Absolute Contraindications of tDCS
The primary absolute contraindication to transcranial direct current stimulation (tDCS) is the presence of metallic implants in the head or neck region, particularly intracranial metal objects, cochlear implants, or deep brain stimulators, as these can alter current flow and potentially cause tissue damage or device malfunction.
Key Absolute Contraindications
While the provided evidence does not explicitly enumerate a comprehensive list of absolute contraindications in a dedicated guideline format, the safety literature and consensus papers 1 establish tDCS as having a well-established safety profile when conventional protocols are used. However, based on the mechanism of action—delivering low-intensity electrical currents through the skull—certain conditions represent absolute contraindications:
Metallic Implants in the Head/Neck
- Intracranial metal plates, clips, or electrodes pose the highest risk as they can concentrate current density unpredictably
- Cochlear implants can malfunction or cause tissue damage
- Deep brain stimulators or vagal nerve stimulators may interact with the electrical field
- Medication pumps in the head/neck region represent a contraindication
Skull Defects
- Large skull defects or breaches (from surgery or trauma) alter current distribution unpredictably, potentially causing focal areas of excessive current density that could lead to tissue damage
Important Safety Context
The evidence demonstrates that conventional tDCS protocols (≤40 min, ≤4 mA, ≤7.2 Coulombs) have not produced any serious adverse effects across over 33,200 sessions in 1000+ subjects 2. Animal models indicate brain injury occurs at current densities (6.3-13 A/m²) over an order of magnitude above those produced by conventional tDCS 2.
Relative Contraindications vs. Absolute
The literature emphasizes that many conditions previously considered contraindications are actually relative contraindications requiring careful consideration rather than absolute exclusion:
- Epilepsy: Not an absolute contraindication; safety data from vulnerable populations including epilepsy patients show no serious adverse effects 2
- Pregnancy: Insufficient data but not listed as absolute contraindication
- Skin conditions at electrode sites: Relative contraindication (can cause discomfort but not serious harm)
Special Populations Studied Safely
Evidence specifically addresses theoretically vulnerable populations that have been studied without serious adverse effects 2:
- Children and elderly
- Patients with mood disorders
- Stroke patients
- Home users
Critical Caveat
One notable adverse effect documented is treatment-emergent mania or hypomania (0.88 [0.62-1.13]) 3, which while not an absolute contraindication, requires careful screening and monitoring in patients with bipolar disorder or predisposition to mania.
Clinical Decision Algorithm
When evaluating a patient for tDCS:
- Screen for metallic implants in head/neck → If present, exclude from tDCS
- Assess skull integrity → If large defects present, exclude from tDCS
- Evaluate for active implanted medical devices in head/neck → If present, exclude from tDCS
- All other conditions require risk-benefit assessment but are not absolute contraindications
The evidence base 1, 2 consistently demonstrates that when standard safety protocols are followed, tDCS has a remarkably safe profile, with absolute contraindications limited primarily to conditions that would alter current flow in unpredictable and potentially dangerous ways.