But what then of the ability to recharge? Does this result in significant benefits to the patient? Several factors merit discussion.
Device Size Comparison
First, rechargeable batteries result in smaller volume implantable pulse generators (IPGs). These may result in more comfort for patients with low body mass index (BMI), and intuitively at first glance, the much smaller size will be more attractive to the patient than the current InterStim II IPG. Currently available SNM systems include the recharge-free InterStim II system (14 cm3 volume) and the rechargeable Axonics system (5.5 cm3 volume) . The emerging InterStim Micro technology (2.8 cm3 volume) reduces the size by about 80% when compared with the InterStim II and will be approximately 49% smaller than the current available Axonics rechargeable SNM device.
However, much smaller rechargeable SNM devices will not be a benefit to all patients because about 40% of the global adult population has body weight issues . In a large, multicenter, prospective study of 272 patients with overactive bladder only 7% reported implant site pain with the current recharge-free InterStim II device . For comparison, with smaller, rechargeable SNM devices pain at the neurostimulator site has been observed in less than 2% of the patients . Therefore, although the smaller size of a rechargeable device does matter for some patients, the vast majority of patients will do equally well with the InterStim II device, which is about the size of a heart pacemaker. Moreover, the correct implantation of a small rechargeable device with the necessity for frequent recharges may be more challenging in obese patients, since the angle and distance between the superficially implanted IPG and recharger may change significantly between recharging sessions, thus making the recharge more cumbersome for obese patients. Furthermore, the stability of the IPG inside the fat tissue could be compromised and/or the patient might be more likely exposed to twiddler’s syndrome . The latter occurs when patients manipulate or rotate their device leading to a dislodgment of the leads with subsequent malfunction of the modulation system.
A second issue relates to the requirement for battery (IPG) changes. The battery life of rechargeable devices has been estimated as 15 years compared to the longevity of the current InterStim II (IPG), which in clinical practice is about 5–7 years [11,12,13]. As a result of the claimed longer battery life of a rechargeable IPG, it has therefore been suggested that this therapy is associated with a reduced need for reoperation . This assumption, however, ignores other important facts. For various reasons some patients will not need device longevity of 15 years. In a long-term study of 325 patients with a mean follow-up of 7.1 years it was shown that up to 39% of patients drop out because of loss of follow-up, death, dementia, lack of efficacy, device problems, or infections, thereby eliminating any need for future battery replacements . Thus, life expectancy based on the biological age will be an important determinant when considering rechargeable devices. Furthermore cognition and patient’s dexterity will also be important to consider in the aging population when deciding between rechargeable versus recharge-free systems. Additionally, many revision surgeries are due to lead issues (i.e., lead breakage, lead migration, or loss of effectiveness). In the same large cohort, a new lead was required in 37.8% of patients over a mean follow-up period of 7.1 years . A longer-lasting IPG will have no bearing on lead-related surgeries (although it must be acknowledged that in real-life practice it may often be difficult to predict up-front which patients will need an extended longevity of more than a decade). Although the lead revision rate has been reduced to 13% in other studies, it cannot be neglected . Finally, the recent adoption of an optimized tined lead placement technique allows for lower amplitudes and thus even longer battery life could be expected from rechargeable and recharge-free systems .
Finally, patient expectations of having only one surgery over a period of 15 years for rechargeable devices may also turn out to be an illusion. Battery life may be shorter, if battery fade is taken into account. In that respect different manufacturers may have different technologies  and industry deserves merit for the advancements in battery technology. However, only future observation (long-term studies) can ultimately show if battery fade will eventually be an issue.
Treatment Compliance and Disease Awareness
There are also other less obvious but significant pitfalls associated with rechargeable devices. First, patients must be compliant and have the cognitive capability and the manual dexterity to recharge their IPG on a weekly or biweekly basis  over a period of 15 years. Although the recharging process with modern technologies can be done conveniently at home without being connected to a power socket, the therapeutic non-compliance of patients has been an issue for decades . Typically, the compliance rate of long-term medication is between 40% and 50%, while the compliance for lifestyle changes is low at 20–30%. Since a lack of compliance will lead to a loss of effectiveness and/or an increased burden for the healthcare professionals, careful screening of the patient before implantation of a rechargeable device is imperative. One neuromodulation device manufacturer reported that one of the most frequent patient questions addressed to its helpline related to charging issues in rechargeable devices (Medtronic, private communication). In a well-selected study population (that may differ from real life), patients were recruited and followed in a strict and intensively controlled protocol: 17% of patients did not agree at 1-year follow-up that recharging their IPG was moderately or very easy . In a real-life study with recharge-free SNM devices only 50–63% of the implanted patients had a good understanding of the previous InterStim patient programmer (iCon 3037) or acceptable skills in handling the device . With simpler patient programmers and an increased usage of smartphones (even in an elderly patient population), familiarity with technical devices may improve significantly over time. Nevertheless, recharge-free devices require no regular or frequent interactions with the patient programmer and patients with a poorer compliance may better qualify for recharge-free SNM.
Another factor is one of disease awareness. One of the greatest benefits that patients describe is their ability to forget about their medical condition once an SNM is implanted. A recharge-free system allows the patient to set and forget their SNM system. In a rechargeable SNM system, the patient is reminded of their condition every 1–2 weeks. While it has yet to be studied formally, the authors believe the psychological and patient perception of disease will be experienced differently between rechargeable and recharge-free populations.
Although there are currently no patient preference studies of SNM in terms of rechargeable versus recharge-free devices, some conclusions can be drawn from patient surveys in spinal cord stimulation (SCS) or deep brain stimulation (DBS) therapies, where rechargeable devices have been used for more than 10 years. In a survey of 30 patients with movement disorders visiting a pre-DBS clinic (mean age 65, range 45–79 years), 63% chose the recharge-free device compared to 37% for the rechargeable device, even though the battery longevity of the recharge-free device was estimated at only 3–5 years . In a multicenter, retrospective study with 352 explanted SCS patients it was reported that patients with rechargeable devices terminated their therapy earlier than patients with recharge-free devices . This observation may be consistent with an increased burden for therapy maintenance, which could be related to a higher probability of device removal . Additionally, it seems that industry’s attitude towards rechargeable devices has changed over the years from an initial technical enthusiasm towards a sobering experience with nowadays preferring a more patient-centered approach . Age may be a factor in this. Lam and Rosenow reported that patients in whom the recharging burden outweighed benefits of increased battery life were significantly older (74 years) than those who felt that the tradeoff was worthwhile (56 years) . However, in other surveys an age or gender dependency has not been found .