Patient selection for thymectomy begins with a referral from a neurologist managing the patient with myasthenia. The practice parameter reported by the American Academy of Neurology (Gronseth, 2000) states that "for patients with nonthymomatous autoimmune myasthenia gravis, thymectomy is recommended as an option to increase the probability of remission or improvement." The strongest association between thymectomy and an improved clinical course has been seen in female myasthenics with systemic weakness. Interested readers are encouraged to read that practice parameter and its cited primary references to guide decisions about the use of thymectomy, especially in controversial groups such as elderly patients and those with only ocular myasthenia symptoms.
|Table 1: Modified Osserman classification|
|1||ocular signs and symptoms|
|2||mild generalized weakness|
|3||moderate generalized weakness|
|4||severe generalized weakness respiratory
dysfunction or both
These patients are evaluated and given preoperative staging based on a modified Osserman classification as depicted in Table 1. Patients with generalized weakness are treated with plasmapheresis to improve their overall fitness for general anesthetic and operation. While it is common for patients to require plasmapheresis to prepare them for surgery, it is extremely uncommon for a patient to be so debilitated by myasthenic symptoms that surgery remains contraindicated after aggressive pre-operative therapy. In many patients, this pre-operative therapy also allows tapering or complete withdrawal of corticosteroids prior to surgery. Computed tomography scans of the chest are performed in order to exclude the possibility of a thymoma. It is our feeling that patients with a thymoma are best treated with a sternotomy and the transcervical approach is reserved for patients with thymic hyperplasia or normal thymus by CT scan. However, once a surgeon is comfortable with this exposure and these techniques, resection of masses or cystic lesions completely contained within the thymus is feasible. Deeb and colleagues (2001) have reported results with expanded indications for this procedure. Finally, although the authors do not consider preoperative pulmonary function testing a routine requirement, such testing may help assess risk of postoperative respiratory failure in patients with severe systemic weakness or with comorbid pulmonary conditions.
Other contraindications are relative and should be considered in light of a surgeon's experience with thymectomy and with this exposure. Previous median sternotomy will make the substernal dissection more difficult and the suspension of the patient by the sternum hazardous. Severe osteoarthritis or previous cervical spine surgery may create added risk for extreme hyperextension of the neck. Previous thyroid surgery may obscure the upper poles and create difficulty in starting the dissection.
The patient is taken to the operating room and given a general anesthetic with a single lumen endotracheal tube. The anesthetic plan should be reviewed with the anesthesiologist to reach agreement on the use of muscle relaxants. Some programs, our own included, prefer a deep general anesthetic without the use of muscle relaxants. Others prefer the judicious use of nondepolarizing muscle relaxants with careful monitoring to ensure complete reversal at the end of the surgical procedure.(Baraka, 2001) Typically the intravenous line is placed in the right arm and an arterial line is not routinely necessary. If the arterial line is used, the anesthesiologist is advised to use the left arm, as intermittent compression of the innominate artery throughout the operation may lead to false determinations of a low blood pressure. An inflatable bag is placed behind the shoulders to allow hyperextension of the neck once a general anesthetic has begun.
|Figure 1||Figure 2|
The incision is a curved incision along the natural lines of the neck. The incision is approximately 4 cms in width and reaches a distance of 2 cms above the sternal notch at the midline (Figure 1.) The caudal edge of the incision is elevated in the subplatysmal plane until the sternal notch is reached. The ligamentous insertion of the sternocleidomastoid muscles are divided with electrocautery in a vertical manner to allow improved exposure into the substernal plane (Figure 2.) The upper edge of the skin flap is elevated in the subplatysmal plane to the level of the inferior border of the thyroid gland. Gelpe retractors are used to pull apart the upper and lower skin edges and expose the deeper structures. The strap muscles are separated longitudinally along their midline raphae and the upper poles of the thymus gland are detected behind these strap muscles (Figure 3.)
Typically, the left superior pole is identified first and dissected free down towards the point at which it merges with the right superior pole (Figure 4.) The right superior pole is similarly dissected free and ligatures are placed around both upper poles to allow gentle retraction during the subsequent thymectomy. The upper poles meet just above the sternal notch and together pass into the chest, anterior to the innominate vein. With the upper poles fully freed up to the level of the innominate vein, the prethymic plane is created with blunt dissection of an examining finger into the substernal location. This plane is freed up to allow placement of the retractor (Figure 5.) The sternum is lifted upward using a specially designed sternal retractor. (Cooper thymectomy retractor, Pilling Company, Ft. Washington, PA). The sternum is retracted upward to the point at which the patient is nearly lifted off the table. The inflatable bag behind the shoulders is then deflated and the thymectomy retractor is indeed suspending the patient's chest. This allows the head and the shoulders to fall back and allows direct visualization into the mediastinum through the 4 cm incision.
|Figure 4||Figure 5|
Two Parker retractors are placed in the upper corners of the incision to provide counter-traction against the upward pull of the sternal support. These Parker retractors are affixed to the side rails of the operating table using elastic penrose drains. The operation takes place with the surgeon seated and illumination is provided with a headlight on the surgeon (Figure 6.) Once the substernal plane has been freed up, the thymus is gently retracted forward and upward to display the veins draining the thymus directly into the innominate vein. These thymic veins are doubly ligated with silk ligatures and divided. Because this vein acts as a thoroughfare into the mediastinum with multiple instruments and sponge dissectors passing through that thoroughfare, we found that electrocautery and surgical clips are insufficient for hemostasis of these thymic veins.
|Figure 6||Figure 7|
With division of all thymic veins the dissection is then carried along the posterior aspect of the thymus into the mediastinum (Figure 7.) The thymus generally remains completely encapsulated and is separated without difficulty from the pericardium. If adhesions make it necessary, it is quite possible to remove a portion of pericardium along with the gland at the site of the adhesion. As the dissection continues deeper into the mediastinum, it is quite common that the bulging pleura on either side offer some impairment in the visualization. With some coordination, the anesthesiologist can intermittently suspend ventilation for a reasonable period of time to allow better visualization deep into the mediastinum.
Using a Kitner dissector or a larger sponge dissector, it is quite easy to depress the great vessels and allow direct visualization into the aorto-pulmonary window for complete removal of the thymus in this vicinity (Figure 8.) Some direct contributing blood vessels can be encountered in the form of veins draining into the superior vena cava on the right or small branches of the internal mammary artery from the left or right. These can be dealt with using electrocautery, although care should be taken to avoid injury to the phrenic nerves. With this in mind, the patients are generally not given a paralytic agent during the course of the anesthetic in order to allow detection of the proximity of the phrenic nerves when the electrocautery is used in the mediastinum.
In most cases the thymus gland is removed in the form of a complete gland with both upper poles and both lower poles intact (Figure 9.) Careful inspection of the remaining tissue in the mediastinum is performed to identify any possible anatomic anomalies that might result in retained thymic tissue after the operation. The most common anomaly is the unanticipated location of the upper poles of the thymus behind the innominate vein. The aortopulmonary window is also a common location for thymic tissue and this area is sometimes difficult to expose with a transcervical approach. Work by Jaretzki and others have suggested the presence of ectopic thymic tissue throughout the neck and mediastinum, but the clinical significance of this assertion is unclear. Any suspicious bits of fat in the mediastinum are removed, and if necessary sent for frozen section to ascertain whether or not they contain thymic tissue. In rare cases, it will be appropriate to remove a thymus containing a small thymoma or cyst (Figure 10.)
|Figure 9||Figure 10|
A red rubber catheter is generally then placed into the mediastinum and brought out slowly after the wound has been closed with running absorbable suture. If there has been entry into either pleural space, this entry will often be enlarged to allow evacuation with this red rubber catheter under a prolonged positive pressure breath. The skin is closed with a subcuticular suture of absorbable material and steri strips. The patients are then extubated and taken to the recovery room where a follow up upright chest x-ray is performed.
The short and long term outcomes after transcervical thymectomy have been previously reported. (Calhoun, 1999) In our recent report of 100 consecutive patients there was low morbidity and no perioperative deaths. The complication rate was 8% including one seizure, one deep vein thrombosis, one myasthenic crisis and five episodes of postoperative pneumothorax. There were no phrenic nerve or recurrent laryngeal nerve injuries. The mean operative time was 104 minutes and the mean postoperative length of stay just longer than a day. In this report, there were 78 patients who had more than 12 months of follow up and were therefore available for long term analysis of the impact of transcervical thymectomy on myasthenia gravis. When a comparison was made between a neurologist's evaluation at the time of referral to the condition at most recent follow-up, the median Osserman grade improved from 3.0 at the time of referral to 1.0 after surgery. Eighty-five percent of the patients improved 1 or more Osserman grades. Sixty-three percent of the patients (49/78) improved 2 or more Osserman grades. Thirty-five percent were in complete remission with no symptoms and on no medications. Seventy-one percent or (55/78) were free of any generalized symptoms of weakness. Only 1 patient deteriorated by a single Osserman grade and 14% of the patients (11/78) experienced no change in Osserman grade after thymectomy.
In another report describing longer term outcome of patients who underwent transcervical thymectomy by the senior author of this report, similar long term results were experienced. (Bril, 1998) With a mean follow up of 8.4 years, the 52 patients experienced an improvement in Osserman grade from 2.7 at the time of referral to 0.4 at most recent follow up. The percentage of patients who improved by at least one grade was 90.4%. There was an overall remission rate of 59.6% of patients including a "complete" remission with no ongoing requirement for medications in 44.3%. Neither series experienced perioperative mortalities or occurrences of phrenic nerve injuries.
|Table 2. Comparison of Complete Remission Rates for Thymectomy|
|Calhoun, et al. 1999||Transcervical||5.3||35.0|
|Bril, et al. 1998||Transcervical||8.4||44.2|
|Masaoka, et al. 1996||Extended||5.0||45.8|
|DeFilippi, et al. 1994||Transcervical||5.0||43.0|
|Papatestas, et al. 1987||Transsternal||10.0||32.0|
|Papatestas, et al. 1981||Transsternal||5.0||24.0|
|Papatestas, et al. 1981||Transcervical||5.0||24.0|
|Jaretski, et al. 1988||Extended||3.3||46.0|
|Mulder, et al. 1989||Transsternal||3.6||36.0|
Controversy surrounds the selection of operation for patients with myasthenia gravis. Foremost of these controversies is the question of what constitutes a complete thymectomy. Jaretsky and colleagues have demonstrated variations in the distribution of thymic tissue throughout the mediastinum and the neck. The consequence of such variability is the logic that a complete thymectomy would require resection of tissue beyond that which is normally detected on gross inspection of the thymus. Masaoka also demonstrated occasional thymic rests in unusual mediastinal locations and has advocated more extensive resection. It has not yet been demonstrated whether these rests play a role in the prognosis of patients with myasthenia gravis. If indeed these microscopic locations of extra anatomic thymic tissue play a role in the pathogenesis of myasthenia gravis, then one might expect a higher complete remission rate in patients after an extended thymectomy as compared to a standard transsternal thymectomy or a transcervical thymectomy. Table 2 demonstrates a collection of papers describing clinical course after thymectomy. There appears to be no obvious benefit of the extended approach when compared to the standard transsternal approach or the transcervical approach.
Publication Date: 4-Jun-2002
Last Modified: 15-Sep-2008