The safe removal of primary brain tumours depends on an understanding of the relationship of the tumour to functional brain regions such as those that enable movement, sensation and language. This is usually determined relatively easily pre-operatively on imaging modalities such as MRI, sometimes supplemented by advanced imaging techniques such as functional MRI which can readily identify motor and somatosensory cortex.
Fortunately, most brain tumours develop in the brain at a distance from brain regions important for functions such as movement, sensation and language. This allows removal of the tumour with a margin of normal white matter, a so called macroscopically complete resection, with a minimal risk of new neurological problems. This is very important as a maximal removal of gliomas (a type of tumour that occurs in the brain) is thought to be associated with an improved survival from what are often very malignant tumours.
Sometimes the proximity on MRI of a brain tumour to functional brain may potentially restrict resection to either biopsy alone or a limited incomplete resection, both of which may significantly worsen prognosis. In this setting, sometimes utilizing techniques such as cortical stimulation in an awake patient to identify important functional brain during tumour removal can demonstrate a degree of separation of tumour and functional brain not captured on pre-operative MRI that then allows a safe maximal tumour removal.
In an awake craniotomy, a combination of local anaesthetic infiltration of the scalp combined with IV sedation is used during opening of the skull and exposure of the brain. The IV sedation is then ceased and the patient allowed to awaken mid-procedure. As the brain itself has no pain sensation, surgical procedures can be performed on the brain in a conscious patient. Direct stimulation of the brain with a small electrical current then allows identification of functional cortex.
When motor cortex is stimulated, involuntary movements will occur. When somatosensory cortex is stimulated the patient will report changes in sensation to parts of the body. If language cortex is stimulated while the patient is speaking, speech arrest or dysphasic speech errors will occur.