29 Surgical Removal of Pilocytic Astrocytoma from Brain Stem and Thalamus

29

Surgical Removal of Pilocytic Astrocytoma from Brain Stem and Thalamus

VINKO V. DOLENC

Diagnosis Pilocytic astrocytoma

Problems and Tactics Intraaxial brain stem tumors represent a difficult tumorous pathology. Tumors in different locations in the brain stem lead to different clinical problems and corresponding neurological deficits. Despite such new technologies as neuronavigation and endoscopic microsurgery, the outcome in surgical treatment of intraaxial tumorous lesions is difficult to predict. To a certain extent, the only safe way to resect such a tumor is to enter the lesion and resect it piecemeal from the center to the periphery. To enter such a tumor safely is only possible when the lesion is extending to the surface of the brain stem and if there is no normal tissue covering the tumor, at least in a very limited area. Traversing the normal brain tissue to enter the tumor is extremely dangerous and does always result in additional postoperative neurological deficits. Of greatest importance is the approach to the tumor through such a corridor that no additional damage to the normal brain tissue is created. What is also important is that at the borderline of the tumor facing the normal brain tissue no coagulation is allowed because the lesion of the normal tissue following even minor coagulation is usually significant. For the sake of sparing normal brain tissue, it is much better to leave the last piece of the tumor than to risk damage to the normal brain tissue. Such damage usually does happen when the remnants of the tumor at the periphery are to be resected, and, to visualize the region, the brain stem tissue is retracted. That is why the approach is so important, and the central light beam from the microscope to the last corner of the cavity after resection of the tumorous mass should be constantly readjusted to avoid retraction of the brain tissue.

Keywords Brain stem tumors, pilocytic astrocytoma

Case I

Clinical Presentation

A 4-year-old, right-handed male patient was admitted to our neurosurgical department for surgical treatment of an expansive tumorous lesion in the brain stem. Eleven months prior to admission to our department, he suffered from pneumonia that was accompanied by high temperature (up to 41°C). Following this infection, the parents noticed that his left leg was not functioning properly. Six months later his left upper arm was not functioning properly, and then the right nerve III became affected (ptosis) when he would bend his head to the left. After consultation with the pediatrician, the left-sided pyramidal symptoms were confirmed and were found to be more pronounced in the left lower member in comparison with the left upper member. The progression of symptoms and signs was steady, and the patient had more and more difficulties with walking and balance, he was tired, he did not like to cooperate, and he also had occasional headaches. The patient did not vomit, did not have nausea, and did not have epileptic seizures. Two months prior to admission to our institution, slight pyramidal signs and symptoms were also noticed on the right side. Finally, 1 month prior to admission to our department, magnetic resonance imaging (MRI) was performed, and the diagnosis of the brain stem tumor was established. The patient was treated with corticosteroids and gained weight (4.5 kg) as a result, but did not improve neurologically.

On admission to our facility, the patient was conscious, restless, and not fully cooperative. Nerve III palsy was found on the right side. The head was inclining to the left. The functions of the other nerves, apart from nerve III, were normal bilaterally. Severe spastic hemiparesis with Babinski’s sign was found on the left side. The right (upper and lower) extremities were practically normal. MRI performed at our neuroradiological department prior to surgery revealed a large, partially cystic tumor in the posterior part of the right hypothalamus and in the brain stem. The smaller portion of the lesion in the brain stem was also cystic (Fig. 29–1A,C,E).

Despite the fact that the symptoms and signs were already present almost 1 year prior to admission to our institution, and that the tumor (due to its size and location) was declared as inoperable, and that the patient was already bedridden, we nevertheless decided to proceed with surgery. The decision was based on our experiences with hypothalamic and brain stem tumors, as well as on the basis of the MRI scans, which were in accordance with pilocystic astrocytoma.

Surgical Technique

The interhemispheric–transcallosal–transventricular approach was used, as has already been described for hypothalamic and brain stem lesions.¹ After sectioning the corpus callosum in its anterior half, between both pericallosal arteries, the right lateral ventricle was entered. The septum pellucidum was left intact, and the septal and thalamostriate veins on the right side, as well as the choroidal plexus were preserved. The right foramen Monroi was widely dilated and was used as the corridor toward the tumor bulging into ventricle III from the right thalamus. After aspiration of the cerebrospinal fluid (CSF), the brain was slack, and a corridor through the foramen Monroi was wide enough to visualize the bulging tumor into ventricle III. First, the solid part of the lesion was entered and carefully removed piecemeal from the center of the lesion toward the periphery. The tumor was grayish and poorly vascularized. After the solid part of the lesion had been removed, the cystic part was reached and the membrane of the cyst was dissected from the normal brain tissue. The most important guidance for the resection of a pilocytic astrocytoma is the consistency of the lesion and the grayish color of the tumorous tissue, which is distinctly different from the normal tissue, as it was in this case. It was possible to resect the tumor completely and to achieve complete hemostasis at the borderline between the tumor and normal brain tissue.