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Tags: Neuro

Deep brain stimulation

Parkinson's disease

The exact underlying mechanism of action of DBS remains uncertain, but it is likely to result from both stimulatory and inhibitory effects on neural elements that affect the generation and propagation of electrical signals through the cortico-striatal-pallidal-thalamic-cortical (CSPTC) network

DBS also results in changes to glutamate and γ-aminobutyric acid (GABA) pathways within the network

Chronic DBS has also been shown to affect synaptic and neural plasticity, resulting in long-term neuronal reorganisation

Common targets for surgery: globus pallidus, thalamus and subthalamic nucleus

Pre-op

As patients with underlying cognitive impairment are at risk of further decline after surgery, severe preoperative cognitive impairment is a contraindication for surgery

DBS benefits patients with a diagnosis of idiopathic PD. Patients with atypical or secondary Parkinsonism have less favourable outcomes, therefore surgical intervention is not recommended

Successful outcome is also dependent on the patient's response to levodopa

The DBS hardware has three main components:

1. intracranial electrodes
2. a programmable internal neurostimulator
3. an extension cable connecting the DBS electrodes to the neurostimulator.

The basic components of DBS surgery include

• preoperative imaging,
• stereotactic anatomical targeting,
• burr hole placement,
• intraoperative physiological target verification,
• lead implantation
• implantation of a power source (i.e. an implantable pulse generator, IPG)

The surgical procedure consists of two stages:
Stage one involves mapping and placement of the intracranial electrodes.
Stage two involves implanting and connecting the neurostimulator, sometimes referred to as an implantable pulse generator (IPG). The whole procedure may be completed on the same day, or in a 2-stage procedure. The decision to separate the stages is usually based on the patient’s condition, team preference, and local hospital experience.

patients have to be in a “drug-off” state to elicit intraoperative mapping and clinical testing

patients with PD commonly suffer from obstructive sleep apnoea or have an increased risk for aspiration

Stereotactic imaging

frame-based

• gold standard & more accurate
• fiducial markers on frame as reference points
  frameless system
• fiducial markers affixed onto patient's skull
• or surface anatomy features / contours of scalp / face used to register image
• ↑ patient comfort
• ↓ operating time

Direct vs indirect targeting

direct

• relies on imaging resolution
• specific (MRI) sequences may be used for certain nuclei
  indirect
• use anatomical brain atlases as reference
• less reliable due to anatomical variability

Displacement of the brain within the cranial vault relative to preoperative imaging may arise from several causes, including CSF loss, pneumocephalus, and the mechanical process of lead insertion resulting in brain deformation. Such brain shift can cause deviation of the intended trajectory and target, resulting in suboptimal lead placement and increased risk of damage to adjacent structures

Post-op

Device programming is usually deferred for several weeks after surgical implantation to ensure the ‘microlesion’ effect (transient improvement in symptoms as a result of lead insertion) has subsided.

The initial programming session establishes which contacts or pattern of contacts on the DBS lead have the best therapeutic window = difference between clinical improvement threshold & adverse effect threshold

Complications

ICH

• risk factor
  • age
  • Hx of HT
  • use of MER
    Cx related to hardware or stimulation
    Hardware related Cx, typically presenting w/i 3mo of surgery
• infection
  • IPG pocket
  • burr hole site
  • retroauricular connector site
• lead migration
• lead fracture
  • burr hole site
• lead / skin erosion
• hardware malfunction

Anaes Mx of patients w/ DBS

Preoperative preparation includes identification of the device and the severity of the patient’s symptoms when the DBS implant is turned off. Supplementation with oral medications may need to be considered if the device needs to be deactivated and symptoms are severe.

DBS systems may potentially interfere with monitoring and therapeutic equipment in the operating room, often producing artefacts on ECG readings. When diathermy is required, the neurostimulator may need to be disabled before induction of anaesthesia with the DBS Patient Programmer.

Bipolar diathermy should be used where possible, to reduce the risk of thermal injury to brain tissue around the implanted electrode or inadvertent reprogramming of the device. If monopolar diathermy is necessary, the grounding pad should be placed far away from the IPG and the lowest possible level of energy should be used, and only in short pulses. Short wave (microwave and ultrasound) diathermy should be avoided, as it has been linked to significant brain injury in patients with DBS.

Patients may experience rigidity once the device is switched off, and mechanical ventilation is sometimes necessary in the rare situation where severe rigidity interferes with their ability to breathe. Postoperatively, the device should (ideally) be turned back on before awakening.

When needed, defibrillation paddles should be placed as far away from the neurostimulator as functionally possible. Interrogation of the DBS is recommended after external defibrillation.

Electroconvulsive therapy, radiofrequency neuro-ablation, and peripheral nerve stimulation have also been shown to be safe in patients with a DBS in situ, if the stimulator is turned off and the probes are distant from the generator.

ECG

DBS is known to produce ECG artifacts and may make interpretation difficult. Deactivating the DBS system before ECG acquisition can remove such interference, but can sometimes lead to recurrence of severe tremor with electromyographic activity sufficient to affect ECG recording, significant patient discomfort, and inconvenience.
Patients may take up to 1 h to regain the ability to walk safely after even brief inactivation of DBS.

Short wave diathermy

Short wave (microwave and ultrasound) diathermy is commonly used to provide tissue heating for muscle or joint conditions. There have been two case reports of diathermy causing significant brain damage in patients with DBS, with one death. In one case, pulse-modulated radiofrequency diathermy was applied to the maxilla and this resulted in permanent brain damage. The mechanism of interaction is believed to be a result of induction of a radiofrequency current and heating of the electrodes. An in vitro study has suggested that, with typical diathermy power levels, heating may occur at a rate exceeding 2.54°C s−1 at the tip of the electrode.

The manufacturer advises against the use of any short wave diathermy in patients with DBS.

Electrocautery

Potential problems include thermal injury to brain tissue, reprogramming, and damage of the device and its leads. Manufacturer recommendation and literature review encourages preoperative pulse generator adjustment and postoperative interrogation. If the patient can tolerate the tremor and it does not interfere with surgery, the pulse generator can be safely turned off before the operation. Bipolar electrocautery may reduce the potential for electromagnetic interference. If a monopolar device is necessary, haemostasis can be obtained with the aid of a battery-operated heat-generating handheld electrocautery device or with the use of a dispersive plate to direct the current away from the pulse generator and lead system. Surgeons should be reminded to use the lowest diathermy energy possible in short irregular bursts.

Defibrillator

The manufacturer recommends positioning the paddles as far from the neurostimulator as possible, perpendicular to the implanted neurostimulator-lead system, and using the lowest clinically appropriate energy output. The neurostimulator should be checked carefully after any defibrillation.

Since both DBS and ICD generators can be affected by placement of a magnet over them, patients should not use a magnet to adjust the DBS device

Telemetric ICD programmers are able to deactivate the pulse generator of DBS, and the resulting patient tremor (‘pseudoventricular tachycardia’) has the potential to set off the defibrillator

References

Anaesthetic Management of Deep Brain Stimulators Insertion & Perioperative Considerations

Movement Disorder Surgery Part I Historical Background and Principle of Surgery - BJA Ed

Anaesthesia for Deep Brain Stimulation and in Patients With Implanted Neurostimulator Devices - BJA