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|Year : 2012 | Volume
| Issue : 4 | Page : 385-389
The transylvian trans-insular approach to lateral thalamic lesions
Shashwat Mishra, Ramesh C Mishra
Department of Neurosurgery, Neurosurgical Clinics, Kamayani Hospital, Agra, India
|Date of Submission||07-May-2012|
|Date of Decision||28-Jun-2012|
|Date of Acceptance||17-Jul-2012|
|Date of Web Publication||6-Sep-2012|
A-31, Gyandeep Apartments, Mayur Vihar Phase-1, New Delhi-91
Source of Support: None, Conflict of Interest: None
Background: Thalamic tumors are rare intracranial tumors. The most common approaches to the thalamus have been directed through the ventricular system, which surrounds it. The transsylvian trans-insular approach to the lateral thalamus has been infrequently described probably because of the vulnerability of the internal capsule, which skirts this part of the thalamus. Aims: To describe the approach emphasizing its anatomical basis and also to evaluate its safety and efficacy. Settings and Design: Retrospective study conducted at a tertiary hospital. Materials and Methods: Patient population included all the patients who underwent surgery for the lesions in lateral thalamus using the transylvian trans-insular approach between 2005 and 2011. A trephine craniotomy was made, centered over posterior sylvian fissure and the surgical corridor was developed through the insular cortex. Results: During the study period 10 patients (7 tumors and inflammatory lesions and 3 hypertensive bleeds) were treated using this approach. One peri-operative mortality was noted. In patients with lesions other than hypertensive thalamic hemorrhage, there was no postoperative worsening of neurological deficit as comparative to preoperative deficits. Total excision/evacuation of the lesion could be accomplished in all the patients. Conclusions: The transylvian trans-insular approach is safe, effective, anatomical procedure, and can be performed at a peripheral center without the need for navigation and intra-operative monitoring.
Keywords: Surgical approach, thalamus, trans-insular, transylvian
|How to cite this article:|
Mishra S, Mishra RC. The transylvian trans-insular approach to lateral thalamic lesions. Neurol India 2012;60:385-9
| » Introduction|| |
The thalamus (Gk: "meeting place"/"bridal chamber"), as the name suggests, is a secluded part of the brain and is the largest component of the diencephalon. Thalamic lesions are rare and thalamic tumors account for 4% of all intracranial tumors.  Surgical treatment of thalamic lesions has generally been associated with significant morbidity, necessitating a careful evaluation of the risks and benefits before making a decision to operate. A variety of surgical corridors have been utilized for approaching thalamic lesions with transventricular approaches being the most frequent and popular among neurosurgeons. The extra-axial approaches to the thalamus through the subarachnoid space are advantageous in not violating the cortex but often employ a long corridor and sometimes require special positioning of the patient. Though, the posterior transylvian trans-insular approach for thalamic lesions has been described long ago, but is only infrequently mentioned in published literature. We describe our experience with this approach in treating a spectrum of thalamic lesions at a small peripheral center, emphasizing the efficacy and safety of this approach.
| » Materials and Methods|| |
Surgical anatomy of the thalamus
A brief discussion of relevant surgical anatomy of the thalamus is pertinent to the discussion of our technique and results. Thalamus is often represented as a tetrahedron with three free surfaces exposed to ventricular system and one surface in contact with critical neural structures. The posterior, dorsal, and medial faces of this tetrahedron are open to the atrium, body of lateral ventricle, and third ventricle, respectively. Anteriorly, thalamus is bounded by foramen of Monro; dorsally, by the roof of third ventricle and posteriorly it is delimited by the posterior commissure. Inferiorly, the hypothalamic sulcus distinguishes it from sub-thalamic structures. The enlarged lateral and caudal parts of the thalamus overlie the brain stem. Thalamic radiations, emanating from the lateral surface of the thalamus, are the reciprocal connections of thalamic nuclei with cerebral cortex. Upon the dorsal surface of the thalamus, thalamostriate vein delineates it from caudate nucleus anterolaterally. Ventrolaterally, the thalamus is in contact with posterior limb of internal capsule, a structure of critical importance in approaches directed towards the lateral surface. From the insular segment of sylvian fissure, the insular cortex, extreme capsule, external capsule and the retrolentiform limb of internal capsule are sequentially traversed to reach the lateral aspect of the thalamus [Figure 1].
|Figure 1: A diagram displaying the relationship between the structures traversed in the trans-sylvian trans-insular approach (blue arrow) to the thalamic lesions [TO-temporal lobe, PIC-posterior insular cortex, ExC- external capsule and claustrum,RLIC- retrolentiform limb of internal capsule, Pu-putamen, Gp-globus pallidus, ALIC-anterior limb of internal capsule, PLIC-posterior limb of internal capsule, TV-third ventricle, Th -thalamus, HC-head of caudate nucleus]|
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| » Materials and Methods|| |
We analyzed case records of patients with thalamic lesions treated by transylvian trans-insular approach at Kamayani Hospital, Agra, India, between 2005 and 2011. Transylvian trans-insular approach was employed for patients with laterally projecting thalamic lesions approaching the posterior sylvian fissure on imaging. Following the surgery, these patients were closely followed up and outcome information was collected. The clinical presentation, imaging studies and the outcome data for eligible patients were analyzed.
Patients were positioned supine and the head was rotated to the contralateral side with an aim to bring sagittal suture parallel to the floor. Ipsilateral shoulder was propped up with padding to check excessive neck rotation. The head was held in this position with Sugita head holder and a linear incision was marked, centered at a point along the surface projection of the posterior sylvian fissure about 2-3 cm anterior to the parietal eminence [Figure 2]. The incision was deepened through fascia and temporalis muscle and bone exposed. A trephine craniotomy was made as shown in the figure. After opening the dura, posterior sylvian fissure was identified by the presence of prominent superficial middle cerebral vein and the M4 branches entering the fissure along both operculae. Arachnoid over the veins was sharply divided on the frontal side and the sylvian fissure was opened. Sometimes, a minor bridging vein was sacrificed to widen the operating space. An effort was made to open the sylvian fissure along its entire visible length to allow atraumatic separation of the operacula. We adopt an "inside-out" technique for opening the sylvian fissure, separating the opercular arachnoid bands in the depth before slitting the superficial arachnoid. Attention was directed toward the temporal operculum and transverse temporal gyri were identified by their oblique orientation and undulating appearance on an otherwise flat opercular surface. Normally, the postero-medial ends of these gyri converge toward the posterior half of the superior limiting sulcus of the insula, which is superficial to the posterior thalamus, the retro-lentiform internal capsule and the anterior limit of the atrium. , Though the transverse temporal gyri still remain identifiable when the tumor distorts posterior insula, their medial ends are often rendered obscure. Mechanical retraction of the opercula was conscientiously avoided as initial tumor decompression provided the required working space. For the initial approach, the opercula were held apart with cottonoids wedged into the fissure. However, keeping the normal relationship in mind, the posterior insular cortex (the long insular gyri posterior to the central sulcus) was entered and the lesion was found beneath a thin cortical layer. The lesion was then removed using routine microsurgical techniques. Care was taken to remain within the substance of the tumor. Lesion excision was followed by hemostasis and routine closure.
|Figure 2: (a) The relationship between the thalamus, overlying cortex, and the pinna, (b-a). CT scout film exhibiting the trephine craniotomy, (c) showing the operative incision (case 4) and (d) Complete neurological recovery. [STg - superior temporal gyrus, MTG - Middle temporal gyrus, PCG - Precentral gyrus, PoCG - Postcentral gyrus, IFG - Inferior frontal gyrus, SMG - Supramarginal gyrus]|
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| » Results|| |
The clinical data of the patients who underwent this surgical approach are presented in [Table 1]. Seven tumors and three hypertensive thalamic hematomas were treated using this approach. We had one death in the peri-operative period. One patient remains in chronic vegetative state following evacuation of thalamic hematoma. In patients with thalamic lesions other than hypertensive thalamic hematoma, there was no postoperative worsening of the preoperative deficits. Total excision of the lesion could be achieved in all the cases [Figure 3].
|Table 1: Clinical characteristics, pathology and outcome of studied patients.|
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|Figure 3: Respective preoperative (a and c) and postoperative (b and d) images of a pilocytic astrocytoma (Case 6) and Fungal granuloma (Case 7) operated upon using the discussed approach|
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| » Discussion|| |
Thalamic lesions, historically, have been considered to be inoperable due to the complex architectural organization of vital thalamic nuclei and the proximity to critically important structures such as the internal capsule, subthalamus, and basal ganglia. However, with the evolution of microsurgical techniques and improved understanding of microsurgical anatomy, these lesions no longer remain unassailable. Recent studies have demonstrated improved outcomes with aggressive surgical resection of thalamic lesions.  Complete excision of benign tumors can be curative while the resection of high grade tumors confers a survival advantage. ,,,
Three distinguishable patterns of expansion and growth for thalamic tumors have been described by Yasargil.  Tumors arising in one of the thalamic nuclei remain confined to the segment of the origin and distort as well as displace the surrounding structures as they expand. Alternatively, when arising from the thalamus in proximity to the ventricular surface, an exophytic tumor growth beneath the intact ventricular ependyma is often observed. The third group of tumors may expand laterally and superiorly into the white matter of an adjoining gyrus. This is the group, which in our opinion grows toward the sylvian fissure and posterior insula. Thus tumor expansion frequently entails a separation rather than infiltration of critical white fiber bundles and vascular structures.
A variety of surgical approaches have been described for thalamic tumors depending upon the part of thalamus that is predominantly involved. Transventricular approaches are the most frequent because of the intimate relationship of the thalamus with the lateral ventricles. Access to the ventricles and subsequently to the thalamus may be gained either by transcortical or interhemispheric routes. The interhemispheric approaches may jeopardize the bridging cortical veins and may even sometimes be precluded by anomalous venous anatomy. Transcortical approaches, do not deal with veins, but necessitate a potentially epileptogenic cortical incision. The trans-parietal, trans-atrial approach may also affect the superior loop of the optic radiations. Further, in the absence of ventriculomegaly, these approaches may penetrate a substantial depth of the cortex to reach the target. The extra-axial trans-arachnoidal approaches such as the infratentorial, supracerebellar approach (Krause's), and the posterior interhemispheric approach, aim to circumvent the disadvantages of the transventricular approaches. The posterior interhemispheric approach takes advantage of the fact that the midline bridging veins are scarce posterior to the lambdoid suture. Approach to the atrium and posterior thalamus necessitates a small incision in the precuneus, which is inconsequential. However, these approaches are applicable mainly to pulvinar lesions and necessitate a long operative corridor. Moreover, the basal veins often limit the lateral reach of the Krause's approach and its variations and hence are not preferable for lesions in lateral thalamus.
Direct transcortical approaches to the lesions in ventrolateral thalamus are directed through the temporal cortex. Besides, the general disadvantages of the cortical incision, they also place the Meyer's loop and Wernicke's area (in dominant hemisphere) at risk. In contrast, the transylvian trans-insular approach to lateral thalamic lesions offers a natural corridor and avoids an incision in eloquent cortex. The main criticism of this approach is that it apparently traverses the retro-lenticular portion of the internal capsule. However, as our experience has borne out, the white matter tracts are separated and splayed by the expanding tumor and can safely be preserved by confining the surgical dissection to the lesion. Further, the extension of the tumor beneath the insular cortex provides an avenue for atraumatic dissection. This approach was initially described by Yasargil, and has been attempted by others. ,, These authors have affirmed the feasibility of this approach in selected thalamic lesions. However, it must be remembered that the configuration of the thalamic lesion suitable for this approach is usually encountered when the lesion arises from the posterior ventrolateral thalamus, as this part of the thalamus is closest to the retro-lenticular white matter and the posterior insular gyri. Expectedly, the site of origin within the thalamus is difficult to determine when the lesion is sizeable, which is quite frequently seen. In these cases, extension of lesion toward posterior insular cortex and anterior displacement of the lenticular complex renders it suitable for the trans-insular approach.  We have also explored hypertensive thalamic bleeds using the same operative corridor. However, we did not see encouraging results or significant neurological recovery in these cases. This observation is consistent with the view that hypertensive bleeds often destroy the affected white matter, thus reducing the chances for neurological recovery. Hence a more conservative strategy, as advocated in cases of other ganglion bleeds, ,, is also applicable here.
To conclude, in our view the transylvian trans-insular approach is a safe and effective operative plan for lesions arising in the ventrolateral thalamus and extending toward the insular cortex. For these lesions, the chief advantages of this approach are a short operative corridor and avoidance of incision over the eloquent cortex.
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[Figure 1], [Figure 2], [Figure 3]