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Percutaneous electrical nerve stimulation

ISSN 2398-2977

Synonym(s): PENS


  • Trigeminal mediated headshaking is an idiopathic facial pain syndrome in equids.
  • The condition carries a poor prognosis with medical therapy having limited success.
  • PENS therapy offers an alternative and successful therapy for the treatment of trigeminal mediated headshaking.
  • PENS is a minimally invasive neuromodulation therapy that has applications in the human field for neuropathic pain.


  • Sole use for the treatment of trigeminal mediated headshaking Behavior: headshaking and no other conditions.


  • Better success rate than the majority of other medical treatments.
  • Intermittent treatment required rather than continuous medication.
  • Tolerated well with most horses.


  • Incomplete understanding of the etiopathogenesis and therefore treatment success can be limited.
  • Lack of response following treatment in a number of cases.
  • Complication rate of 8.8%, most commonly a transient neuritis usually resolving in 3 days.

Technical problems

  • Relatively simple procedure to undertake once fully trained but it is essential to rule out all non-trigeminal mediated reasons for headshaking.

Alternative techniques

  • Nose nets (70% improvement in about 30% of cases).
  • Medical therapy (gabapentin, carbamazepine, cyproheptadine Cyproheptadine hydrochloride).
  • Electroacupuncture Acupuncture: overview.
  • Platinum coil placement in the infraorbital canal (concerns about adverse effects leading to euthanasia Euthanasia).

Time required


  • 15-30 min.


  • 60 min.

Decision taking

Criteria for choosing test

  • The horse should be diagnosed as a trigeminal mediated headshaker based on exclusion of other causes of headshaking.
  • Horses should show stereotypical signs of headshaking (vertical uncontrolled head tick, rubbing of nose on the ground, etc).
  • A thorough clinical examination should take place prior to administering PENS therapy including upper airway endoscopy Respiratory: endoscopy, ophthalmological examination Eye: examination – direct ophthalmoscopy, exercise testing diagnostic local anesthesia performed bilaterally at the caudal aspect of the infraorbital canal Infraorbital nerve: perineural anesthesia and, ideally, a head CT Computed tomography. Without these, an underlying pathology may be missed resulting in unsuccessful treatment.
Trials with a nose net and NSAID therapy should have been performed prior to treatment.

Risk assessment

  • Horses should be placed in stocks for the procedure and adequately sedated as some horses can violently react at the beginning of therapy. The stocks should not be bolted shut to allow rapid exit from the stocks if required.



Veterinarian expertise

  • Fully trained in EquiPens.

Nursing expertise

  • Fully trained in EquiPens.
  • Competent in administering sedation via an intravenous catheter or direct IV injection.


  • Advisable to have a separate handler for the horse, wearing a helmet.

Materials required

Minimum equipment

  • EquiPens neuromodulation equipment and probes.
  • Ultrasound scanner.
  • Stocks.
  • Headstand.

Ideal equipment

  • Clippers: the procedure can be performed without clipping, however the grounding patch sticks better on clipped skin.

Minimum consumables

  • Catheter and syringes for sedation.
  • Heparinized saline.
  • Sedation (detomidine Detomidine hydrochloride and butorphanol Butorphanol).
  • Probe for the PENS therapy (single).
  • Chlorhexidine scrub.
  • Surgical spirit.
  • 2% mepivacaine Mepivacaine.
  • 2.5 ml syringes and 1.5 cm 25G needles.

Ideal consumables

  • If continuous rate infusion being used, will need 500 ml bag of 0.9% saline and a giving set.



  • Acepromazine Acepromazine may be beneficial in a particularly anxious horse.
  • Loading dose of detomidine Detomidine hydrochloride and butorphanol Butorphanol.
  • Some horses benefit from a continuous rate infusion (CRI). After the loading dose of detomidine and butorphanol attach a 500 ml bag of saline, with 10 mg detomidine instilled, via a giving set; commence infusion at 1 drip per second. 

Site preparation

  • A 5 x 5 cm site dorsal and cranial to each infraorbital foramen should be clipped and aseptically prepared with a chlorhexidine scrub followed by surgical spirit; allow the spirit to evaporate.
  • The aim is to lie the probe, the whole of which is conductive, across the nerve as it exits the infraorbital foramen.
  • Instill 0.25 ml of 2% mepivacaine at the site of entry of the probe; this will be 2 cm caudodorsal to the infraorbital foramen. A small quantity must be used so as to not affect the infraorbital nerve itself.
  • A 10 x 10 cm area over the biceps and triceps should also be clipped on the side closest to the neurostimulator; this second site is for the grounding pad. It is possible for this site to be under where the saddle would fit to make clipping less obtrusive. This site should also cleaned with surgical spirit soaked swabs before allowing it to evaporate.


  • The horse handler should always stand to the side as head movements can be violent, especially when first starting the therapy.
  • When starting the therapy, the horse should be allowed to move its head as it is impossible to stop them throwing their head up.



Step 1 - Preparation

  • Ensure the horse is adequately sedated.
  • Clip and prepare the sites described above.
  • Attach the electrical pad to the site on the triceps or under the saddle (depending on where the clip patch is; occasionally you will need gel to increase electrical contact).

Step 2 - Probe placement

  • All following steps should be performed aseptically.
  • Make a small stab incision using a 14G needle 2 cm caudodorsal to the infraorbital foramen (where the mepivacaine was instilled) to allow easy passage of the blunt probe.
  • The probe should be introduced through the skin at a perpendicular angle until contact with the bone is made. It should then be passed under the nasolevator labii muscle (ensure it does not go through it) and passed over the infraorbital nerve  . The aim is to lie about 1 mm above the nerve.
  • When reaching the nerve some horses will react so care should be taken.
  • The probe placement should be confirmed using ultrasound (≥7.5 MHz linear probe).
  • If the probe is not accurately placed, it should be repositioned.

Step 3 - Stimulation

  • Ensure the system is set to the lowest mV setting.
  • Connect the grounding pad lead from the neurostimulator to the grounding pad, ensuring the metal of the crocodile clip and metal lip of the grounding pad are in contact.
  • Connect the probe to the probe lead from the neurostimulator .
  • Ensure the neurostimulator is positioned close to the horse’s head, as the leads are short, and you need to allow for the horse throwing its head up during the procedure.
  • Hold the wires high so as to reduce the risk of probe removal should the horse move its head.
  • Have the front door of the stocks unlocked in case the horse reacts strongly and needs to be evacuated quickly from the stocks.
  • When switched on, the neurostimulator defaults to 0.5v. You must reduce to the lowest possible (0.2v) before pressing TEST and starting the therapy.
  • Use the test function to ensure a complete circuit.
  • Start treatment for 25 min.
  • The mV should be gradually increased until the horse can just tolerate it although if very heavily sedated this should be done with some caution as it is possible to cause neuritis if overstimulation occurs.
  • The muzzle, and sometimes the ipsilateral eyelid, should twitch when the neurostimulator is stimulating at 2 MHz; the neurostimulator alternates every 3 seconds between 2 MHz and 100 MHz.
  • If there is no twitch, check the plane of sedation and the positioning of the probe.
  • If the horse moves and the probe comes out, simply replace and continue for the remaining period of time.
  • Over the time of the neurostimulation, you usually need to increase the voltage to maintain a twitch. This is better tolerated if voltage is increased when the neurostimulator is at 100 MHz. The twitch only needs to be mild; if too severe the horse will not tolerate the procedure.

Step 4 - Probe removal

  • The stimulation protocol is 25 min; once this has been completed on one infraorbital nerve the protocol is repeated on the other side.
  • Whilst, in theory, the two sides can be done in tandem, this is not recommended as two probes would be needed and the horse would be less likely to tolerate the procedure.
  • The probe should be thrown away following use.


Immediate Aftercare


  • The site should be monitored for any signs of heat, swelling or discharge in case of infection or hematoma although these are very uncommon side effects.
  • If a catheter has been used the site should be monitored for heat, pain or swelling associated with thrombophlebitis.

General care

  • For safety, it is advised to lunge the horse for at least 15 min to assess headshaking signs. If the horse is not headshaking, then the horse can be ridden. This should be repeated each day between the first three treatments.
  • Some horses may benefit from using a nose net after treatment, when previously it did not alleviate clinical signs.
  • If headshaking has resolved the horse can return to ridden exercise.  


Other medication

Wound Protection

  • None as the stab incisions are made with a 14G needle only.

Potential complications

  • Some horses will show worsening signs of headshaking for approximately 1 week and then a gradual improvement.
  • A worsening of the headshaking signs has been reported in one horse which may not have been associated with neurostimulation.

Long term Aftercare

Follow up

  • Repeat PENS therapy should be performed 7 days after the first treatment and then a third treatment approximately 2 weeks after the second unless there has been a dramatic clinical improvement. If there is an improvement the third therapy should be delayed until return of clinical signs.
  • Repeat treatments will be required as the clinical signs return. This can occur within weeks, months or years depending on the case.



  • Rarely local infection or a hematoma can occur at the site of probe placement.
  • 8.8% of cases had the above mild complications but all, bar one, were transient.
  • Incomplete resolution of clinical signs or no improvement.

Reasons for treatment failure

  • Unknown etiology therefore in some instances it may not be the right therapy.


  • Remission after the initial course occurred in 53% of horses.
  • Median time of remission was 9.5 weeks (2 days to 156 weeks ongoing so this may prove to be longer)
  • When signs recurred, most horses would go into a period of remission with repeat treatment.
  • No predictors for outcome were found.

Further Reading


Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Roberts V L H, Bailey M, Equipens™ group et al (2020) The safety and efficacy of neuromodulation using percutaneous electrical nerve stimulation for the management of trigeminal-mediated headshaking in 168 horses. Equine Vet J 52 (2), 238-243 PubMed.
  • Devereux S (2019) Electroacupuncture as an additional treatment for headshaking in six horses. Equine Vet Educ 31 (3), 137-146 VetMedResource.
  • Roberts V L H, Patel N K & Tremaine W H (2016) Neuromodulation using percutaneous electrical nerve stimulation for the management of trigeminal-mediated headshaking: a safe procedure resulting in medium-term remission in five of seven horses. Equine Vet J 48 (2), 201-204 PubMed.
  • Mills D S & Taylor K (2003) Field study of the efficacy of three types of nose net for the treatment of headshaking in horses. Vet Rec 152 (2), 41-44 PubMed.