Brain stem gliomas:

 
Dr. A. Vincent Thamburaj,
Neurosurgeon,  Apollo Hospitals, Chennai , India.

Brainstem was, not long ago, considered as a 'no man's land'.

With recent advances in neuroimaging, and surgical tools, there is a renewed interest in brainstem tumors among neurosurgeons.

All brainstem tumors do not behave in the same manner. In the past, all tumors of the midbrain, pons, and medulla oblongata were regarded as 'brainstem tumors'. Recently, it has been suggested that there is a distinction between the thalamic and midbrain lesions on the one hand and pons and medulla lesions on the other and that the brainstem gliomas are not a homogeneous group with regard to their clinical, pathohistological, or biological features and that their prognosis may be directly related to tumor type and location.

 

Pathology:

 

Brainstem tumors are most commonly diagnosed in children, in whom they account for 10% to 20% of the primary brain tumors.

Pilocytic astrocytomas account for 30% of all infratentorial tumors in childhood, and 80% of pilocytic astrocytomas are found in the posterior fossa. The reported increase in the incidence of primary malignant brain tumors in children by 35% from 1973 to 1994, appears to be brainstem tumor rates, which may due to advances in imaging.

 

Brainstem gliomas have been classified by site, imaging, and pathology.

However, in clinical practice, they are classified into diffuse, and focal ones; both may have exophytic growth.

 

The diffuse type accounts for nearly 80% of them. The majority arising from and causing diffuse enlargement of the pons. Axial and exophytic growth is also found in two thirds of the cases. The medulla is often spared.They are infiltrative in nature with relentlessly progressive. They present with short duration of symptoms, usually consists of multiple bilateral cranial nerve deficits., long tract signs, and ataxia. Raised ICP and hydrocephalus are rare and occurs in 10% of cases.

Histologically, they belong to fibrillary astrocytomas, anaplastic astrocytomas, and glioblastoma multiforme.

 

Focal ones may be discrete (<2cm in diameter) and exophytic variants. They may be cystic or solid.

Cervicomedullary variants arise from upper cervical cord, with typical rostal extension into the cervicomedullary junction. The rostal extension is limited anteriorly by the pyramidal decussations; thus the mass expands posteriorly at the level of the obex and may rupture into the fourth ventricle. The caudal part is identical to an intramedullary tumor. They usually present with lower cranial nerve deficits, long tract signs, and occasionally torticollis.

 

The dorsal exophytic variants arise from the floor of the fourth ventricle, usually filling it. Clinically, there is marked by an insidious failure to thrive and signs and symptoms of raised ICP in the young. Cranial nerve deficits are found in 50% of them; but long tract signs are rare.

Histologically, they mostly belong to pilocytic astrocytomas.

Imaging:

 

Radiological findings associated with low grade tumor include, the presence of exophytic protrusion into the fourth ventricle through the dorsal brainstem and cervicomedullary location, and surgery has been recommended.

 

On MRI, the focal tumors are small, well circumscribed without associated edema or evidence of infiltration. Cystic changes may occur. The diffuse ones are hypodense on TI and hyperdense on T2.

 

Differential diagnosis include,

Exophytic type with necrosis

Cervicomedullary cystic type

        focal:   ependymomas, tuberculosis, metastasis, lymphoma, abscess, vascular malformations, and infarction.

        diffuse: vascular malformations, demyelinating lesions, and encephalitis.

 

Some of them may disseminate. Neuroaxis imaging and CSF analysis, if possible, have been recommended.

 

Management:

 

Surgery, despite all the recent advances, has limited role in diffuse ones.

Resection of well circumscribed, low grade tumors has been shown to improve survival.

Cystic components, and focal tumor appearance are thought to be favorable features for surgery.

Exophytic ones may be amenable for surgery; patient selection is critical.

 

A midline suboccipital approach is commonly preferred. Lateral exophytic ones may need a CP angle approach.

The floor of the fourth ventricle, or the lateral brainstem in CP angle approach, is inspected for bulges and discoloration.

Cyst, if any, should be aspirated .

Exophytic area, if any, is incised. Methylprednisolone may be given just before incision, and continued for 24 hours (30mg/kg for the first hour followed by 5.4mg/kg/hr.

If no exophytic area is seen, one of the 'safe areas' is incised longitudinally, after mapping out eloquent areas by direct electrophysiological stimulation of the cranial nerve nuclei. Continuous intraoperative EMG monitoring from certain muscles of the head is an alternative and can provide information about the status of the respective cranial motor nerves.

 

The tumor is excised in piecemeal with meticulous microsurgical techniques; recent tools, such as laser, irrigation bipolar,and ultrasonic aspirator, facilitates tumor excision. Tumor rim may be left behind, if there is no satisfactory cleavage is seen.

 

In view of the wide variety of brainstem lesions, a tissue diagnosis is warranted for appropriate treatment.

The use of open biopsy, and more recently, image guided stereotactic biopsy has been advocated for tissue diagnosis. In addition, cyst, if any, may be drained which will help symptomatically.

 

Stereotactic biopsy is indicated in focal enhancing masses, especially when open surgery was not contemplated, for whatever reason.

 

Stereotactic procedures make use of

              a transfrontal trajectory, through the axis of brainstem, for biopsies of the diencephalon, midbrain, and rostal pons, and

              a suboccipital transcerebellar trajectory, through the middle cerebellar peduncle, for biopsies of the lateral pons.

 

Steretactic biopsy is generally not advised in dorsally exophytic tumors, cervicomedullary tumor, and focal midbrain tumors.

Open surgery has been suggested as a better option.

A stereotactic biopsy is unreliable in diffuse non enhancing tumor. Diffuse ones has image characteristics which are almost diagnostic, and the stereotactic biopsy, especially in children, is not reliable in the diffuse variety. Direct radiotherapy is acceptable without biopsy.

Aggressive radiotherapy and chemotherapy are recommended as alternatives, in diffuse ones; the standard therapy has been 55Gy to the tumor area with weekly dose of 800-1000 cGy.

 

Prognosis:

 

Prognosis is generally poor in children. Adults do better.

Small size, lack of diffuse appearance, accessible location, and absence of preoperative neurological deficit suggest favorable outcome. Some have questioned the value of surgery; they claim the better prognosis is due to histology which is mostly pilocytic in focal and exophytic variants.

Most authors agree that there is little improvement in survival, with the attempted resection of a high grade tumor, even though in specific instances current advanced techniques allow for the performance of such surgery with acceptable morbidity.

Two year survival is reported to be 18% to 46%  with radiotherapy in diffuse ones.

Chemotherapy, when added to radiotherapy results a median survival of 44 weeks and a 1 year survival of 47%.

Other modes of radiotherapy do not add to it. Experience with radiosurgery is limited. SRT is, usually, preferred.

 
 
 

 

 

 

 

 


 

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