Cerebral: surface

Transcranial Magnetic Stimulation (TMS) : TMS in a repetitive mode (rTMS) is a noninvasive technique with a benign side effect profile and is easy to administer. Standard TMS consists of a brief, high-intensity current pulse in the coil of a wire which produces a magnetic field that can reach up to 2 T and lasts for approximately 100 ms. Parameters that modulate the geometry of the induced electric field include the type and orientation of the coil and the waveform of the magnetic pulse as well as pulse intensity. Patients are awake, alert, and sit in a comfortable chair during treatment sessions, which are approximately 20–30 min in duration. Cardiac or neural pacemakers, metal clips in the brain and previous seizures are a contraindication. Side effects are transient and include scalp discomfort, headache, sleepiness, dizziness, or an increase in pain. Ear plugs are recommended to avoid hearing damage. The target (e.g., the primary motor cortex—M1—or the dorsolateral prefrontal cortex—DLPFC) is located either with conventional craniometric techniques or image-guided neuronavigation. The motor threshold of the hand area of the unaffected hemisphere is often assessed to calculate the intensity of therapeutic stimulation, but its significance is moot when the target is nonmotor or outside the hand area in M1. Effects can be immediate or build up over several sessions (days). After effects, that is, symptom relief beyond the actual period of stimulation, are highly variable, either being absent or lasting up to 2 weeks; they are unassociated with entrainment of specific frequencies. A hand-held machine for home use is available, but expensive.

Transcranial Direct Current Stimulation (tDCS) : The benefits of tDCS are its low-cost, ease of use, safety, tolerability, and portability. Patients are awake, alert, and sit in a comfortable chair during treatment sessions. The power supply is 10 mA delivered via two surface conductive electrodes covered with saline or gel soaked sponges to reduce impedance. The sizes are 5 × 7 cm (but can be larger or smaller). The anode carries the positive charge and is considered excitatory, the cathode conveys the negative charge and is considered inhibitory; however, subtle variations in technique may have substantial effects. Various parameters of stimulation in different settings have been used, with intensities up to 5 mA (generally 2 mA), in single to repeated sessions on consecutive days, and a duration of 5′-30’. The safe current density delivered is 0.029–0.08 mA/cm2. The target is generally located by using the EEG 10–20 system (e.g., M1: C3/4); the reference is placed on the contralateral supraorbital area or shoulder. However, given the large electrodes used, tDCS is expected to engage nearby areas (e.g., both M1 and S1). In psychiatric conditions, one electrode is placed over the left DLPFC, and the second electrode is placed either over the right DLPFC or over the left supraorbital region. A more focal high-definition tDCS is under study. Side effects include redness under the electrode, tingling, lightheadedness, headache, fatigue, insomnia, anxiety, confusion, nausea, among others: these are generally transient. Exclusion criteria include prior head trauma, pregnancy, and epilepsy.

Invasive Cortical Stimulation (ICS) : ICS requires a neurosurgical approach to place one or more stimulating paddle electrodes on the dura mater overlying the target area (extradural ICS, e-ICS)—or, infrequently, underneath it (subdural ICS, s-ICS). No stimulation leads specifically designed for ICS are available at the present time: quadripolar electrode arrays (5 mm contacts with 1 cm spacing), singly or in combination (for wider coverage), are borrowed from neurosurgical spinal cord stimulation; 8, 16, and even 32 contact paddles are also used. Both constant current or constant voltage stimulators are used. In current-controlled devices, output (measured in Amperes) is kept constant while the voltage is adjusted to keep the current intensity at the desired level. Conversely, in voltage-controlled devices, the output (measured in Volts) is kept constant while the output current is adjusted according to the tissue impedance. The patient either lies down supine or in lateral decubitus (Park Bench). Electrodes are inserted via one or two burr holes or via a flap craniotomy ( Fig. 2.1 ), either under local anesthesia with conscious sedation(neuroleptoanalgesia) or general anesthesia (e.g., TIVA). Clinical outcomes are similar, but burr holes are less traumatic to the patient. If M1 is elected as a target, paddles can be positioned parallel to the central sulcus or perpendicularly, with similar outcomes, at least for chronic pain. ICS does not require stereotactic frames or microelectrode recording. Pulse generators are implanted in the subclavicular region, similar to DBS procedures. Devices that allow sequential testing of multiple contact configurations at very short intervals are available, but are bound to miss effective configurations for some conditions (e.g., pain, tinnitus), given the lack of a straightforward stimulus–effect relationship; also, after effects are not factored in. ICS allows for stimulation with increased spatial specificity than NICS. Absolute exclusion criteria are similar to other neuromodulation techniques and include major depression accompanied by suicidal thoughts or gestures (unless specifically targeted for palliation), major psychosocial stressors (job dissatisfaction, marital problems …), major personality disorders, dementia, substance abuse, poor motivation, malingering. Although referred to as invasive, actually ICS is by far safer than deep brain stimulation (DBS). Complications of DBS can be serious, for example, venous infarction caused by the transection or coagulation of large draining veins at the site of the burr hole, deep infarcts, subdural hemorrhage, intracerebral bleeding, brain abscesses, cognitive and neuropsychiatric disturbances, and suicide. Mortality, although rare, is nontrivial (average: 0.4%). e-ICS, the commonest form of ICS, virtually eliminates the possibility of intracranial hemorrhage; extradural hematomas are exceptional and easily manageable (s-ICS, however, carries morbidity and mortality similar to DBS, with potentially more frequent cerebrospinal fluid—CSF—leaks). Postoperative MRI, including high-field (3T) MRI, can be performed safely [ ].