Chapter 20. Cerebral Microdialysis

Introduction

Cerebral microdialysis involves placement of a small catheter with a semipermeable membrane in the parenchyma of the brain, so that a dialysate fluid can be instilled into the catheter, allowed to equilibrate and withdrawn for analysis (Figure 20-1). Neurochemical levels such as lactate, pyruvate, glucose, and glutamate can then be measured to determine metabolic and neurotransmitter activity in the area of interest.

Definitions and Terms

• ▪  Microdialysis: A technique used to determine the chemical composition of extracellular fluid in a tissue/organ of interest.
• ▪  Lactate: Chemical by-product of anaerobic metabolism.
• ▪  Glutamate: Amino acid and excitatory neurotransmitter.
• ▪  Glycerol: A marker of cell membrane breakdown.

Techniques

• ▪  The insertion technique for cerebral microdialysis is similar to the technique described for ventriculostomy:
  • —Shave and prepare insertion site
  • —Small skin incision and burr hole
  • —Incise dura and advance catheter into area of interest
• ▪  The tip of the catheter can be positioned in various areas depending on the nature of the underlying lesion, but microdialysis is primarily used to monitor neurochemical changes in tissue at risk for secondary injury following a primary neurological event:
  • —The catheter may be placed in the nondominant hemisphere to monitor global cerebral events in diffuse axonal injury.
  • —The catheter may be implanted in the less injured hemisphere of a brain-injured patient to determine the effect of whole brain interventions, such as hyperventilation on vulnerable tissue.
  • —The catheter may be positioned in the penumbral area around infarcted or contused brain to evaluate the effects of treatments on the at-risk tissue surrounding irretrievably injured brain.
  • —The catheter may be placed in an area of tissue perfused by arteries susceptible to vasospasm to monitor the effects of interventions designed to treat vasospasm.

Clinical Pearls and Pitfalls

• ▪  Low glucose levels in microdialysate indicates reduced cerebral glucose supply and/or cerebral hypoxia and ischemia.
• ▪  An increase in the lactate-pyruvate ratio may result from hypoxia-ischemia, reduction in cellular redox state, mitochondrial dysfunction, or low cerebral glucose supply and is probably the most reliable indicator of local ischemia.
• ▪  Increased glycerol level indicates hypoxia/ischemia and cell membrane degradation, although it may be due to systemic events rather than local cell breakdown.
• ▪  Increased glutamate levels have traditionally been suspected of a causal role in “excitotoxicity,” wherein glutamate release after injury, but there is significant variability in glutamate both within and among patients with brain injury.

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