending laterally to both phrenic nerves, and also incorporating the mediastinal fat in the aortopulmonary window. In this way ectopic thymic tissue will be removed to a maximal extent.


Table 52.2
Surgical approaches to the thymus

































Open procedures

 Partial median sternotomy

 Full median sternotomy

 Lateral thoracotomy

 Hemi-clam shell incision

 Clam shell incision

Minimally invasive techniques

 Cervicotomy ± sternal retractor (increases exposure)

 VATS: left, right, combined left and right

 RATS: left, right

Combined procedures

 Cervicotomy + VATS

 Cervicotomy + RATS

 Subxiphoid incision + VATS


VATS video-assisted thoracic surgery, RATS robotic-assisted thoracic surgery

In case of combined myasthenia and suspected or proven thymoma it is important to completely remove the thymic lesion. When present, its capsule should not be breached in order to avoid spilling of malignant cells into the mediastinum or pleural cavities.

Unfortunately, no randomized trials exist that compare the different surgical approaches to determine mortality, frequency of postoperative complications and evaluate long-term results. So, no high-quality evidence is available.

No doubt, sternotomy remains the current gold standard allowing an extended thymectomy by an anterior approach with complete removal of the thymus and surrounding fatty tissue, allowing opening of both pleural cavities, control of major mediastinal blood vessels and extensive dissection along the pericardium into the cardiophrenic recesses [14]. On the other hand, it requires a large incision necessitating an extensive osteotomy which may result in increased pain, higher morbidity, slower recovery and prolonged hospitalization time. However, a direct comparison of sternotomy with alternative incisions for similar patient groups is not available. A partial upper sternotomy provides good visualization of the upper mediastinum but evaluation of more caudal regions is not feasible [12]. Large mediastinal tumors invading major mediastinal structures may be approached by a lateral thoracotomy or combined incisions. The latter include hemi-clam shell or clam shell approaches, consisting of partial sternotomy combined with an anterolateral thoracotomy or bilateral anterior thoracotomy with transverse sternotomy, respectively. Especially for patients with myasthenia gravis and generalized complaints including respiratory problems, less invasive methods were developed to decrease morbidity in relation to the incision but still allowing an extended thymectomy.

By a cervicotomy it is possible to remove the entire thymus and surrounding fatty tissue, especially when a sternal retractor is used [13]. With the advent of video-assisted thoracic surgery (VATS), new methods became available to perform a thymectomy by small thoracoports. Recently, robotic-assisted thoracic surgery (RATS) was introduced providing optimal three-dimensional visualization facilitating dissection with the aid of highly flexible robotic arms [1517].

Within the surgical community there is an ongoing discussion whether a right-sided, left-sided or bilateral thoracoscopic approach is indicated to perform a complete thymectomy. In some centers combined procedures are used as VATS in combination with a cervicotomy or a subxiphoid approach, in this way avoiding a scar in the cervical region [18]. The thymus may thus be approached from different angles allowing complete removal.

Regarding the incision no definite recommendations can be made due to the lack of randomized trials. Median sternotomy remains the gold standard. So, the experience of a surgeon and a specific center but also the preference of the patient after all relevant information has been provided, will determine the final approach.

Another controversial topic remains the role of a minimally invasive approach in patients with myasthenia and suspicion of thymoma. Depending on the location and size of the thymoma recent data indicate that well-encapsulated tumors without invasion into the mediastinum or large vessels can safely be removed by a thoracoscopic or especially, a robotic technique [15, 17, 18]. Invasion in large mediastinal vessels is a contra-indication for a VATS or robotic approach. The size limit is usually considered to be 4–5 cm [19]. However, as thymomas are slowly growing tumors, long-term results are still awaited for before definite conclusions will be reached regarding long-term oncological outcome.


Extended Thymectomy vs Standard Thymectomy


The role of thymectomy in the treatment of myasthenia gravis has not been elucidated yet [20]. Until recently, variable patient selection, timing and type of surgery and analytical methods rendered the conclusions of the most important retrospective studies inconsistent. Moreover, there are no controlled prospective studies and the unique randomized trial comparing thymectomy versus non-thymectomy in patients treated by steroids is still ongoing as outlined earlier [9].

Before dealing with the extent of the thymectomy to be performed for myasthenia gravis, it is crucial to emphasize that the thymus is a functional entity not limited to the gland itself and thymic cells may be found outside the main capsule. As outlined before, surgical and anatomical studies already showed many years ago that the thymus frequently consists of multiple lobes in the neck and mediastinum, often separately encapsulated and not necessarily contiguous [21, 22]. This results in the recommendation that as much mediastinal thymic tissue as possible should be removed for the treatment of non-thymomatous myasthenia gravis. This statement is supported by a review of published papers on the results of thymectomy [4, 20, 2225]. Further proof comes from the presence of residual thymic tissue in most of the re-operations after previous transcervical and standard transsternal thymectomy, with improvement or remission of the myasthenic symptoms [26].

The original standard transsternal thymectomy used by Blalock was limited to removal of the thymic gland with its cervical and mediastinal lobes [27]. Unrelated to the surgical approach, the resection is currently more extensive than originally described and includes, at least, removal of all visible mediastinal thymic lobes and also part of the mediastinal and low cervical fat [4, 2325].

The extended transsternal thymectomy also called “aggressive transsternal thymectomy” and “transsternal radical thymectomy” consists of en-bloc resection of all fat and thymic tissue in the neck and mediastinum. Dissection starts at the inferior part of the thyroid lobes proceeding caudally to the diaphragm and extending laterally from one phrenic to the contralateral one. Removing cervical tissue by a VATS or robotic procedure starting from below without a neck incision, may result in an incomplete resection which is less radical than a transcervical and transsternal thymectomy [20, 22]. However, because it is less invasive with low morbidity, this approach has been adopted by many thoracic surgeons dealing with myasthenia gravis. Retrospective studies comparing extended thymic resections with standard thymectomy, also supported the premise that the more thymic tissue is removed, the higher the remission rate will be [20].

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Dec 30, 2016 | Posted by in CARDIOLOGY | Comments Off on Extended Versus Standard Thymectomy for Myasthenia Gravis
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