The beginning of e.s. clinical application to the hearing organ appeared after observation of tinnitus disappearing after the implantation of a single-electrode cochlear implant. In 1973, House (The House Ear Institute) reported total disappearing of tinnitus after the implantation of the single-electrode cochlear implant (using electrical current to stimulate auditory nerve). Such an effect was noticed later by other authors . This fact was a fundamental observation which resulted in the idea of suppressing tinnitus with electrical current. In this way the idea of the e.s. in tinnitus treatment appeared. Tinnitus suppression was obtained when cochlea, round window, promontorium, preauricular skin or mastoid was subjected to stimulation. In 1974, House suggested a method of evaluation of the hypothetical benefit of the cochlear implantation based on the transtympanal e.s. of the promontory. In case of a sound sensation (reported by a patient during the stimulation), peripheral impairment of the hearing organ (outer, inner hair cells) was considered, whereas when the patient reported hearing no noise central. Similar research was conducted by Bochenek et al. . During e.s. in more than half of the patients, a sound sensation was observed. The authors suggested that in such cases, clinical diagnosis indicated central VIII-th nerve dysfunction, rather than the peripheral one. They admitted, however, that some VIII-th nerve’s fibers, which were not impaired, could have been enough to evoke sound.
Skarzynski et al.  and Bochenek et al.  proved the usefulness of non-invasive alternative extratympanic ear canal e.s., as a test in prediction of post-operative profits before cochlear implantation. The authors observed reception of the tones as well as speech signal, by some completely deaf patients in whom e. s. via external auditory canal (with a ball-shaped electrode dipped in saline solution) was conducted. In this way, they claim to stimulate the fibers of the auditory nerve, obtaining hearing sensation as an evidence. Despite numerous researches on tinnitus disappearance after cochlear implantation, its mechanism seems to remain unexplained conclusively. As many patients benefit from hearing aid (experiencing tinnitus suppression) we may suspect that the enhancement of the signal in the auditory pathway is the factor responsible for this phenomenon . Sensorineural hearing loss is one of the most apparent risk factor for tinnitus, probably resulting from maladaptive attempts at cortical reorganization due to peripheral deafferentation . As in patients with tinnitus and single-sided deafness (SSD) therapies based on acoustic input (retraining, masking) are impossible, the restoration of peripheral sensory input may be a method of masking/relieving tinnitus. There are some data showing good effects of binaural integration of acoustic (unilateral normal hearing) and electric stimulation (via cochlear implant), which appeared to be superior to the alternative rehabilitation methods of SSD and tinnitus [bone-anchored hearing aid (BAHA), contralateral routing of signal (CROS)] . Although groups of patients implanted with tinnitus and SSD were not numerous, there were studies demonstrating significant improvement reaching 100 % [27–29]. In effect, SSD with severe tinnitus is considered a new indication for cochlear implantation; however, appropriate patients selection is required [27, 29, 30]. Arts et al.  state that cochlear implant should be considered as a treatment for tinnitus resulting from SSD (from peripheral-cochlear deafferentation). Furthermore, there may be some predictors of the degree of improvement after such procedure. Song et al.  collecting quantitative electroencephalography found positive correlation between increased activity of auditory posterior cingulate cortex and dorsolateral prefrontal cortex and slight tinnitus reduction after cochlear implantation.
In the research, the application of the current was based on our experience. The intensity of current was applied according to patient’s sensation and tolerance, whereas the frequency according to tinnitus pitch (frequency)—selective e.s. As in the majority of cases, the cochlea may be a trigger or ignition site for subjective tinnitus, authors adopted the theory that the frequency of tinnitus might be consistent with the area of damaged outer hair cells in the basilar membrane [33, 34]. For that reason those parameters were similar as well as the tinnitus matching (pitch) method was used in the research. It was done based on ‘Adaptive Method’ according to Tyler’s indications [35, 36]. Despite the fact that psychoacoustic measurements may not correspond to tinnitus severity, such quantification is needed in clinical trials for evaluation of treatment, but it requires standardization of techniques.
Tyler et al.  summarize the state of the art knowledge of extra- and intracochlear e.s. in tinnitus. The authors state that the optimal parameters of stimulation are likely different for different subjects. Offut claims that auditory stimulation with specific frequencies within the area of loss of hearing in pure tone audiometry can reduce tinnitus, by suppressing the inner hair cells . However, Dauman et al. , using cochlear implant for e.s. of hearing organ, observed that the effectiveness of the stimulation depended on the stimulating frequency and was optimal using 125 Hz. Morawiec-Bajda et al.  performed e.s. via external auditory meatus with the active electrode placed on tympanic membrane and the other on the forehead. The improvement was obtained in 46.6 % of cases. Furthermore, using for stimulation frequencies close to tinnitus frequencies, an increase in otoacoustic emission’s amplitude (more distinct in DPOAE than TEOAE) was obtained, as well as increase in amplitude and shortened latencies in auditory brainstem responses.
The theories on the cochlea as an ignition site for tinnitus, together with hypothesized ways of influencing its structures by e.s., may indicate the need for individually modified parameters of e.s . The promontory stimulations were conducted by Aran and Cazals  who achieved satisfactory effects (complete or partial improvement of tinnitus condition) in 43 % of cases, compared with 60 % of improvement cases when oval window was stimulated. Ito and Sakakihara  via stimulation of cochlea directly with cochlear implant received better outcomes (77 %) than via stimulation of promontory (69 %). The results of invasive (direct) transtympanal e.s. are better when compared with that of non-invasive methods.
As far as an improvement is concerned, our outcomes are comparable to the results of non-invasive e.s. conducted by other authors. The total number of tinnitus disappearance is more apparent in presented research; however, the tendency to decrease with time is observed. Application of hydrotransmissive method in our research greatly simplifies the technique of e.s. This non-invasive procedure allows the doctor to perform it in any outpatient clinics, and as a result, the patient does not have to stay under the medical observation directly after stimulation. Furthermore, hydrotransmissive stimulation allows application of cycle of such stimulations, improving the chances of relieving tinnitus, as well as helping to boost and maintain the improvement of hearing. Recently, a range of device delivering transcutaneous mastoid e.s. has been constructed. The idea was to stimulate in a simple, non-invasive way, giving the patient possibility to perform ‘self’-stimulations at home. However, most reports do not support efficient success rates.
Subsequent improvement of the results after a month is a fact worth noticing. The number of stimulation applications (15), performed regularly, might have had an influence on the subsequent improvement together with its stabilization. In our research, in both treated groups, we observed the change in the nature of tinnitus (permanent to temporary). The change was most noticeable in group one (treated with e.s.)—the number of cases with permanent tinnitus decreased by around 50 %, but in both stimulated groups (groups one and two), the improvement was statistically significant (p < 0.05). As patients referred, the change from permanent to temporary tinnitus was meaningful for them, and it allowed to experience some silent periods (often after months or years of presence of continuous, chronic noise in the ear) again. That is why most of the patients considered it an apparent improvement. In literature, such evaluation of the tinnitus treatment has not been found.
In one of the earliest publications concerning e.s. in the tinnitus treatment, Portman et al.  described the dependence of the result of stimulation on the polarization of the current. Using direct negative current, a sound sensation was evoked, proving that the VIII-th nerve was stimulated. In case of positive polarization suppression of tinnitus was observed, but only for the time of stimulation. After the procedure was finished, tinnitus appeared again . In our research, in most cases a sound perception was observed during stimulation with positive current, so it may be possible that the factor responsible for such auditory reception is the condition of the hearing organ rather than current polarization. At early studies conducted by Portmann et al.  and Aran et al. , direct current appeared to be more harmful to the inner ear (than alternating); however, it was more efficient in suppressing tinnitus.
Konopka  in his study on the influence of direct current on the hearing organ of guinea pigs demonstrated no pathologic effect on auditory pathway based on the evaluation of summation potentials, cochlear microphonics and auditory brainstem potentials. In present research, we did not notice any destructive influence of direct current based on the hearing evaluation, but in the literature there are some reports on damaging effect of direct current on the cochlea [41, 42]. Furthermore, in our research, pure tone audiometry revealed statistically significant improvement in hearing threshold (p < 0.05) in group one, which can mean hypothetically that the function of outer hair cells improved (but in present study, objective measurements such as otoacoustic emissions or auditory brainstem responses were not performed after the treatment). Subjective evaluation of hearing in pure tone audiometry revealed the best effect in group one. Although the degree of the hearing improvement seems not to be clinically meaningful, the hearing level reminded stable and this measurement was repeatable during control period. This might also be the result of disappearance or decreasing tinnitus severity, which could have had a masking effect. Having in mind that sensorineural hearing loss is difficult to treat, especially in chronic course, this improvement appeared more significant, nevertheless it needs further confirmation in objective measurements. In group one we observed a perception of sound in most ears while using direct positive current, with its frequency adopted according to the tinnitus frequency parameters (selective e.s.). In the majority of cases those frequencies corresponded with impaired frequencies in pure tone audiometry—peripheral cochlear impairment. This observation might be coherent with a hypothesis that the sound perception during e.s. proves normal functioning of the auditory nerve.
Many authors highlight the need for placebo-controlled studies to assess placebo effect in tinnitus treatment. The placebo effect is known to cause neurobiological changes comparable to those resulting from pharmacotherapies; however, its mechanisms are not fully understood. According to Benedetti et al.  placebo responses can be attributed to the two phenomena: conditioning and expectation of therapeutic benefit. The authors state that expectation can best stimulate a placebo response. Neuroimaging [positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)] and neurotransmitter release measurement have contributed to explanation of some placebo underlying mechanisms. The differences of the placebo effect in patients may reflect variations in the activity in some neurotransmitter systems (dopamine, serotonin, cholecystokinin, opioid). Neuroimaging of placebo analgesia pointed to decreased activity in thalamus, insula and somatosensory cortex .
With respect to tinnitus, the placebo effect was mainly assessed in pharmacotherapy (dexamethasone, lidocaine, paroxetine, betahistine, vasodilators, diuretics) and complementary medicine therapies (ginko biloba, acupuncture, massage, meditation) showing no advantage of therapy over placebo . The comparison of transcutaneous electrical stimulation and placebo stimulation resulted in similar outcomes [46, 47]. Duckert  assessed placebo effect in tinnitus after saline solution injection instead of lidocain. He obtained improvement in 40 % of tinnitus concluding that each uncontrolled clinical trial may by biased by placebo effect. McFerran and Phillips  point to difficulties in evaluating different tinnitus treatment methods as they lack blinding. They cite the Duckert’s experiment (aforementioned) stressing that placebo effect “is often regarded pejoratively by practitioners of modern medicine, overlooking the fact that a placebo is not the same as no effect”. Kapkin et al.  assessed placebo effect in transcutaneous e.s. of preaurical region. The rate of improvement in e.s. group was 42.8 % and in placebo group 28.5 %. The rate of worsening was respectively: 16.6 and 42.8 %. The authors used lidocain to deliver local infiltration anesthesia to the stimulated region of the preaurical skin to create identical circumstances during e.s. and placebo e.s. procedure. According to them, substantial difficulty in placebo-controlled study is to establish comparable conditions for treated and placebo group. In case of e.s. the patient will feel the electric current. On the basis of our research and previous experiences only 26 % (31 ears) felt electric current during e.s. In those cases patients reported sensation of pricking, tingling, warming and pain. All patients were informed that during e.s. they can experience some sensations like above, but not necessarily.
According to Hoare et al.  most research on tinnitus lack blinding, thus those therapeutic methods remain to be demonstrated conclusively especially those most commonly used: hearing aids, maskers, tinnitus retraining therapy. On the other hand many patients with ear problems/diseases suffer from tinnitus as well, but only in few cases underlying etiology can by clarified. For that reason each therapy, also symptomatic, may be effective especially for those with constant, severe tinnitus.