Safety and tolerability of Transcutaneous Vagus Nerve stimulation in humans; a systematic review

BACKGROUND

Transcutaneous Vagus Nerve stimulation (tVNS) may be an alternative to surgically implanted VNS for epilepsy and other diseases. However, its safety and tolerability profile is unclear.

OBJECTIVE

We performed a systematic review of treatment harms from tVNS in humans.

METHODS

A systematic published and grey literature search was carried out to identify studies which deployed tVNS in human subjects. Study authors were contacted for safety/tolerability data if these were not available in the publication. Databases were searched from 1966 to May 2017. We noted study type, population, stimulation parameters, type and prevalence of side effects and/or serious adverse events (SAE). We also noted whether side effects/SAE were considered to be related to the tVNS and the proportion of participants dropping out of studies due to side effects.

RESULTS

51 studies were included comprising a total of 1322 human subjects receiving tVNS. The most common side effects were: local skin irritation from electrode placement (240 participants, 18.2%), headache (47, 3.6%) and nasopharyngitis (23, 1.7%). Whilst heterogeneity in overall side effect event rates between studies was not accounted for by the frequency (Hz) or pulse width (ms) of stimulation, a minority (35 participants (2.6%)) dropped out of studies due to side effects. Overall, 30 SAE occurred but only 3 were assessed by the relevant researchers to be possibly caused by tVNS.

CONCLUSION

tVNS is safe and well tolerated at the doses tested in research studies to date.

The economic cost of stroke-associated pneumonia in a UK setting

Introduction Stroke-associated pneumonia (SAP) is common, however, data on the economic impact of SAP are scarce. This study aimed to prospectively evaluate the impact of SAP on acute stroke care costs in a UK setting. Methods Prospective cohort study of 213 consecutive patients with stroke (196 ischemic, 17 hemorrhagic) was admitted to a UK hospital over 1 year. Socio demographic and clinical characteristics were recorded along with all treatments and rehabilitation activity. Patients were classified as having SAP if they fulfilled criteria for "probable" or "definite" respiratory tract infection according to the Centres for Disease Control and Prevention definition, within the first seven days following stroke. Resource use was calculated using a "bottom up" approach of cumulative unit costs. Univariate and multivariate regression analyses were used to establish independent predictors of direct costs. Results Probable or definite SAP occurred in 13.2% (28/213) of patients. Patients with SAP experienced greater inpatient stays (31 days vs. 9 days, p ≤ 0.001) and higher in-hospital mortality (29.2% vs. 10.2%, p = 0.007). Mean (SD) acute care costs per patient was £7035 (6767), but costs were significantly greater for patients with SAP than without [£14,371 (9484) versus £6,103 (5,735); p ≤ 0.001]. SAP was an independent predictor of costs along with increasing stroke severity (NIHSS) and age. Occurrence of SAP resulted in an adjusted incremental additional cost of £5817 (95% CI 4945-6689; p = 0.001) per patient. Conclusions SAP increased acute care costs for stroke by approximately 80%. This provides further impetus for research aimed at reducing SAP, and will inform cost-effectiveness analyses of potential therapeutic strategies.

Does sodium intake modify red cell Na+ transporters in normal and hypertensive subjects?

Abnormalities in sodium transport in red blood cells (RBC) have been reported in hypertensive subjects. Uncertain from previous studies is whether the level of sodium intake per se might influence the level of these transporters. This study was designed to answer that question by evaluating the level of activity of three RBC sodium transporters in fourteen normotensive and twenty-six hypertensive subjects studied twice--once while in balance on a 10 mEq and once while in balance on a 200 mEq sodium intake. There was no significant difference in the maximal velocity (Vmax) of the sodium pump or the cell sodium and potassium content in normotensive or hypertensive subjects, nor did sodium intake influence these factors. Li-Na countertransport activity was significantly (P less than .01) higher in the hypertensive than in the normotensive subjects. However, sodium intake did not influence the Vmax of this transporter in either group. In contrast, sodium efflux through the Na-K-Cl cotransporter was significantly influenced (P less than .03) by the level of sodium intake, but only in the hypertensive subjects. On the high sodium intake, both the Km and the Vmax were significantly elevated (P less than .03) compared to their levels on the low sodium diet. Additionally, only on the high salt diet did hypertensives have significantly higher levels of activity than normotensives (P less than .03). Thus, Na-K-Cl cotransport activity is influenced by the level of sodium intake, suggesting that caution needs to be exercised in interpreting the level of activity of this transporter unless the level of sodium intake is fixed and known.

Red blood cell lithium-sodium countertransport in non-modulating essential hypertension

Abnormalities in erythrocyte Li-Na countertransport have been reported in hypertensive subjects, and the available evidence favors familial aggregation and striking heritability of this marker. It is uncertain, however, whether the abnormalities are associated with hypertension per se or whether they may be concentrated in a particular subset of hypertensive subjects. In the present study, maximal rates of Li-Na countertransport were measured in red blood cells of 82 white subjects, including 37 normotensive subjects and 45 normal- or high-renin hypertensive subjects previously classified as non-modulators (n = 21) or modulators (n = 24). Mean countertransport activity was significantly higher in non-modulators compared with normally modulating hypertensive or normotensive subjects (0.475 +/- 0.044 vs. 0.309 +/- 0.028 or 0.249 +/- 0.012 mmol/l cell x hr, respectively, p less than 0.001). Modulators did not differ significantly from normotensive subjects with regard to mean countertransport activity. Red blood cell sodium pump and Na-K-Cl cotransport were not significantly different in modulating and non-modulating hypertensive subjects. These relations remained unchanged after adjusting for age, body weight, and plasma cholesterol levels by analysis of covariance. A countertransport value exceeding 0.50 mmol/l cell x hr occurred in 40% of the non-modulators but in only one of the other subjects. In contrast , while one half of the modulators and normotensive subjects had a countertransport value less than 0.235 mmol/l cell x hr, none of the non-modulators did. Thus, elevated countertransport appears to aggregate in the non-modulating subset of essential hypertensive subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal renal sodium handling in essential hypertension. Relation to failure of renal and adrenal modulation of responses to angiotensin II

This study assessed renal sodium handling in a group of patients with essential hypertension in whom control of the renal blood supply and aldosterone release by angiotensin II is abnormal ("non-modulating") because of recent evidence that these patients have sodium-sensitive hypertension. Sixty-one patients were studied, 25 as balance was achieved with a daily sodium intake of 10 meq and 36 after a shift from a 10 meq to 200 meq sodium intake for five days. Renal and adrenal responsiveness to angiotensin II was assessed by measurement of para-aminohippurate clearance and plasma aldosterone prior to and during the infusion of 3 ng/kg per minute of angiotensin II, to identify the non-modulator group (n = 32). The half-time of the exponential function relating sodium excretion to time during the three to five days when external balance was being achieved with a 10 meq sodium intake was 23.9 +/- 0.3 hours in 60 normal subjects, 24.5 +/- 1.8 hours in the patients with essential hypertension in whom renal responsiveness to angiotensin II was normal, and prolonged (p less than 0.001) to 36.6 +/- 2.1 hours in the non-modulating patients. A prolonged half-time suggests that, with a shift to a high sodium intake, more time will be required to achieve external sodium balance and at the expense of more retained sodium. During the shift from a 10 to 200 meq sodium intake, the non-modulator group showed a delayed rate at which external sodium balance was achieved, greater cumulative positive sodium balance, more weight gain, and a greater frequency of blood pressure rise. The abnormality in the rate at which external sodium balance is achieved in non-modulation results in a difference in total body sodium that varies with sodium intake and that may well contribute to, or cause, sodium-sensitive hypertension.

Correction of abnormal renal blood flow response to angiotensin II by converting enzyme inhibition in essential hypertensives

In 40-50% of patients with essential hypertension, a high sodium intake does not increase renal blood flow (RBF). These patients have been defined as nonmodulators because sodium intake does not modulate renal and adrenal responsiveness to angiotensin II (AII). To define the role of AII in mediating this altered responsiveness, we assessed the effect of a converting enzyme inhibitor (enalapril) on RBF and its responsiveness to AII in 25 patients with essential hypertension--10 modulators and 15 nonmodulators--and 9 normotensive controls. After 5 d of a 200-meq sodium intake, the nonmodulators did not increase RBF, whereas the normotensives (79 +/- 28 ml/min per 1.73 m2) and modulators (75 +/- 26 ml/min per 1.73 m2) did (P less than 0.025). Arterial blood pressure did not change in the modulators with the salt loading, whereas in the nonmodulators, blood pressure rose (P less than 0.004). After enalapril administration for 66 h, there was a significant difference (P less than 0.01, Fisher Exact Test) in the blood pressure response in the two hypertensive subgroups. In the modulators, there was no change; in the nonmodulators, despite the high salt diet, a blood pressure reduction occurred. In parallel, basal RBF and RBF responsiveness to AII were not changed after converting enzyme inhibition in the normotensive control (n = 9) or the hypertensive modulators (n = 10). Conversely, in the nonmodulators (n = 14), the basal RBF increased significantly (83 +/- 25 ml/min per 1.73 m2; P = 0.01), the increment being indistinguishable from the response to salt loading in normal subjects. Furthermore, renovascular responsiveness to infused AII was also significantly enhanced (P = 0.027) in the nonmodulators, suggesting that enalapril-induced increase in RBF reflected a fall in intrarenal AII levels, and not an increase in prostaglandins or kinins, which would have blunted the renal response to AII. Thus, short-term converting enzyme inhibition corrected abnormalities in sodium-mediated modulation of renal vascular responsiveness to AII. The close quantitative relation of the increase in RBF with sodium loading in normal subjects and modulators, and with converting enzyme inhibition in nonmodulators, viewed in the context of the effectiveness of enalapril only in the latter, and parallel shifts in sensitivity to AII, raises the intriguing possibility that converting enzyme inhibition reversed the failure of the renal blood supply to respond to sodium loading. Thus, converting enzyme inhibitors may reduce blood pressure specifically in this subset of patients with essential hypertension, who are sodium sensitive by way of mechanisms more closely related to local than systemic activity of the renin-angiotensin system.

Effect of the phorbol ester TPA on PTH secretion. Evidence for a role for protein kinase C in the control of PTH release

Parathyroid hormone (PTH) secretion is stimulated by low extracellular calcium (Ca2+) in association with a reduction in cyosolic Ca2+, indicating that this cell type does not conform to classical models of stimulus-secretion coupling. We used the phorbol ester TPA (12-O-tetradecanoyl phorbol 13-acetate), which directly activates protein kinase C, to investigate the possible role of this enzyme in the unusual secretory properties of the parathyroid cell. TPA causes a dose-dependent stimulation of PTH release inhibited by high extracellular Ca2+ (EC50 = 10 nM) but has relatively little effect on secretion stimulated by low Ca2+. This effect was mimicked by the beta 4-isomer of phorbol 12,13-didecanoate which also activates kinase C, but not by the alpha 4-isomer, which has no effect on this enzyme. TPA does not modify cellular cAMP or cytosolic Ca2+ in the parathyroid cell indicating that its effects on PTH secretion are not mediated indirectly via changes in these second messengers. These results suggest that inhibition of PTH release at high Ca2+ might be related to a reduction in protein kinase C activity which can be overcome when the enzyme is directly activated by TPA.