Functional procedures and microelectrode recording
- DBS
- Neurophysiology
- Functional procedures re frequently performed in awake subjects.
- Stimulation for
- Localization of the target,
- Determining the therapeutic window
- Determining the proximity of adjacent structures
- Avoidance of stimulation- induced side effects
- Such as proximity to internal capsule from thalamus giving muscle contraction.
- Impedance measurement
- Microelectrode recordings
- For guidance of tissue traversed by the electrodes.
- Give information about the activity of the target area
- Extracellular microelectrodes are mainly used during DBS surgery to record action potentials from adjacent neurons.
- White matter shows only occasional spikes,
- Grey matter shows more continuous activity with patterns that vary from one location to another.
- This is a useful technique to determine the precise location of targets such as the
- Parkinson’s disease: subthalamic nucleus (STN)
- Multicellular activity as recorded with a microelectrode passing through the subthalamic nucleus (STN).
- Each trace is recorded in sequence as the electrode descends, thus top traces originate deep to STN, with some regular firing cells (top right) probably arising from substantia nigra: increasing activity in the centre where rapidly firing STN cells make a sound through the amplifiers like ‘ripping velcro’.
- Deep brain stimulating lead should be positioned to straddle this zone
- Spontaneous discharges with frequencies at about 40 Hz which are normally recorded from the STN are increased in Parkinson’s disease and reduced in Huntington chorea.
- The increased firing rate of excitatory glutamatergic STN neurones are responsible for the increased firing in inhibitory GABA- ergic output of GPi resulting in inhibiting thalamic or brainstem targets and thus reduced motor activity in Parkinson’s disease.
- Dystonin: Globus pallidus
- Subject asleep,
- As vigorous movement gives technical difficulties.
- GPi recording in dystonia
- Shows diminished spontaneous discharges evident as change from a tonic to a phasic pattern of the pallidal neurones discharges. → reduced inhibition of the thalamus → increased activity in the motor and premotor cortex.
- The discharge pattern of the advancing activating electrode changes with its location from the putamen, GPe and GPi.
- Flashes of light can evoke responses from visual pathways and can be recorded with the microelectrode from the optic tract below globus pallidum.
- Tremor: Thalamic Vim
- Thalamic recordings from Vim may show may show related rhythmic burst activities.
- These neurophysiological techniques were essential to locate targets when imaging technology was not able to demonstrate them clearly.
- Advances in imaging have resulted in successful DBS implantations without neurophysiological information, although such methods remain controversial
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