Percutaneous Electrical Nerve Stimulation for Facial Pain...Percutaneous electrical nerve stimulation (PENS) is a novel, minimally invasive and useful treatment modality. Its use in

AbstractPercutaneous electrical nerve stimulation (PENS) is a novel, minimally invasive and useful treatment

modality. Its use in complex facial pain has been on the rise, and its utility will further increase with the

advances in the technology and renewed interest in the field of peripheral neuromodulation. PENS

therapy can be used both as diagnostic and therapeutic option. The precise mechanism of action is not

known, although a combination of electrical neuromodulation and release of endogenous morphine-

like substance in the central nervous system appears as plausible explanation. We analyse the various

studies in the literature and discuss the Southampton data regarding facial pain treatment with PENS

therapy. We believe that PENS therapy for facial pain and headache is currently underutilised. It is safe,

economical and should certainly be part of the armamentarium in the treatment of complex facial pain

and headache. © 2020 S. Karger AG, Basel

Peripheral electrical nerve stimulation (PNS) to control chronic pain is not novel. The remarkable work of Patrick Wall and William Sweet provided the proof of concept of PNS in 1967, when they stimulated their own infraorbital nerve by a needle electrode, thereby experiencing the effects of electrical stimulation first-hand [1]. Following this landmark work, PNS was enthusiastically employed for chronic pain relief in various forms by dif-ferent workers. The results however were not optimal due to several reasons, and eventu-ally there was a decline in the use of peripheral neuromodulation, despite its awe-inspiring beginning. Recent advancements in technology and understanding the mechanism has

Slavin KV (ed): Neuromodulation for Facial Pain. Prog Neurol Surg. Basel, Karger, 2020, vol 35, pp 1–15 (DOI: 10.1159/000509621)

Percutaneous Electrical Nerve Stimulation for Facial Pain

Girish Vajramani Centre for Functional Neurosurgery, Wessex Neurological Centre, Department of Neurosurgery, University Hospital Southampton, Southampton, UK

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resulted in resurgence of interest in peripheral stimulation. Various devices are now avail-able on the market for both permanent implants as well as temporary stimulation [2]. The field of peripheral neuromodulation is ever increasing. It is important to understand the various nomenclature that is used in this field as there is still lack of standardised terms that are used in publications.

Nomenclature of Peripheral Nerve Stimulation

Peripheral Nerve Stimulation and Peripheral Nerve Field StimulationPeripheral nerve stimulation (PNS) is electrical stimulation of a specific nerve that supplies a very distinct area of body. The stimulation results in changes in the function of the particu-lar nerve. PNS provides unidirectional paraesthesia along that selected peripheral nerve with a better stimulation quality [3]. This can be achieved by open method, wherein the nerve is exposed surgically, and the electrodes are placed overlying it or by a minimally invasive per-cutaneous technique. In case a specific nerve is not stimulated, the procedure is called periph-eral nerve field stimulation or peripheral subcutaneous field stimulation (PSFS).

Percutaneous Electrical Nerve Stimulation and Percutaneous Neuromodulation TherapyPercutaneous electrical nerve stimulation (PENS) and percutaneous neuromodulation therapy involves percutaneous insertion of electrodes for temporary stimulation for a specified period of time. It is similar to TENS – transcutaneous electrical nerve stimula-tion – and electroacupuncture and can be performed in various ways. Acupuncture acts by mechanical stimulation, but electroacupuncture employs electrical stimulation (2–100 Hz) for analgesic effects mediated through opioid receptors. TENS can be applied with either low (2 Hz) or high (50–100 Hz) frequency stimulation on the skin; but not at the same time. PENS therapy utilises both high and low frequencies in a rapidly alternating rhythm to achieve similar effects of stimulation as above [4, 5].

PENS Therapy

There has been a resurgence of interest in the use of temporary peripheral stimulation for short-term control of pain in various neuropathic pain conditions. As it does not involve implantation of permanent and expensive pulse generators, it offers attractive prospects. PENS does not require the complex surgical implantation, since the needle electrode for stimulation is removed after the therapy. It does not demand great technical skills. Be-sides, it can be performed in the outpatient settings. Selection of the area of stimulation is also not particularly difficult since there is often an area of derangement marked out clearly by the patient. PENS therapy can be used both as a therapeutic measure as well as a trial stimulation before PNS, along with electrophysiological studies and nerve blocks to make better selection of indications [2]. The earliest method of PENS therapy involved


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PENS for Facial Pain 3

insertion of electroacupuncture needle-like electrodes [6]. There have been recent tech-nological advancements in the delivery of PENS therapy and probes.

Currently, there are several devices available on the market for PENS therapy. Electroacupuncture PENS (Fig. 1a): It involves insertion of 32-gauge 0.2-mm stainless

steel acupuncture-like needles (Ito Co, Saitama, Japan) to stimulate peripheral nerve fi-bres in the dermatomal distribution corresponding to patients’ pain [4, 6, 7]. The concep-tual basis for PENS is related to both TENS and electroacupuncture. PENS bypasses the resistance of cutaneous nerves and delivers electrical stimulus in close proximity to the nerve endings located in the soft tissue muscle and periosteum of the involved derma-tomes [7]. The stimulation is typically performed at alternate frequency of 15 and 30 Hz, although various low and high frequencies can be used as well [5].

Biowave PENS (Fig. 1b): BioWave Percutaneous Electrodes (Biowave Corporation, Norwalk, Conn., USA) are sterile, single-use and comprised of an array of over 1,000 needles within a 2.5-inch diameter patch. The needles are 0.74 mm in length and feel like Velcro to the touch. Percutaneous electrodes are implanted through the epidermis di-rectly over locations that pain presents and/or over the origin of pain for the duration of the 30-min treatment. The BioWavePRO neurostimulator utilises high-frequency signal-mixing technology to deliver electrical signals through the skin directly to nociceptive pain fibres for inhibiting pain transmission.

BioWavePENS can be used to treat nociceptive as well as neuropathic pain in numer-ous locations including the lumbar and thoracic area of the back, cervical area of the neck,

a

b c

Fig. 1. a Electroacupuncture apparatus (Ito). b Biowave PENS (Biowave Corporation). c Accustim PENS (Bioampere Research) (with permission).


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hip, groin, knee, shoulder, ankle, foot, elbow, wrist and hand. However, it has not been used in facial pain [8]. Besides, the needle is 0.74 mm in length, which means it cannot be used for deeper targets.

Accustim PENS (Fig. 1c): This method of PENS utilises 27-gauge flexible electrodes (P-lead) attached to external neurostimulator (Accustim-P, Bioampere Research, Conselve, Italy). The electrode is inserted via Touhy needle and can be left for the duration of treat-ment. It can be used for both and diagnostic and therapeutic purpose [9]. Currently, the literature available regarding its usage is limited.

Algotec PENS (AlgostimTM, Algotec Research and Development Ltd, Crawley, UK) (Fig. 2): PENS therapy can be used to stimulate a single peripheral nerve, or the most pe-ripheral branches of a peripheral nerve in order to control pain. It uses disposable 21-gauge probes and a stimulation paradigm based on published evidence [10–12]. There are three program options available, program A, B and C. The default program is program C, which is continuous alternating/cycling between program A and program B, alternating every 3 s for a total therapy duration of 1,200 s. Program A has a pulse width of 0.2 ms and fre-quency of 100 Hz, whereas program B has a pulse width of 1.0 ms and frequency of 2 Hz. The stimulation is delivered for 25 min at alternating frequency of 2 and 100 Hz every 3 s (program C). Amplitude varies between 0.1 and 3 V. The cylindrical percutaneous type electrode has improved the access to the sensory afferents in head and face regions as well as extremity peripheral nerves. Algotec PENS is the most common type of PENS therapy in current use and is well suited for use in facial pain.

Mechanism of Action

The precise mechanism of PENS-induced analgesia is not known. It has been speculated that alterations in neural modulation produced by electrical stimulation [7] as well as an increase in endogenous morphine-like substance within the central nervous system (CNS)

Fig. 2. Algotec PENS therapy. Neurostimulator with probes of different length (with permission).


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contributes to PENS-induced analgesia [10, 11, 13, 14]. Studies have shown that elec-troacupuncture-induced analgesia can be blocked by an opioid receptor antagonist [13, 15]. Three types of CNS opioid receptors – mu, sigma and kappa – are important media-tors of analgesia produced by electroacupuncture and TENS [11, 14]. It is also plausible that PENS works similar to spinal cord stimulator and dorsal root ganglion stimulator by involving gate mechanism.

Studies have shown that mixed frequency (alternating 15 and 30 Hz) of PENS is more effective than either low (4 Hz) or high (100 Hz) frequencies alone in improving pain. Us-ing a rat model for studying electroacupuncture, Chen and Hans [14] reported that the dense-disperse mode of electrical stimulation (alternating 2 and 15 Hz) is more effective than a fixed frequency of stimulation at either 2 or 100 Hz. According to Sun and Han [4] and Han et al. [11], the enhanced effect is due to the differing effects of the frequency of stimulation on the pattern of neurotransmitter release within the CNS. At 2-Hz frequen-cy, analgesia was thought to be mediated by stimulation of µ- and δ-opioid receptors, whereas, at 100 Hz, analgesia was reportedly mediated by activation of κ-opioid receptors in the CNS [10, 16]. Hamza et al. [17] using TENS found that mixed-frequency electrical stimulation at 2 and 100 Hz produced greater post-operative analgesic-sparing effect than either 2 or 100 Hz alone.

The number of treatment sessions and period of treatment required to sustain the an-algesia is unclear. A cumulative analgesic effect of PENS has been reported by previous authors [5, 6, 18]. Weiner et al. [19] reported 3-month pain relief with twice-weekly elec-troacupuncture PENS and physical therapy for 6 weeks in patients with low back pain. Yokoyama et al. [18] believe that there may be a cumulative effect as well as a ceiling effect in PENS treatment, although the mechanism is not clear. Accumulation of anti-opioid substance is known to cause tolerance to electroacupuncture in experimental animals im-plicating that long-term PENS therapy in humans can lead to tolerance as well [15, 16]. However, clinical studies so far have not revealed any instances of tolerance to PENS therapy.

PENS for Facial Pain

PENS therapy has been used for facial pain and headache disorders. Algotec PENS in-volves placement of 21-gauge PENS probes (Fig. 2) (Algotec Research and Development Ltd., Crawley, UK) of various lengths in the vicinity of nerves or area of the pain. Typi-cally, a 20-mm probe length is used for facial PENS, whereas for greater occipital nerve (GON) and lesser occipital nerve (LON) a 50-mm probe would be preferable. The probe is attached to the neurostimulator. Stimulation at 100 Hz is initially performed to ascer-tain the proximity of the nerve to the probe. Stimulation is typically carried out at alter-nating frequency (2 and 100 Hz) alternating every 3 s. The amplitude varies from 0.1 to 3 V, and the therapy duration is about 25 min. The stimulation paradigm is based on pub-lished evidence for an increased release of enkephalins, beta-endorphin and endomor-


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phin with 2-Hz stimulation and dynorphin at 100-Hz stimulation, with the combination of the two frequencies accelerating the release of all four [10–12]. PENS for facial pain targets various nerves (Table 1) but can also be used as a field stimulation in the area of pain. PENS for facial pain have been used in various conditions typically causing neuro-pathic pain (Table 2).

Technical Nuances

Greater and Lesser Occipital Nerve (GON and LON) PENS Procedure (Fig. 3)The GONs arise from fibres of the dorsal primary ramus of the second cervical and to a lesser extent from the fibres of third cervical nerve. It pierces the fascia below the superior nuchal along with occipital artery. The emergence point is approximately 2–3 cm below the superior nuchal line and 1.5–2 cm lateral to the midline. It courses medial to the oc-cipital artery. The LON arises from the ventral primary rami of the second and third oc-cipital nerve. It passes along the posterior border of sternocleidomastoid, dividing into cutaneous branches, which supply the cranial surface of the pinna and the lateral poste-rior scalp.

The GON PENS procedure is done in a sitting position with patient’s forehead resting on the padded bedside table. A hand-held Doppler ultrasound is used to locate the oc-cipital artery. The GON usually runs medial to the artery. The point is marked, and a 50-

Table 1. PENS for facial pain – nerves targeted

– Supraorbital/supratrochlear nerve PENS– Infraorbital nerve PENS– Mental nerve PENS– Inferior alveolar nerve PENS– Auriculotemporal nerve PENS– Focal PENS procedures for trigger spots/nummular headache– Greater occipital and lesser occipital nerve PENS

Table 2. PENS for facial pain and headache – usual indications

– Occipital neuralgia– Migraine– Trigeminal neuropathic pain– Nummular headache– Post-traumatic trigeminal neuropathic pain– Post-herpetic trigeminal neuropathic pain– Atypical facial pain– Post-dental extraction trigeminal neuropathic pain– Cluster headache and trigeminal autonomic cephalalgia


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mm PENS probe is then inserted along the occipital nerve site either from medial to lat-eral or lateral to medial. If the LON is to be targeted, then a medial to lateral approach targeting the GON and LON is preferable. If the procedure is being done bilaterally, then it is imperative to measure the distance between the two GONs to decide whether a 50-mm probe is adequate to capture both nerves. If the nerves are further apart, then two probes are needed to stimulate, one for each of the nerves. Once the probe is inserted across the nerve, stimulation is commenced. Initially program A, utilising high frequency, is used to confirm the stimulation-induced paraesthesia across the back of the head radi-ating forwards. If the paraesthesia is adequate, then program C is used to stimulate the nerves between 1.5 to 2.5 volts for 25 min. The probe is then removed. Patients usually experience immediate pain relief, although it can take several days before the relief is noted.

Supraorbital/Supratrochlear Nerve (Fig. 4) and Infraorbital Nerve (Fig. 5) PENS These are generally performed in supine position. The supraorbital and supratrochlear nerves are branches of the frontal nerve, a branch of the ophthalmic division of trigeminal nerve. The supraorbital nerve, which is the larger lateral branch, runs along the supraor-bital artery. A hand-held Doppler ultrasound is used to locate the artery. The nerve is in close vicinity of the artery. The supratrochlear nerve is located at a point where the bridge of the nose abuts the supraorbital ridge.

The infraorbital nerve is a branch of the maxillary division. It runs in the infraorbital groove and exits the orbit via the infraorbital foramen, along with the infraorbital artery. A hand-held Doppler ultrasound is used to locate the artery. The nerve is in close vicinity of the artery.

Fig. 3. Greater occipital nerve PENS therapy.


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A 20-mm probe is preferable for these procedures if the nerve can be localised using the hand-held Doppler ultrasound. However, if it is not possible to locate the nerve, then a 50-mm probe across the area will provide the necessary stimulation.

Mental Nerve/Inferior Alveolar Nerve (Fig. 6) PENSThe inferior alveolar nerve is a branch of the mandibular nerve. After branching from the mandibular nerve, the inferior alveolar nerve travels behind the lateral pterygoid muscle. It gives off a branch, the mylohyoid nerve, and then enters the mandibular foramen. While

Fig. 4. Supraorbital PENS therapy.

Fig. 5. Infraorbital nerve PENS therapy.


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in the mandibular canal within the mandible, it supplies the lower teeth. Anteriorly, the nerve gives off the mental nerve at about the level of the mandibular 2nd premolars, which exits the mandible via the mental foramen and supplies sensory branches to the chin and lower lip. The inferior alveolar nerve continues anteriorly as the mandibular incisive nerve to innervate the mandibular canines and incisors.

The mental nerve can be approached via extraoral or intraoral approach in supine po-sition. A hand-held Doppler ultrasound is used to locate the nerve as it exits the mandible via the mental foramen at the level of second premolar. A 20-mm PENS probe is inserted, and the paraesthesia testing is performed to ascertain the proximity of the probe to the nerve.

Inferior alveolar nerve is approached intraorally at the level of mandibular foramen. A 20-mm probe would be suitable for this approach with the patient supine on a dental chair. It may be advisable to seek help from maxillo-facial surgeon for probe insertion.

Auriculotemporal Nerve PENS (Fig. 7)The auriculotemporal nerve, a branch of the mandibular nerve, courses over the zygoma along with superficial temporal artery. The nerve is deep and posterior to the superficial temporal artery. It courses along the artery and divides into anterior and posterior tem-poral branches.

With the patient supine, a hand-held Doppler ultrasound is used to locate the superfi-cial temporal artery just above the zygoma. The 20-mm PENS probe is then inserted pos-terior to the artery, and paraesthesia testing is done to confirm the proximity to the nerve, before using program C for the treatment.

PENS for Focal Pain (Fig. 7, 8)PENS probes can be inserted in the focal areas of pain such as scar and trigger spots. In our series, we have used PENS therapy to treat nummular headache (Table 3). Nummular headache is a rare condition [20]. It is poorly understood and involves severe pain in the scalp with the pain restricted to a small area the size of a coin. PENS can be used to treat

Mental nerve

Inferior alveolar nerve

Fig. 6. Mental nerve is ap-proached intra- or extra-orally at the level of mental foramen, whereas the inferior alveolar nerve is approached intraorally at the level of mandibular fora-men.


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this pain. The probe, 20 or 50 mm, is inserted either at the periphery of the area of head-ache or within the area of headache and standard stimulation at alternating frequency performed. The results have been variable, with 2 patients obtaining significant pain relief lasting up to 6 months.

Fig. 7. Auriculotemporal nerve and trigger point PENS for post-craniotomy scar pain.

Fig. 8. PENS therapy for nummular headache.


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Results and Discussion

The duration of pain relief with PENS has been variable in the literature. In our own se-ries, 60 patients with an age range of 22–93 years (average, 54 years) underwent 119 PENS procedures. The patients were suffering from various conditions mentioned in Table 3. Repeat PENS therapy was performed in 31/60 patients of which 14 patients underwent twice, 9 patients thrice, 5 patients underwent 4 sessions and 3 patients underwent 5 ses-sions of the therapy (Table 4). The average duration of pain relief following the PENS therapy has been 9 weeks, and the maximum duration 52 weeks (Fig. 9). PENS therapy does not work in all the cases. In our series of 119 PENS sessions, the treatment failed to obtain any meaningful reduction in pain in 19. Very rarely, the pain deteriorates follow-ing the PENS therapy.

Various workers have used PENS therapy for facial pain and headaches. Bhasker et al. [21] have used Algotec PENS therapy in 8 patients with trigeminal pain following cancer treatment. A total of 17 treatments were carried out in these 8 patients. They noted pain relief in 7 patients and the pain relief lasted 14–175 days. In one patient, PENS made no difference to his pain. All 7 patients had good relief from symptoms of allodynia and hy-perpathia allowing them to reduce their opioid medication. The target was diffuse field

Table 3. Southampton data

Diagnosis Patients, n

Atypical facial pain 8Migraine 11Complex headache 1Occipital multiple sclerosis-related central neuropathic pain 2Craniotomy scar pain 1Nummular headache 3Occipital Neuralgia 19Post-herpetic trigeminal neuropathic pain 6Sphenopalatine neuralgia 1Post-traumatic trigeminal neuropathic pain 7Cluster headache 1

Total 60

PENS repeats Patients, n PENS procedures, n

1 29 292 14 283 9 274 5 205 3 15

Total 60 119

Table 4. Southampton data


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stimulation with the aim of stimulating the trigeminal ganglion, sphenopalatine ganglion and trigeminal nerve branches. No attempt was made to target the individual nerve branches.

Cobianchi et al. [22] used Algotec PENS treatment in 10 patients with occipital neu-ralgia. All patients noted reduction in pain, and the effect lasted up to 6 months.

Weatherall and Nandi [23] reported on 36 patients with various headache syndromes where Algotec PENS therapy was used. In 6 out of 9 chronic cluster headache patients, there was significant improvement. PENS therapy converted chronic cluster headache to episodic cluster headache, and the benefit lasted for almost 2 years. The duration of pain relief following PENS therapy has ranged from no relief to 6 months of pain relief. In only 3 out of 14 migraine patients was there significant improvement. In 6 patients with chron-ic migraine, PENS therapy resulted in some exacerbation of the pain. The outcome in other headache syndromes has not been reported.

Simpson and Nannapaneni [12] reported a 70-year-old woman with severe neuro-pathic pain in the left side of face and scalp which had developed soon after an opera-tion on the ipsilateral temporomandibular joint 20 year earlier. She underwent 26 ses-sions of Algotec PENS therapy over 5 years and 2 months. The PENS probe was in-serted at the site of the greater auricular nerve and the LON over the left mastoid region. The stimulation was felt throughout the area affected by pain. The degree of reported pain relief increased over the first eight sessions (from 60% to 80–100%) and the duration of relief increased over the first three sessions (from 3 weeks to a maxi-mum of 9 weeks).

0PENS sessions (n = 119)

Dur

atio

n of

pai

n re

lief,

wee

ks

15

30

45

60

Fig. 9. Duration of pain relief following PENS therapy (average, 9 weeks).


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Rossi et al. [24] treated 25 patients with occipital neuralgia and 12 patients with post-herpetic trigeminal neuropathic pains. They noted complete disappearance of pain fol-lowing PENS therapy in 11 patients (6 occipital neuralgia and 5 trigeminal neuropathic pain), and the pain relief lasted about 6 months.

Ahmed et al. [25] reported on the use of electroacupuncture PENS treatment for man-agement of headache. They randomised 30 patients with headache to receive PENS or “needles alone” according to a crossover study design. They found PENS therapy to be significantly more effective in decreasing VAS scores. PENS therapy also produced great-er improvement in patient’s physical activity and quality of sleep.

Algotec PENS has also been used in veterinary practice. Roberts et al. [26] have used Algotec PENS in management of trigeminal-mediated headshaking, which is thought to be a disease akin to trigeminal neuralgia. Six of the 7 horses showed a positive response, and the median remission time increased with repeat procedures with almost 20 weeks of remission time with fourth PENS treatment.

The success of PENS therapy depends upon various factors. Robust patient selection is extremely important. PENS generally works if the pain is restricted to dermatomal region and based on specific nerve pathway. Anecdotal observation has been made of poor re-sponse to PENS if the patient has concurrent infection/inflammation such as sinusitis, dental infection etc. at the time of the procedure. We would recommend avoiding the procedure until the infection is treated. Rossi et al. [24] believe that finding a trigger spot is important as the pain relief obtained immediately after PENS therapy relies on this. It is also important to ensure that the PENS probes are close to the nerve. Paraesthesia test-ing using programme A of the neurostimulator device at 100 Hz is advisable to confirm the proximity of the probe to the peripheral nerve or its branches. The action of PENS on the peripheral sensitisation of nociceptors is a primary element of the beneficial effects of PENS on peripheral neuropathic pain. The length of the probe could correlate to a posi-tive outcome. The longer the probe the greater is the area of stimulation. However, if the probe is targeted and placed close to the nerve using ultrasound or Doppler ultrasound, then the length may not matter. In their series pain relief lasting almost 6 months could be achieved with single application of PENS. We however feel that multiple applications are needed to achieve a greater therapeutic result, although the exact mechanism of this is not clear.

Adverse effects of PENS therapy have been reported but are extremely rare. In one of our patients, there was severe pain at the site of the dental plate following PENS therapy. We would recommend that the patients remove any dental plate prior to the procedure. In some patients, worsening of the pain has been reported. Reported incidents in the lit-erature include needle site haematoma and contralateral dysaesthesia in a patient treated for intercostal pain [24]. PENS therapy is contraindicated in patients with pacemakers, metallic implants, neuromodulation devices and epilepsy.


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Conclusion

PENS therapy is effective and less expensive treatment option for facial pain. It has been used in various facial pain conditions as well as headaches. Though the effect is not long lasting, it can provide temporary pain relief and alongside analgesics can be a useful tool in the management of the patients with complex facial pain.

Conflict of Interest Statement

The author has no conflicts of interest to declare.

Funding Sources

None.


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19 Weiner DK, Rudy TE, Glick RM, Boston JR, Lieber SJ, Morrow LA, Taylor S: Efficacy of percutaneous electrical nerve stimulation for the treatment of chronic low back pain in older adults. J Am Geriatr Soc 2003; 51: 599–608.

20 Pareja JA, Caminero AB, Serra J, Barriga FJ, Baron M, Dobato JL, Vela L, Sanchez del Rio M: Numular head-ache: a coin-shaped cephalalgia. Neurology 2002; 58:

1678–1679.21 Bhasker A, Patel K, White A, England J: Trigeminal field

stimulation using percutaneous electrical nerve stimula-tion (PENS) therapy – Preliminary case series of 8 pa-tients. Neuromodulation 2011; 14: 558.

22 Cobianchi MR, Cesaroni A, Colini Baldeschi G: PENS therapy for the treatment of intractable occipital neural-gia. Preliminary data. Neuromodulation 2014; 17:e18.

23 Weatherall MW, Nandi D: Percutaneous electrical nerve stimulation (PENS) therapy for refractory primary headache disorders: a pilot study. Br J Neurosurg DOI: 10.1080/02688697.2019.1671951.

24 Rossi M, De Carolis G, Liberatoscioli G, Lemma D, Nosella P, Nardi L: A novel mini-invasive approach to the treatment of neuropathic pain: The PENS study. Pain Physician 2016; 19:E121–E128.

25 Ahmed HE, White PF, Craig WF, Hamza MA, Ghoname ES, Gajraj NM: Use of percutaneous electrical nerve stimulation (PENS) in the short-term management of headache. Headache 2000; 40: 311–315.

26 Roberts VLH, Patel NK, Tremaine WH: Neuromodula-tion using percutaneous electrical nerve stimulation for the management of trigeminal – mediated head shak-ing: a safe procedure resulting in medium-term remis-sion in five of seven horses. Equine Vet J 2016; 48: 201–204.

Girish VajramaniCentre for Functional Neurosurgery, Wessex Neurological CentreDepartment of Neurosurgery, University Hospital Southampton NHS FTMailpoint 101, Tremona Road, Southampton, SO16 6YD (UK)girish.vajramani @ uhs.nhs.uk


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