Transcranial magnetic stimulation (TMS)

Concept: TMS uses an electrically induced magnetic field to induce electrical currents that stimulate regions of the cerebral cortex. 156

• Repetitive pulses of electrical current are called repetitive TMS (rTMS)
• Can be delivered in high frequency (10 – 20Hz) that typically leads to increase in brain activity or low frequency (≤ 1Hz) which typically has the opposite effect 157
  • In theory, depressed patients have left prefrontal hypoactivity and right prefrontal hyperactivity, thus treatments would be to aim to stimulate the left with high frequency rTMS or inhibit the right with low frequency rTMS 157
• There are a variety of rTMS forms: priming low-frequency, bilateral, high-frequency, low-frequency, and theta burst have been found to be more effective than sham for treatment of MDD 157

Adverse effects 156

• There is a very low risk of seizure associated with TMS
  • Patients with various neurological disorders (TBI, stroke, AD, myoclonus dystonia) did not have subclinical EEG abnormalities with application of TMS 158
  • Patients with epilepsy did demonstrate induction of epileptiform abnormalities with TMS, and thus patients at high risk of seizures (such as those with uncontrolled epilepsy) would require continuous EEG monitoring during TMS session 158
• Vasovagal syncope sometimes occurs in initial treatments.
• Due to an audible clicking sound, ear plugs are recommended to both the patient and the clinician administering the TMS.
• Mild scalp discomfort, though generally patients develop tolerance after 1-2 weeks
• Patients with implanted devices require extra caution 158
  • TMS with figure-8 coils is considered safe in patients with pacemakers, VNS systems, and spinal cord stimulators, as long as the coil is a safe distance away (< 10cm)
  • TMS can be safely conducted in patients with implanted electrodes in CNS that are not connected to an implanted pulse generator (MRI compatible implants are more likely to be TMS safe)
  • In patients with DBS or cortical stimulation electrodes, TMS can induce currents causing unintended stimulation

Treatment for MDD

• Eligible patients 156
  • Insurance typically requires history of failing 2 – 4 antidepressant trials (due to lack of efficacy or intolerability of side-effects)
  • Moderate to severe MDD w/o psychosis
• Standard treatment regimen involves high frequency, left prefrontal rTMS 5 days / week over 4-6 weeks 156
• There has been concern for relapse rates in patients who achieve remission (Carpenter et al – TMS for major depression, a multisite naturalistic observational study of acute treatment outcomes in clinical practice 2012; Dunner et al – A multisite naturalistic observational study of TMS for patients with pharmacoresistant MDD, durability and benefit over a 1 year follow-up 2004; Janicak et al Durability of clinical benefit with TMS in the treatment of pharmacoresistant major depression, assessment of relapse during a 6 month multisite open label study 2010).

References

Perera, T. et al. The Clinical TMS Society Consensus Review and Treatment Recommendations for TMS Therapy for Major Depressive Disorder. 24 (2017). https://pubmed.ncbi.nlm.nih.gov/27090022/