Neurosurgery notes/Procedures/Cranial procedures/Neurophysiology/Brain mapping

Brain mapping

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Neurophysiology

General

  • Shift from Localizationist to Network Model:
    Connectome
    • Historically, functional anatomy focused primarily on the cortex in a rigid, localizationist framework, largely neglecting subcortical connectivity.
    • Hodotopical account (connectomics), where brain functions arise from the dynamic interactions of large-scale, distributed, and parallel sub-networks.
  • Requires a large team
    • Neuropsychologist
    • SALT

Direct Electrical stimulation (DES)

General

  • Perform real-time anatomo-functional correlations in awake patients during brain surgery.
  • DES is considered the gold standard in brain mapping because of its optimal sensitivity, identifying eloquent structures (cortical or axonal) that, if disturbed, induce a functional consequence.
  • Accuracy:
    • DES can identify crucial structures—including white matter tracts—with high accuracy (about 5 mm) and reproducibility in vivo in humans.
  • Comparison to DTI:
    • Non-invasive methods like Diffusion Tensor Imaging (DTI) investigate structural connectivity in vivo, but DTI cannot provide direct functional data, making DES the only current way to directly investigate the functional role of subcortical fascicles.
  • Non-Local Effect:
    • Stimulation acts as an input gate into a functional network;
    • The perturbation is small enough to propagate only within a "sub-circuit," inhibiting a specific component of the tested function.

DES Technique

  • DES involves intraoperative electrical mapping in awake patients, utilizing a biphasic electrical current (typically 60 Hz) to create a "virtual transient lesion" in discrete cerebral structures

Anatomical mapping

Parameter
Cortical Mapping
Subcortical Mapping
Stimulation type
Bipolar, biphasic constant current
Monopolar or bipolar, often cathodal
Frequency
25-60 Hz
250-500 Hz (monopolar train pulses)
Pulse width/duration
0.2-0.5 ms (200-500 microseconds)
0.2-0.5 ms (200-500 microseconds)
Stimulation intensity
1-15 mA, gradually increased
5-20 mA, intensity varies with proximity
Train duration
1-10 seconds (language longer)
Typically shorter trains (brief pulses)
Goal
Identify functional cortex areas
Identify critical white matter tracts
Methods
Penfield (50 Hz, bipolar), Taniguchi (high frequency monopolar)
Taniguchi monopolar cathodal high-frequency trains
Afterdischarge monitoring
Yes, to avoid seizures
Yes, intensity adjusted to avoid adverse effects
Subcortical mapping
  • Subcortical mapping is achieved through DES mapping of myelinated tracts in awake patients undergoing surgery
  • Stimulation parameters:
    • Trains of 5 pulses
    • Cathodal stimulation
    • ISI 2.0 ms
    • Pulse width 0.5 ms
    • 1 Hz
  • The rule of thumb: 1 mA = 1 or 2mm (only for subcortical mapping)
  • Procedure:
    • While the surgeon temporarily disrupts discrete cerebral structures using DES, the awake patients perform several sensori-motor, visuospatial, language, cognitive, or emotional tasks. If the stimulation causes the patient to stop moving, stop speaking, or produce an incorrect response, the surgeon avoids removing the stimulated site.
  • Accuracy:
    • DES is able to identify the cortico-subcortical structures essential for brain functions, including white matter tracts, with great accuracy (about 5 mm) and reproducibility in vivo in humans
Subcortical mapping Stimulation parameters: • Trains of 5 pulses • Cathodal stimulation • ISI 2.0 ms • Pulse width 0.5 ms • 1 Hz The rule of thumb: I mA = I or 2 mm 10 mA 15 mA 9 mA
Functional Mapping of Myelinated Tracts
  • Sensorimotor Function:
    • DES confirmed the organization of the corticospinal tracts (pyramidal tracts) within the corona radiata and internal capsule.
    • It evidenced a "modulatory motor network," likely the fronto-striatal tract, which elicits movement arrest or acceleration when stimulated.
    • DES data suggests a wide fronto-thalamo-parietal sensory-motor network.
  • Visual Tract:
    • Axonal DES of the optic radiations can generate transient visual field deficits, described as "negative effects" (e.g., blurred vision) or "positive effects" (e.g., phosphenes).
  • Language Tracts (Dual Stream Model):
      • Speech centre
    • DES supports a dual model of language processing involving distinct ventral and dorsal streams.
    • Dorsal Stream (Phonological/Articulatory):
      • Mediated by the Superior Longitudinal Fascicle (SLF)/Arcuate Fascicle (AF) complex. Stimulation of the AF evokes phonemic paraphasias and repetition disorders, supporting its role in phonological processing. A sub-part of the lateral SLF (SLF III) constitutes the articulatory loop and, when stimulated, can elicit anarthria.
    • Ventral Stream (Semantic): Connects posterior temporal/occipital areas to the frontal lobe.
      • The Inferior Fronto-Occipital Fascicle (IFOF) (direct pathway), particularly the superficial layer, is crucial for verbal semantics; its stimulation elicits semantic paraphasias.
      • The Inferior Longitudinal Fascicle (ILF) and Uncinate Fasciculus (UF) (indirect pathway) are generally thought to be compensable when unilaterally damaged, though injury to the posterior ILF can generate alexia.
  • Visuospatial and Vestibular Processing:
    • DES of the right ILF induced contralateral visual hemiagnosia, suggesting its specialization for visual recognition and visuospatial processing in the right hemisphere.
    • Stimulation of the right SLF II caused spatial cognition problems (e.g., line bisection deviation).
    • A subcircuit in the right SLF was found to subserve the vestibular network, causing vertigo when stimulated.
  • Cognitive Control and Consciousness:
    • Executive Function: The Frontal Aslant Tract might control language planning; DES of this tract can evoke stuttering. A cortico-striatal loop involving the caudate nucleus participates in language control (selection/inhibition), where stimulation generated perseverations.
    • Amodal Semantics: Stimulation of the deep layer of the IFOF reliably induced non-verbal comprehension disturbances, suggesting its role in amodal semantic processing and awareness of knowledge (noetic consciousness).
    • Consciousness: Disruption of the subcortical connectivity of the left posterior cingulate cortex reliably induced a transient breakdown in conscious experience, characterized by loss of external connectedness.
Cortical mapping
  • Goal:
    • To define the functional boundaries of the cortex surrounding a lesion (like a brain tumour) to optimize the extent of resection without causing permanent neurological deficits.
  • Methodology:
    • The procedure involves temporarily disrupting discrete cerebral structures using a biphasic electrical current (typically 60 Hz, 1 msec, 1 to 4 mA) while the patient performs specific tasks (sensori-motor, language, cognitive, or emotional).
  • Identification of Eloquent Structures:
    • If the stimulation applied to a cortical area causes a functional disruption (e.g., stops movement, stops speaking, or produces a wrong response), the site is considered "eloquent" and is preserved.
  • Historical Context:
    • Mapping the cortex using electrical stimulation has been a specific paradigm elaborated for both adult and childhood surgery (noting that stimulation parameters may need adjustment for nonmyelination of fibers in young children).

Result of stimulation

  • Negative mapping
    • If you stimulate the brain and if nothing happens then you can take it out
    • Can have false negative if one is not using a stimulation that is high enough current
  • Positive Mapping
    • Map an area (motor cortex) to see at what amps needs to stop the brain working , then do it in the operative site to see if we there is a deficit

Implications for Neuroplasticity

  • Constraint on Plasticity:
    • The connectomal account emphasizes that post-lesional neuroplasticity requires the preservation of white matter fibers to allow communication and synchronization among distributed networks.
  • Low Subcortical Plasticity:
    • While cortical areas exhibit "huge plastic potential," subcortical plasticity is low.
  • "Minimal Common Brain":
    • A probabilistic atlas based on DES data shows that regions with the highest probability of residual (non-resectable) tumor are located in deep white matter tracts.
    • This supports the idea that long-range axonal pathways are less subject to inter-individual variability and reorganization than cortical sites, implying the existence of a "minimal common brain" necessary for basic cognitive functions.
  • Clinical Value:
    • The understanding provided by DES mapping is crucial for surgical planning, minimizing the risk of permanent neurological deficit, and predicting functional recovery and rehabilitation needs following brain damage.

Evidence

  • Pallud 2021
    • Awake surgery group had significantly improved overall survival and higher extent of resection compared to asleep or biopsy groups.
    • Seizure control rates were higher after awake resection (89% vs 69% preoperatively).
    • Permanent motor deficits were rare (3/61 patients).
    • Cognitive outcomes: Despite temporary postoperative deficits, long-term function was preserved in most cases.
De Witt Hamer et al., 2012
  • Intraoperative neuromonitoring has been found to be associated with
    • Greater degree of resection (gross total resection of 75% vs. 58%)
    • Lower rate of permanent neurological deficit than in surgeries without DEBS (3.4% vs. 8.2%)
      • But INOM had high early and severe deficit
          • Outcome Type
          With Stimulation Mapping (%)
          Without Stimulation Mapping (%)
          Any Early Neurological Deficit
          47.9%
          14.2%
          Severe Early Deficit
          7.1%
          5.1%