Chapter 14. Functional neurosurgery

Functional interventions include neurosurgical interventions aimed at normalizing the impaired function of the CNS by either interrupting pathological impulses or stimulating structures that inhibit pathological impulses.

Functional neurosurgery is mainly aimed at the treatment of the following types of pathology:

• involuntary movement disorders (hyperkinesis) and muscle tone — Parkinson's disease, Parkinsonism, torsion dystonia, etc.;
• chronic pain;
• epilepsy;
• some psychiatric conditions.

Methods of functional neurosurgery include those that destroy (destructive) and modulate the functions of the CNS (neuromodulation, previous term — neurostimulation). Along with open surgery performed primarily for the treatment of epilepsy (see below), in the treatment of “functional” diseases of the CNS, a stereotactic method is widely used based on the introduction into certain brain structures of special tools for stimulation or destruction of brain structures involved in the pathological process (thalamic nuclei, pallidum, etc.) using stereotactic technology (Fig. 14.1, 14.2).

Fig. 14.1. The Riechert−Mundinger stereotactic apparatus: a — the working part fixed to the patient's head; b — the phantom, on which the surgical intervention was simulated

Fig. 14.2. Modern stereotactic devices (a, b)

Historically, the first destructive methods were used to treat pain syndromes, extrapyramidal disorders, epilepsy and psychiatric diseases by openly crossing the pathways in the brain or spinal cord. Some techniques (for example, prefrontal lobotomy) were abandoned due to inefficiency and traumatism, but destructive interventions have shown effectiveness in extrapyramidal forms of tremor and torsion dystonia. An important role in the development of functional neurosurgery was played by the use of the stereotactic method, which allowed for the low-traumatic destruction or stimulation of brain structures involved in the pathological process.

Stereotactic technology meant fixing the patient’s head at standard points in the frame of the stereotactic apparatus (see Fig. 14.1, a); the target point (thalamic nucleus or pallidum) was determined from the atlas in comparison with the patient’s ventriculograms; the cannula insertion trajectory was calculated on a “phantom” (see Fig. 14.1, b). Then, under local anesthesia, a burr hole was created, and a surgical cannula was inserted into the patient’s brain along a calculated trajectory to a given depth. An electrode was inserted through the cannula, and it was observed whether there was a tremor inhibition or a decrease in muscle tone when exposed to a weak anode current. With a positive result of the test stimulation, brain tissue was destroyed within a radius of several millimeters from the tip of the cannula by freezing (cryodestruction) or heating (thermal destruction).

With the advent of CT, MRI and the improvement of stereotactic devices (see Fig. 14.2), the accuracy of the method has increased, the number of complications has decreased.

However, the functional outcomes of destructive stereotactic surgery remain insufficiently satisfactory, primarily due to the impossibility of performing bilateral interventions due to the high frequency of complications, as well as due to the irreversibility of the destruction performed if it has spread to areas adjacent to the planned areas. Today, the method is used for limited indications. Prospects for destructive interventions may be associated with the development of non-invasive technologies — radiosurgery or focused ultrasound under the control of MRI. The latest methods are also used in Russia, but their widespread implementation requires continued research on effectiveness.

Neuromodulation implies correction of the CNS function with the help of chronic electrical or drug effects on its structures. The advantage of the method is the reversibility of the impact, the disadvantage is the need for implantation of foreign bodies and the high cost of implantable devices.

Chronic electroneuromodulation (traditionally also called electroneurostimulation, neurostimulation and electrical stimulation, or deep brain stimulation) is a long-term effect on certain brain structures by electrical impulses of a given frequency, amplitude and shape.

The mechanism of action of electrical stimulation is not completely clear. It is assumed that it suppresses the pathological activity of neurons, in particular, which occurs when the inhibitory effect of dopaminergic structures is eliminated. Along with this, under the influence of electrical stimulation, changes in neurotransmitter metabolism occur in subcortical structures, as evidenced by the preservation of the clinical effect for a sometimes significant period after the termination of the supply of impulses.

Implantation of the system for intracranial neurostimulation is performed in several stages. First, a temporary electrode (electrodes) is inserted at the specified points using stereotactic technology, and the accuracy of getting into the necessary structure is verified by neuronal activity, test stimulation is also performed, and the clinical effect is evaluated. In some cases, the test stimulation is carried out for several days, in this case the electrode is fixed under a sterile bandage (Fig. 14.3).

Fig. 14.3. Test period of spinal cord stimulation. The electrode is implanted into the spinal canal — the posterior epidural space — and connected to an external stimulator

If a positive clinical effect is achieved as a result of the test stimulation, a permanent electrode (electrodes) and a pulse generator (neurostimulator) are implanted and fixed, which is usually located in the subcutaneous tissue (Fig. 14.4).