Neurosurgery notes/Anatomy/Cortex/Hemispheric dominance

Hemispheric dominance

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Cortex

Numbers

  • Lurito JT, Dzemidzic M.
    • 95% of right-handed people show left-sided hemispheric localization,
    • 15% of left-handed people show right sided localization.
      • Left handedness: 70% left-hemispheric dominance

Test to determine dominant hemisphere

fMRl

Wada test

  • Non-invasive functional MRI is replacing Wada, the historic "gold standard" for pre-operative age lateralisation
  • Aka
    • Unilateral Amytal Hemispheric Anaesthesia
    • Intra-carotid amytal test
    • lntracarotid sodium amobarbital procedure (ISAP)
  • Aim
    • Establish hemispheric language dominance
    • Establish hemispheric memory dominance
      • This is not able to be done on fMRI
  • Method
    • Intra-carotid sodium amytal injection during DSA
      • Hemi-anaesthesia during which language and memory function of the non-anaesthetised contralateral temporal lobe, planned for temporal lobectomy, are tested
        • Side of resection is injected first to determine the functioning of the nonresection side.
        • Each cerebral hemisphere is transiently inactivated by injecting the respective supplying ICA
    • Neuropsychology testing is performed in awake patient
      • Patients perform language or recall tests
      • Hemiplegia will occur on the side of amobarbital injection
  • Cons:
    • DSA
      • Invasive → Risk of stroke
    • Patients can be confused, drowsy, agitated → unable to comply with assessments.
    • PCA supply of hippocampus-
      • Are medial temporal structures adequately anesthetized by intra-carotid injection?
    • Crossflow into the contralateral hemisphere can complicate interpretation;
      • Intra carotid HMPAO injection reflects distribution of amytal and has shown crossflow between hemispheres not appreciable on DSA;
      • Potential anaesthesia of both hemispheres from a unilateral injection.
  • Diagnostic accuracy of Wada
    • Schmid E al. 2018 Meta-analysis:
      • Sensitivity estimate of 0.79
      • Specificity estimate of 0.65

Clinical significance

  • Patients at
    • High risk of postoperative memory impairment are those
      • Without MRI lesion
      • Average-low average memory on preoperative neuropsychology
      • Good memory on side of resection
      • Poor memory on side to be preserved postoperatively
    • Lower risk postoperative memory impairment are those
      • With a unilateral mesial temporal sclerosis (MTS)
      • Poor material-specific memory on neuropsychological testing (i.e. poor verbal memory if left MTS, or visual memory if right MTS)
      • Good memory on side to be preserved
      • Poor memory on side to be resected