Vagal nerve stimulation is nowadays considered a safe surgical and neuromodulatory technique. Positive results in seizure control and quality of life improvement have been observed in children [1, 8, 19, 21, 23]. In the literature, the rates of surgical complications and adverse effects are reported to be between 2.5 and 16.8 % [1, 12, 14, 24, 26].

However, as far as hardware complications are concerned, the literature reports a rate ranging from 0 to 21 % [1, 6, 8, 11–14, 24, 26].

After our analysis of the abovementioned studies, it was clear that the most common and relevant complications concerning VNS implantations are: infections, vocal cord palsy, and hardware-related inconveniences.

In these studies the infection rate was reported to be between 0 and 10.9 %. In most cases, infections could be considered as a minor complication, only requiring oral and/or local antibiotics. Sometimes infections become a major event and explantation of the device is needed. The risk of infection is undoubtedly due to the implantation of the prosthetic device. One comparable neurosurgical procedure is deep brain stimulation (DBS), where the literature reports infection rates between 2.5 and 8.5 %, values that are similar to those reported for VNS [12]. In our series we have not had any case of infection so far. This could be explained by several factors; in particular: the careful draping of the sterile field, the surgeon’s behavior (reduction of operative time, changing gloves, use of disposable materials, etc), the administration of intravenous antibiotics (usually cefazolin) 1 h before surgery, and the care of the wound during hospitalization and at home, after accurate training of the care-givers. An increased infection rate seems to be a consequence of the number of pulse generator replacements, for battery exhaustion [12] and the rate could be reduced by the introduction of new stimulator models s with longer battery life or rechargeable generators, as employed for DBS devices.

As far as vocal cord palsy is concerned, an incidence between 1 and 5.6 % emerged from the aforementioned studies. This kind of complication appears to be a direct consequence of the manipulation of the vagus nerve, or it may be due to recurrent laryngeal nerve injury; usually, it is a transient adverse effect. A similar incidence of vocal cord palsy was also reported following a second surgery, performed for the replacement or explantation of the electrode; commonly, in these cases, the palsy may last longer [12, 27]. The reason could be that fibrosis and scar near the previous lead insertion would make dissection more troublesome. However, it seems evident that vocal cord palsy is a complication that affects adults more than children [26].

Left vocal cord adduction due to stimulation of the left recurrent laryngeal nerve could be the cause of laryngeal and respiratory symptoms that might develop in these patients. It has been demonstrated that stimulation of the vagus leads to an increase in respiratory rate and to a decrease in respiratory amplitude, tidal volume, and oxygen saturation. These respiratory alterations could cause sleep apnea or exacerbate it in patients with a preexisting diagnosis. Moreover, as the VNS device is usually implanted in patients who could, potentially, have abnormal central nervous system (CNS) development and who generally receive active CNS drugs, it has to be kept in consideration that these patients are at higher risk of apnea than the normal population, even in the absence of the usual risk factors, in that the stimulation of the vagus could influence respiratory drive and the opening of the upper airway during sleep [22].

Cardiac complications (arrhythmias) are uncommon, with an incidence lower than 1 % [3, 12]. The left vagus nerve has fewer cardiac efferent fibers than the right vagus nerve and this could potentially limit the arrhythmogenic effects of left vagal stimulation. Patients with a history of bradyarrhythmias and asystole are at greater risk of cardiac effects [10].

Given the risk of cardiac and respiratory complications, it is mandatory for potential VNS patients to undergo a careful preoperative screening.

Transient side effects of VNS, such as cough (6–45 %), headache, nausea/vomiting, and neck spasms are usually dependent on the stimulation parameters and occur during stimulus. These effects are reported to be reversible after parameter adjustment and, if persistent, are usually well tolerated by the patients and do not impinge on the efficacy of VNS [24].

Since many of the patients who have undergone VNS for drug-resistant epilepsy are mentally disabled and could not well describe some types of adverse symptoms, their frequency could be underestimated and, in particular, vocal cord palsy and consequent hoarseness could be more frequent, as reported in the literature [12].

A very uncommon surgical complication described in our series is a case of lesion of the thoracic duct with subsequent lymphorrhea; the duct was immediately surgically sealed, without any systemic postoperative complication. To the best of our knowledge, this complication has never been reported in the literature. Possible reasons for the surgical accident might depend on anatomical variants of the vagal nerve and variants of its relationship with the internal jugular vein and carotid artery. In our series we observed two cases of plexiform nerve and one case of vagal nerve positioned lateral and more posterior to the internal jugular vein.

These anatomical variants required a more challenging and extensive dissection of the nerve trunk, to adequately isolate the nerve; in particular in the caudal portion, where it is more likely that the thoracic duct, which runs posterior and medial to the internal jugular vein, below the clavicle, will be found and damaged, .

Other complications, such as lesion of the jugular vein or carotid artery, neck hematoma, dysesthesia due to sensory nerve damage, keloid development, and Horner’s syndrome are reported more rarely, with an incidence of less than 2 % [12].

Hardware complications most commonly consist of breakage of the connecting cable or deterioration of its isolating shield. In both such cases, replacement of the lead is required [1, 2, 6–8, 11–14, 24, 26]. The failure of VNS leads can be determined clinically, with the loss of efficacy of the stimulation, i.e., the recurrence of seizures, and failure may be highlighted by interrogation of the device, verifying the presence of very high lead impedances.