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Video-assisted thoracic surgery: thymectomy for myasthenia gravis

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Abstract

The description of the video-assisted thoracic surgery: thymectomy for myasthenia gravis covers all aspects of the surgical procedure used for the management of myasthenia gravis.
Operating room set up, position of patient and equipment, instruments used are thoroughly described. The technical key steps of the surgical procedure are presented in a step by step way.
Consequently, this operating technique is well standardized for the management of this condition.

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Publication

2002-04

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E-publication

WeBSurg.com, Apr 2002;2(04).
URL: http://www.websurg.com/doi-ot02en260.htm

Video-assisted thoracic surgery: thymectomy for myasthenia gravis

1. Introduction

Thymectomy is now an established therapy in the management of generalized myasthenia gravis (MG) in conjunction with medical treatment. Although a randomized prospective study comparing thymectomy to medical treatment alone has never been undertaken, a recent meta-analysis of 28 controlled studies showed that MG patients undergoing thymectomy were twice as likely to attain medication-free remission, 1.6 times as likely to become asymptomatic, and 1.7 times as likely to improve (Gronseth and Barohn, 2000). While several surgical approaches are available, video-assisted thoracic surgery (VATS) represents a new, minimally invasive approach to thymectomy with additional benefits to patients with MG over the other approaches.

2. Anatomy

• General anatomy

The thymus is embryologically derived from the 3rd and 4th branchial pouches. It weighs 10 to 35 g at birth, grows to 20 to 50 g during puberty and after that, slowly involutes to 5 to 15 g in the adult.
With the process of involution, the thymic parenchyma is gradually replaced by fibrous adipose tissue. The fully developed gland is bilobed, resembling a H-shaped configuration, but its exact shape is largely moulded by adjacent structures and is highly variable.
The thymus occupies the anterior mediastinum, with its superior horns often extending into the neck, lying deep to the sternothyroid muscle. The body of the gland is related:
- anteriorly to the sternum and the upper four costal cartilages;
- posteriorly to the pericardium, the ascending aorta, the brachiocephalic veins and superior vena cava (SVC);
- laterally with the mediastinal pleura.
Its relationship with the veins is of great surgical importance. Its fibrous capsule merges with the pretracheal fascia.
1. Thymus
2. Internal mammary vein
3. Pulmonary artery
4. Superior pulmonary vein
5. Left bronchus
6. Inferior pulmonary vein

• Vascular supply

The arterial supply is derived laterally from branches of the internal mammary artery and venous drainage is through two to three tributaries posterior to the left brachiocephalic vein.
1. Inferior thyroid artery
2. Right primitive carotid artery
3. Subclavian artery
4. Subclavian vein
5. Right internal mammary artery and vein
6. Thymic artery
7. Left primitive carotid artery
8. Inferior thyroid vein
9. Left subclavian artery
10. Left brachiocephalic venous trunk
11. Vagus nerve
12. Aorta

3. Preop period

• Preoperative management

Myasthenia gravis causes weakness of voluntary muscles, including those involved in breathing, so patients are at risk of developing postoperative respiratory failure. If bulbar palsy is present, they may also develop aspiration pneumonia. Medical treatment is associated with its own complications. Anticholinesterase treatment increases vagal tone, enhances oral secretion, and potentiates laryngeal spasms. Prolonged steroid use can result in electrolyte imbalance and increased susceptibility to infection.
Before elective surgery:
- distribution and severity of muscle weakness should be carefully assessed;
- respiratory function and nutritional status should be documented and medical treatment optimized.
Patients with severe weakness may require preoperative plasmapheresis, together with steroid and anticholinesterase therapy.
Admission to the intensive care unit for ventilatory support is indicated for patients with respiratory failure, but it is not necessary to wait until the patient is extubated before surgery can proceed.
Intravenous immunoglobulin is an alternative to plasmapheresis, but there is no clear evidence that one is better than the other.
Patients should be warned of the possibility of postoperative ventilation.
Premedication is appropriate, but respiratory depressant drugs are avoided.
“Stress” doses of steroids may be required.

• Anesthesia

General anesthesia with a left-sided double-lumen endobronchial tube is used and confirmed with a fiberoptic bronchoscope. Selective one-lung ventilation to the left lung is required to facilitate the operation (Yim et al., 1999). Hypoxemia during one-lung ventilation is usually caused by shunting of blood. In case of hypoxemia, the position of the double-lumen endobronchial tube and hemodynamic stability should be confirmed. A low level of continuous positive airway pressure (CPAP) applied to the collapsed right lung may improve saturation. Applying positive end expiratory pressure (PEEP) to the ventilated lung can also raise oxygen saturation during one-lung ventilation (Low, 2000).
Patients with myasthenia gravis are usually more susceptible to the neuromuscular blocking effect of volatile anesthetics so nondepolarizing muscle relaxants are usually not required (El-Dawlatly and Ashour, 1994).
Patients with MG are usually also very sensitive to nondepolarizing muscle relaxants (Smith et al., 1989; Nilsson and Meretoja, 1990).
If muscle relaxation is necessary during the course of anesthesia, a reduced dose of an intermediate-acting nondepolarizing muscle relaxant should be used followed by a carefully titrated intravenous infusion. Monitoring neuromuscular transmission is mandatory to adjust the dose of muscle relaxant used and to ensure complete reversal of neuromuscular blockade after surgery (Baraka, 1992).

• Continuous monitoring

- electrocardiogram, non-invasive blood pressure, pulse oximetry;
- end-tidal CO2, airway pressure, ventilatory volume, inspired oxygen, and neuromuscular transmission.
An arterial line and a central venous catheter for invasive pressure monitoring may be required for coexisting medical conditions.

4. Operating room set-up

• Patient

- full left lateral decubitus position;
- flexion of the operating table to 30° to open up the intercostal spaces.

• Team

The team remains in the same position during the procedure.
1. Surgeon
2. Assistant
3. Scrub nurse
4. Anesthesiologist

• Equipment

1. Operating table: flexed at 30° just inferior to the level of the nipples, to open up the upper intercostal spaces for thoracoscope insertion and instrumentation (Yim, 1995).
2. Anesthetic unit
3. Video-thoracoscopy unit (monitor, video image printer, video recorder, light source)
4. Monitor
5. Electrocautery
6. Instrument trolley

5. Basic principles

The chest is the most suitable body cavity for the minimal access approach, because once the lung is collapsed (with selective one-lung ventilation), there is plenty of room for instrument maneuvering. The use of CO2 insufflation and valved trocars is therefore unnecessary. In fact, there is evidence that thoracic CO2 insufflation during VATS has an adverse effect on the patient’s hemodynamics compared with selective one-lung ventilation (Brock et al., 2000).
There are additional strategies in VATS that can help minimize chest wall trauma and hence postoperative pain:
1) avoiding the use of trocars by introducing instruments directly through the incisions;
2) avoiding torquing of the thoracoscope by visualizing with an angled lens (30° scope);
3) using smaller telescopes (5 mm) when clinically allowed;
4) delivering specimens through the anterior trocar because the anterior intercostal spaces are wider (Yim, 1995).
We advocate a right-sided approach because:
- the superior vena cava is a clear landmark;
- it facilitates dissection of the brachiocephalic vein junction;
- there is an ergonomic advantage for right-handed surgeons (start dissection at the inferior pole and work cephalad).
Under general anesthesia, selective one-lung ventilation should be confirmed with the anesthesiologist prior to trocar insertion.

6. Trocar placement

The 3-trocar technique is utilized for the procedure:
- one thoracoscope trocar is inserted in front of the tip of the scapula along the posterior axillary line for a 0° telescope:
- two instrument trocars are inserted under direct thoracoscopic vision:
- in the 3rd intercostal space, midaxillary line,
- in the 6th intercostal space, anterior axillary line.

Additional trocars are inserted for lung retraction as necessary. The trocar sites should be at suitable distances from the target lesion to provide space for manipulation. Furthermore, the instrument and camera trocars should be sufficiently far apart in a “triangulation” manner to prevent instrument “fencing”, and should be within the same 180° arc to avoid mirror imaging.
In young female patients, instrument trocars should be strategically placed in the inframammary fold for cosmetic considerations.

7. Instruments

• Optical device

1. 0° thoracoscope

• Operating instruments

We use mostly conventional instruments:
1. Sponge-holding forceps (for retraction);
2. Dental pledget mounted on a curved clamp (for dissection);
3. Right-angled clamp (for dissection of vascular branches);
We also use a few dedicated endoscopic instruments:
4. Scissors (for incising the mediastinal pleura)
5. Grasper
6. Clip applier

8. Dissection

• Exploration

The entire hemithorax is carefully examined, with particular attention to the mediastinum.
Blunt instruments may be used to help collapse the lung, and for manipulation to complete the exploration. The major structural landmarks should be identified, including the SVC, the brachiocephalic vein and the right phrenic nerve. Pleural adhesions may be present and require adhesiolysis to facilitate complete lung collapse and achieve a good operating field.

• Dissection

The right phrenic nerve is identified and carefully preserved throughout the dissection. The right inferior horn of the thymus can be seen draping over the pericardium.
The mediastinal pleura over the free edge of the right inferior thymic horn is incised.
The thymus can then be lifted up and bluntly dissected off the underlying pericardium extending onto the aorta in a cephalad manner until the left brachiocephalic vein is exposed.
We have found it useful to apply deliberate and gentle traction on the thymus to allow blunt dissection using a pledget.
The thymic venous tributaries (usually two or three) draining into the left brachiocephalic vein are then identified, clipped, and divided.
It is important to obtain vascular control before further manipulation of the thymus.

• Left inferior thymus

Dissection is then carried out behind the sternum. With gentle traction on the thymus using a sponge-holding forceps, the left inferior horn is identified and likewise dissected up to the isthmus of the thymus.

• Superior horns

The most difficult part of the operation is the dissection of the superior horns. The right internal mammary vein is divided in most cases to facilitate exposure. With gentle but deliberate inferior traction on the thymus, the superior horns are carefully dissected to free them from their fascial attachments.
The left superior horn may occasionally pass behind, instead of in front of, the brachiocephalic vein, and this anatomical variation has to be looked for.
1. Right internal mammary vein

9. End of procedure

• Extraction

The thymus, as a free specimen, is removed in a plastic bag through the most anterior trocar, because the intercostal space is wider anteriorly.
After thymectomy, the anterior mediastinal soft tissue including the pericardial fat is separately removed.
The specimen is inspected for completeness of resection.
In small children with hyperplastic thymus, we have found it useful to retract part of the gland out of an anteriorly placed wound. The maneuver creates more room for further dissection.

• End of the procedure

The thymic bed is inspected for hemostasis and completeness of resection. The brachiocephalic veins should have been skeletonized and the junction with the superior vena cava clearly visualized.
Chest drainage is optional. The lung is then reinflated under direct vision, and layered closure of the stab wounds completes the operation.

10. Complications

There are relatively few contraindications to VATS. In addition to general contraindications such as recent myocardial infarction and severe coagulopathy, specific contraindications include pleural symphysis, severe underlying lung disease or poor lung function. Patients with severe underlying lung disease or poor lung function may not be able to tolerate the selective one-lung ventilation during general anesthesia.
Prior operation in the ipsilateral chest should not be viewed as a contraindication (Yim et al., 1998). Adhesions can usually be freed using a combination of sharp and blunt dissection under videoscopic vision. However, an open procedure may be more suitable for patients with adhesions difficult to take down.
Concerns have been raised about the use of the VATS approach for thymoma with or without associated MG. We are careful to restrict this technique to small, completely encapsulated thymoma (Masaoka stage I). Clinical judgement is of paramount importance in thymic surgery, and any sign of tissue plane invasion mandates conversion to an open dissection (Yim et al., 1999).

Possible postoperative complications include:
- prolonged ventilatory support due to MG;
- bleeding;
- wound infection;
- pneumothorax;
- surgical emphysema;
- intercostal neuralgia;
- phrenic nerve palsy: it is essential that the right phrenic nerve is clearly identified and protected at all times during dissection. Phrenic nerve palsy represents a major complication for patients with MG.

11. Benefits of VATS

Patients who underwent thoracoscopic thymectomy have significantly less analgesic requirement and shorter hospital stay compared with a historical group who underwent transternal thymectomy (Yim et al., 1995).
Superior cosmetic appearance of VATS, especially in young females should also be considered.
Pulmonary function is significantly better preserved in the immediate postoperative period following VATS compared to the median sternotomy approach (Rückert et al., 2000).
Compared with the conventional transcervical approach, VATS has the advantage of better visualization and less instrument crowding.
A meta-analysis comparing nine published series performed by various approaches showed no difference in clinical improvement after thymectomy between series (Mack, 1997).

12. Thymus resection?

Regardless of the technique, it is generally agreed that thymectomy for MG should be complete. The Columbia-Presbysterian group advocated “maximal” thymectomy involving a combination of median sternotomy with cervical incision to achieve en bloc thymectomy and anterior mediastinal exenteration, which includes mediastinal pleura from the level of the thoracic inlet to the diaphragm, pericardial fat pad, and all the mediastinal fat. However, despite this radical approach, when compared with sternotomy alone (Olanow, 1987) or the transcervical approaches (Cooper et al., 1988), results in terms of clinical improvement did not seem to be significantly different.
In addition, a detailed autopsy study identified ectopic thymic tissue in areas (such as the retrocarinal fat), which are not accessible via a median sternotomy (Fukai, 1991).
Although it may seem intuitive to remove as much mediastinal soft tissue as possible to avoid leaving behind ectopic thymus, these remnants have never been conclusively shown to be clinically relevant, and even the most radical surgical approach does not result in a remission rate greater than 40%.
We believe that we are performing the same operation thoracoscopically compared with the transternal approach by examination of the thymic beds and the resected specimens (Yim et al., 1995).

13. Patient selection

Thymectomy is accepted as standard for young patients with generalized MG. In our hospital, since 1985, we have adopted a policy of offering thymectomy to all patients younger than 70 with generalized MG, and as soon as possible after the diagnosis is established (Kay et al., 1994). It is vital that the thoracic surgeons work closely with the neurologists and anesthesiologists to achieve optimal results.
Uncertainties remain over the role of thymectomy for patients with:
- late onset of disease;
- purely ocular symptoms: arguments have been put forward not to operate on ocular symptoms alone because ocular MG is not only less likely to respond to thymectomy, but also carries a better prognosis, compared with generalized MG. On the other hand, it has been shown that between 30% to 70% of patients with initial ocular symptoms will eventually develop generalized myasthenia (Oosterhuis, 1989; Sommer et al., 1997). Although some patients with purely ocular symptoms improve following thymectomy, the patients have to clearly understand that the rationale for surgery here is not based on symptomatic improvement, but rather on the expectation of disease progression.

14. Other approaches

The most commonly adopted surgical approach to thymectomy is via a median sternotomy. Other approaches include:
- transcervical;
- transcervical and median sternotomy (“T” incision);
- video-assisted thoracic surgery (VATS) (unilateral);
- partial sternotomy :
- upper sternum (Milanez de Campos et al., 2000);
- lower sternum (Granone et al., 1999);

- video-assisted thoracoscopic extended thymectomy;
- bilateral thoracoscopic approach combined with a cervical incision (Novellino et al., 1994).

15. Conclusion

VATS thymectomy is a safe operation in experienced hands and represents a new, viable approach for patients with MG. The right-sided approach is preferred because visualization of the venous anatomy for dissection is essential. Experience shows that this approach produces results comparable to other conventional surgical techniques. By minimizing chest wall trauma, the thoracoscopic approach causes less postoperative pain, shortens hospital stay, better preserves lung function in the early postoperative period (which may be particularly important for patients with MG), and gives superior cosmesis. It is hoped that this patient-friendly approach will lead to wider acceptance by MG patients and their neurologists of earlier thymectomies.

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